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 cholesteryl acetate, we have performed a semi-empirical quantum calculation. In a previous work, among a set of nine configurations, two minima of the total energy of this cholesteryl alkanoate (corresponding to the two configurations labeled Conf. II and Conf. VII) have been observed. We have suggested that the transition from one configuration to the other is made via an intermediate configuration (New Conf.). In recent calculations, the passage from configuration II for this molecule to the intermediate configuration has been established via a proton transfer and a rotation of the acetyl group. In this paper, we study the passage from configuration VII to the intermediate configuration. For this process, three radical substitutions and a rotation of the iso-octyl group are necessary. From these new calculations, the activation energy of this reaction path is evaluated as the total activation energy which permits the passage from configuration II to configuration VII. This activation energy has been compared with the energy provided by the incident laser on the cholesteryl acetate in the case of the micro-Raman experiments.