Methyl torsional barriers in different electronic states. Simultaneous determination from the rotationally resolved fluorescence excitation spectrum of a large molecule

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Abstract
Two types of splittings have been observed in the rotationally resolved fluorescence excitation spectrum of the 000 band in the S1 ←S0 electronic transition of 1-methylnaphthalene at ∼315 nm. One type is independent of J and K−1 and is produced by small differences in the A/E methyl torsional level spacings in the two electronic states. The second type is independent of J but dependent on K−1. It has its origin in a coupling of the methyl torsion with overall molecular rotation, which produces even smaller differences in the spacings of the S0 and S1 E levels. A simultaneous fit of both types of splittings yields the threefold barrier heights in the zero-point vibrational levels of both states, V3 (S0 )=295.2 and V3 (S1 )=292.6 cm−1. The ground state value of V3 is very different from that which has been reported earlier.