Approximate minimum energy reaction surfaces for the cyclobutene–cis-butadiene thermal isomerization

Abstract

A simple procedure is used for obtaining approximate minimum energy sections of potential energy hypersurfaces for reactions of polyatomic molecules. Choice of a couple (a,b) of internal co-ordinates (bond-angles or -lengths) which undergo large changes from reactants to products allows the calculation for a sufficient number of points of the plane (a,b) of all the other angles and bond lengths by use of a modified version of Del Re's Criterion of Maximum Localization. This criterion had been previously shown to produce accurate equilibrium (i.e. minimum energy) geometries for hydrocarbon molecules. Energies are then calculated for the selected geometries. For the thermal ring-opening of cyclobutene to cis-butadiene, approximate minimum energy maps have been drawn for the conrotatory and disrotatory modes. Energies have been calculated simply as sums of contributions due to the localized σ bonds, to the π systems, and to the non-bonded interactions. The two maps are quite different: that of the ‘symmetry allowed’ conrotatory mode shows a connection path between the product valley and the reactant valley which corresponds at every point to a minimum potential energy with respect to the perpendicular direction (reaction path), whereas that corresponding to the ‘symmetry forbidden’ disrotatory mode does not. The ‘forbiddenness’ of the disrotatory mode appears to be due more to this feature than to the fact that the energy barrier is higher than in the favoured mode.