Structural Information on Large Amplitude Motions

Abstract

Several molecular structure concepts based on differing approximations (e.g. r _e ,〈r〉)are summarised and a case is made for the chemical suitability of r _p, the most probable structure. This structural concept is particularly useful when considering molecules that undergo large-amplitude motions (LAMs). Structural studies on various cumulenones and related molecules are used as illustration.

The value of using the bender approximation for analysis of rotation-vibration spectra of molecules that have LAMs is illustrated. When the semi-rigid bender approximation is employed we gain the further insight of having values for some semi-rigidity parameters. These display flexing effects of the structures that are well predicted by using simple arguments based on resonance structures. Ab initio molecular orbital calculations provide rough quantitative predictions of the semi-rigidity parameters.

Some problems remain in this field of structural chemistry: even quite elaborate molecular orbital calculations occasionally do not correctly predict the shapes of flexible molecules, in spite of using large basis sets and including allowance for electron correlation, as illustrated by data on butatrienone; cases of molecules that undergo two or more LAMs are proving a challenge although progress in analysing spectra has been made in some instances.