vibrational spectroscopy

Question 1

what type of EM radiation is used in vibrational spectroscopy?

Answer 1

IR

Question 2

why can IR radiation be used for vibrational spectroscopy?

Answer 2

vibrational energy levels are further apart (~2000cm-1, ΔE ~ 5-50 kjmol^-1), and so higher energy photons are needed match ΔE + promote molecules to higher energy levels

Question 3

give 6 types of bond vibration

Answer 3

symmetrical stretching
asymmetrical stretching
rocking
bending/scissoring
twisting/torsing
wagging

Question 4

which bond vibrations are high in energy?

Answer 4

symmetrical and asymmetrical stretching - as these change the bond lengths the most

Question 5

which bond vibrations are lower in energy?

Answer 5

rocking and wagging

Question 6

what is the function of a diffraction grating?

Answer 6

this is a part in the IR spectrometer which separates wavelengths - determines the resolution of the spectra

Question 7

how does an IR spectrometer work?

Answer 7

half the beam of IR radiation passes through the sample, the other half passes through a reference, they both pass through the diffraction grating and detector, the detector alternates the source and reference signals allowing the effect/signals from solvent/air to be filtered out
- the signal measured + displayed on the spectra is the difference between the sample and the reference

Question 8

what is harmonic motion?

Answer 8

as a system moves from equilibrium (its lowest energy state), a restoring force pulls it back

Question 9

how is harmonic motion applied to IR spectrometry?

Answer 9

consider for chemical bonds - as they are stretched/compressed by vibrations, a restoring force F is generated which returns bonds back to equilibrium
- this is described by hookes law: F=-kx
where k = force constant of bond
x = extension

Question 10

what does the classic simple harmonic motion equation tell us about bond vibration?

Answer 10

frequency of vibration depends on the stiffness/strength of bonds + mass/heaviness of atoms
- molecules with double/triple bonds have very large force constants

this equation can be used with E = hv to find energy needed for the vibration

Question 11

how does mass/weight of atoms affect energy needed for vibration?

Answer 11

lighter atoms need higher wavenumber vibrations = more energy

Question 12

how does bond strength affect energy needed for vibration?

Answer 12

stronger atoms need higher energy radiation with higher wavenumbers to cause vibration

Question 13

how does isotopic substitution affect energy needed for bond vibration?

Answer 13

isotopic substitution doesn’t affect bond strength/force constant as adding neutrons has little affect on electron density
however it does affect reduced mass
- increase in reduced mass means lower frequency/energy is needed for bond vibration
- decrease in reduced mass means higher frequency/energy is needed for bond vibration

Question 14

what types of atoms does isotopic substituion affect most?

Answer 14

lighter atoms, as this changes the reduced mass more

Question 15

how are vibronic energy levels determined?

Answer 15

by solving schrodinger equation for energy using simple harmonic potential

Question 16

what rules are necessary for the wavefunction to describe the system as a function of x (=bond displacement)

Answer 16

• wave must be smooth
• wave must be continuous, and each subsequent wave must pass through 0
• wave must tend to ~ 0 at large values of +/- x
• each wavefunction can be thought of an an energy level

Question 17

what formula is used to determine vibrational energy levels?

Answer 17

E = (v+1/2)hv
- where 1st v = vibrational QN = no. energy levels
- 2nd v = vibrational frequency

Question 18

give 3 features of vibrational energy levels, based on the harmonic approximation

Answer 18

• all equally spaced
• all have degeneracy = 1, as vibrational isn’t directional like rotation
• lowest possible energy isn’t 0, molecules can never have 0 vibrational energy as atoms can never be completely at rest relative to each other - called 0 point energy

this has been determined by solving E = (v+1/2)hv

Question 19

give a limitation of the harmonic approach

Answer 19

assumes that the potential energy changes in the same way as the bond is compressed/extended - therefore harmonic/perfect parabla shape

Question 20

how does energy realistically change depending on bond compression/extension?

Answer 20

at short distances, atoms repel - causes steep curve/destabilisation
at large distances, bonds break completely - causes potential energy curve to tend towards 0
- this gives a lennard jones potential like shape, known as a morse curve = antiharmonic

Question 21

how is the E = (v+1/2)hv equation adapted to fit antiharmoniticity?

Answer 21

E = (v+1/2)hv - A(v+1/2)^2 hv
where A = antiharmonicity constant

Question 22

give one gross selection rule for vibrational spectroscopy

Answer 22

the dipole moment of a molecules must change during the vibration (necessary for the molecule to have rotational spectra)

Question 23

give one specific selection rule for vibrational spectroscopy

Answer 23

only transitions between adjacent energy levels can occur, Δv = +/- 1

Question 24

how does the anharmonic approximation affect vibrational energy levels?

Answer 24

vibrational levels are not equally spaced under this approximation, separation decreases with increasing v - this means there is a finite number of levels
- assume equally spaced anyway

Question 25

what happens when ΔE between adjacent energy levels = 0

Answer 25

point where bond breaks

Question 26

how does asymmetric bond vibration affect peaks on the spectra?

Answer 26

asymmetry of bond vibration can allow Δv = +/- 2 or 3, these appear as overtones on the IR spectra, weaker signals at 2hv (1st overtone) or 3hv (2nd overtone)

Question 27

are homonuclear diatomics IR active?

Answer 27

no, no permanent dipole

Question 28

are heteronuclear diatomics IR active?

Answer 28

yes, as they have permanent dipoles, meaning that vibration can cause dipole moment to change

Question 29

if Δv for vibrational energy levels is always constant, how do we know which Δv transition has occured?

