Chem 112b- Spectroscopy and Analytical Chemistry Flashcards
1
Q
what can spectroscopy determine?
A
the arrangement of atoms, bond lengths, angles and strengths
2
Q
what real world procedure can be accomplished due to spectroscopy?
A
MRI uses same technology as NMR, the technology can be used in imaging, PET scans and X-rays.
3
Q
what is spectroscopy in terms of radiation?
A
the study of the interaction of EM radiation with matter
4
Q
what is electromagnetic radiation?
A
a form of energy consisting of oscillating electric and magnetic fields, which are orthogonal to each other and the direction of travel.
Low frequency= long wavelength for example, microwaves and radio waves. High frequency= short wavelength for example, gamma and x-rays.
5
Q
what is the equation relating speed of light, wavelength and frequency?
A
c (speed of light) = wavelength (λ) x frequency (v)
c= m/s, wavelength = m, frequency = Hz (s^-1)
6
Q
draw out the order of the electromagnetic spectrum, with approximate wavelengths relating to each unit
A
7
Q
which colour of visible light displays the lowest energy and which one displays the highest (relate them to frequency + wavelength)
A
red light is lowest energy therefore lowest frequency and highest wavelength
violet light is highest energy, therefore highest frequency and lowest wavelength
8
Q
what does a monochromatic wave mean?
A
single wave, all photons have identical energy
9
Q
order electronic transitions, vibrational transitions, electron spin, rotational transitions, and nuclear spin in order of highest to lowest energy and for each give the radiation type associated with them
A
- electronic: UV and visible
- vibrational: infrared
- rotational: microwave
- electron spin: microwave
- nuclear spin: radiowave
10
Q
describe what wave-particle duality is
A
while we say EM radiation is a wave, there are other situations where it is better described as a particle. Planck proposed that EM radiation can only be transferred in discrete packets or ‘quanta’ of energy called photons.
this is known as wave-particle duality- EM radiation is a wave (light shadows) and also a particle (photoelectric effect)
11
Q
how can you calculate energy of a photon from its frequency and wavelength?
A
E = freq. x wavelength = (freq. x c) / wavelength
we can think of a ray of light as a stream of photons with the energy related to the frequency, and the intensity related to the number of photons.
Brighter light is not more energy but simply more photons.
12
Q
why is energy sometimes referred to as eV?
A
electron volt- energy relevant to charge on electron
13
Q
describe emission and absorption spectrums
A
atomic absorption and emission spectra are ‘line spectra’ and demonstrate the quantisation of energy.
only specific wavelengths of light are absorbed or emitted as electrons move between energy levels- only certain electronic energies are allowed
14
Q
if a unit was in grams, how much would you have to multiply it to get to: kilograms (kg), megagrams (Mg), gigagrams (Gg), and teragrams (Tg)?
A
kilo: x10^3
mega: x10^6
giga: x10^9
tera: x10^12
15
Q
if a unit was in litres, how much would you have to multiply it to get to:
deci, centi, milli, micro, nano, angstrom, pico and femtolitres?
A
deci: x10^-1
centi: x10^-2
milli: x10^-3
micro: x10^-6
nano: x10^-9
angstrom: x10^-10
pico: x10^-12
femto: x10^15
16
Q
what is the equation for energy relating freq., c and wavenumber?
A
E= freq. x c x wavenumber (v with a squiggle above it)
17
Q
describe what transitions between energy levels are
A
an atom or molecule can absorb a photon of EM radiation and be raised from one energy level to another. this only happens if the photon energy corresponds to the exact energy difference between the 2 energy levels (E=hv).
Different types of molecular transition involve different energy gaps, therefore absorbing different parts of the EM spectrum.
18
Q
draw a diagram showing how electron, vibrational and rotational energy levels relate
A
19
Q
what does (delta) E mean?
A
energy between translational levels
20
Q
why do electrons move faster than nuclei?
A
they’re lighter
21
Q
describe the born-oppenheimer approximation
A
a molecule will possess different forms of internal energy- translational, rotational, vibrational and electronic energies, which are ALL QUANTISED.
because atoms and molecules exhibit wave-particle duality, we can solve the Schrodinger equation which links the wave functions that describes the molecule to its energy (E).
however, electrons move faster than. nuclei so an electronic transitions occurs within fixed nuclei. vibrations in bonds occur on a shorter timescale than rotations, which is in turn shorter than translations.
Hence, the Born-Oppenheimer approximation allows us to treat each form of energy independently:
Total energy: electronic + via + rot + trans
22
Q
how does wave-particle duality give rise to quantisation of molecular energy?
A
for rotational, vibrational and electronic transitions, the differences between the energy levels are such that we can observe the transitions and they give rise to spectra.
while these transitions occur in different regions of the EM spectrum, you can have simultaneous transitions (e.g. via fine structure can be seen in UV-vis spectra sometimes).
translation is also quantised, however the energy gaps are so small that this is essentially a continuum
22
Q
what is the basic feature of all forms of spectroscopy in terms of how the sample is prepared and how a result is gathered?
A
the sample is irradiated with EM radiation and the energy gradually changed. whenever the energy exactly corresponds to the difference between 2 energy levels, radiation may be absorbed (E=hv).
Lowest energy state= ground state, higher one= excited state.
23
Q
which 2 ways can a spectrum obtained from absorption spectroscopy be displayed?
A
either an absorption spectrum (like UV-vis
0 or a transmission spectrum (like IR).
an alternative form of spectroscopy is emission spec, where radiation is emitted when a species in an excited state falls back down to the ground state- this is how NMR is produced.
24
describe how a single-beam spectrometer works generally + state one issue
in a single-beam one, a wavelength sector (diffraction grating) selects individual wavelengths that pass through the sample and the detector produces an electrical signal which is in proportion to the intensity of the radiation coming through the sample.
one issue can be background absorption, for example the solvent being using in UV-vis, or atmospheric CO2 and H2O vibrations in IR spectroscopy. this is compensated for by first running a background scan, before subtracting it from a sample scan
25
describe how a double-beam spectrometer generally works + state what dispersive and continuous means
the incident radiation is split into 2 paths. one of them passes through a reference cell, while the other path passes through the sample cell.
the difference between the 2 beams is recorded by the detector and the spectrum recorded eliminates any background absorptions.
this type of instrumentation is called dispersive or continuous as the spectrum is recorded by varying the wavelength of radiation and recording the output from the detector as a function of wavelength.
it's more common today to see Fourier transform (FT) instruments where the entire frequency range is recorded simultaneously. they're more sensitive, and the spectra is quicker to record.
26
what is the position of a peak determined by?
the energy of the transition
27
the intensity of a peak is related to a number of absorption (or emission) events which are determined by:
1. the concentration of the sample
2. the distance the radiation travels through the sample (path length)
3. how many molecules within the sample are of the correct energy state to absorb or emit at that frequency (population of energy levels)
4. how likely the transition will occur (probability)
28
what is the Boltmann distribution thing purpose?
to see the number of molecules in the correct energy state.
it applies to any system at thermal equilibrium
29
what is the purpose of the Beer-Lambert law?
to see the effect of concentration and distance in a spectrum sample
30
what are allowed and forbidden transitions?
allowed: only certain transitions between certain energy levels can take place
forbidden: low probability of occurring
31
what are selection rules?
most molecules have a large number of energy levels and in principle, a large number of different transitions are possible. however not all of the possible transitions give rise to peaks in the spectra.
selection rules tell us which transitions are allowed.
32
what are gross selection rules?
Gross= molecule need permanent dipole moment.
