A study of nonrigid aromatic molecules by supersonic molecular jet spectroscopy. II. Propyltoluenes

Abstract

Dispersed emission (DE) and time of flight mass spectra (TOFMS) are presented for supersonic molecular jet-cooled o-, m-, and p-n-propyltoluene. The spectra exhibit multiple origins which are assigned to anti and gauche conformations of the propyl group relative to the aromatic ring. The TOFMS of m-n-propyltoluene rules out an eclipsed propyl conformation as a populated conformation. Empirical force field (EFF) calculations are presented which support these assignments. The spectra also exhibit features, typically within 100 cm−1 of the origin, which are assigned to transitions associated with the internal rotation of the ring methyl group. A model which treats this methyl motion as a one-dimensional rigid rotor is used to solve for the rotational constant B of the methyl rotor, and for the size and shape of the barrier to rotation. The barrier for p-n-propyltoluene is found to be small in both the ground (S0−V6=5 cm−1) and excited (S1−V6=20 cm−1) electronic states. For m-n-propyltoluene, the ground state is again found to have a low barrier (V6=23 cm−1), but the excited state has a potential barrier of V3=75 cm−1. The barrier for o-n-propyltoluene in the ground state is observed to be higher (V3=64 cm−1) than for the other two isomers. The first excited electronic state of this latter compound displays a multitude of levels in the TOFMS which are explained by a double-rotor model involving the ring methyl group (V3=72 cm−1, V6=−14 cm−1) and terminal methyl group of the propyl chain (V3=106 cm−1, V6=−13 cm−1). The results are indicative of an interaction between the ring methyl group and the propyl chain. In all three n-propyltoluene isomers, more than one origin peak is present in the TOFMS, indicating the presence of multiple stable propyl conformations for each isomer. The propyltoluenes thus demonstrate two types of nonrigid molecular behavior: internal rotational motion associated with the ring methyl group, and potentially three stable conformations of the propyl chain with respect to the aromatic ring.