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Ab initio and LMO studies on the integrated intensities of infrared absorption bands of polyatomic molecules. Part III. NH3

par Figeys, Hubert ;Berckmans, Didier ;Geerlings, Pau̧l
Référence Journal of molecular structure, 57, C, page (271-281)
Publication Publié, 1979

Article révisé par les pairs

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Résumé :

Minimal basis set (STO-3G) ab initio calculations are performed for the integrated intensities of the four fundamental infrared absorption bands in NH3. The agreement between experimental and theoretical values is excellent except for the asymmetric bending. The results are interpreted via analysis of the dipole moment derivatives into (a) molecular contributions (point-charge, hybridization and homopolar terms), and (b) bond-dipole contributions obtained using a Localized Molecular Orbital (LMO) method. The large difference between the symmetrical stretching and bending intensities is essentially due to hybridization effects. At the LMO level, these effects can be localized in the NH bonds. The unexpectedly low contribution of the nitrogen lone pair to the bending intensity results from a cancellation between the hybridization and homopolar terms. In the asymmetric vibrations, the contribution of the hybridization terms turns out to be appreciably lower for the bending mode, as compared to the stretching vibration. The influence on the intensity difference between the symmetric stretching and bending vibrations of "incomplete orbital following" in the NH bonds is analyzed in detail. This effect is also detectable in the asymmetric modes, although less easily visualized. © 1979.