Question

(A) How does a change in mass (for example, swapping a hydrogen with a deuterium) affect the frequency of a molecular vibration?

(B) How does a change in bonding strength (for example, comparing a C-C single-bond to a C-C double bond) affect the frequency of a molecular vibration?

Answer 1

The frequency of vibration of a bond can be calculated using Hooks law

V = (1/pi.C)(sqrtK/mu)

V = frequency of vibration

C = velocity of light

K = force constant

Mu = reduced mass

From the equation, it is shown that frequency of vibration of a bond is directly proportional to force constant and inversely proportional to its reduced mass

1) if mass of the bonded atom increases, vibrational frequency decreases

Ex: C-H v = 3000 cm-1 and C-Cl, v = 650 cm-1

2) if force constant or bond strength increases, vibrational frequency increases

Ex: C-C, v = 1100 cm-1 and C=C, v = 1640cm-1