DENSITY-FUNCTIONAL STUDIES OF REPRESENTATIVE PERICYCLIC-REACTIONS

Density functional theory (DFT) has traditionally been shunned by comp utational chemists, but has long seen widespread use in the physics co mmunity. Recently, however, DFT has been adopted by the ab initio quan tum chemistry community and much activity has been devoted to refining the methodology and exploring the range of its applicability. We inve stigate the use of DFT (both local (LDF) and non-local (NLDF) spin den sity approximations) to calculate transition and equilibrium structure s for three representative pericyclic reactions: the electrocyclic rin g opening reaction of cyclobutene, the [1,5] sigmatropic hydrogen shif t reaction in (Z)-1,3-pentadiene, and the Diels-Alder cycloaddition re action between ethylene and butadiene. LDF theory tends to overemphasi ze the stability of the ringed structures in each of these reactions. For example, LDF predicts a very low (6 kcal mol-1) barrier to reactio n for the Deils-Alder reaction. NLDF theory substantially improves the calculated reaction barrier (20 kcal mol-1), but it is still low with respect to experiment.