Vibrational Spectroscopy in Solutions

Vibrational spectroscopy is a powerful tool for studying solvation in polar solvents. These solvent molecules have permanent dipole moments and therefore absorb radiation in the infrared region of the electromagnetic spectrum due to the vibrational modes of motion. In the present section, the principal experimental techniques used in vibrational spectroscopy are outlined with emphasis on methods for analyzing solvent structure and solvation. 

Estimate the force constant for the C-C bond in acetonitrile using the harmonic oscillator approximation, given that the frequency of the C-C stretching mode expressed in wave numbers is 918cm .  

In the harmonic oscillator approximation the vibrational frequency is given by 

There are two kinds of vibrational spectroscopy commonly used to study solvent structure and solvation, namely, infrared and Raman spectroscopy. These methods differ significantly with respect to the nature of the vibrational features which may be observed. Therefore, some fundamental concepts and experimental methods are introduced before specific results are presented. 

Infrared spectroscopy is normally carried out in the mid-infrared region, that is, from 4000 to 200 cm. The near infrared region is at higher frequencies (12,500 to 4000cm ) and the far infrared, at lower frequencies (200 to 10cm ). Experiments in these regions require special techniques and are not considered further here. 

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