Answer 29

vibrational energy levels have much larger ΔE values so mostly the transition observed is v=0 to v=1 this can be shown mathematically using equation: n(upper)/n(lower) = exp(-ΔEKbT)

Question 30

degrees of freedom definition

Answer 30

an independent mode of position in a molecules, assigned across translational, rotational and vibrational modes = all the ways molecules can move

Question 31

give the formula for degrees of freedom

Answer 31

N atoms has 3N degrees of freedom

Question 32

how are degrees of freedom in a molecules assigned?

Answer 32

- first 3 degrees of freedom for any molecule regardless of no. of atoms = translational (x y z) - next we assign rotational: if non-linear, check how many axes the molecule can rotate about (=moments of inertia), usually = 3 if linear, molecule has 2 moments of inertia because the one going through the bond doesn't count as atoms don't change place, = 2 - all left over degrees of freedom are vibrational

Question 33

large molecules with many atoms have many vibrational degrees of freedom, how is this accounted for on the IR spectra?

Answer 33

- not every possible vibration will be IR active - most vibrations will be recorded in the fingerprint region - as number of atoms increases, number of peaks increases rapidly, often not possible to assign all peaks

Question 34

why do peaks overlap so much in the fingerprint region?

Answer 34

each vibration has its own energy gap + set of energy levels, some may be very close in energy so these peaks will be touching / overlapping eachother

Question 35

why do we usually not focus on the fingerprint region?

Answer 35

most vibrations are recorded here, all peaks are overlapping and hard to assign, and vibrations/frequencies which are most characteristic of specific functional groups tend to appear outside the fingerprint region

Question 36

give 1 use for the fingerprint region

Answer 36

can be compared to databases to identify unknown compounds - fingerprint so unique for every compound

Question 37

how does resolution of IR spectrometer affect the spectra?

Answer 37

higher resolution spectrometers are able to detect overtones better, so instead of one peak for a certain bond, there might be many split peaks transitions occur simultaneously between rotations and vibrations so higher resolution spectrometers will also be able to detect rotational transitions

Question 38

how does the fact that rotational and vibrational transitions occur simultaneously complicate things?

Answer 38

both sets of selection rules must be obeyed, for example ΔJ and Δv must both = +/-1 this affects what transitions are allowed

Question 39

what are the 2 branches seen on a spectra?

Answer 39

R branch and P branch

Question 40

what is the R branch?

Answer 40

made up of transitions with a larger ΔE than approximations based only on vibrations, meaning vibrational and rotational energy has been gained (ΔJ = +1)

Question 41

what is the P branch?

Answer 41

made up of transitions with a smaller ΔE than approximations based on only vibrations, meaning vibrational energy gained, rotational energy lost (ΔJ = -1)

Question 42

what is the Q branch/peak?

Answer 42

the forbidden transition: v=0, J=0 -> v=1, J=0 - this is never seen, appears as a space in between the R and P branches

Question 43

when would you see R, P and Q branches?

Answer 43

only on a very high resolution spectra

Question 44

give 3 ways IR spectra allows the determination about molecular structure

Answer 44

- obviously via peaks caused by characteristic functional groups - line spacing (high res only) allows calculation of bond length by determining B - decoupling of energy changes due to vibration + rotation allows calculation of force constants, via bond length

Question 45

what is raman spectroscopy?

Answer 45

a type of vibrational spectroscopy based on scattering of photons rather than absorption - means incoming photon doesn't have to match in energy transition

Question 46

what are the 3 possible things that can happen when a molecule is irradiated, in raman spectroscopy?

Answer 46

- energy can be absorbed/transmitted - energy can be scattered at the same energy (elastic), small amount only, this = raman scattering - energy can be scattered with a different energy (inelastic), very small amount only, means light comes back out having either lost or gained energy, energy difference can be measured

Question 47

stokes raman scattering definition

Answer 47

when wavelength of scattered light > wavelength laser, meaning scattered photon emerges with lower energy

Question 48

anti-stokes raman scattering definition

Answer 48

when wavelength of scattered light < wavelength laser, meaning photon hits an already excited molecule, causing it to drop to a lower energy state so extra energy is given to scattered photon

Question 49

how is raman scattering seen?

Answer 49

on a raman spectra plot - intensity against raman shift, ṽ - raman shift is specific to bond vibration

Question 50

how does stokes raman/ anti-stokes raman scattering appear on a spectra?

Answer 50

3 peaks: a main peak, representing the case where the photon does not exchange energy with the molecule, with 2 smaller peaks on either side, representing stokes raman and anti-stokes raman scattering - distance between main peaks and side peaks = Δṽ if jump up to excited state and jump down the same, then Δṽ for both peaks should be equal

Question 51

what are the selection rules for raman spectroscopy?

Answer 51

- vibration needs cause a change polarisability that leads to induced dipole moment, not just a change in dipole moment - this is necessary for the vibration to be raman active

Question 52

give 1 reason that raman spectroscopy is useful?

Answer 52

some vibrational modes that aren't IR active are raman active, e.g. homonuclear diatomics are raman active because electron cloud distorts as bonds are stretched/vibrated

Question 53

how can you tell if a substance is raman active for a type of vibration?

Answer 53

most vibrations cause distortions in the electron cloud, for the vibration to be raman active, at each extreme of the vibration the electron clouds must not be mirror images, they must have completely different shapes - usually complements IR, if something is IR inactive its usually raman active (+vice versa)