Capillary column gas chromatographic method for the study of dynamic intramolecular interconversion behaviour

Abstract

Reversible reactions which proceed unimolecularly and are mediated by temperature may be suited to gas chromatographic (GC) study. This paper describes the study of a series of intramolecular sterically-hindered isomerisations of some novel aromatics in which the isomers possess different capacity factors on the column, and where by appropriate selection of operating temperature and gas flow-rate, the extent of interconversion can be altered. Restricted rotation about the CN bond in oximes can also be readily observed and quantified as it occurs on the GC column, leading to derivation of rate and activation parameters (for forward and reverse processes). Capillary columns enhance the resolution of isomers and interpretation of data. Some equations are presented to describe the on-column phenomenon in a simplified way. The route to activation parameters afforded by GC, especially at high temperatures, is a possible adjunct to the use of variable-temperature NMR studies. Activation energies in the order of 100–140 kJ mol −1 were obtained for the sterically-hindered aromatics, and the oximes gave energies of about 70 kJ mol −1 by the GC method, but coalescence of NMR signals was not observed even up to 150°C.