Electronic spectroscopy of the cis and trans isomers of 1-methyl-2-phenylcyclopropane

Vibrationally resolved electronic spectra of jet-cooled cis and trans isomers of 1-methyl-2-phenylcyclopropane (cis-MPCP and trans-MPCP) were recorded using resonant two-photon ionization. Each isomer was found in a single conformation. Spectroscopic evidence reveals that in the trans isomer the plane of the phenyl ring bisects the 60° internal bond angle of the cyclopropyl ring in both the ground and excited electronic states. Extensive Franck–Condon activity in the low-frequency phenyl torsion mode is observed in the spectrum of cis-MPCP, suggesting that a substantial change in torsional geometry is induced by electronic excitation. Excited state CIS calculations predict that the phenyl ring rotates over 20° toward a bisected conformation in the excited state.