Question 2 - IR Spectra

Question 1

Describe the nature of the change in the molecule when an IR photon is absorbed.

Answer 1

The molecule makes a transition from a lower to a higher vibrational state.

Question 2

Name the model used to understand the transition from lower to higher vibrational state.

Answer 2

Simple harmonic oscillator.

Question 3

What is the equation for calculating the reduced mass of an atom combination?

Answer 3

μ = m1 x m2 / m1 + m2

μ = reduced mass
m1 = mass of atom 1
m2 = mass of atom 2

Question 4

How would you use the reduced mass equation?

Answer 4

Input values and multiply by one atomic mass unit - 1.66E-027

Question 5

What is the equation to calculate the natural frequency of a bond?

Answer 5

f = 1/(2π) (k/μ)^1/2

f = natural frequency
k = force constant
μ = reduced mass

Question 6

How is the wavenumber where high absorption is expected to be found on an IR spectrum calculated?

Answer 6

c = f λ
WAVENUMBER = 1 / λ

Question 7

If the absorption band is sharp how would you explain in terms of energy levels generated why the simple harmonic oscillator model is useful in this case?

Answer 7

For true simple harmonic oscillation the energy for all allowed transition is E = h f, therefore there is sharp absorption.