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–rotation 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 torsional barrier) decreases from 809 cm−1 in 00 1MN to 128 cm−1 in 00 2MN. D (the largest first-order torsion–rotation perturbation term) increases from 0.03 MHz in 00 1MN to 406 MHz in 00 2MN, a change of more than 4 orders of magnitude. The V3 values of 00 and 8̄1 1MN are 563 and ≤ 373 cm−1, respectively. A full discussion of these dynamically relevant effects and their dependence upon both electronic and geometric factors is given.

A unique view of the nascent acid-base reaction between 2-naphthol and ammonia along the proton transfer coordinate is provided by analyses of the rotationally resolved S1←S0 electronic spectra of their hydrogen bonded complexes cis- and trans-2HNA in the gas phase. Both complexes, in both electronic states, have structures in which ammonia, acting as a base, forms an in-plane hydrogen bond with the hydroxy hydrogen atom of 2-naphthol. The ground state O–H⋅⋅⋅N heavy atom separations are R=2.77 Å in cis-2HNA and R=2.79 Å in trans-2HNA. Electronic excitation of the significantly more acidic S1 state of 2-naphthol produces large decreases in R in both complexes. S1 cis-2HNA has R=2.62 Å and S1 trans-2HNA has R=2.57 Å. Comparing these results to the Lippincott–Schroeder potential for the hydrogen bond shows that there is little change in the vibrationally averaged position of the hydroxy hydrogen atom. But decreasing R produces significant decreases in the barrier to proton transfer, in the distance from reactant to product along the reaction coordinate, and in the energy difference between them. We thus conclude that whether or not such transfer occurs is primarily dependent on the ability of the two heavy atoms to come into close proximity during the early stages of the reaction, a condition that is not satisfied in either cis- or trans-2HNA, in either electronic state. This view is supported by observed changes in the shapes of the potential surfaces along the NH3 torsional coordinate that occur on S1←S0 excitation of the two complexes.

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High resolution core-level electron spectroscopy on free molecules and atoms AIP Conf.Based on a study of the high resolution SIr-SO fluorescence excitation spectrum of p-toluidine (p-methylaniline) and related G 12 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 SIr-SO spectrum of p-toluidine, we have determined the sixfold barrier heights V 6 (So) = ( -) 5.6 and V 6 (Sl) = ( -) 43.9 cm-I, values that are similar to those of toluene and other 4-substituted toluenes.

Rotationally resolved fluorescence excitation spectra of the 0(0)(0) bands of the S1<--S0 electronic transitions of 2- and 5-methylpyrimidine (2MP and 5MP, respectively) have been observed and assigned. Both spectra were found to contain two sets of rotational lines, one associated with the sigma=0 torsional level and the other associated with the sigma=+/-1 torsional level of the attached methyl group. Analyses of their structure using the appropriate torsion-rotation Hamiltonian yields the methyl group torsional barriers of V6''=1.56 and V6'=8.28 cm(-1) in 2MP and V6''=4.11 and V6'=58.88 cm(-1) in 5MP. Many of the lines in both spectra are fragmented by couplings with lower lying triplet states. Analyses of some of these perturbations yield approximate values of the intersystem crossing matrix elements, from which it is concluded that the sigma=+/-1 torsional levels of the S1 state are significantly more strongly coupled to the T1 state than the sigma=0 torsional levels.