CONFORMATIONAL STUDIES BY VIBRATIONAL SPECTROSCOPY - A REVIEW OF VARIOUS METHODS

The more important methods employed in conformational studies by infra red and Raman spectroscopy have been discussed. They include experimen tal methods to identify the observed vibrational bands with one or mor e 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 sca nning calorimetry is included, and certain molecules which have two or more solid phases each with a separate conformer are mentioned. An ad ditional technique is spectral recordings of the compound as a solute in solvents of different polarity. Sometimes the formation of clathrat es may favour one conformer. Experimental determinations of enthalpies and barriers using variable temperature studies and the analysis of t orsional or puckering potentials are described. Computational methods including normal coordinate calculations, molecular mechanics and ab i nitio quantum chemical methods are included. The methods have been ill ustrated with compounds from the literature including substituted etha nes, cyclohexanes, cyclobutanes, neopentanes and related molecules.