Spectroscopic analysis demonstrates methyl rotor precession during hindered internal rotation in p-toluidine, highlighting torsional barrier dynamics comparable to toluene analogs.
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.
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Tan et al. (1994) studied this question.
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