Geometry changes in excited states

Abstract

A brief discussion of the experimental methods of obtaining geometrical information about the equilibrium geometry of molecules in their electronically excited states is followed by a sampling of geometrical information available at present. The significance of simple orbital and symmetry arguments exemplified by Walsh diagrams is stressed. The geometrical tendencies of alkane excited states are studied. We find an excited state of methane that is planar. The lowest excited state of ethane should have longer C—H bonds and a shorter C—C bond than the ground state, and should also prefer an eclipsed conformation. Excited cyclopropane breaks one C—C bond. The lowest n, π* state of benzophenone is computed to be more planar than the ground state, and apparently does not become pyramidal at the carbonyl group. The isocyanide-cyanide rearrangement potential surface cautions us not to assume necessarily that excited state reaction is facilitated by a geometry change bringing closer the geometry of reactant to that of product. © 1970, Walter de Gruyter. All rights reserved.