A study of nonrigid aromatic molecules by supersonic molecular jet spectroscopy. I. Toluene and the xylenes

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Abstract
Dispersed emission and time of flight mass spectra are presented for jet-cooled toluene, and o-, m-, and p-xylene. The spectra exhibit features, typically within 100 cm−1 of the S1⇄S0 origins, which are assigned to transitions associated with the internal rotation of the ring methyl groups. A model is developed which treats this methyl motion as that of a one-dimensional rigid rotor. The spacings of the peaks in the spectra are used to solve for the rotational constant B of the methyl rotor, and for the size and shape of the n-fold barrier to rotation (i.e., V3, V6, etc.) within this model. For toluene and p-xylene, the barrier is found to be small in both the ground (S0) (V6∼10 cm−1) and excited (S1) (V6∼25 cm−1) electronic states. For m-xylene, the ground state is again found to have a low barrier (V6∼25 cm−1), but the excited state has a potential barrier of V3=81 cm−1, V6=−30 cm−1. The barrier to rotation of the ring methyl groups is observed to be the highest for o-xylene. In this case the ground state is found to have a rather large barrier V3=425 cm−1, V6=18 cm−1 which changes to V3∼166 cm−1, V′3 ∼−25 cm−1, and V6∼0 cm−1 in the excited state. The V3 term represents a potential cross term between the two methyl rotors. The use of a kinetic energy cross term with a weighting coefficient of 0.72 in the Hamiltonian is also required for an accurate description of the excited state of this isomer. Empirical force field (EFF) calculations are performed for toluene and the three xylenes using a molecular orbital–molecular mechanics (MOMM) algorithm. The EFF-MOMM calculations are in essential agreement with the spectroscopic results and the one-dimensional rigid rotor model.