Gross selection rules say what properties of the molecule are needed for it to absorb the particular type of radiation, e.g. for rotational, the molecule must have a dipole
33
what are specific selection rules?
say which energy levels transitions are allowed between (usually related to a change in quantum number), e.g. rotational spec have a change of +/- 1
34
what does absorption of a photon result in, how is it affected + how does it relate to Boltzmann distribution (degeneracy)?
the promotion of a molecule from a lower to higher level. the more molecules in the lower level relative to the higher level, the more likely it is that a photon will be absorbed and the molecule promoted .
the relative population of 2 energy levels is described by the Boltzmann distribution (degeneracy means how many states exist in the same energy)
35
what is the Boltzmann distribution equation?
36
what does it mean if (delta) E is small, and what if it is big?
is small, the value of the exponent becomes lose to 1 and the ratio of the populations is approximately equal (NMR + insensitive technique)
if large, the ratio of the populations is now close to 0 and hence, very few molecules are in the upper level (UV-vis, sensitive technique)
37
what 3 factors are true for a system at thermal equilibrium at room temp?
1. all atoms and molecules are in the electronic ground state
2. most molecules will be in their ground vibrational state with a small percentage existing in the 1st vibrationally excited state
3. a range of rotational states are populated, and the rotational ground state is typically not the most highly occupied level due to degeneracy
38
what does the Boltzmann distribution say about states of unequal energy?
they will be unequally populated and at higher temperatures, there will be more population of higher energy states (and therefore, less population in the ground/ lower energy states)
39
how does population size affect peak intensity?
the bigger the difference in population between 2 levels, the stronger intensity the peak will be, and the less sample is needed (more sensitive)
40
describe laser light
laser (light amplification by stimulated emission of radiation) light is:
1. monochromatic
2. coherent (photons all have the same phase)
3. emitted in a narrow beam in one direction
4. intense short pulses
41
how do lasers exist?
because molecular energy is distributed among discrete energy levels i.e it is quantised!
42
lasers work through population inversion, what does this mean?
atoms are promoted to level 4 before rapid vibrational decay to level 3. light emission occurs to level 2 before fast vibrational decay back down to level 1.
43
where does infrared radiation lie on the electromagnetic spectrum?
between visible light and microwave regions
44
which state changes are photons in the mid-infrared range associated with?
vibrational state changes (E= 100-1000 cm^-1).
If we absorb infrared radiation, we can move between different vibrational states.
45
photons in the mid-infrared range region have energies of 10^-19- 10^-20, convert this to wavelength, wavenumber, and frequency
wavelength: 2.5-25 um
wavenumber: 4000- 400 cm^-1
frequency: 10^13- 10^14 Hz
46
what is vibration in terms of bonds?
the motion of atoms in individual chemical bonds the can be approximated by a spring joining the 2 atoms.
47
what kind of transitions does vibrational spectroscopy involve + what can it be used to identify?
Vibrational spectroscopy involves transitions between quantised vibrational states due to the absorption of IR RADIATION- it can be used to identify functional groups, confirm the identity of compound and determine concentration.
From the spectra, the vibrational frequencies and force constants of the bond are found.
48
are IR spectra recorded as absorption or transmission spectra, and in which units?
transmission- recorded in wavenumber (cm^-1)
49
what do the peaks (dips) in IR spectra correspond to?
a particular bond (functional group) absorbing IR radiation
50
what is wavenumber proportional to?
energy- E=hcv
51
describe how older IR spectrometers worked, and how is it still relevant today
they worked on the principle of transmission- the sample was held in a sample holder (Cell) and IR radiation was passed through. this is still how the spectra of gasses are recorded.
the cell cannot be made from glass as this absorbs the IR region, leading to additional bands in the spectrum.
52
what kind of cells are liquid or gas samples held in for IR spectra, and why?
cells with windows made from alkali metal halide plates (most often KBr), as metal salts such as NaCl or KBr absorb outside the range of most organic compounds.
they can be compressed under high pressure to make plates that are transparent to IR radiation.
53
how are solid samples prepared or IR spectroscopy?
the solid must be ground up and mixed with Nujol to create a 'mull' (thick suspension) which is placed between 2 KBr disks.
Nujol is a mineral oil with a long hydrocarbon chain, and has a relatively simple IR spectrum with absorptions between 2800-3000 + 1300-1500 cm-1 due to C-H stretches and bends.
54
why must samples recorded using a Nujol mull be stated that Nujol was used?
so the peaks corresponding with Nujol (2800-3000 + 1300-1500) can be ignored.
55
describe the ATR (Attenuated Total Reflectance) technique used for modern IR spectrometers
it is where IR radiation passes through a crystal at such an angle that it is totally reflected from the face. the sample is pressed tightly onto the crystal which is made from a material with high refractive index such as germanium.
some of the radiation passes through the surface of the crystal and interacts with the sample, being absorbed by the bonds, before passing to the detector. this technique can be used for liquids or solids ground into a fine powder.
56
why must a background scan be recorded first for IR spectrometers?
they're single beam
57
describe the only vibration possible for a diatomic molecule
stretching/ compressing, known as the oscillation of the bond. it can be considered as a spring connecting 2 atoms stretching about an equilibrium position corresponding to an equilibrium bond length r0.
58
how does mass of atoms + stiffness of spring affect oscillation of the spring between?
the greater the mass, the slower the spring will oscillate. if the spring is stiffer, it deforms less easily + vibrates faster.
59
what can the natural form/ length of a bond be described as?
equilibrium bond length
60
what is the stretching/ compressing motion of a spring between atoms also called?
simple harmonic motion
61
why does a spring vibrate between atoms?
there is a restoring force when it is displaced from its equilibrium position
62
what is the simple harmonic oscillator (SHO) proportional to, and which law does it give rise to?
the restoring force is proportional to the displacement (how far we've moved it) + gives rise to Hooke's law:
F=-kx, where k is the force constant of the bond (characteristic of different bonds, 'stiffness')
63
how can the potential energy of the simple harmonic oscillator be found?
integrating the force expression, hence the potential energy curve is a parabola of the type y= x^2.
64
what is the expression of the frequency of the spring's vibration as a harmonic oscillator?
reduced mass is used since the oscillation frequency depends on the atoms masses.
65
what does the reduced mass equation simplify to with a homonuclear diatomic like Cl2?
u= 1/2 m(Cl)
66
for a heteronuclear diatomic like HI, m(I) is over that of m(H), and u ~ m(H), what does this mean?
during a vibration, the heavy atom stays almost still, while the light one does almost all the moving
67
by solving the schrodinger equation for SHO, what do we find (+ equation if u can)?
vibrational energy is quantised.
V= vibrational quantum number (not freq.). It can be 0 or any positive integer.
Ev= the energy of the Vth level
68
draw a diagram of quantisation of vibrational energy levels just like generally
Energy levels evenly spaced.
69
describe the ground state in a vibrational energy level diagram
there is no energy at the bottom of the well. the ground state is V=0 + has a 0-point energy of 1/2 hv.
Even at absolute 0, the bond still vibrates, however classical mechanics would say the 0-point energy is 0 which is wrong.
Vibrational levels for a diatomic molecule are non-degenerate (g=1), only one state per Ev.
70
what is the equation for vibrational energy in terms of force constant + reduced mass of the diatomic molecule?
71
describe the consequence of quantum mechanic tunnelling in terms of vibrational energy levels + the pictures shown
the solutions to the particle in a 1D-box (left) don't exactly match the solutions shown on the right. the wave functions have non-zero values outside the potential energy well.
this is a consequence of quantum mechanic tunnelling, where there is a probability of finding a particle in a region that is forbidden by classical mechanics by penetrating the barrier (potential well walls).
this doesn't occur in the particle in a 1D-box approximation as the well walls are infinity
72
what is the gross selection rule for IR spectroscopy?
there must be a change in dipole moment of the molecule during a vibration. for diatomic molecules, this means there molecule must possess a permanent dipole.
homonuclear diatomic such as H2 don't have a dipole moment, so don't give IR spectra (but can be seen thru a complimentary technique called Raman spectroscopy for vibrational).
