B - Spectroscopy - IR

Question 1

What is a vibrational mode?

Answer 1

Atoms vibrate around their equilibrium position in well-defined patterns = vibrational modes

Question 2

What is a harmonic oscillator?

Answer 2

The model for the vibration of a diatomic molecule -> like a spring

Question 3

What is Hooke’s Law?

Answer 3

The force needed to extend or compress a spring by some distance is proportional to that distance:

F = -kx

where k = force constant, which is related to the bond strength

units Nm^-1, usually around 100 - 1000 Nm^-1

Question 4

What do r and v₀ stand for?

Answer 4

r is the bond length; at rest r = r₀, but during the vibration r fluctuates periodically. v₀ is the vibrational frequency; atoms oscillate about r₀ with a characteristic frequency.

Question 5

What is the equation for the reduced mass, ν?

Answer 5

ν = (m1 x m2) / (m1 + m2)

Question 6

What is the purpose of calculating the reduced mass?

Answer 6

It removes translations and rotations; only stretched and compressions will be considered.

Question 7

How do you convert from atomic mass units to kg?

Answer 7

Multiply by the mass of a proton

1.67 x 10^-27 kg

Question 8

What is the equation for v₀ in s^-1 (harmonic oscillator equation)?

Answer 8

v₀ = 1/2π x (k/ν)^1/2

Question 9

How is v₀ proportional to k and ν?

Answer 9

v₀ increases when k increases

v₀ decreases when ν increases -> heavier atoms

Question 10

How do you convert to frequency in s^-1 to wavenumber, cm^-1 ?

Answer 10

cm^-1 = v₀ / speed of light (3 x 10¹⁰ cm/s )

Question 11

What is the equation for v₀ in cm^-1 ?

Answer 11

v₀ = 130 x (k/ν)^1/2

Question 12

What is the effect of isotopes on reduced mass, vibrational frequencies and k?

Answer 12

Isotopes give rise to different reduced masses, so this will change the v₀; heavier isotopes reduce the vibrational frequency. k, however, stays the same.

Question 13

What is the isotope substitution formula?

Answer 13

v_0(heavy) / v_0(light) = (ν_light/ν_heavy)^1/2

Question 14

What kind of vibrational modes are there?

Answer 14

Symmetric and asymmetric stretching, symmetric and asymmetric bending, twisting.

Question 15

What is the fingerprint region?

Answer 15

It is the IR absorption specific to that molecule, it can be matched to a known database of spectra given that the sample is pure. It is mostly used for functional group analysis

Question 16

What is the equation for turning IR transmission into absorbance?

Answer 16

A = -log(T%/100) = -log(It/Io) = log(Io/It)

Question 17

What are IR invisible compounds?

Answer 17

Molecules that don’t have a dipole moment, so vibrating does not change the electric dipole moment, ν₀ -> eg diatomic molecules. these are suitable materials to use as a medium

Question 18

Are all molecules without a dipole moment IR invisible?

Answer 18

No, because some vibrations cause a change in ν₀, and these will be IR active.

Question 19

Which bends or stretches are IR active?

Answer 19

Ones that cause a change in ν₀ eg in CO₂ symmetric stretching is IR inactive, but bending and asymmetric stretching are both IR active.

Question 20

Where do O-H stretches occur?

Answer 20

3700 - 3600 cm^-1 lowered to 3200 - 2500 cm^-1 by H bonding

Question 21

Where do N-H stretches occur?

Answer 21

3500 - 3300 cm^-1 less affected by H-bonding than O-H

Question 22

Where do C-H stretches occur?

Answer 22

saturated = 2960 - 2800 cm^-1 where the intensity is related to the number of C-H bonds present

unsaturated = 3050 - 3020 cm^-1 -> can be weak or broad

Question 23

Where do double bond stretches occur?

Answer 23

1800 - 1600 cm^-1

Question 24

Where do triple bond stretches occur?

Answer 24

2300 - 2100 cm^-1

Question 25

Where does the fingerprint region occur?

Answer 25

1600 - 400 cm^-1 This region contains C-C stretching, bending, torsion etc

Question 26

What can a peak at 1670cm^-1 mean?

Answer 26

Either a C=O stretching or O-H bending. To distinguish between them, we must take into account that the C=O peak is usually very sharp and strong.

Question 27

What does an IR peak at 3200cm^-1 correspond to?

Answer 27

H-bonded OH or NH.

Question 28

Why does H-bonding lower the IR vibration frequency of OH groups?

Answer 28

Forming H-bonds reduces the strength of the bond. This reduces k, which is directly proportional to frequency. Frequency also decreases because the second atom the H is attached to starts to move, causing the reduced mass to increase slightly.

Question 29

How does IR work?

Answer 29

The atoms in molecules are not static: they vibrate around their equilibrium positions. The vibrational frequencies correspond to light energies in the IR region of the spectrum. Some of the vibration cause a change in the electric dipole moment of the molecule. This results in a fluctuating molecular electric field associated with the vibration. This ‘resonates’ with the fluctuating electric field of the light wave, causing absorption of the light energy at each vibrational frequency which results in a peak at that frequency in the IR spectrum.

Question 30

How can IR be used for functional group analysis?

Answer 30

Functional groups will always have approximately the same IR peaks in different molecules, so they can be used to work out features of the structure of an ‘unknown’ molecule.

Question 31

What is the IR frequency of a bond proportional and inversely proportional to?

Answer 31

Proportional to the electronegativity and the strength of the bond Inversely proportional to the weight of the atoms

Question 32

Which gives a higher wavenumber (lower wavelength), stretching or bending vibrations?

Answer 32

Stretching vibrations

Question 33

What type of molecule can IR not identify?

Answer 33

Homonuclear diatoms because there is no dipole moment.

Question 34

Which of these has the highest and lowest absorption frequencies?

Symmetric stretching

Bending

Asymmetric stretching

Answer 34

Highest = asymmetric stretching

Symmetric stretching

Lowest = bending