spectroscopy Flashcards
order of electromagnetic radiation in order of decreasing wavelength
radio waves, microwaves, infrared radiation, visible light, ultraviolet light, x rays, gamma rays.
visible spectrum wavelength range
390nm to 700nm
ground state electrons
electrons which have absorbed no energy and are in their lowest energy shell.
excited electrons
electrons which have absorbed energy to promote them to higher energy shells.
wavelength and frequency equation
C = vλ
speed of light = frequency x wavelength
energy of a photon equation
E = hv
energy = plancks constant x frequency
Planck’s constant
6.626 x 10 ^-34 Js
Avogadro’s constant
The number of atoms/ species in one mole
6.02 x 10^23 mol^-1
wavelength and wavenumber ratio
λ = 1/v*
v* in cm^-1 and λ in m so also use conversion rate.
frequency and period relationship
v = 1/T
frequency = 1 / period
period definition
the number of seconds per wave
wavenumber
the number of waves per cm
electron volts to joules
1eV = 1.602 x 10 ^ -19 J
relating energy and wavelength/ frequency equations
C = vλ and E =hv
v = C/λ
E = h(C/λ)
v* = 1/λ
E = hCv*
photon definition
Electromagnetic radiation which behaves as a particle.
frequency and energy rule for electromagnetic radiation
as the frequency increases the energy of electromagnetic radiation will increase
rule for energy absorbed by an electron
the energy an electron will absorb is not the energy of the energy shell it is currently in or promoted to, it the the energy gap, or difference in energy between the two energy shells.
Rydberg equation
E = RH/n^2
energy = Rydberg’s constant / (principal quantum number)^2
Rydberg’s constant
RH = 13.6 eV
energy gap of an electron equation
ΔE = RH( 1/n^2 - 1/ n^2)
where n = n1 and n = n2
n1< n2
basically the energy must be positive because its an energy gap
series of energy transitions
lyman : n =1
balmer : n = 2
paschen : n = 3
bracket : n = 4
pfund : n = 5
where n is the lower energy shell
basic quantum numbers of electrons
n , l , ml, ms
n quantum number
the principal quantum number referring to electron energy shell
l quantum number
the angular momentum quantum number shows the position and the momentum ( mass x velocity)
ml quantum number
the magnetic quantum number - refers to the orbital orientation that the electron is occupying
ms quantum number
the spin magnetic quantum number - shows the electrons spin direction
orbital selection rule for promotion of electrons
electrons can only be promoted to orbitals with higher energy than their ground state with an angular momentum quantum number of l+1.
(this means if it is an s orbital it can only be promoted to higher energy p orbitals, and a p can only be promoted to higher energy d orbitals, ect)
resolving power of a spectrometer equation
R = λ/Δλ
resolving power = wavelength of light measured / the smallest change in wavelength measurable
resolving power definition
the measurement of how well a spectrometer can differentiate between different wavelengths
fine structure promotion explanation
when there is a very high resolving power spectrometer we can see orbital energy levels will fragment into smaller sub - energy levels.
This is because elections have a mass and a charge meaning they will produce a tiny magnetic field.
this magnetic field is shown by the ms quantum number.
The fine structure energy shell can be shown by the quantum number J
fine structure quantum number equation
J = MS + L
total angular momentum for fine structure = spin magnetic quantum number + angular momentum
hyperfine structure explanation
if you have an incredibly strong resolving power spectrometer you would see the transitions of fine structure energy levels will also fragment into smaller sub - sub shells of orbitals.
this is due to the nucleus which the electrons are attracted to having a mass and a charge meaning they will have a magnetic field.
this magnetic field will be shown by the I quantum number.
The hyperfine structure energy shell can be shown by the equation F = J + I
nuclear angular momentum quantum number
I
hyperfine structure quantum number equation
F = J + I
beer lamber equation
A = ε[M]l
absorption = molar absorbance co efficient x concentration of absorbing species x path length
no units = L mol^-1 cm^-1 x mol L-1 x cm
transmittance equation
T = I/I0
transmittance = intensity detected /source intensity
transmittance to absorption
absorption to transmittance
A = -log(T)
T = 10^-A
limitations of beer lambert calculations accuracy
this equation is based on the assumption that all light is either absorbed or transmitted, and that neither of the container of the solution, or the solution will refelct any light.
note on absorption and transmittance calculations
absorption cannot be directly transferred from a percentage to a decimal, must be converted to transmittance using T = 100 - %A. Then converted back using A = -log T
beer lambert equation of two different species which absorb the wavelength
A1 = l(ε1b1 x [B] + ε1c1 x [C])
A2 = l(ε2b2 x [B] + ε2c2 x [C])
then use simultaneous equations
atomic absorption spectroscopy
a range of uv and visible electromagnetic radiation will be directed at a sample and specific wavelengths of light will be absorbed to promote electrons from to higher energy orbitals. this will reduce the transmission of these specific wavelengths which will detected by the spectrometer. this will cause black lines at absorbed wavelengths.
