CH-stretching overtone spectra of a fast rotating methyl group. II. Toluenes C6D5CH2D and C6D5CHD2

Infrared spectra of dimethylphenanthrenes in the gas phase

Infrared spectra of atmospherically and astronomically important dimethylphenanthrenes (DMPs), namely 1,9-DMP, 2,4-DMP, and 3,9-DMP, were recorded in the gas phase from 400 to 4000 cm(-1) with a resolution of 0.5 cm(-1) at 110 °C using a 7.2 m gas cell. DFT calculations at the B3LYP/6-311G** level were carried out to get the harmonic and anharmonic frequencies and their corresponding intensities for the assignment of the observed bands. However, spectral assignments could not be made unambiguously using anharmonic or selectively scaled harmonic frequencies. Therefore, the scaled quantum mechanical (SQM) force field analysis method was adopted to achieve more accurate assignments. In this method force fields instead of frequencies were scaled. The cartesian force field matrix obtained from the gaussian calculations was converted to a nonredundant local coordinate force field matrix and then the force fields were scaled to match experimental frequencies in a consistent manner using a modified version of the UMAT program of the QCPE package. Potential energy distributions (PEDs) of the normal modes in terms of nonredundant local coordinates obtained from these calculations helped us derive the nature of the vibration at each frequency. The intensity of observed bands in the experimental spectra was calculated using estimated vapor pressures of the DMPs. An error analysis of the mean deviation between experimental and calculated intensities reveal that the observed methyl C-H stretching intensity deviates more compared to the aromatic C-H and non C-H stretching bands.

CH-stretching Overtone Spectra of a Fast Rotating Methyl Group: 2-CH3 and 2-CHD2 Pyridines

The CH-stretching overtone spectra of the methyl group in gaseous 2-CH(3) and 2-CHD(2) methylpyridines are recorded with conventional Fourier transform near-infrared spectroscopy in the Deltav(CH) = 1-4 regions and by intracavity laser photoacoustic spectroscopy in the Deltav(CH) = 5 and 6 regions. All spectra exhibit a complex structure. They are analyzed with a theoretical model that incorporates, within the adiabatic approximation, the coupling of the anharmonic CH-stretch vibrations described by Morse potentials with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving the six angle deformation modes of the methyl group. The molecular vibrations are calculated in terms of redundant internal coordinates in an unambiguous canonical form. A simultaneous analysis of different isotopic derivatives is thus achieved. The Fermi resonance coupling parameters are those previously determined for toluene. The technique of diabatic rotations is used to disentangle the multiple avoided crossings occurring along the internal rotation coordinate theta in the calculated spectra, which become rapidly very dense owing to the low symmetry of the system. This simulation is successful in reproducing the experimental spectra. In addition, the transferrability of the Fermi resonance coupling parameters between two parent molecules is demonstrated.