Conformational studies by vibrational spectroscopy: a review of various methods

Abstract

The more important methods employed in conformational studies by infrared and Raman spectroscopy have been discussed. They include experimental methods to identify the observed vibrational bands with one or more conformers: crystallization under low temperature and high pressure conditions, variable temperature spectroscopy in the vapour and liquid states and matrix isolation spectroscopy with the hot nozzle method, including the effects of annealing and of photolysis. Differential scanning calorimetry is included, and certain molecules which have two or more solid phases each with a separate conformer are mentioned. An additional technique is spectral recordings of the compound as a solute in solvents of different polarity. Sometimes the formation of clathrates may favour one conformer. Experimental determinations of enthalpies and barriers using variable temperature studies and the analysis of torsional or puckering potentials are described. Computational methods including normal coordinate calculations, molecular mechanics and ab initio quantum chemical methods are included. The methods have been illustrated with compounds from the literature including substituted ethanes, cyclohexanes, cyclobutanes, neopentanes and related molecules.