physical chemistry (Walker) Flashcards
1
Q
Define spectroscopy
A
Spectroscopy is the study of the interaction between radiation
(electromagnetic radiation, or light, as well as particle radiation) and
matter.
2
Q
which instruments are used for spectroscopy
A
spectrometer or spectrograph
3
Q
what happens when a heated gas undergoes spectroscopy
A
bright lines characterise the gas, hot substances emit light on the spectrum
4
Q
what happens when a cold gas undergoes spectroscopy
A
dark absorption lines characterise the gas. cold substances absorb light on the spectrum
5
Q
How can the spectra be measured
A
Spectra can be measured in either absorption or emission. The
spectrum is characteristic of the substance.
6
Q
in spectroscopy what properties should the light have
A
Should be as bright as is possible, convenient
and safe. Modern experiments can use lasers.
7
Q
what do the spectrometers usually use to hold the sample
A
Modern spectrometers
often use cuvettes to hold liquids.
8
Q
define dispersive element
A
A dispersive element which separates white light into its constituent colours. Can be a prism or a diffraction grating.
9
Q
what is usually used as the detector in spectroscopy
A
Can be as simple as a white screen, with your eyes used
to monitor the result. Modern experiments use
optoelectronic components such as CCD cameras.
10
Q
define frequency
A
The number of waves per
second.
11
Q
define wavelength
A
the distance between two peaks or two troughs of the waves
12
Q
how do you calculate speed of light
A
wavelength multiplied by frequency
13
Q
describe how light acts as a wave
A
Describing light as an electromagnetic wave, electron dipole that is oscillating as it propagates through space. Perpendicular to the oscillating electrical signal, we also have a magnetic dipole that oscillates as it moves through space
14
Q
what does oscilates
A
Oscilates – means has a wave shape
15
Q
units of wavenumber
A
cm-1
16
Q
what is constructive interference
A
constructive interference – the two waves are coincidence meaning they are orverlaying of each other so they add together, so the resulting wave have bigger peaks than the two individual waves
17
Q
what is destructive interference
A
Desctructive interference – positive region of 1 wave interferes with the negative region of another, so the effect is that the waves almost cancel each other out and give a smaller wave as an output
18
Q
on what surface is light diffracted on
A
Light is diffracted by a finely grooved surface.
19
Q
what does diffraction depend on
A
wavelength
20
Q
what is diffraction
A
Where monochromatic light passes through a slit of width narrower than the light wavelength, it is diffracted. A sequence of alternating bright and dark spots or lines are observed. White light is a mixture of different colors, the angle in which a particular cooler is diffracted depends on wavelength. The spacing between bright and dark spots depends on the wavelength of light which is being used.
21
Q
that is the photoelectric effect
A
When we take UV radiation and shine it on a metal surface, when we do this electrons are emitted from a surface. And if we think light is only described as a wave this shouldn’t happen
22
Q
why is the hydrogen atom important
A
Hydrogen atom is abundant all over space so if we can measure spectroscopic lines coming from hydrogen we can use it to map out the densities of atoms and molecule in space
Allowed us to identify the first link between spectra and atomic and molecular structure
23
Q
what happens when an electron moves from a higher energy level to a lower one
A
When an electron moves from a higher energy level to a lower one it emits a photon and the energy that the photon has is determined by the difference in energy between the higher and lower energies. The energy of these photons is proportional to there frequency
24
Q
what is balmer
A
Balmer – electrons falling from higher levels into the n=2 energy level
25
what is lyman
Lyman – electrons falling from higher levels into the n=1 energy level
26
what is paschem
electrons falling from higher levels into the n=3 energy level
27
what is brackett
Brackett – electrons falling from higher levels into the n=4 energy level
28
define excited state
Excited state - any state of a system with an energy higher than the ground state
29
define ground stste
Ground state – the state of a system with the lowest possible energy
30
define photon
Photon – the transition from an excited state to a lower state results in emission of a photon where energy, E=hv
31
what happens as frequency gets bigger
As frequency gets bigger wavelength gets smaller.
