Effective potential function of internal rotation in the nonrigid approximation

Abstract

The feasibility of the construction of potential functions of internal rotation in the framework of semi- and nonrigid one-dimensional models is considered. The nonrigid approach under discussion is based on using ab initio quantum mechanical calculations to obtain information on molecular geometry relaxation during internal rotation and to find a physically meaningful starting approximation of the potential. Furthermore, the theoretical potential obtained is refined according to the criterion of the best agreement between the calculated and experimental frequencies of the torsional vibrations. During this latter optimization, some additional conditions may be imposed which support the maximal closeness of the results to the starting approximation. The refined potential is sensitive to the accuracy of determination of molecular geometrical parameters. For this reason, the quantum mechanical calculation should be performed at a rather high level of theory (accounting for electron correlation) to obtain reliable results. The comparison of semi- and nonrigid models of internal rotation is performed using nitrobenzene, fluoroacryloyl fluoride, 1,3-butadiene and acryloyl fluoride as examples.