Vibrational Spectroscopic Studies of Intermolecular Dynamics in Organic Liquids

Abstract

Vibrational band shapes in liquids are affected principally by two types of dynamic process. These are reorientational motion and vibrational (phase) relaxation. However, as table 1 shows, these are not the only possible process. They are the ones which are expected to modulate the vibrational co-ordinate on the timescale of the experiment since conventional absorption or light scattering techniques probe the 0.1 to 200 psec regime. Other techniques may, of course, be employed to study these and other processes and they are the subject of other chapters in this volume. As far as this chapter is concerned, the important concept is that of a “dynamic probe” — a “tagged” molecule used to “probe” the intermolecular potential which is modulated by the dynamic processes involved. The oscillator (vibrator) of interest is thus also modulated by the fluctuating intermolecular potential of the surrounding molecules (fig. 1). At least in principle, therefore, it is possible to extract information about the intermolecular potential from the resulting spectral band shape. In a quantum mechanical sense this concept is described [1-3] through the transition probability (B) for the mode involved (fig. 2). For the infrared transition of a particular normal mode, Q_ℓ, this is,

$$ {{\rm{B}}_{{\rm{fi}}}} \propto {\left[ { < {\rm{f|}}\hat \mu {\rm{|i}}} \right]^2} $$

(1)

where \hat \mu can be expanded using,

$$ \hat \mu = {\hat \mu _{\rm{o}}} + {\left( {{{\partial \mu } \over {\partial {Q_\ell }}}} \right)_o}\hat Q + {1 \over 2}{\left( {{{{\partial ^2}\mu } \over {\partial Q_\ell ^2}}} \right)_o}{\hat Q^2} + ...... $$

(2)