32
what so we know about the beer lambert law
Light can be absorbed by substances. Some substances absorb more strongly than others. The higher the concentration of an absorber, the less light is transmitted. The greater the path length of light through an absorber the less light is transmitted
33
what is the molar absorption coefficient epsilon and what affects it
Molar absorption coefficient (epsilon) – measure of the absorption probability. Depends in wavelength and type of molecular transition. Affected by aggregation of the sample
- Units are M-1cm-1 or dm3mol-1cm1
34
what is I0
intensity of incident light - usually 100%
35
what is I or It
intensity of transmitted light
36
what hapens if the curvette contains a weak absorber vs a strong absorber
If the curvette contains a weak absorber then the transmitted light will be almost the same as the incident. However if the curvette contains a strong absorber then the transmitted light will be very different to the incident light
37
how do we represent absorption spectra and describe the results
We represent the spectra by putting either wavelength, frequency or wavenumber on the x axis with either the molar absorption coefficient or the log of this on the y axis, but most instruments will provide an absorbance because this can be accurately calibrated. In the gas phase where the collisons are infrequent between molecules we often have sharp lines. Yet in solution where molecules are colliding more frequently then we have more broad lines in spectra
38
how does a typical absorption spectrometer work
Light Source: Emits a broad spectrum of light (e.g., tungsten lamp for visible light, deuterium lamp for UV light).
Monochromator: Isolates specific wavelengths of light using a diffraction grating or prism.
Sample Holder: Contains the sample in a cuvette; the sample absorbs light at specific wavelengths.
Detector: Measures the intensity of transmitted light after it passes through the sample.
39
what is the davisson germer experiment
Davisson and germer fired a beam of electrons at the surface of the nickel crystal. The separation between adjacent layers of the crystal is of the order of angstroms. This is about the same as the wavelength of the electrons. The distributions of angles over which electrons are scattered is characteristic of a diffraction pattern, showing electrons display the properties of waves.
40
define standing wave
Standing waves – combination of 2 waves moving in opposite directions, each having the same amplitude and frequency, result of interference
- Amplitude must be zero where x<0 ad X>L
- A general solution for allowed wavelengths n x wavelength/ 2 = L (standing waves)
41
define boundary condition
Boundary conditions – the electron is able to move freely within a box ‘but’ cannot leave the box
42
describe conjugated chains
Conjugated chains allow electrons to move freely along but not outside the length of the chain. The longer the conjugated chain the smaller the wavenumber of the transition between the HOMO and LUMO.
43
where do electronic transitions occur
Electronic transitions occur between atomic orbitals in atoms. Electronic transitions occur between molecular orbitals in molecules. Different molecular orbitals have different spatial distributions.
44
what is a chromophore
A chromophore is a region in a molecule where the energy difference between two different molecular orbitals falls within the range of the visible spectrum. Chromophores almost always arise in one of two forms: conjugated π systems and metal complexes
45
what are the different types of electronic transition
There are several types of electronic transitions available to an organic molecule including: σ to σ* (alkanes) σ to π* (carbonyl compounds) π to π* (alkenes, carbonyl compounds, alkynes) n to σ* (oxygen, nitrogen, sulphur, halogen compounds) n to π* (carbonyl compounds).
46
describe the energy changes between the electronic transition
- The energy required to go from sigma to sigma star is much greater than the energy to go from pi to pi star. To go from n to pi * these are sometimes smaller that the energy fro pi to pi star but sometimes larger
47
what do gas phase spectra show
vibrational structure
48
what does solution phase spectra show us
. Solution phase spectra contain broad bands with no resolved structure.
-We can learn what components are in a solution.
-We can learn the relative concentrations of those components.
-The sample is not consumed by the measurement.
49
features in spectra of conjugated molecules
n-pi* transitions are generally found at longer wavelength than pi to pi* transitions. They also have a lower intensity. The special distribution of the orbitals explains this. N to pi* undergos more of a rearrangement the spacial distribution changes. N to Pi* star are significantly weaker than pi to pi* which are quite strong. When we have a pi to pi * transition which involves a very long conjugated chain it might have quite a long wavelength.