73
what is the specific selection rule for IR spectroscopy?
transitions can only take place between adjacent levels such that V= +/- 1.
due to the whole Ev= (V+1/2)hv formula, the energy gap spacing between adjacent vibration levels is constant at hv.
hence you would expect the same peak in the spectrum regardless of the fact transition. for most molecules at room temperature, only the V=0 state is populated.
74
why is the peak seen in IR spectra for diatomics is always due to the V=0 + V=1 transition?
e.g. HCl.
the IR spectrum of HCl has a peak at 2990 cm-1 which corresponds to the transitions between the V=0 + V=1 vibrational levels. E= 5.94 x 10^-20 J
with the Boltzmann formula, the relative populations of these levels are 5.3 x10-7.
Hence, at room temp, virtually all HCl molecules are in the ground vibrational state, with only 1 in every 2 million in the V=1 state.
75
one of the major applications of IR spectroscopy is measuring the force constant (k) of bonds (the stiffness). it is related to the bond strength. what else does it depend on?
electron density between the atoms, it can also give information about the nature of the bond
76
changing the mass of one of the atoms will change the reduced mass u, but what about the force constant k?
since the force constant is related to the electron density between the atoms and isotopes only differ in the number of neutrons, the key assumption we make is that the force constant would not change during an isotopic substitution.
77
convert a frequency of 6.42 x10^13 Hz to a wavenumber
c= freq. x wavenumber ---> wavenumber= c/ freq. = (3x10^8)/ (6.42x10^13)= 1/214000
x10^2= 2140 cm-1
78
what is the true variation of potential energy with bond length
+ describe with a diagram?
Morse potential- not an exact parabola as or the SHO approximation.
At low vibrational quantum numbers, the 2 curves match, but at higher quantum numbers the Morse potential is anharmonic- this is a consequence of if the atoms are pulled far apart enough apart, the bond would break.
this also means that the vibrational levels are not equally spaced, the spacing decreases with higher quantum numbers, such that the levels converge.
79
describe overtone peaks in IR spectra?
the specific selection rule for SHO that V= =/- 1 still applies for anharmonic true potential, hence the peaks we observe are due to V=0 + V=1 transition.
however, in real systems, it is possible to see overtone bands corresponding to V=0 + V=2, or even V=0 + V=3 transitions. these occur at almost 2x and 3x the fundamental frequency of the V=0 to V=1 transition
80
a recorded IR spectrum of HCl shows a peak at 2886 cm-1, corresponding to the fundamental frequency (V=0 to V=1 transition).
at what frequency would you expect the overtone band for the V=0 to V=2 transition to occur at? also, why would it not appear as that in reality?
2886 x 2= 5772 cm-1, as it'll occur at approx twice the fundamental frequency.
however, the first overtone peak is actually 5668 cm-1, due to the anharmonic nature of the potential energy curve, meaning that the vibrational energy levels get slightly closer together as the energy increases
81
what are the 3 vibrational modes (basically different movements it can do) of a linear molecule e.g. CO2?
82
what are the 3 vibrational modes (different movements) of a bent molecule such as water?
83
the gross selection rule for IR spec that the vibration must cause a change in the dipole moment of a molecule can be used to aid in structure determination e.g. cis/ trans of [PdCl2(NH3)2].
describe the stretches + resulting peaks of both the cis and trans isomers for the molecules
for the cis, both the symmetric and asymmetric Pd-Cl stretches cause a change in the dipole moment of the complex- 2 Pd-Cl stretching peaks.
for the trans, only the asymmetric stretch causes a change in the dipole moment of the complex- only 1 Pd-Cl stretching peak seen.
same effect for cis/trans alkenes.
84
how do you calculate the number of vibrational modes for linear and non-linear molecules, and why?
linear: 3N-5, rotation along the bond axis doesn't change the atom positions so only 5 modes subtracted.
non: 3N-6, if we just want vibrations we must subtract the translations and rotations along the x,y,z axes (-6 modes)
85
are asymmetric stretches higher than symmetric ones for vibrational levels?
yes, they are also both higher than bends for wavenumber/ energy
86
how many vibrational modes are expected for HCl?
linear, so (3x2)-5= 1 (oscillation)
87
how many vibrational modes are expected for CO2?
(3x3)-5=4
88
how many vibrational modes expected for benzene?
non-linear: (3x12)-6= 30
89
would you see all vibrational modes possible on an IR spectra, and why(2)?
no- some vibrational modes will not be IR active (no change in dipole moment), and some modes will be degenerate (occur at the same energy- 1 peak seen)
90
which bond does a medium 1650-1800 cm-1 stretch on an IR spec correspond to?
C=O stretch, but also C=C and C=N
91
what does a short 2000-2300 cm-1 stretch on an IR spec correspond to?
nitrile or C (triple bond) C
92
which bonds do stretches from 2850-3100 cm-1 correspond to on an IR spec?
C-H stretches.
before 3000, it'll most likely be an alkyl stretch.
after 3000, it'll most likely be an alkenyl stretch
93
which bond does a broad stretch from 3200-3400 cm-1 correspond to?
O-H bond
94
which bonds does a medium stretch from 3300-3500 cm-1 correspond to?
N-H, however alkyne C-H stretches can occur here around 3300 cm-1
95
how do O-H bonds in alcohols compare to O-H bonds in carboxylic acids on an IR spec?
in alcohols, the stretch is typically well separated from an C-H stretches.
in carboxylic acids, the O-H stretch occurs at a slightly lower wavenumber and often overlaps with C-H stretches, due to the effect of resonance delocalisation within the carboxylic acid unit
96
what are the position and line shape of O-H and N-H stretches sensitive to on an IR spec?
the extent of hydrogen bonding present e.g. conc and temperature, as H-bonding changes the length (hence, strength) of the bond.
the stronger the H-bond, the longer the O-H/N-H bond
97
why do different functional groups give different stretching frequencies on an IR spec?
the wavenumber is determined by the energy separation of the vibrational energy levels. both the bond strength (force constant) and reduced mass determine the position of the band.
98
do strong bonds (e.g. multiple bonds) absorb at high or low wavenumber like above 1600?
above
99
why do bonds involving light atoms (e.g. H) absorb at very high wavenumber like above 2500 cm-1?
the very small reduced mass
100
below which wavenumber do single bonds not to h give stretches?
1400 cm-1
101
around which wavenumber do bending modes occur on an IR spec?
1600 cm-1
102
does intensity of an IR band increase or decrease the more polar the bond is?
increasing intensity e.g. a C=O peak is more intense than a C=C peak
103
the probability of a vibration absorbing a photon is related to what?
the change in dipole moment that occurs during the vibration
dipole moment= charge x distance
104
why do more polar bonds result in a more intense band on an IR spec, and why do non-polar bonds not?
the polar bond creates an oscillating electric field as it vibrates, which can couple with the electric vector of a passing photon of the same frequency + give a mechanism for photo absorption- gives an intense band
in contrast, a non-polar bond creates little or no oscillating electric field as it vibrates. therefore, a passing photon (even of correct energy) has a low probability of being absorbed- giving a weak or even absent band.
105
would the fingerprint regions of propan-1-ol and propan-2-ol be different?
yes
106
give 2 real-world applications of IR spec
1. aiding in determination of different types of plastic in the ocean or different types of fabric from mixed samples to help with better separation for recycling. e.g. difference between polystyrene + polyethylene
2. forensic science- e.g. examination of inks (document forgery) as well as biological samples such as sweat prints + blood stains, fibre identification + paint analysis (e.g. car accidents).
