No abstract

The harmonic frequencies of acetaldehyde, acetaldehyde-dl , and acetaldehyde44 were obtained experimentally via a comprehensive analysis of the molecules' infrared vibrational spectra. Specifically, the observed fundamental frequencies were corrected for anharmonicity by applying an extensive list of anharmonic factors derived from the overtone and combination bands assigned to features in the infrared spectra of CH3CH0, CH3CD0, and CD3CD0. An ab initio harmonic force field calculated at the MP2/6-31 l++G(3df,p) level of theory yielded fundamental frequencies that were an average of 1.6% too large relative to the experimentally obtained harmonic values. The theoretical force constants were subsequently refined through a least squares fit to the experimental harmonic frequencies. The results of the normal coordinate analysis were then used to convert the acetaldehyde-do, -dl, and -d4 isotopomers' measured absolute intensities to atomic polar tensors.The final potential function was successfully used to predict the observed vibrational spectrum of acetaldehyded3 and led to the reassignment of the v5, v12, and vi3 vibrational bands in the CD3CH0 infrared spectrum.

A method for analyzing asymmetric top rovibrational bands displaying both blended and resolved features is described. The two-phase computational procedure uses a modified version of the asymmetric rotor-band contour program FASTPLOT to generate a preliminary set of upperstate spectroscopic constants. The parameters are subsequently refined by employing the assigned line-fitting formalism of the ASYROT program using both resolved and blended features. The technique is detailed in a comprehensive analysis of the Y17 band of pyridine. Inclusion of quartic centrifugal distortion constants was found to satisfactorily model a high-resolution (0.004 cm-') spectrum of this band, yielding a standard deviation of 0.00137 cm-'. The variation in the rotational parameters with vibrational quantum number is examined in terms of possible weak rovibrational perturbations to the v17 state. An ab initio calculation of the v17/u27 Coriolis coupling constant indicates the observed results are consistent with the interaction of these two vibrational states.

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