50
where else might we intense electron transitions
we may see intense electronic transitions are where transition metals are involved for example crystals, these gemstones have different matrixes which have effect on electronic structure so the electronic spectra is based on octahedral field splitting, the energy gap between them determines the energy of a photon when an electron undergos a transition
51
what does the born-oppenheimer approximation assume
The Born-Oppenheimer Approximation assumes that the motions of electrons and nuclei within a molecule can be treated independently. Provides the conceptual foundation that allows us to independently consider and model the electronic, vibrational and rotational motions of molecules
52
what is internuclear separation
- Internuclear separation – distance between the two atoms in the bond
53
what is Re
the bottom of the curve
54
what does the simple harmonic oscillator predict and describe the shape
The simple harmonic oscillator predicts as u increase the separate the atoms the force gets stronger so that you cannot separate the two atoms, however in a real life scenario it is possible to put enough energy in where the molecule will dissociate and separate the two atoms at an infinite distance. The shape of the curve is based upon the morse potential and is more accurate way of modelling the chemical bond that is modelled by the simple harmonic oscillator. The way that the morse potential differs from the simple harmonic oscillator is that, close to the equilibrium distance the simple harmonic oscillator follows the predictions of the morse potential as you get to higher energy and stretch the bond it doesn’t give a good prediction
55
what does the vibrational frequency of diatomic molecules depend on
- The stiffness (force constant) of the bond
- The masses of the atoms within the molecule
56
what is zero point energy
Zero point energy – it is impossible to completely take all of the vibrational energy out of a molecule. So even in the lowest energy state the molecules still have a small amount of vibrational energy. The zero point energy is the distance that this energy is to the bottom of the well. Vibrational energy is quantised. The spacing between vibrstional between vibrational levels is much less than then spacing between electronic energy levels in molecules.
57
what is the De
energy measuring from bottom of well to the dissociation limit
58
what is D0
measured from v=0 vibrational level to the dissociation limit
59
how do we study vibrational motions
by looking at infrared light
60
how do we study rotational energy levels
by looking at microwave radiation
61
what must be present for a vibrational mode to absorb or emit infrared radiation
For a specific vibrational mode to absorb or emit infrared radiation it much have a dipole that changes during the period of molecular vibration for example
62
do diatomic molecules such as H2,O2 and N2 absorb or emit infrared rasiarion
H2, O2 and N2 no dipole moment therefore no infrared spectrum, any diatomic molecule has a single vibrational mode and that is the molecule can stretch and that is it. During the stretching and compressing there is no change in the dipole moment therefore the molecule cannot be observed during infrared spectroscopy.
63
can HBr, HCl or CO absorb or emit infrared radiation
- HCl, HBr, CO each have a dipole moment and therefor an infrared spectrum can be measured. Heteronuclear diatomic have observable infrared spectra.
64
how do we calculate the number of vibrarional modes for a linear molecule
The number of internal coordinates that we need to describe should equal the number of X,Y,Z coordinates that we have in total which is 3N where N is the number of atoms
If you have a linear molecule there will be 3 translations and 2 rotations (for rotations we are not including the one about the axis. So the number of vibrational modes = 3N-5
65
describe the vibrational modes for carbon dioxide
it is a linear molecule we discover that is has 4 vibrational modes
1)One is a symmetrical stretch where both of the oxygen atoms are being stretch and the carbon dioxide remains the same position
2)Asymmetric stretch where one c-o bond is stretching and the other c-o bond is shortening, we could describe this as the carbon bouncing between the two oxygens
3)Bending motion where both oxygens are moving upwards whereas the carbon is moving dowards, oxygen in y direction carbon in -y direction
4)There is another bending motion but this time the oxygens are going into the page whereas the carbon is coming out of the page
66
which of carbon dioxides vibrational modes are infrared active
-to absorb or emit infrared radiation it much have a dipole that changes during the period of molecular vibration.
-Symmetric stretches are not infrared active as they are symmetric the dipole is not changing
-In the asymmetric stretch it is considered to be infrared active and the dipole changes between the two oxygen atoms
-If we think about bending it is infrared active
67
describe the number of vibrational modes for non-linear molecules
The number of internal coordinates that we need to describe should equal the number of X,Y,Z coordinates that we have in total which is 3N where N is the number of atoms. For a non-linear molecule the number of vibrational modes is given by 3N-6, non-linear molecule there is 3 translations and 3 rotations.
68
describe the vibrational modes for water and state if they are IR active
-Water has a symmetric stretch an asymmetric stretch and a bending motion, however for water we do not have a bending motion of hydrogens going into the pages whereas oxygen moving out as this would lead to a rotation which we have already counted in the 3 rotations
-For the symmetric stretch the dipole moment does change so it is infrared actibe
-All 3 vibrational modes for water are infrared active.