atomic emmision spectroscopy
where electrons are excited to promote them to higher energy orbitals, this will allow the electron to relax and emit photons which will be detected by a spectrometer. the photons emitted will have an energy equal to the energy of the downwards transition of the electron.
reason for a broad range of absorption for UV Vis spectrometry
a molecules electrons will have hyperfine and fine electron structures due to electron magnetic influence and nuclear magnetic influence on an orbital. These will cause a large variety of different energy gaps between the the different orbitals, meaning electrons can absorb a wider range of wavelengths of uv/ visible light to promote electrons to higher orbitals.
solvent rule for ranges of absorptions within UV and visible spectroscopy
solvents will bombard a solute molecule with electrons which will result in transitions of electrons to a broader range of absorption of wavelengths of UV / visible wavelengths of light.
chromophore definition
a series of atoms within a molecule which will allow the molecule to absorb visible wavelengths of light.
types of chromophore
conjugated system and weak field ligands in transition metal complexes.
transition metal complexes defintion
a transition metal atom or ion which will accept non bonding electron pairs from a ligand by allowing them to occupy its d orbitals
why do transition metals absorb visible light
Ligands form dative covalent bonds by filling unoccupied d orbitals with lone pairs, these lone pairs will repel the d orbitals which will split them from degenerate orbitals to higher and lower energy, which will result in visible wavelengths of light being absorbed to allow d - d transitions to occur.
lower energy d orbitals
dxy, dxz, dyz
higher energy d orbitals
d x2 - y2
dz2
strong field ligands
a ligand which creates a large energy gap between the lower and higher d orbitals when splitting the d orbitals, resulting in UV light being absorbed by the metal complex.
weak field ligands
a ligand which will create a small energy gap between the lower and higher energy d orbitals when splitting the d orbitals, which will result visible light being absorbed by the metal complex.
rule for metal complexes colour
transition metal complexes will be coloured due to occupied d orbitals, other metal complexes will have no colour due to no d orbitals present.
difference in geometric isomers absorption for UV visible spectroscopy
the E configuration will be a lower energy configuration than the Z configuration due to the groups with more electron density being on opposite sides of the molecule. this will result in there being a larger energy gap between the bonding and antibonding MO in E than in Z, which will result in a higher frequency and lower wavelength being absorbed by E than Z.
cis = Z trans = E
electron transition feasibility rule
electron transition feasibility is based on the molar absorption co efficient ε
ε = [10^4, 10^6] allowed
ε = [10^3, 10^4] weakly allowed
ε = [0,10^3] forbidden
key features affecting the molar absorption co efficient ε
the symmetry of the molecule
bathochromic shift
where there is a shift to a larger wavelength and lower frequency (bathochromic)
hypsochromic shift
where there is a shift to a lower wavelength and higher energy/ frequency (blue)
hyperchromic shift
an increase in the molar absorption co efficient
hypochromic shift
a decrease in the molar absorption co efficient
effect of an electrophilic substituent to a symmetric molecule in UV visible spectroscopy.
the substituent will reduce the symmetry which will cause a hyperchromic shift.
the electrophile will remove electron density from the original molecule which will result in a bathochromic shift.
nucleophilic substituent added to the symmetric molecule in UV / visible spectroscopy
the substituent will reduce symmetry which will increase the molar absorption co efficient causing a hyperchromic shift.
the nucleophile will add electron density to the molecule which will cause a hypsochromic shift.
pH effect
increasing the pH means adding a base or adding a nucleophile - so hypsochromic shift.
decreasing the pH means adding an acid or adding an electrophile - so a bathochromic shift.
IR spectrometry principle
bonding electrons will absorb IR radiation to enter different vibrational states allowing for the bonds to vibrate at a higher frequency
diatomic molecules vibration modes
stretching
polyatomic molecules vibration modes
bending and stretching
factors affecting the frequency of IR absorbed by the bond
bond strength
masses of atoms involved in the bond
bond strength effect on frequency of IR absorbed
as the bond strength increases the frequency of IR will increase
mass of different atoms involved in bond effect on IR absorbed
as the masses of the atoms increases the frequency of IR absorbed will decrease.
