Chapter 3, Problem 29P

Interpretation Introduction

Interpretation:

The IR spectra of salts of ${\left[{\text{AlF}}_{\text{6}}\right]}^{\text{3-}}$ ($O_h$) exhibit absorptions around 540 and 570 ${\text{cm}}^{\text{-1}}$. Using a group theory approach, only one of these absorptions arises from a stretching mode is to be confirmed.

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.