Analyse de la rotation interne de systèmes moléculaires à deux rotateurs fortement couplés : oxyde et sulfure de diphényle

Laboratorio de Química Cuántica, Instituto de Estructura de la Materia, CSIC, Serrano, 119 Madrid-6, Spain.

Résumé

Dans ce travail on étudie deux molécules présentant deux rotateurs non-coaxiaux fortement couplés. Dans ce but. on calcule, au moyen de la méthode CNDO/2, les surfaces d'énergie potentielle. On développe la fonction potentielle en série de Fourier, et analyse la nature des barrières ainsi que les interactions entre atomes liés et non-liés. On trouve que les surfaces doivent leur origine à trois types d’interactions : les effets stériques, la résonance et finalement des forces attractives du type de Van der Waals.

Abstract

In this work, the internal rotation problem for molecules showing two non-axial, strongly coupled rotators is investigated. For that purpose, the potential energy surfaces of the diphenyl oxide and sulphide are calculated using the CNDO/2 method. In both cases, the preferred conformations are found to have a skew structure, and the surfaces are smooth enough to allow, in accordance with the experimental data, a non- rigid structure in which both benzene rings can rotate like cog-wheels.

In order to analyse the intramolecular forces, the two potential energy functions are developed in Fourier expansions, ft is seen that the sinus terms do not converge satisfactorily because of the coupling between the rotating groups. On the other hand, the nature of the barriers and the significance of the interactions between bonded and lion-bonded atoms are investigated. It is found that the potential energy surfaces rise essentially from three kinds of interactions. The first one. repulsive nature, is due to steric effects, the second one, attrac tive. is due lo the conjugation between the benzene rings, and finally the third one, of attractive nature also, is due lo Van der Waals forces.