CH stretch excitation causes conformational interconversion in ground state methyl vinyl ether but not in methyl nitrite

Abstract

Molecules exhibiting conformational isomerism have been studied by spectrally resolving the infrared laser induced fluorescence from CH stretch fundamental excitation of methyl vinyl ether and from CH stretch fundamental and N=O overtone excitation of methyl nitrite. By comparison of the resolved fluorescence spectra of methyl vinyl ether with its absorbance spectrum, we have observed the first case of state selected ground electronic state intramolecular conformational interconversion in an isolated molecule. The distribution of conformers is altered drastically by the deposition of energy. The cis/trans ratio of the laser excited molecules may also be sensitive to the laser excitation frequency. Excitation of the N=O overtones of both cis methyl nitrite and trans methyl nitrite, which are well separated in frequency, gives resolved fluorescence spectra which mimic the absorbance spectra for cis methyl nitrite and trans methyl nitrite, respectively, thereby placing a lower bound for the conformational interconversion energy barrier from trans to cis of 3325 cm−1. The experiments were performed by exciting either molecule cooled in a pulsed nozzle expansion with light from an infrared optical parametric oscillator. The fluorescence spectra were recorded with a cryogenic Michelson interferometer.