reduced mass formula
(m1 x m2)/ (m1 + m2 )
frequency of IR absorbed formula
v = 1/2π x sqrt(k/μ)
frequency = 1/2π x sqrt(force constant / reduced mass)
force constant units
kg N m^-1
normal modes of linear polyatomic molecules
3N -5
normal modes of non linear poly atomic molecules
3N -6
IR active normal mode definition
a normal mode which can be detected on an IR spectrometer due to there being a change in the dipole moment of the molecule.
normal mode definition
where a bond within a molecule will absorb infrared radiation of a specific wavenumber to produce a unique vibration.
vibrational energy level equation
Ev = (V + 1/2)hv
vibrational energy = (vibrational energy level + 1/2) x Plancks constant x frequency
boltzmanns equation
n1/n0 = exp[-(E-E)/kT)
important property of nuclei for NMR
nuclei have a charge, mass and angular momentum meaning they will produce their own magnetic moment.
magnetic moment definition
The magnetic field produced by a particle
NMR process
- nuclei will have different orientations.
- an external magnetic field will cause the nuclei to align parallel or antiparallel to the external magnetic field, due to their magnetic moment.
- antiparallel is the high energy conformation and parallel is the low energy conformation.
- radio frequency electromagnetic radiation will be directed at a sample and specific frequency will be absorbed to allow nuclei in the parallel conformation to flip to the antiparallel orientation.
- the frequency absorbed will be detected by the spectrometer and recorded.
- The chemical shift will be calculated by measuring the radio frequency absorbed by the standard chemical TMS and using the formula
δ = (vsample - v standard)/ v standard
this is done to remove the variable of different strengths of magnetic field.
nuclear angular momentum quantum number
I
identifying the nuclear angular momentum
nuclear angular momentum will have a specific value for each different type of nuclei, which is a fixed property.
magnetic moment of a nucleus requirement
I ≠ 0
important nuclei angular momentums
12C = 0
1H = 1/2
13C = 1/2
MI quantum number
the magnetic nuclear quantum number - which refers to the orientation of the nucleus.
MI values
MI goes up in 1/2 from -I and I
energies of nuclei with different MI
nuclei of the same type with different MI quantum numbers will be degenerate in normal environments but have different energies in an external magnetic field.
external magnetic field symbol
B(naught)
magnetic moment formula
μ = Yn x I*
number of MI values
number of MI = 2I + 1
bold I formula
I* = h*(1+I)I
energy of a nucleus equation
E = -μ x B(naught)
change in energy between nuclear conformations equation
ΔE = h*B(naught) x yn
local magnetic field
the magnetic field which is acting on the nucleus
electron effect on local magnetic field
electrons also have a mass charge and angular momentum so will also have a magnetic moment (Bind) which will reduce the effect of the external magnetic field on the nucleus
change in energy between nuclear conformations equation including shielding
ΔE = h* yn x B(nought) x (1 - δH)
reason for chemical shift
to remove the variable of magnetic field strength to allow for universal comparison of spectroscopies.
chemical shift formula
δ = (vsample - vstandard)/v spec
chemical shift units
parts per million because the difference on the top will be approximately 1 million times smaller than the number on the bottom.
range of chemical shift
from 0 - 12ppm
greater electron shielding effect on chemical shift
greater electron shielding will result in a weaker local magnetic field, which will reduce the energy gap between the parallel and antiparallel conformation, which will result in a lower frequency being absorbed and a smaller chemical shift.
weaker electron shielding effect on chemical shift
weaker electron shielding will result in a stronger local magnetic field, which will increase the energy gap between the parallel and antiparallel conformation, which will result in a higher frequency being absorbed and a larger chemical shift chemical shift.
electronegativity effect on chemical shift
an electronegative group will remove electron density from the nucleus reduce the electron shielding and increase local magnetic field, increasing the energy gap between nuclei conformations increasing frequency absorbed and chemical shift.
number of substituents effect on chemical shift
as the number of substituents increases the original effect on the chemical shift will increase.
effect of the distance of a substituent from the nucleus
as the substituent gets further from the nucleus the original effect on the chemical shift will decrease.
hybridisation effect on chemical shift
As the distance between electron density in the molecule increases the deshielding effect will increase which will mean a stronger local field and a stronger chemical shift.
sp2>sp>sp3
delocalization effect on chemical shift
delocalized pi electrons will create a current due to the free flow of electrons. this will produce a magnetic field which acts in the same direction as the external magnetic field increasing the chemical shift.
intermolecular forces effect on chemical shift
hydrogen bonding will remove electron density from a nucleus using an electronegative atom, which will result in a deshielding effect and stronger local magnetic field, meaning a larger energy gap and greater chemical shift.
intramolecular forces effect on chemical shift
covalent bond formed between the electrophile and the nucleus will remove electron density, which will increase chemical shift
covalent bond formed between a nucleophile will add electron density which will decrease chemical shift.