107
describe how IR spec is used in breathalysers and such
test for conc of ethanol in the blood.
the intensity of the C-H stretching vibration at 2950 cm-1 is recorded from the suspect's breath is compared to a blank sample of air with no alcohol.
other organic compounds also absorb at 2950 cm-1 (e.g. propane in breath) but these can be subtracted and accounted for using other peaks (e.g. C=O).
108
which transitional state changes is microwave radiation associated with?
rotational state changes (delta E= 1-10 cm-1)
109
why is pure rotational spec only obtained in the gas phase?
molecules in the gas phase are free to rotate and hence, we can cause transitions between quantised rotational energy states by absorbing EM radiation.
so, pure rotational spectra are obtained only in the gas phase as in the solid phase, molecules aren't free to rotate and in the liquid, collisions occur too frequently to allow the molecules to rotate freely enough (heat up instead).
110
what is rotational spectroscopy used to determine?
bond lengths as well as identification
111
for linear motion (straight line), how do you define kinetic energy from mass and velocity?
energy= 1/2 m x v^2
112
for rotational motion (in a ring), how do you define rotational energy from moment of inertia (l) and angular velocity (curly w)?
energy= 1/2 I w^2
113
what is the equation for moment of inertia, and therefore rearranged equation for rotational energy?
where r0= fixed bond length
114
what would the mass of a 12-C atom be in kg?
the atomic mass is 12 g mol-1, so would be 12 x10-3 kg/mol.
then divide by avagadros, to get 1.993 x10^-26 kg
115
what are the 2 factors affecting peak position on an IR spec?
1. Bond strength (k)
2. Reduced mass
Both together: energy separation of vibrational energy levels. If reduced mass is small, the wavenumber will probably be very high.
116
what does it mean when a diatomic molecule is treated as a rigid rotor?
means the bond length is fixed and doesn't change during rotation.
rotation occurs about the centre of mass (CoM)
117
how does classical mechanics describe the mechanics of a diatomic molecule considered as a rigid rotor, and why is it wrong xxxx
they are identical to those of a single particle of mass u rotating on the surface of a sphere of radius r0 at an angle (phi).
this description shows there are no restrictions on the rotational energy i.e. the rotational energy isn't quantised.
this isn't true and we must apply quantum mechanics
118
what occurs when we cause a molecule to rotate?
a transition between rotational energy states
119
what is the simplified equation for Ej in rotational energy levels?
B= rotational constant in Hz.
120
each rotational level has a degeneracy, what does this mean and what is the formula?
number of states with the same energy.
g= 2J+1
121
the J=0 level in rotational spec has 0 rotational energy, how does this contrast with IR and vibrational?
the 0-point energy is non-0
122
what is the gross selection rule for rotational spec?
for a molecule to exhibit a pure rotational spectrum, it must possess a permanent dipole moment. this allows the EM radiation to exert a force on the diatomic and cause it to rotate.
123
which of heteronuclear and homonuclear diatomics show pure rotational spectra?
heteronuclear ones, as they possess a permanent dipole moment
124
what is the specific selection rule of rotational spec?
only deltaJ +/- 1 transitions allowed.
+1= absorption, -1= emission
125
are rotational spectra recorded as absorption or emissions spectra?
absorption
126
describe how a rotational state population is temperature dependent with Boltzmann
the energy difference is much less than kBT at room temp, hence the population ratio is largely determined by the degeneracy ratio. as J increases, g increases, but so does deltaE (which eventually becomes bigger than kBT)- it is temp dependent
hence, the intensities of lines increases, passes through a maximum, then decreases and the population of CO molecules is spread across many levels.
127
what is the difference between the way rotational and vibrational levels generally look on a graph?
Rotational diverges, whereas the vibrational levels converge (2 hb difference every time though).
128
explain the appearance of this rotational spectrum of a heteronuclear diatomic such as CO
radiation is absorbed when it corresponds to the exact energy gap (delta E) between adjacent rotational states where delta J +/- 1.
due to quantisation of rotational energy levels, absorptions occur at 2hB, 4hB, 6hB, etc- energy difference between 2 adjacent states is always 2hB, meaning we see regularly spaced absorptions and due to Boltzmann population formula, we see rise, peak and fall in intensity
129
how do we calculate bond lengths with rotational spec?
delta E= 2hB, and delta E= hv, so B=v/2.
wavenumber needs to be converted to frequency first tho with the given equations (ignore random calculations they're not there)
130
would a pure rotational spectra for a linear polyatomic molecule e.g. HCN look similar or different to ones for diatomics?
similar
131
would a non-polar polyatomic molecule such as carbon dioxide give rotational spectra?
no, there is no permanent dipole moment to interact with the EM field.
132
what can you deduce from vibrational-rotational spectra?
bond length, moment of inertia and force constant
133
what kind of spectra can the effects of rotational transitions within vibrational states be seen with?
HIGH RESOLUTION infrared spectra
134
what do vibrational-rotational ir spectra look like kinda (hint: high resolution)?
molecules can vibrate and rotate simultaneously, and each vibrational level has rotational levels within it- hence we can see equally spaced absorptions corresponding to V=+/-1 and J=+/-1 simultaneous transitions.
135
how did rotational spec help in understanding CFCs?
rotational spec can be used to identify species in interstellar space using radio-telescopes, and quantitative analysis of atmospheric gases.
emission of CFCs was observed at frequencies around 280 GHz, corresponding to a rotational transition of ClO.
the emission has a width and shape dependent on temp and pressure, collisions of ClO with nitrogen and oxygen molecules broaden the line shape.
as atmospheric pressure decreases with altitude, the shape is different at different altitudes (broader at high pressure = low altitude)
136
what kind of transitions is visible light radiation associated with?
electronic state changes
137
which unit is peak position in UV-vis spectroscopy recorded in?
nanometers
138
is UV radiation higher energy than visible light?
yes (lower wavelength)
139
what is another term for electronic spectroscopy?
UV-vis spectroscopy
140
not just organic molecules can undergo electronic spectroscopy, what else can and what would their absorption be due to?
transition metals e.g KMnO4 and CoCl4-. their absorptions will most likely be due to d-d transitions and charge transfer transitions
141
if a yellow colour was observed, which colour would've been absorbed by the compound?
violet
142
the a blue colour is observed from an object, which colour must it have absorbed?
orange
143
what is usually on the axes of absorption spectra?
absorbance of light (arbitrary units on the y-axis) against wavelength (x-axis)
144
which 2 important pieces of data can we obtain from an absorbance spectrum?
peak wavelength + absorbance at the peak wavelength
145
what is the basic equation for transmittance?
It/ I0 (lower intensity emitted/ incident radiation)
146
give an equation describing the non-linear relationship between absorbance and transmittance
A= log (I0/It)
147
what is the equation for the beer-lambert law?
E= molar extinction coefficient- how effectively the compound absorbs light at a particular wavelength. (mol-1 dm3 cm-1)
148
describe what a graph of absorbance vs conc will look like
initially linear but then at higher concentration it will become non-linear. typical of A values higher than 1.
149
how is the beer-lambert law useful?
it can be used to determine the unknown concentration of a solution by comparing it to standard solutions of the same compound (calibration curves and such)
150
draft a graph of absorbance and conc, and label the limit of detection, limit of linearity, dynamic range and detector overload
151
are most UV-vis spectrometers single- or double-beam, and what does this mean for the process of gaining a spectrum?
single-beam instruments
this means the solvent must be recorded first, before the sample in the same solvent is then recorded.
the solvent background is then subtracted to give the true sample spectrum
152
when recording a UV-vis spectrum, why must the value of the solvent be subtracted first?
this will absorb light too (although they're typically in the UV-region)
153
why is cuvette choice important for UV-vis spectroscopy?
plastic ones are cheap and disposable, but can only be used between 380-780 nm as the plastic absorbs UV and near-IR radiation.
quartz ones are transparent to UV-radiation so can be used at lower wavelengths, but they are much more expensive and NOT disposable
154
how do photons contribute to a UV-vis spectrum being seen?
