Chapter 3, Problem 27P

Interpretation Introduction

Interpretation:

Six of the nine vibrational degrees of freedom of ${\text{SiF}}_4$ are inactive. IR absorption observed only at 389 and 1030 ${\text{cm}}^{\text{-1}}$ for this compound, is to be explained.

Concept Introduction:

Raman and infrared (IR) spectroscopies are branches of vibrational spectroscopy.

For a mode of vibration to be infrared (IR) active, it must give rise to a change in the molecular electric dipole moment.

For a mode of vibration to be Raman active, it must give rise to a change in the polarizability of the molecule.

Vibrational spectroscopy is concerned with the observation and determination of number of the degrees of vibrational freedom.

The degrees of vibrational freedom depends on the number of atoms in molecule and shape of the molecule.

The number and nature of the symmetry elements of a given molecule are conveniently represented by its point groups and give rise to labels such as ${\text{D}}_{\text{3h}}$, ${\text{T}}_{\text{d}}$, $O_h$, ${\text{I}}_h$, ${\text{D}}_{\text{2d}}$, ${\text{C}}_{\text{2}}$, ${\text{C}}_{\text{3v}}$ etc.