Vibrational Spectroscopy

Question 1

Where do transitions occur + at what energy?

Answer 1

Transitions occur in the infrared region ~10 ^-20 J

Question 2

State what is meant by the “classical idea”.

Answer 2

The bond has a natural stretching frequency, Vo, and can be made to vibrate at this frequency when it interacts with the electric field of the electromagnetic radiation.

Question 3

What is the gross selection rule?

Answer 3

There must be a change in the dipole moment of the molecule during the vibration in order to be observed in the spectrum.

Question 4

What is the specific selection rule?

Answer 4

∆v = ± 1

Question 5

State the equation for defining the dipole moment along with units.

Answer 5

u = qr

Units in coulomb metre. Direction (u) pointing from negative (-q) to positive (+q).

Question 6

State what is on the x and y-axis of a vibrational spectrum.

Answer 6

x-axis (horizontal): wavenumber/cm-1

y-axis (vertical): % transmittance

Question 7

What is a harmonic oscillator?

Answer 7

An oscillator in which the displacement from the equilibrium position is proportional to the force causing the oscillation.

Question 8

State the equation for the frequency of oscillation and annotate.

Answer 8

v = 1/2pi x square root: kf/u

kf: force constant in Nm-1, shows bond strength
u: reduced mass of the molecule kg molecule-1

Question 9

What are the limitations of the harmonic oscillator? (3).

Answer 9

1. Only good at equilibrium bond length.
2. Real bonds don’t vibrate with a simple harmonic motion.
3. It shows that at long distances, molecules fully dissociate and have no interaction which is NOT true of a real bond.

Question 10

What is the “morse potential” of anharmonic vibrations?

Answer 10

Morse potential accounts for the dissociation and unequal spacing between energy levels.

Question 11

Vibrational energy is quantised. What is the equation that gives the allowed energy levels?

Answer 11

Ev = (v, + 1/2) hv

v, : vibrational quantum number (+ve integer values)
v: frequency of oscillation

Question 12

State the vibrational modes of diatomic and triatomic molecules.

Answer 12

1. Symmetric stretching mode.
2. Bending mode.
3. Antisymmetric stretching mode.

Question 13

Define the degrees of freedom for a polyatomic molecule.

Answer 13

Number of molecules x 3

Question 14

Why do all molecules display 3 translational modes?

Answer 14

Due to the 3 axis molecules are displayed on; x y z.

Question 15

Draw the table for type of molecule, degrees of freedom and modes.

Answer 15

See lecture notes

Question 16

Which mode normal appears at lower wavenumber; bend or stretch?

Answer 16

Bend.

Question 17

State the order of energies of characteristic groups on IR spectra, from highest to lowest energy.

Answer 17

O-H/N-H stretch
C-H stretch
C- - - N stretch (triple bond)
C=O stretch
C-H bend
C-O stretch

Question 18

In what region of the spectrum is the fingerprint region?

Answer 18

The region below 1500-1400 cm-1.