UV-vis spectra are observed due to transitions between electronic energy levels (atomic or molecular orbital) when a photon with the exact energy matching the energy gap is absorbed (quantisation).
Energy gap is typically 200 kJ/mol (using Boltzmann) meaning all molecules are in the ground electronic state.
155
draw a little diagram showing how we can represent the 2 electronic energy levels in UV-vis spectroscopy using an anharmonic Morse potential energy curve for each level, explain a little about it
Promotion of an electron to a higher energy level often changes the geometry of the molecule (typically the bonds become longer as anti bonding orbitals are being populated)- equilibrium bond distance in the 2 levels is different.
156
when an electron is promoted to a higher energy in UV-vis spectroscopy, which state does it go from and to?
excitation always occurs from the ground vibrational state of the ground electronic state.
157
how do broad peaks usually form on UV-vis spectrum?
so when electrons are excited they go from vibrational to electronic, but the molecule can be promoted to a number of vibrationally excited states within the 1st electronic- a number of different absorptions with slightly different energy will be seen on the spectrum, and if they're close enough in energy they'll just merge.
158
what is the Franck-Condon principle?
the absorption of a photon occurs on a much shorter timescale than that for nuclear motion.
basically, electronic transitions are faster than nuclear motion because nuclei are heavier i.e. electronic transitions will occur with stationary nuclei.
159
how does the structure of an atom change with the absorption of a photon?
it first changes the electron distribution around the atoms, which then respond by moving to new equilibrium positions (vibrational relaxation).
in some cases, enough energy can be put in to break the bond (photodissociation)
160
describe which orbitals an electron would be promoted to if it were in: sigma, pi, non-bonding from the ground state?
sigma- sigma antic-bonding (high energy, low wavelength)
pi- pi anti-bonding (lower energy, longer wavelength)
non- pi anti-bonding (low energy, long wavelength)
in reality, sigma- sigma AB are too high energy to be seen on a UV-vis spectra
161
what do the following transitions usually show up as on a UV-vis spectrum: non-pi AB, pi- pi AB, non- sigma AB
non- pi AB usually show up as around 10-100 M-1 cm-1 in carbonyls
pi- pi AB usually show up as carbonyls, alkenes, aromatics etc, over 10,000 M-1 cm-1.
a non-sigma AB is way weak, usually forbidden, but it could be scanned so idk
162
describe what a chromophore is?
the part of a molecule that is responsible for the absorption of a photon. typically a conjugated system or functional group containing pi-bonds or lone pairs.
due to Huckel theory- increased conjugation (more alternating single/double bonds) leads to longer peak wavelength and E values
163
as conjugation increases, what will happen to a HOMO->LUMO sigma-> sigma AB gap?
decreases.
for conjugated polymers, a band gap is created
164
how is optical spectroscopy used in medicine with pulse oximeters?
they're devices which fit onto a patient's fingertip or ear lobe and measures their pulse rate and conc of oxygen in the blood.
'a percentage of Hb oxygenated' reading is given.
165
describe what vibrionic transitions are
some molecules (such as poly aromatics) have large enough spacing between their vibrational energy levels in UV-vis to give individual peaks for the different transitions.
166
how can rotational fine structure be obtained from vibrational?
if higher resolution spectrometers are used along with gaseous samples.
167
how can vibrational energy level spacing be obtained from vibronic transitions in UV-vis spec?
calculating wavelength difference between the peaks
168
draw the general positions of naphthalene, anthracene and tetracene on a UV-vis spectra
169
what is the spin selection rule for electronic spectra, and why is there no gross one?
an allowed transitions is between the spin states of the same multiplicity (delta S= 0).
no gross one as all atoms and molecules give rise to electronic spectra
170
describe what spin multiplicity is
its relate to the number of unpaired electrons, where the multiplicity is given by (2S+1) where S is the total spin of the system.
basically number of unpaired electrons (with the same spin) +1
171
give an example of a molecule with a triplet ground state (UV-vis)
oxygen
172
describe briefly the lore of the symmetry selection rule for electronic transitions, and why sigma-sigma AB things are allowed but not non-sigma AB in carbonyls
symmetry of the molecular orbitals- the sigma and sigma AB alkene orbitals are in the same plane and when an EM field interacts with the electron in the bond it causes an oscillation between the 2 orbitals which promotes the e-.
for the oxygen lone pair (non-bonding), its technically a sp2 hybrid orbital, which is perpendicular to the plan of the pi-star alkene orbital. so the electron can't be pushed.
173
when molecules absorb photons they go into a short excited electronic state, which really quickly loses the excess energy and returns the the electronic ground state. why and how does that relate to fluorescence and phosphorescence xxx
more commonly by vibrational relaxation during collisions with other molecules and the energy ends up as heat.
however, in some systems, excited state molecules also lose energy via the emission of a photon, called fluorescence if from a singlet excited state, or phosphorescence if from a triplet excited state.
174
what is fluorescence in terms of like photons
the emission of a photon in a singlet state. takes around 10^-15 seconds.
the emission always comes V=0 level of the first excited state, typically not the state that the electron is promoted to, so vibrational relaxation occurs before photon emission
175
describe what Stokes' shift is and how it relates to absorbance and fluorescence
the absorbance and fluorescence emission spectra are usually mirror images, especially when the vibrational fine structure is resolved within the electronic bands e.g. anthracene.
the shift between the absorbance and fluorescence maximum is due to a Stokes' shift (the exact wavelengths difference will be solvent dependent)
176
how are fluorescence emission spectra recorded- how is it different to UV-vis?
with a fluorimeter spectrometer.
similar because single beam and everything but here, the detector is at right angles to the light source (erroneous), but in normal absorbance its direct
also no background recorded
177
what is phosphorescence in terms of photons and such
the emission of a photon from a triplet excited state.
takes around 10^-3 s.
the photon emitted is of a lower energy than the photon absorbed + lower energy than any fluorescence emission.
as most excited states are singlet, a process called intersystem crossing has to occur to generate a triplet one.
178
some molecules show fluorescence and phosphorescence emission. how xxxx
phosphorescence can be recorded on a fluorimeter in 'phosphorescence mode', as phos is way slower and can usually be hidden under any fluorescence that occurs, so the machine is set up only to record spectra after a long enough time that the fluorescence has decayed.
179
what kinda diagram shows like all of the different photo physical processes together (on same energy level)
jablonski xxx
180
draw a diagram depicting ms (spin quantum number): the +1/2 spin and -1/2 spin
181
how can nuclei have a spin state?
Like electrons have a spin (+1/2 or -1/2), protons and neutrons do as well, so nuclei have a spin state (can also be +1/2 (spin up) or -1/2 (spin down).
the nuclei spin quantum number= I
182
what would be the integer value for the nuclei spin of 12-C, 6-Li, 1-H, and 13-C?
183
how do nuclei, with their spin states, come to have an energy gap?
with no magnetic field, the 2 spin states are degenerate.
a spinning charge has an associated magnetic field. when it's placed into an external magnetic field, it will either align with or against it.
this causes the energy of the 2 spin states to split= energy gap.
