AN ABINITIO STUDY OF THE STRUCTURES AND THE INVERSION BARRIERS IN 1,3-CYCLOHEXADIENE, 1,2-DIHYDRONAPHTHALENE AND 9,10-DIHYDROPHENANTHRENE

Ab initio 3-21G geometry optimization calculations performed for both the half-chair and planar conformers of the title compounds are reported. Vibrational analysis shows the nonplanar conformers to be the minimum energy structures, whereas the planar forms represent the transition states for ring inversion. The calculated geometries are in very good correlation with the experimental data available. The effects of the basis set quality up to 6-311G** level in addition to the electron correlation corrections (to the MP4 level) on the calculated barriers for inversion are addressed. The latter effect seems to be more important for the proper description of the barriers in the systems in question. Benzannelation of the 1,3-cyclohexadiene ring increases slightly the barrier for its inversion. The "best theoretical estimations" of the barriers are 3.7, 4.8 and 8.6 kcal mol-1 for the three title compounds, respectively. Even at the relatively high level of the calculations, the ab initio method tends to overestimate, to some extent, the barriers considered.