Internal Methyl Rotation in the CH Stretching Overtone Spectra of ortho-, meta-, and para-Xylene

Abstract

The room-temperature vapor phase overtone spectra of o-, m-, and p-xylene have been recorded in the CH stretching region corresponding to Δv CH = 2−6 with conventional near-infrared spectroscopy (Δv CH = 2 and 3), intracavity titanium:sapphire (Δv CH = 4 and 5) and dye laser photoacoustic spectroscopy (Δv CH = 6). Absolute oscillator strengths have been measured from the conventional spectra and relative oscillator strengths within a given overtone from the conventional and photoacoustic spectra. The aryl region of the spectra can be interpreted simply in terms of a number of nonequivalent and independent local modes. The methyl region of the spectra is more complex. The methyl band profiles in the overtone spectra of m-xylene and p-xylene are very similar to each other and to that of toluene and differ significantly from the methyl band profiles in the spectra of o-xylene. We use a simple anharmonic oscillator local mode model with ab initio calculated dipole moment functions to calculate oscillator strengths of the aryl transitions. The methyl band profiles are simulated on the basis of a model that incorporates the harmonically coupled anharmonic oscillator local mode model for the CH stretching modes and a rigid rotor basis for the methyl internal rotation (torsion). The model parameters are calculated ab initio. The dipole moment function is expressed in a series expansion in both the CH displacement coordinate and the torsional angle whereas the frequency, anharmonicity, and torsional potential are expressed in Fourier series of the torsional angle. The difference in the methyl band profiles in o-xylene, and in m- and p-xylene are ascribed mainly to differences in the torsional potential and the dipole moment function. Our simulations have successfully reproduced the methyl band profiles in the xylene spectra and the relative aryl to methyl intensities.

Vibrational Overtone Spectroscopy of Phenol and Its Deuterated Isotopomers

“…Our ICL-PAS setup has been described in detail previously. 49 The cell was filled with argon buffer gas at pressures of 180-200 Torr to increase the photoacoustic signal. Absolute intensities are not determined from our ICL-PAS spectra; however, accurate relative intensities can be obtained when several bands are accessible with the same set of optics.…”

Measurement of ultraweak transitions in the visible region of molecular oxygen

“…Our version of ICL-PAS has been described previously [13]. Briefly, a Coherent Innova Sabre argon ion laser running at all lines was used to pump a Coherent 890 titanium:sapphire tunable laser with short-wave optics.…”

Calculated Band Profiles of the OH-Stretching Transitions in Water Dimer

“…This grid size has previously been shown to provide well-converged DMF derivatives and hence intensities. 37 Terms diagonal in q were calculated for each of the OH-stretching grids associated with the different OO displacements. The off-diagonal expansion coefficients are found by standard numerical techniques as the first derivative (j ) 1) of the diagonal OH-stretching dipole moment expansion coefficients with respect to s. 48 We have inspected the magnitude of the matrix elements, 〈υ′|q i |0〉 and 〈n′|s j |n〉 to select the cutoff value of i and j.…”

“…[34][35][36][37][38][39][40][41][42][43] In this article we have used a similar approach to calculate the OH-stretching band profiles in water dimer. We use a local mode model that includes an OH-stretching vibration and the lowfrequency OO-stretching vibration, which takes place in an effective potential that depends on the OH-stretching state.…”