Short and Long Range Vibrational Coupling in Molecules

In the past, in order to describe the normal modes much use has been made of the so-called potential energy distribution (PED). Torkington [31] has first proposed and Morino and Kuchitsu [32] have later generalized the concept of PED defined 

The advantage of PED is that it can be expressed either in internal or group coordinates, thus allowing a more direct description of the main characteristic of the nomal modes. 

A typical case, famous in organic vibrational spectroscopy, is represented by the work on tron -planar n-alkanes by Schachtschneier and Snyder [33] and by Snyder et al. [34] the latter has extended his study to branched paraffins [35], to n-alkanes in the liquid phase [36] and to oligo and poly-ethers [37]. The use of PED has allowed to distinguish clearly between band progressions and the evolution of the normal modes by changing the phase-coupling (see later in this chapter). 

4 Short- and Long-Range Vibrational Coupling in Molecules 

As in this chapter the aim is to understand the vibrations of large and highly disordered and structurally irregular molecules, the basic problem to be faced is whether and to what extent normal vibrations are able to probe the molecular intramolecular environment, i.e., whether normal vibrations are mostly localized or extended over a large portion of the molecular system. 

---