METHYL-GROUP TORSIONAL DYNAMICS FROM ROTATIONALLY RESOLVED ELECTRONIC-SPECTRA - 1-METHYLNAPHTHALENE AND 2-METHYLNAPHTHALENE

Rotationally resolved fluorescence excitation spectra of three vibronic bands in the S1<--S0 transitions of 1- and 2-methylnaphthalene (1 and 2MN) have been obtained. Each band exhibits perturbations that are produced by an interaction between the restricted torsional motion of the attached methyl group and the overall rotational motion of the entire molecule. A complete analysis of these effects yields values of the torsional barrier heights, the rotational constants, and the torsion-rotational perturbation coefficients of all vibronic levels that participate in the transitions. These values depend significantly on the position of the methyl group attachment, on the electronic state of the naphthalene chromophore, and on its vibrational state, as well. For example, V3 (the threefold barrier) decreases from 809 cm-1 in 0(0) 1MN to 128 cm-1 in 0(0) 2MN. D (the largest first-order torsion-rotation perturbation term) increases from 0.03 MHz in 0(0) 1MN to 406 MHz in 0(0) 2MN, a change of more than 4 orders of magnitude. The V3 values of 0(0) and 8BAR1 1MN are 563 and less-than-or-equal-to 373 cm-1, respectively. A full discussion of these dynamically relevant effects and their dependence upon both electronic and geometric factors is given.