Ring expansion and isomerization in methyl indene and methylene indene radicals. Quantum chemical and transition‐state theory calculations

Abstract

AbstractRing expansion and isomerization in 1‐ and 2‐methylene indene radicals and isomerizations among the three isomers of methyl indene were studied by the Becke three‐parameter hybrid method with Lee‐Yang‐Parr correlation functional approximation (B3LYP). Structure, energy, and frequency calculations were carried out with the Dunning correlation consistent polarized doubleζ, (cc‐pVDZ) basis set. In contrast to methyl cyclopentadiene, transition states for ring expansion starting from the molecule methyl indene could not be located. The potential energy surfaces for ring expansion in methylene indene radicals consist of several intermediates and transition states and involve two principal pathways. One pathway is associated with cleavage of the five‐membered ring adjacent to the methylene group and formation of a six‐membered ring. In the second pathway, the transition states of the first stage consist of a newly‐formed three‐membered ring fused to the original cyclopentadiene ring. In all cases, the reaction pathways leading to ring expansion include an intermediate that, via an additional transition state, produces an α‐ or (β‐hydronaphthalene radical. The latter, by a fast H‐atom ejection, forms naphthalene. The structure, energetics, and additional parameters on the potential energy surfaces are shown. Several transition states and intermediates are common to both ring expansion and isomerization, so that there are competing parallel pathways that determine the final distribution among the isomerization and ring expansion products. Potential energy surfaces for interisomerization among the various isomers of methyl indene were calculated, and several reaction pathways were suggested.