A global study of proton transfer mechanisms and structural changes in cholesteryl acetate

In order to verify a hypothesis which could explain the difference between our micro-Raman spectroscopy investigations and the X-ray results on the ch olesteryl acetate, we have performed a semi-empirical quantum calculation. In a previous work, among a set of nine configurations, two minima of the t otal energy of this cholesteryl alkanoate (corresponding to the two configu rations labeled Conf. II and Conf. VII) have been observed. We have suggest ed that the transition from one configuration to the other is made via an i ntermediate configuration (New Conf.). In recent calculations, the passage from configuration II for this molecule to the intermediate configuration h as been established via a proton transfer and a rotation of the acetyl grou p. In this paper. we study the passage from configuration VII to the interm ediate configuration. For this process. three radical substitutions and a r otation of the iso-octyl group are necessary. From these new calculations, the activation energy of this reaction path is evaluated as the total activ ation energy which permits the passage from configuration II to configurati on VII. This activation energy has been compared with the energy provided b y the incident laser on the cholesteryl acetate in the case of the micro-Ra man experiments. (C) 2001 Elsevier Science B.V. All rights reserved.