184
describe how NMR spectroscopy works
a compound is placed into a strong external magnetic field, if the compound has nuclei with unpaired spins, 2 spin states with different energies are created.
the sample is irradiated with EM radiation of the appropriate energy, to promote the nucleus from the lower energy spin state to the higher one.
when the source of applied EM radiation (radio frequency) is removed, the nucleus relaxes back down, releasing another radiofrequency, which is detected to give a signal
185
why are powerful magnets needed for NMR spec?
the energy differences between the nuclear spin states are very small, so powerful magnets are needed to give noticeable difference
186
for NMR, which kind of energy on the EM spectrum do the nuclear spin state changes correspond to?
radio waves. long wavelength= small energies
187
why is it easier to record a H-NMR spectrum than a C-NMR one?
like 99% of naturally-occurring C is 12-C, but the integer thing is 0 so not NMR active.
only like 1% of C is 13-C, which is NMR active with I=1/2.
however, over 99.9% of H is 1-H and NMR active.
hence why its easier, less sample is required + the scan is quicker
188
what would a value of 1 mean if you calculated Boltzmann distribution for 2 spin states in NMR, and what does this mean about how easy it is to find a transition?
equal population between the 2 spin states, so very low probability of observing the transitions + very sensitive equipment required.
189
which is more sensitive, NMR or UV-vis?
UV-vis wayyyyy more
190
if the magnetic field is really large, will the value of delta E be larger or smaller in NMR spec?
larger
191
if the magnet is aligned exactly with the magnetic field, and if the magnet is aligned exactly against the field, which will be of the lowest or highest energy?
if the magnet is aligned exactly with the field= lowest energy
if the magnet is aligned exactly against the field= highest energy
192
what does the Larger frequency mean in terms of nuclear spin?
according to quantum theory + quantum physics and all that, the nucleus can only align at certain quantised angles to the magnetic field.
the nucleus spins like in a spinny top way about the magnetic field.
193
what is resonance?
transition from lower to higher state when energy is absorbed to promote the nucleus (only when EM radiation matches Larmor frequency).
194
is NMR an absorption or emission spec thing?
emission
195
what needs to happen experimentally for solid-state NMR to be possible?
most samples need to be dissolved in a deuterated solvent, as protons in h atoms will mix with the protons in the sample + fuck with the spectrum
196
why do solvent residual peaks show up?
even with the deuterated solvent, a small %. will still be the non-deuterated version e.g. CHCl3, and hence, a residual solvent peak is observed (7.26 ppm for CDCl3)
197
describe what y (gyromagnetic ratio) is in terms of nuclear spin states
the value is different for different nuclei, so the energy difference depends on the specific nucleus involved.
we can also see that the energy difference is directly proportional to the magnetic field strength (B) + gyromagnetic ratio (y), with the equation shown
198
why do different proton environments exist in a molecule?
different H-nuclei in a molecule experience slightly different magnetic fields + will absorb slightly different energy at different frequencies.
this is because the precise magnetic field experienced by an atom in a molecules is influenced by neighbouring atoms + is therefore no longer exactly equal to the applied magnetic field B
199
how does shielding come about in nuclei for NMR?
the applied magnetic field induces the electrons in the molecule to circulate, generating a small local magnetic field which acts against the applied magnetic field.
the overall magnetic field experienced by the nucleus is reduced + is said to be shielded.
200
what does shielding depend on?
the electron density around the nucleus.
the precise value of the energy absorbed is then very sensitive to the electron density around the nucleus, which depends on its position in a molecule.
201
what does an electron -donating group do in terms of shielding?
an electron-donating group near an H nucleus increase shielding. the nucleus feels a smaller magnetic field and hence, the frequency needed for the spin energy gap is reduced (nucleus is shielded)
202
what does an electron-withdrawing group do in terms of shielding?
it decreases shielding. the H nucleus feels a greater magnetic field and hence, the frequency needed for the spin energy gap is increased (nucleus is desuhielded)
203
what is the use of tetramethylsilane (TMS) in NMR?
it is a reference compound- the spectra are recorded as the difference in energy between the particular nucleus and the reference compound (chemical shift).
204
how does the structure of TMS make it an epic reference compound?
it has 12 1-H nuclei all symmetric, strongly shielded + the single is well separated from most organic compounds. 1 intense peak, gives a chemical shift value of 0 ppm.
205
what does energy to bring a nucleus into resonance depend on?
applied magnetic field strength, + hence operating frequency
206
epic equation
207
why is the proton on the -OH the furthest left?
all groups experience slightly different local magnetic fields + are shielded to different extents.
the H connected to O is the most deshielded (highest chemical shift, downfield), as its' the closest to the electronegative oxygen, which withdraws electron density away
208
why do OH bonds not really split in NMR spec?
O-H bonds don’t really split because of lone pairs on the oxygen and fields and stuff shielding the hydrogen from anything.
209
what decides chemical shift mainly?
electronegativity
210
what is integration in NMR?
the area under each peak, related to the number of nuclei giving rise to it.
it is a RATIO, not the absolute number
211
multiplicity of peaks in NMR is due to spin-spin coupling, what does this mean?
the magnetic fields of different nuclei interacting with each other if they're close enough (usually 3 bonds or less).
it is transmitted through the electron density in the chemical bonds joining the nuclei.
212
C-NMR is way less sensitive than H-NMR, what does this mean in terms of its ability to couple + integrate?
no coupling + no integration, due to the decoupling effect
213
describe what SNR (signal to noise ratio) is in terms of NMR
n= number of scans performed, which are then averaged to give the resulting spectrum.
with more scans, SNR increases which is good because more intense peaks, but this takes longer.
214
how many scans does a typical H-NMR + C-NMR use, and what does this mean in terms of SNR?
usually 16 scans, giving a 4 times increase in SNR ratio.
usually thousands of scans for C-NMR due to lower abundant isotope
215
NMR spec for inorganic ions can also be shown, how would the P-NMR + F-NMR look for the ion [PF6]-?
216
what is an absolute technique?
measured response is directly related to the measured variable (no calibration needed)
217
what is an instrumental technique?
measured response is arbitrary, requires prior calibration with a standard e.g. UV-vis.
218
label this epic diagram with the labels sample, analyte, matrix
219
what is a selective method?
a technique that responds only to the analyse, as properties + species in the matrix can interfere with and influence analysis
220
what is a blank measurement?
basically a control experiment, similar to a background scan for spectroscopy like a scan of air for IR
221
what does carrying out analyses on lods of replicate samples ensure?
reproducibility
222
what is the difference between accuracy + precision?
accuracy- how close is the result to the true value
precision- how close are the numbers to each other
223
what is the difference between systematic + random errors?
systematic- affects all readings in the same way e.g. incorrect calibration or equipment setting. affects ACCURACY
random- happens in all experiments + causes results to scatter. affects PRECISION, usually called anomalies.
if the errors are truly random, the results follow a normal distribution
224
which % of measurements will lie within 2 standard deviations of a mean?
95%
225
a measure of looking at the spread of data is to consider coefficient of variation (CV), what is it defined as?
ratio of standard deviation to the mean.
226
what is the equation for coefficient of variation (CV)?
227
what is limit of detection, limit of quantification, limit of linearity + dynamic range?
228
what is the equation of limit of detection (LOD)?
response to 0 analyte concentration (a) + 3 standard deviations (need to use the equation for standard deviations given)
229
how do you do a type of calibration called standard addition?
a standard at fixed intervals of concentration are added to the sample + the measured response recorded. the data is plotted + line of best fit extrapolated back to the x-axis intercept.
the x-axis intercept= unknown concentration
230
what is the equation for H+ for strong acids?
[HA]=[H+]
231
what is the equation for a weak acid's [H+]?
square root of Ka x [HA]
232
if pH=-log[H+], what is the equation for pKa?
pKa= -logKa
233
if an acids pKa is very negative and small, is the acid strong or weak?
strong
234
draw a titration curve for a strong acid + strong base
235
what does a neutral pH mean?
