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Ab initio and localised molecular orbital studies of the integrated intensities of infrared absorption bands of polyatomic molecules. Part 5. - Minimal- and extended-basis-set results for the pyramidal AX3 series (A = N, P; X = H, F)

par Figeys, Hubert ;Berckmans, Didier ;Geerlings, Pau̧l
Référence Journal of the Chemical Society. Faraday transactions II, 77, 11, page (2091-2103)
Publication Publié, 1981

Article révisé par les pairs

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Résumé :

Ab initio SCF-GTO type calculations on the dipole-moment derivatives with respect to the normal coordinates of vibration (∂μ/∂Qi)0 for pyramidal AX3 molecules (A = N, P; X = H, F) are reported. A comparison between theoretical and experimental infrared intensity values Ai shows that double-zeta (DZ) values are generally worse than those obtained with a minimal (STO-3G) basis set, which may be qualified as satisfactory. For NH3 and PH3 calculations with more extended basis sets, including polarization functions, (DZ + P) were also performed. Finally, diffuse functions were included in the calculations for NH3; this leads to a substantial improvement of the DZ or "DZ + P" results. A "DTZPD" basis for NH3 (double-zeta for N, triple-zeta for H, including polarization and diffuse functions) was set up; this gives (∂μ/∂Qi)0 values which are close to recently published near-Hartree-Fock results. The sign instability of the stretching derivatives upon change of atomic basis is discussed in terms of derivatives with respect to symmetry coordinates. The "DTZPD" signs for the (∂μ/∂Qi)0 in NH3 are identical to those obtained in the STO-3G case, except for Q3. The STO-3G dipole-moment derivative calculations reproduce the experimental sequence for the A1, A2 and A3 integrated intensities along the AX3 series; this sequence is interpreted via a combined investigation of vibrational and electronic factors. The smaller A1 value for NH3 as compared with NF3 is vibrational in origin, whereas the smaller A1 intensity of NF3 as compared with PF3 is essentially due to electronic factors. The A2 sequence is analysed in terms of the N or P lone-pair behaviour during the bending mode; the lone pairs of the fluorine atoms present in the molecule are seen to have a drastic influence on the intensities; incomplete orbital following of the AX bonds is described. The NH3 LMO results in the minimal basis for (∂μ/∂Q1)0 and (∂μ/∂Q2)0 are quite comparable with those obtained with the larger "DTZPD" basis set. For the asymmetric stretching vibration (Q3), the higher calculated (and experimental) intensity in PF3 as compared with NH3 again mainly resides in an important contribution of the fluorine lone pairs to the intensity of this mode, but also in an inversion of sign in the contributions of the corresponding P-F and N-H bonds.