69
compare the frequencies of bending and stretching modes
Stretching modes generally have higher frequency than bending modes
70
define vibronic transition
Where the energy of an emitted photon results from a change in both the vibrational and electronic state of a molecules
71
define stokes shift
The stokes shift – the difference in wavelength between the max wavelength in the fluorescence spectrum and the max wavelength in the absorption spectrum
72
how does fluorescence occur
Fluorescence occurs when absorption of a photon is followed by radiation less transfer of energy into the solvent or matrix. This leads to emission of a photon that has lower energy and longer wavelength than that which was originally absorbed
73
what is the frank condon principle
The franck-condon principle – an electronic transitions happens so quickly that we can consider the nuclei to be static during the transition
74
contrast the differences in wavelength between microwave radiation and infrared radiation
microwaves have Longer wavelength and lower frequency then infrared radiation
75
what is microwave spectroscopy used for
Microwave spectroscopy talks about pure rotational spectroscopy, there isn’t a change in vibrational or electronic energy that molecules have.
-Microwave spectroscopy is good for accurate bond lengths and bond angles for gas phase molecules
-It is also very good for astrochemistry, this is done through a process of fingerprinting.
76
what are the selection rules for rotational spectroscopy
Molecule must have a permanent dipole moment to have a pure rotational spectrum. Molecules of Cnv, Cn symmetry. The dipole moment we are considering is the averaged out dipole moment that the molecule has and we are not worried about the vibrational motion that the molecule has.
77
describe which molecules have a microwave spectrum and therfore rotational spectrum
Homonuclear diatomic molecules do not have a permanent dipole moment and therefore doesn’t have a microwave spectrum. To give a permanent dipole moment the molecule must be polar
-O2, H2 do not have microwave spectrum whereas HCN and CO do
-Molecules that are free to rotate have quantised energy levels which give rise to a rotational spectrum of a molecule.
78
define the moment of inertia
Moment of inertia – the inertia of the object against rotation, the closer together that two objects are the lower their moment of inertia.
The moment of inertia is directly measurable from microwave spectra
79
what is B and what are the units
B – rotational constant units s-1
80
describe rotational energy levels and how they are spaced
Rotational energy level are spaced according to E=BJ(J+1)
j- is the notation that we use to label different rotational levels
the rotational energy levels within molecules, the spacing increases as you go up in J
change in energy
1->0 = 2B
2->1 = 4B
3->2 = 6B
4->3 = 8B
5->4 = 10B
81
what is the interval between lines in a rotational spectrum
2B
82
why are isotopes useful in determining bond length
When you change an isotope you do not change the structure of the molecule, so the bond lengths and the bond angles stay exactly the same, but the rotational constant changes because the moment of inertia changes, so if you measure the rotational isotopes for enough different isotopic combinations, you can use these in a series of simultaneous equations to tell you all of the different bond lengths and bond angles are in that particular molecule
83
define fundamental transition
Fundamental transition – transition between v=0 and v=1 vibrational levels
84
define overtone transition
Overtone transition – a transition between v=0 and any vibrational level higher than v=1
85
define a hot band transition
Hot band transition – a transition between two levels that each have v>0, this is called a hot band because at room temperature most molecules are in the v=0 state, so in general to see a hot band you would have to heat up a chemical sample in order to detect transitions between v>0
86
what are the important selectional rules for linear and diatomic molecules on an infrared spectrum
Important selectional rule applies for linear and diatonic molecules for example CO,HCL and OCS the selection rule states that we can have transitions where delta j is = to + or -1 but not where J=0. Delta J is defined by J upper – J lower
87
define p branch and explain the pattern
- P branch is any transition where J=-1
- However as we climb the ladder every line in the p branch, the transition requires less energy, that is because as we climb the ladder, the spacing between the levels increases
88
define R branch and explain the pattern
- R branch is any transition where J = 1
- We find that going from left to right the arrows are getting longer, as we go to higher rotational levels the energy is getting larger. And since energy is proportional to wavenumber we are going to a higher frequency
- This is what gives the double hump
89
what does the pattern of intensitites in infrared spectroscopy depend on
temperature
90
what is the q branch
- Q-branch: ∆J = 0. Only allowed when there is angular momentum around the axis of a linear molecule
91
what does the doubling up of lines on an infrared spectrum tell us
presence of an isotope
92
what did boltzman express
- Boltzman expressed the relationship between temperature and the relative populations of different energy levels
- If you change the temperature does that change the number of molecules are in the excited states on average in a given temperature vs the number of molecules that are in the ground state at a certain temperature
93
describe boltsmans findings
At room temperature in terms of electronic states all the molecules are in the lowest available state, not many molecules in excited states. More molecules are vibrationally excited at room temperature then they are electronically excited, the reason for this is the energy gap between vibrational levels is much smaller then that of the energy gap between electronic levels. When it comes to rotational levels there about the same number of molecules in the excited states then there are in the ground states at room temperature. So molecules can be rotationally quite excited in room temperature.
