Methyl rotor dependent vibrational interactions in toluene

Abstract

The methyl rotor dependence of a three state Fermi resonance in S 1 toluene at ∼460 cm −1 has been investigated using two-dimensional laser induced fluorescence. An earlier time-resolved study has shown the Fermi resonance levels to have different energy spacings at the two lowest methyl rotor states, m = 0 and 1 [J. A. Davies, A. M. Green, and K. L. Reid, Phys. Chem. Chem. Phys. 12, 9872 (2010)]. The overlapped m = 0 and 1 spectral features have been separated to provide direct spectral evidence for the m dependence of the resonance. The resonance has been probed at m = 3a 1 for the first time and found to be absent, providing further evidence for a large change in the interaction with m. Deperturbing the resonance at m = 0 and 1 reveals that the m dependence arises through differences in the separations of the "zero-order," locally coupled states. It is shown that this is the result of the local "zero-order" states being perturbed by long-range torsion-vibration coupling that shifts their energy by small amounts. The m dependence of the shifts arises from the m = ±3n (n = 1, 2, . . . ) coupling selection rule associated with torsion-rotation coupling in combination with the m 2 scaling of the rotor energies, which changes the E for the interaction for each m. There is also an increase in the number of states that can couple to m = 1 compared with m = 0. Consideration of the magnitude of reported torsion-rotation coupling constants suggests that this effect is likely to be pervasive in molecules with methyl rotors.

Vibrations of the low energy states of toluene ($\tilde X$X̃ 1A1 and $\tilde A$Ã 1B2) and the toluene cation ($\tilde X$X̃ 2B1)

“…3 A corresponding feature in the Tol-d 3 spectrum may be seen to appear at 529.1 cm -1 . We note that Gascooke and Lawrance 44 have been able to show that this feature is made of slightly displaced components arising from m = 0 and m = 1, to which we are not sensitive in the ZEKE spectrum.…”

“…There is no obvious fundamental or combination band which would appear in this region, but it is possible that vibration-internal rotor combination bands may contribute. We note that Gascooke and Lawrance 44 calculate that the M 14 M 20 (m = 4) band should appear in this region at ∼457 cm -1 , which has the correct symmetry (totally symmetric). This would place it very close to the M 11 band and so it also could be responsible for the "filling in" of the rotational profile.…”

“…As will be discussed later, when referring to the time-resolved experiments of Davies et al 41 it has been hypothesized that the Fermi resonance feature is, in fact, made up of three main vibrational contributors, each of which has m = 0 and m = 1 components. Furthermore, Gascooke and Lawrance 44 have shown that these vibration-torsion levels are each interacting with other vibration-torsion levels, but to differing extents. As discussed therein, in the case of band B, this is made up of m = 0 and m = 1 components, which have been shifted enough to lead to an obscuring of the band gaps of each feature.…”

“…This feature could then interact with the other states, in a manner described by Gascooke and Lawrance. 44 Since we recorded a ZEKE spectrum for band E, presented as the bottom trace in Figure 4, we examine this for further insight.…”

“…We shall also compare our ZEKE results to the dispersed fluorescence spectra of Hickman et al, 3 gaining further insight into the assignment of the S 1 levels. Additionally, in an accompanying paper, Gascooke and Lawrance 44 present their results obtained from a detailed two-dimensional laser-induced fluorescence (2D-LIF) study of the Fermi resonance features at ∼460 cm -1 . As will be seen, there is excellent agreement between the two sets of results regarding those features, as well as with the results of the time-resolved photoelectron study.…”