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High resolution electronic spectroscopy of p -toluidine. A precessing rotor model for G 12 molecules

1994; American Institute of Physics; Volume: 100; Issue: 10 Linguagem: Inglês

10.1063/1.466906

1520-9032

Xue-Qing Tan, David W. Pratt,

Spectroscopy and Laser Applications

Based on a study of the high resolution S1←S0 fluorescence excitation spectrum of p-toluidine (p-methylaniline) and related G12 molecules, we propose that the threefold axis of the methyl group is tilted slightly with respect to the symmetry axis of the molecular frame, and exhibits a kind of precessional motion in the course of its hindered internal rotation. We derive a new Hamiltonian to describe this motion and show that it is consistent with previous modifications of the traditional torsion–rotation Hamiltonian first proposed by Wilson, Lin, and Lide [J. Chem. Phys. 23, 136 (1955)]. Applying the new Hamiltonian to the S1←S0 spectrum of p-toluidine, we have determined the sixfold barrier heights V6(S0) = ( − ) 5.6 and V6(S1) = ( − ) 43.9 cm−1, values that are similar to those of toluene and other 4-substituted toluenes.