Pyridine: High-resolution analysis of the frequency(17) band, and, Acetaldehyde: Harmonic frequencies, force field, and infrared intensities

A method for analyzing asymmetric top rovibrational bands displaying blended and resolved features was empirically established. The two phase computational procedure uses a modified version of the asymmetric rotor band contour program FASTPLOT to generate a preliminary set of upper state 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 was applied in the analysis of the {dollar}nusb{lcub}17{rcub}{dollar} band of pyridine. Inclusion of quartic centrifugal distortion constants satisfactorally modeled a high resolution (0.004 cm{dollar}sp{lcub}-1{rcub}{dollar}) spectrum of this band, yielding a standard deviation of 0.00137 cm{dollar}sp{lcub}-1{rcub}{dollar}. An ab initio calculation of the {dollar}nusb{lcub}17{rcub}{dollar}/{dollar}nusb{lcub}27{rcub}{dollar} Coriolis coupling constant indicated the variation in the {dollar}nusb{lcub}17{rcub}{dollar} rotational parameters with vibrational quantum number was consistent with an interaction between these two vibrational states.; The anharmonic constants of acetaldehyde, acetaldehyde-d1, and acetaldehyde-d4 were determined from a comprehensive analysis of the molecules' infrared vibrational spectra. The observed fundamental frequencies of these isotopomers were subsequently corrected for anharmonicity. The ab initio harmonic frequencies calculated at the MP2/6-31G{dollar}sp{lcub}*{rcub}{dollar} level were found to be approximately 3% too large when compared with the experimental harmonic values. The MP2/6-31G{dollar}sp{lcub}*{rcub}{dollar} ab initio force field served as a starting point in a normal coordinate calculation. The theoretical force constants were adjusted via a least squares fit to the experimentally obtained harmonic frequencies. The absolute infrared intensities of the d0, d1, and d4 acetaldehyde derivatives were measured and converted to atomic polar tensors with the results of the normal coordinate analysis.; The harmonic frequencies of acetaldehyde-d3 were derived from the revised force field. The anharmonic fundamental frequencies of the d3 species were generated by applying the appropriate anharmonic constants to the calculated harmonic energies. The predicted and observed vibrational spectra correlated well. The results of this study led to the reassignment of the {dollar}nusb5{dollar}, {dollar}nusb{lcub}12{rcub}{dollar}, and {dollar}nusb{lcub}13{rcub}{dollar} vibrational bands in the acetaldehyde-d3 infrared spectrum.