Abstract
Abstract : The sterically hindered styrene derivative, alpha-methylstyrene (2- phenyl-propene), is studied by 1-color time-of-flight mass spectroscopy (TOFMS). In contrast to styrene, which has an intense spectral origin transition in the TOFMS, alpha-methylstyrene exhibits a weak origin transition. A progression in a low frequency torsional mode, with an energy level spacing of approx. 69/cm, is built on the origin. The intensity maximum of this progression occurs at the eighth peak position, indicating that the ground and excited state geometries are displaced from one another. The torsional progression is assigned to the hindered rotation of the propenyl group with respect to the aromatic ring. Potential parameters derived from an analysis of the spectra are V2 = 0.0 and V4 = 150.0/cm for S0 and V2 = 4867, V4 = -500 and V6 = -80/cm for S1. Based on hot band transitions in the region of the origin, this torsional mode is assigned and energy level spacing of approx 32/cm in the ground state. In the ground state, the propenyl group is calculated to be at a approx. 31 degree angle from the plane of the aromatic ring. Analysis of the Franck-Condon intensity profile yields a displacement in the excited state, for the angle between the aromatic and ethylenic groups, of 30 degrees relative to the ground state. Alpha- Methylstyrene is suggested to be nearly planar in its first excited pi* state.
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