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Ab initio and LMO studies on the integrated intensities of infrared absorption bands of polyatomic molecules. 7. The formic acid dimer. Influence of hydrogen bonding and isotopic substitution

par Berckmans, Didier ;Figeys, Hubert ;Maréchal, Yves;Geerlings, Pau̧l
Référence Journal of Physical Chemistry (1952), 92, 1, page (66-73)
Publication Publié, 1988

Article révisé par les pairs

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Résumé :

Moderately sized ("3.1" basis) ab initio SCF calculations are performed on the integrated intensities of the IR absorption bands of the formic acid dimer. A finite perturbation approach is used within the framework of the double harmonic approximation. The force field used in the normal-coordinate calculations is a refined Kishida force field yielding an average deviation of less than 1% between theoretical and experimental frequencies. The overall agreement between theoretical and gas-phase experimental intensity values is very satisfactory: an average deviation of 1.3, i.e., almost the same value as for the monomer in the preceding paper, is found. The experimental dimer/monomer intensity ratios are also accounted for in our calculations, the increase in the OH(D) stretching intensity being one order of magnitude larger than the two other stretching modes for which experimental data exist, namely the C=O and CH(D) stretching modes. A vibrational origin is attributed to the experimentally found large deviation from the localized harmonic value for the isotopic intensity ratio ACD/ACH. A comparative study of theoretical and experimental intensities suggests a reconsideration of the obtention of the AOD intensity for (DCOOD)2 from various experimental data. The LMO decomposition of dipole moment derivatives suggests a dynamic assistance effect to be responsible for the large OH intensity increase upon dimerization. The DHA cannot account for the anomalous isotope effect in the OH(D) bands. Exploratory calculations beyond the DHA indicate an important low-frequency (O-H⋯O)-high-frequency (O-H) mechanical coupling whose order of magnitude confirms previous estimates. An electrical anharmonicity effect is also found, its importance, however, being smaller than that proposed earlier. © 1988 American Chemical Society.