Chapter 11, Problem 11.8DQ

Interpretation Introduction

Interpretation:

It is important to obtain both IR absorption and Raman spectra to characterize the normal modes of benzene in the gas phase. The reason behind this has to be given.

Concept introduction:

Gross selection rule for a normal mode to be infrared activity:

A normal mode is infrared active, if the motion causes the electric dipole moment of the molecule to change.

Selection rule for vibrational spectrum:

During the vibration, the electric dipole moment of the molecule should change.

Gross selection rule for vibrational Raman spectra:

A normal mode is vibrational Raman active, if the motion causes the change in polarizability of the molecule.

According to exclusion rule, no modes can be both infrared and Raman active in a molecule when if it contains a centre of inversion.