Supersonic jet electronic spectroscopy of benzylidenecyclopentane and benzylidenecyclohexane

Cable, J. R.; Westrick, N. R.
October 1994
Journal of Chemical Physics;10/15/1994, Vol. 101 Issue 8, p6455
Academic Journal
The resonance-enhanced ionization spectra of the S1←S0 transitions in jet-cooled benzylidenecyclopentane and benzylidenecyclohexane are presented and analyzed to characterize the torsional conformation of these two compounds in their ground and excited electronic states. Both spectra display considerable vibronic activity in low-frequency modes which are associated with torsional motion. In the cyclopentane derivative, a single conformation is found in the ground electronic state with a torsional angle of 23° which is consistent with a symmetric double-well torsional potential in which the torsional coordinate also involves inversion of the cyclopentane ring. A planar excited-state conformation is suggested by a hindered rotor analysis of the torsional energy levels. The cyclohexane compound is found in two ground-state conformations in a He expansion but only a single conformation in an Ar expansion, suggesting that conformer interconversion can occur over a relatively low barrier. Much larger steric interactions are found in this compound which are consistent with a chair conformation of the cyclohexane ring as suggested by semiempirical molecular orbital calculations. Both electronic states of benzylidenecyclohexane appear to have nonzero torsional angles which are considerably larger in magnitude than in the cyclopentane derivative.


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