94
where does blackbody radiation come from
Blackbody radiation is what we get from molecules that are solid or plasma.
95
describe blackbody radiation from the sun
-The higher the temperature of the source, the shorter the wavelength (on average) of emitted blackbody radiation. (the higher the temperature the more intense the blackbody radiation it emits)
-Emission from the sun is most intense at visible (yellow) wavelengths.
-Intense UV light is also emitted and near IR.
-There is negligible emission at microwave wavelengths.
96
describe the blackbody radiation from the earth
-With an average surface temperature of 287 K, Blackbody radiation from the Earth is much less intense than that from the sun.
-Blackbody emissions are in the infrared.
-Some of this is reabsorbed by “greenhouse” gases such as CO2 and CH4 in the atmosphere. Therefore the energy is retained inside the earths atmosphere instead of being allowed to escape into space, greenhouse effect
97
how and why is thermal imaging used
-Humans cannot see blackbody radiation at ambient, Earth surface temperatures but our instruments can.
-Detect heat lost from a building.
-Detect people in the dark.
-Detect a missile from the heat of its exhaust.
98
key facts about blackbody radiation
-The higher the temp of the source the more intense the blackbody radiation the objects emits
-The higher the temperature of the source the bluer the light that you get from it, as an object temperature increases the light that it emits moves to a shorter wavelength.
99
what is the hertzsprung russel diagram
A Hertzsprung-Russell diagram is used to connect stars with their physical properties and also shows pathways for stellar evolution. Temperature determines both brightness and colour because stars emit Blackbody radiation. The most massive stars will tend to be hotter (and therefore bluer in colour) and brighter. Smaller stars, and those which are running out of fuel, appear red and faint.
100
what does spectroscopy tell us about stars
- We know the temperature of stars. We know the elements they contain.
- We know which molecules are in space, light years away. If it is ever proven that there is life on other planets, it will be a spectroscopist that tells you.
101
how we obtain NMR spectrum and why
In order to obtain an NMR spectrum we need to excite molecules using radio wave light
- Radio waves have longer wavelength than micro waves infrared or visible light this also means they have a lower frequency then these
102
what equipment does NMR need
- Superconducting magnet
- Radiofrequency ( antenna and receiving equipment)
- Liquid nitrogen the superconducting magnets require cryogenic medium to produce strong magnetic field
- Liquid helium
103
when do or dont we see an NMR spectrum
When do we see a NMR spectrum
- If a nucleus has a spin of ½ or greater e.g proton NMR
- Odd Protons and/or Neutrons 1H, 31P, 11B, 17O, 14N, 2H = Deuterium
We do not see NMR spectra if
- Even Protons and Neutrons 12C, 16O
104
what do we mean by spin
When we are saying that a nucleus has a spin we are saying that is has a magnetic orientation, a nucleus of spin I has a 2L+1 possible orientations in a magnetic field we represent that using mI – the nuclear magnetic quantum number. We can also assign a spin angular momentum to tht nucleus which can be calculate by multiplying the nuclear magnetic quantum number by h bar where h bar = h/2pi
105
how do we acquire an NMR spectrum
Apply a magnetic field, then measure the frequencies of transitions between different mI levels (will be at radio frequencies of the EM spectrum).
106
what affect does the size of the magnet have on NMR
- The size of the magnet in the NMR spectrometer is very important, the bigger the magnetic field than can be applied,the bigger the spacing between the energy levels. Have to also apply slightly higher frequency radiation to excite transition
107
define chemical shift
If we have a nucleus which has a spin of ½ or greater and that is in an applied magnetic field then we can get different energy levels created by the different orientations of the nuclear spin of that nucleus.