[H+]=[OH-]
236
draw a titration curve for weak acid + strong base
237
draw a titration curve for a weak base + strong acid
238
what are indicators?
weak acids or bases which exhibit colour changes on protonation/ deprotonation
colour change= end point (same as equivalence point)
239
what is a buffer solution?
it resists change in pH when small amount of acid/ base are added- for an acidic buffer, it is made from a weak acid + the salt of a weak acid e.g. ethnic acid + sodium ethanoate.
240
how do you get [H+] in a buffer solution?
[H+]= (Ka x [HA])/ [A-]
241
draw up an titration curve for a weak acid/strong base curve, but with an acidic buffer
generates a curve with an equivalence point above 7 due to salt hydrolysis.
242
what would a titration curve for a poly functional acid look like (multi-hydrogen acids)?
number of equivalence points= how many hydrogens there are
243
what are potentiometric methods?
electrochemical methods which measure concentration from cell potentials
244
does the liquid phase the 2 electrodes are between in potentiometry need to be aqueous or not?
doesn't need to be but usually is
245
what does a pH meter consist of + how was it calibrated?
a probe immersed in a solution + connected to a high-resistance voltmeter which measures the voltage produced by the probe
the meter is calibrated with 3 buffer solutions of known pH
246
how would you measure pH with a pH probe with like silver + silver chloride?
it has a tube containing 2 electrodes.
1: silver wire coated with solid AgCl immersed in a saturated Cl- solution= reference Ag/AgCl electrode
2: glass electrode also containing the Ag/AgCl inside.
H+ from the solution diffuse to the outermost layer of the glass membrane, influencing arrangement + movement of Na+ ions in the glass, chasing charge distribution= induces a membrane potential.
membrane potential creates a potential difference between the electrodes, measured by the voltmeter.
the bigger the [H+] difference inside + out the membrane, larger potential difference between the electrodes
247
what is an ion-selective electrode (ISE)?
these are electrodes which respond selectively to a particular ion in solution (H3O+ for the glass electrode).
248
what is the key component of ion-selective electrodes?
the ion-selective membrane- this can be made out of different material (doped inorganic crystals, polymers, etc), but will contain an ion-selective ionophore
249
what is the active ion-selective reagent called in ion-selective electrodes?
the ionophore
250
how would a fluoride electrode (solid-state electrode) work?
the barrier is a crystal of LaF3. selectivity is due to the crystal structure of Laf3- other anions such as Cl- + Br- are too big to fit into the lattice. F- ions can fit into the structure.
there is a difference in ion concentration on each side of the barrier
251
how does a calcium electrode (liquid-membrane electrode) work?
consists of a polymer film containing a compound that allows the selected ion to be transported into the film to generate a membrane potential
252
what are ion-selective electrodes used for in terms of pH, and how are they displayed?
used similarly to pH meters to display the concentration of an ion e.g. pF = -log[F-]
they're usually displayed with calibration graphs of V vs log[ion]
253
name an application of ion-selective electrodes
measuring partial pressures of things like carbon dioxide in blood
254
the affinity of receptor molecules in ion recognition systems (ligands) is defined by a binding constant K, what is the equation?
K = [complex] / [ion] [ligand]
255
describe how crown ethers form a ring of polarity with the atoms
crown ethers. the lone pairs on oxygen atoms form a ring of 'negative polarity'
256
how are crown ether molecules named?
described like X-crown-Y, where X= total numbers of atoms + Y=number of heteroatoms e.g. 12-crown-4
257
how does rigidity of macrocycles in ligands affect stability of the complex?
if its more rigid, the binding to the ion in the middle is way stronger + the complex is more stable (higher logK value)
258
what are the 2 distinctive regions of ionophores?
1. ion-selective molecular cavity, based on a geometric arrangement of atoms with lone pairs (like O). they can be cyclic (crown ethers) or acyclic
2. hydrophobic groups- prevents the ionophore dissolving out the PVC polymer matrix + into the sample .
usually long aliphatic chains
259
what does a PVC membrane do for ion-selective electrodes?
binds to Ca2+ ions from the analyse solution and transports them into the membrane
260
what is conductimetry?
where a product is formed which is more weakly ionised than the reactants
261
what is the unit of conductivity?
measured in S cm-1
262
sketch what a cheeky graph looks like for a conductimetric titration with the pH probe + stuff for a strong acid/strong base, describe why it look like that x
before the equivalence point there's loads of H+ ions, making water (more weakly ionised). excess OH- ions after it.
263
sketch a graph for a conductimetric titration between a weak acid/ strong base, describe it x
shallow increase before the end point because of buffering effect of weak acid (conjugate base ions forming).
after the equivalence point its the excess oH- ions
264
sketch a graph for a conductimetric titration with a strong + weak acid/ strong base
2 endpoints
Mixture: initial sharp decrease= strong acid being used up, then strong acid endpoint. Still have weak acid in there tho, 2nd endpoint= weak acid being used up. There will be a big shoot-up after with excess OH- ions.
265
what is chromatography?
methods that separate out the components of a mixture by their affinity to either the mobile phase or stationary phase
can also involve identifying them (mass spec), or determining their conc (UV-vis)
266
for pen inks for example, if the component has higher affinity for the mobile phase (liquid), will it be carried higher up or not?
it will be carried up the paper + appear higher up
267
there is a constant for how much a solute will be adsorbed to the stationary phase, what is it?
K = [stationary phase]/ [mobile phase]
268
what are the stationary and mobile phases for paper chromatography?
stationary= water absorbed in cellulose fibres
mobile= solvent in which paper dip; solvent rises up the paper
269
what are the stationary + mobile phases for TLC?
stationary= silica-backed plate- POLAR
mobile= solvent in which plate dips- NON POLAR
270
what is the equation for Rf value?
Rf = distance moved by spot/ solvent front
271
describe column chromatography
basically same as TLC, but upside down I fear.
the sample is loaded at the top of the column onto the silica stationary phase, and then the mobile phase is added to the top + runs down
detectors can be placed at the bottom of the column to record when the samples are eluting
272
what are gradient/ isocratic elution in chromatography?
gradient: altering the mobile phase composition, helps when the components have different Rf values, allows good separation without taking ages so more cost effective
isocratic: mobile phase remains same
273
how are silica column chromatography + TLC used together?
TLC first to find a solvent system which separates out the reaction components, before the column is performed to separate them out.
274
describe the basic instrumental set up + the steps of how GC + HPLC work
1. instrument delivers a constant flow of mobile phase
2. sample is injected into mobile phase
3. mobile phase carries the sample over the stationary phase + the sample components interact to differing degrees + are eluted at different times
4. detector produces a signal proportional to the analyte's concentration
5. signal feeds directly into a computer to calulcate the concentrations
275
what is tr (retention time)?
time taken from when the mixture was injected for the component to reach the detector
it is a characteristic of a particular compound under a states set of conditions- can be used for identification
276
what is resolution in terms of a chromatogram?
how well 2 adjacent peaks are separated
<1 = not good. 1-1.5= partially. >1.5= well resolved
277
what shape are the peakson a chromatogram described as?
Gaussian
278
what is the equation for resolution with chromatography?
279
what is tm (minimum retention time)?
time take for an un-retained component to pass through the column + be detected.
e.g. here, methane has no affinity for the stationary phase + is un-retained
280
what is tr' (adjusted retention time)?
additional time (beyond tm) that a solute requires to travel through the column
tr' = tr - tm
281
the measure of column efficiency is called the effective number of theoretical plates (Neff), what is the equation for this?
smaller number of theoretical plates= been eluted faster (basically shorter column)
282
which kind of analytes can be used for GC?
volatile ones- can be vaporised + transported by a gaseous mobile phase
the gas must be gaseous or volatile enough to be transformed into the gas phase by heating
283
would low or high polarity compounds be suitable for GC?
low to moderate polarity
284
do compounds used in GC need to be thermally stable?
yes
285
will a high density or low density gas give better separation in GC?
high density, but slower
286
describe the steps of how GC is performed
a VERY SMALL amount of sample is injected into a heated zone where it evaporates into a stream of inert gas, which flows over the stationary phase contained in a column in a heated oven.
