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Ab initio and LMO studies on the integrated intensities of infrared absorption bands of polyatomic molecules. 6. The formic acid monomer. Influence of isotopic substitution

par Berckmans, Didier ;Figeys, Hubert ;Geerlings, Pau̧l
Référence Journal of Physical Chemistry (1952), 92, 1, page (61-65)
Publication Publié, 1988

Article révisé par les pairs

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Résumé :

Ab initio calculations are performed on the integrated intensities of the in-plane vibrations of the four H,D isotope combinations of the formic acid monomer. Within the framework of the double harmonic approximation and the finite difference approach for calculating the (∂μ/∂Qi)0 derivatives, the STO-3G minimal basis yields intensity results which deviate a factor 6.7 from the experimental values, known for the C=O, CH(D), and OH(D) stretching vibrations. Consequently this basis is inadequate for further work in this direction (influence of dimerization). The moderately sized "3.1" basis yields highly satisfactory results (average deviation factor 1.2) even better than the extended DTZPD basis (deviation factor 1.6). The CH(D) and OH(D) stretching intensities are analyzed in detail. Both theory and experiment indicate a ACD/ACH ratio of about 1.5 whereas the AOD/AOH ratio is of the order of 0.5, the value expected on the basis of a perfectly localized oscillator. The reason for the large deviation from this value in the CH(D) vibration is found to be related to the much larger vibrational impurity of the CH(D) mode as compared to the OH(D) case, the effects in the L-1 or Lτ matrices being reinforced in the intensity calculations as seen when the (∂μ/∂Qi)0 is decomposed in terms of the contributions of the various internal coordinates (∂μ/∂Rj)0. A LMO decomposition of (∂μ/∂QOH(D))0 shows that the most important contributions to the intensity of this mode originate from electronic rearrangements in the OH bond, where an incomplete orbital following effect upon stretching can be observed. © 1988 American Chemical Society.