108
describe the inductive effect
Electrons surrounding a nucleus circulate setting up a magnetic field opposed to the applied field. Nuclei in region of high electron density experience a field weaker than those in a region of low electron density and thus a higher field has to be applied to bring them into resonance.
High electron density-shielded nucleus → high field (low values of δ)
Low electron density-deshielded nucleus → low field (high values of δ)
109
what is the shielding constant
Shielding constant σ is the sum of three contributions: σ = σ (local) + σ (neighbour) + σ (solvent)
σ(local): the electron behaviour around the nucleus of the atom in question: Broadly proportional to electron density of atom in question Shielding decreased – e density decreased - if electronegative atom nearby
σ(neighbour): the electron behaviour from the groups of atoms in molecule Anisotropy of aromatic compounds - deshielded
σ(solvent): contribution from the solvent molecules H-bonding, Aromatics, Sterics
110
what is the standard used in 1H NMR and why
tetramethylsilane – provides the commonly accepted standard reference signal. This molecule has 12 H atoms that are all equivalent (in respect of their chemical environment). From this molecule we get a really strong magnetic resonance and we only get one peak which is very distinctive within the spectrum, good benchmark
111
describe chemical shift
- Nuclei are shielded by the electron cloud surrounding them
- Different degrees of shielding result in different chemical shifts
- Will depend on (for a given nuclide): σ = the shielding constant
112
what effect does electronegative atoms have on chemical shift
Electronegative atoms will deshielded adjacent nuclei
113
what is chemical anisotropy
Chemical anisotropy - Chemical bonds are regions of electron density that can set up magnetic fields. Fields are stronger in one direction than another (anisotropic), and the effect of the field on the chemical shift of nearby nuclei is dependent upon the orientation of the nucleus in question with respect to the bond.
114
describe spin coupling in NMR
- Each magnetic spin of 1H for example affects the magnetic environment of nearby 1H’s
- Coupling of nuclear spin occurs via the intervening bonding electrons
- This effect is not normally important beyond three bonds unless delocalisation in present
- We must consider all the nuclei in the same chemical environment together. They all have the same resonant frequency and they display the same splitting pattern
115
what are the rules for spin coupling in 1H NMR
If we have 3 nuclei that are in an identical chemical environment and they have a spin of ½. The number of peaks in the splitting pattern would be 2nI+1
- Where I is the spin of the nucleus for 1H I=1/2
- And n is the number of neighbouring coupled nuclei
For I = ½ you get the n+1 rule
- N=1 doublet, n=2 triplet, n=3 quartet, n=4 quintet, n=5 sextet, n=6 septet
The relative intensities of the peaks of a multiplet also depends on n and is given by pascal’s triangle
If the chemical shift difference in HZ is much larger than the coupling constant J the simple pattern of 2 doublets appear
116
describe 13NMR
-12C is not magnetically active I=0 no angular momentum, no magnetic moment
-13C has I=1/2
-The natural abundance of 13C is a lot lower than the abundance of 12C abundance of 13C is about 1.1%. which means the signals for 13C NMR are generally very weak due to the low concentration of 13C in the sample
117
how do we usually run 13C NMR
- Usually we run in a proton decoupled mode, which means we a apply a radio frequency which put all the protons into resonance such that we see no coupling effects to protons and we are exclusively looking at the splitting pattern generated by 13C nucleus
118
what is ESR spectroscopy
Electron spin resonance spectroscopy – ESR spectroscopy
-Also called electron paramagnetic resonance spectroscopy
-Concerned only with compound having unpaired electrons (e.g metal ion or free radical) as its spin state will change
-Intensity is proportional to concentration
-The resonance frequency depends on the strength of an applied magnetic field
119
describe ESR spectroscopy
- Unpaired electron within sample has 2 spin states, so 2 orientations of magnetic moment, so when u place these under a magnet the 2 different spin states have 2 different energies. If you then apply microwaves/ electromagnetic field you can induce a transition between the spin states and by measuring the frequency or the wavelength of the radiation absorbed or emitted you can determine something about the structure of the molecule
- Represented by a derivative spectrum
