A [1,5] sigmatropic hydrogen shift induced by high vibrational overtone excitation: The isomerization of 2-methylcyclopentadiene

Abstract

Laser excitation of high CH stretching overtone transitions has been used to induce the isomerization of 2-methylcyclopentadiene to 1-methylcyclopentadiene. The reaction is a [1,5] sigmatropic hydrogen shift and therefore the reaction coordinate involves significant hydrogen atom motion. This provides a reasonable experimental test for the possibility that overtone excitation of CH stretching motions may lead to enhanced unimolecular reaction rates if there is strong coupling between the reaction coordinate and the nuclear motions which are excited by photon absorption. Values of the unimolecular reaction rate constant, k(E), have been deduced from photochemical quantum yields for excitation of the methylenic, methyl, and olefinic fourth CH stretching overtone transitions of 2-methylcyclopentadiene. No rate enhancement is observed for excitation of the methylenic transition compared to the methyl or olefinic transitions. The values of k(E) scale only with total energy and are in reasonable agreement with values calculated from RRKM theory. © 1983 American Chemical Society.