287
how does the polarity of compounds used in GC affect how much sample is allowed?
if its more polar, less sample is allowed- more polar compounds may be less thermally-stable + less volatile, hence why liquid chromatography may be more suitable in some cases
288
in GC, which 2 things is separation determined by?
boiling point + affinity, but mostly boiling point because of the heated oven
when the analytes have similar polarity, volatility (boiling point) will determine the elution order.
where analytes have similar boiling points, affinity for the stationary phase will determine the elution order
289
will most volatile compounds elute first or last in GC?
first
290
polychlorinated biphenyls (PCBs) are widely used for GC, but concerns were raised about toxicity so they're banned, why are they used?
break down very slowly
291
why will more polar analytes be have a greater affinity for the liquid stationary phase in gas-liquid chromatography?
the liquid stationary phases are commonly based on long chain siloxane polymers, with cahins of alternating Si + O atoms. they are polar.
the R group on the compound will determine the polarity of the column.
292
why does the liquid polymer need to be stable + non-volatile in gas-liquid chromatography?
so it doesn't get carried with the carrier gas
293
mixed polymers can be used as a stationary phase for GC, what does the polarity depend on?
proportions of each co-polymer
294
why is it cool to change the stationary phase sometimes in GC separation?
changes the elution order + time of separation
295
how does temperature programming help GC (3)?
1. optimum separation temp for high, low + intermediate volatility compounds
you can start with low temp, end with high temp + use a linear temp ramp
296
why is HPLC used instead of GC sometimes?
it's for compounds that aren't volatile, but the samples must be soluble in the mobile phase
297
what is separation of compounds determined by in HPLC?
affinity
298
what is needed in a HPLC column to achieve good separation (3)?
a very large solid surface area + high pressures + room temp (don't have to be thermally stable)
299
how much analyte is needed for HPLC?
only a little bit like GC, but depends on the extinction coefficient of the analyte
300
what are the units of chromatograms obtained from HPLC?
absorbance against time
301
what is usually the HPLC stationary phase?
based on silica, so polar compounds will have a higher affinity for it
302
what is the polarity of the compounds analysed in HPLC?
non-polar/ moderately-polar
303
will non-polar or polar compounds elute last in HPLC?
non-polar ones will elute last
304
which compounds is reverse-phase HPLC usually best suited to in terms of polarity?
polar ones
305
why do we ideally want sharp intense peaks in HPLC?
affects detector sensitivity
306
what does a fronting peak look like?
307
what does a tailing peak look like?
308
how does a fronting peak occur with HPLC?
usually be the column being overloaded
309
how does a tailing peak occur in HPLC?
usually when some sites on the stationary phase retain the solute more strongly than others e.g. polar analytes adhering to the polar stationary phase
310
what can be varied in HPLC to speed up elution?
mobile phase velocity
311
some epic similarities + differences between HPLC + GC
312
which technique can be coupled with GC OR LC as a detection method?
mass spec- allows separation + identification of the sample components
called GCMS or LCMS together
313
what does mass spectrometry tell us about a compound (3)?
Mr, molecular ion fragmentation, relative isotope abundance
314
describe the 4 main components of a mass spectrometer
1. sample inlet- where sample is injected
2. ion source- produces molecular ions from the sample + accelerates ions so that they all have the same kinetic energy
3. analyser- this separates ions based on their mass-to-charge ratio
4. detector- records the m/z value + relative abundance of each ion
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does mass spec have low or high sensitivity?
very high (sub-picogram level)
316
how do magnetic analyser machines separate out ions based on their m/z ratio in mass spec?
a magnetic field is applied to the machine which deflects the ions towards the detector. the magnetic field strength is varied to deflect ions of different mass
317
are smaller or larger ions deflected more in mass spec?
smaller m/z ions are deflected more (small m/z= light ion, high m/z= heavy ion)
318
what is the most common type of analyser in mass spec?
time-of-flight analyser
319
in a time-of-flight analyser, do high m/z ions move slowly or quickly?
slowly
they travel through a drift tube + their times of arrival at the detector are recorded.
320
how are samples made into positive ions for mass spec?
electron impact ionisation- it is bombarded with a high energy beam of electrons to produce positive ions
321
do EI mass spectra have high or low fragmentation?
very high
322
what happens in chemical ionisation for mass spec?
sample molecules are mixed with an excess of reagent gas (NH3/ CH4), which is ionised by EI and some other stuff, which transfers a proton to the sample molecule
forms a quasi molecular ion [M+H]+
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is chemical ionisation a hard or soft technique?
soft, as an electron isnt removed from the sample, a proton is added
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is there low or high fragmentation with chemical ionisation?
low, as its a soft technique
325
what are the differences between EI + CI mass spec in terms of: how big the molecular ion peak is, fragmentation + what the molecular ion peak means
EI = M+, so molecular ion peak is the mass of the compound
CI = [M+H]+ quasi molecular ion, mass seen on spectrum is 1 Da above the actual one
326
describe MALDI (matrix-assisted laser desorption ionisation)
the matrix efficiently absorbs the laser light, while the sample is ionised
suitable for biopolymers
multiply charged ions often seen + very little ion fragmentation
327
what happens in electro spray ionisation (most popular ionisation technique for mass spec)?
little bit of sample in solution pumped through a steel nozzle. intense electric field disperses the liquid into a spray of +ve-charge droplets where the sample has picked up protons from the solvent
soft ionisation technique
suited to polar molecules
328
are +ve or -ve ions produced in ESI?
both. ES+ is more common, to be seen in ES- mode, the compound must be able to hold a negative charge
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is ES+ ionisation, positive ions are created. how will the molecular ion peak correspond to the mass of the compound?
the molecular ion peak will be 1 Da above the actual mass
330
in ES- ionisation, negative ions are created= [M-H]-. how will the molecular ion peak correspond to the actual weight of the compound?
the molecular ion peak will be 1 Da below the mass of the compound
331
epic thing showing which technique will be suited in mass spec
332
do multiply charged peaks occur at lower or higher m/z ratios?
lower, usually in MALDI + ESI
333
what is the base peak in mass spec?
highest one, usually a fragment for EI but may be the molecular ion in sifter stuff like CI + ESI
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there will usually be a peak at +1 Da next to the molecular ion peak in mass spec, why?
presence of the 13-C isotope
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how do isotope patterns work in mass spec e.g. with Cl + Br?
since the ratio of 35-Cl: 37-Cl isotopes are 3:1, there is a 3:1 ratio of molecular ions separated by 2 Da.
79-Br: 81-Br are in a 1:1 ratio, so there are 2 M+ peaks in a 1:1 ratio
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what does low + high resolution mean in mass spec?
low: m/z to the nearest Da/ 1 d.p.
high: m/z to 4 d.p.
337
how do multiply charged. ions affect peak spacing in mass spec?
the top will have like the recorded spectrum on the thing, and the bottom is a stimulated spectrum for the peptide with only quasi molecular ion shown
[M+H]+= 688.4/1
[M+H]2+= 688.4/2
[M+3H]3+ = 688.4/3
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how do GCMS spectra come about?
each peak in the GC trace gives an individual mass spectrum which can be analysed to determine the identity of the component
339
does LCMS use soft or hard ionisation techniques?
soft, usually ES
