AB-INITIO MOLECULAR-ORBITAL STUDY OF TOLUENE AND ITS DERIVATIVES

Geometries of toluene and eight of its heavy atom derivatives (3-fluorotoluene, 3-chlorotoluene, 2,6-difluorotoluene, 2,6-dichlorotoluene, benzotrifluoride, benzotrichloride, 2,6-difluorobenzotrifluoride, and 2,6-dichlorobenzotrifluoride) were calculated using ab initio molecular orbital techniques. The sixfold barrier of toluene was calculated using various HF optimizations and single point MP2 and MP4SDTQ methods. Among them, one value of the barrier calculated by HF/6-31+G** method is close to the experimental value observed by microwave spectrum. Except that bond angles are affected by internal rotation, most bond lengths and angles of the internal rotational isomer are similar to the original geometry. The more the substituents, the more the deformations found in the benzene ring. HF energies of these molecules and their internal rotational isomers show that not only the 2,6-substitution effect affects the stability of the molecules but also the steric effect.