Torsion–rotation analysis of OH stretch overtone–torsion combination bands in methanol

Abstract

We report rotationally resolved spectra of jet-cooled methanol for the OH stretch overtones, 2v 1 and 3v 1 , and for the torsional combinations, 2v 1 ϩv 12 , 2v 1 ϩ2v 12 , 3v 1 ϩv 12 , and 3v 1 ϩ2v 12 . The spectra are obtained by direct excitation from the vibrational ground state with an infrared laser pulse. Population in the resulting upper state levels is detected by infrared laser assisted photofragment spectroscopy ͑IRLAPS͒. Global fits of the spectra to the Herbst Hamiltonian yield the torsional and rotational parameters, including F, , V 3 , and V 6 , for each OH stretch excited state. For each quantum of OH stretch excitation, we find that the torsional barrier height V 3 increases by 40.9Ϯ1.9 cm Ϫ1 and the torsional inertial F decreases by 0.89Ϯ0.02 cm Ϫ1 . With reference to ab initio calculations, we explain the increase in V 3 in terms of changes in the electronic structure of methanol as the OH bond is elongated. For ⌬v 12 ϭ1 we observe only transitions with ⌬KϭϮ1, and for ⌬v 12 ϭ2 we observe only ⌬Kϭ0. We present a Franck-Condon model to explain these apparent selection rules and the overall pattern of intensity.

Doppler-limited CW infrared cavity ringdown spectroscopy of the ν1+ν3 OH+CH stretch combination band of jet-cooled methanol

“…Therefore, we expect the Herbst Hamiltonian to do as well for this band as for other ''normal'' bands. This expectation is supported by comparison of m 1 þ m 3 with other bands: the residuals are less than those for the 2m 1 and 3m 1 bands [11] and the perturbations are fewer in number and weaker in strength than in the m 1 band ( Table 5). …”

“…In the OH stretch overtones, the ranges of the torsion-vibration energies vary considerably from 13 cm À1 in the vibrational ground state and 2 cm À1 for 6m 1 [10]. When one quantum of torsion is excited in addition to a number of OH stretch quanta, the range increases to about 70 cm À1 and the pattern is inverted [11]. When two quanta of torsion are excited, Table 7 Torsion-vibration energies (in cm À1 ) for the m 1 þ m 3 band of methanol the range increases to about 160 cm À1 [11].…”

“…Kleiner et al [36] implemented the Herbst Hamiltonian in a fitting program that was later applied to methanol by Xu and Hougen [34,37]. The Hamiltoninan correctly predicts the torsional structure of the ground state of methanol [34] and of some vibrationally excited states including the OH stretch fundamental m 1 [28] and its overtones [11]. However, the fundamentals that correlate to the degenerate E species in the C 3v conformation (in which the C-O-H group is linear) exhibit a non-trival geometric phase.…”

“…This was necessary because F 0 , the torsional barrier V 0 3 and the band origin m 0 are strongly correlated and cannot be determined independently from the present data set, which contains no information from torsionally excited levels in the upper state. However, previous work on methanol OH stretch overtones [11] has shown that the changes in F 0 caused by vibrational excitation can be estimated reliably as a linear extrapolation from lower levels.…”

“…Fehrensen et al [4] have shown that this kind of inverted torsional structure occurs when a small amplitude vibration accumulates a geometric phase of e ip when the torsional angle is changed by 2p. Infrared laser enhanced photodissociation spectroscopy (IR-LAPS) has shown that the torsional barrier increases substantially with OH stretch overtone excitation [10,11].…”