An effective Hamiltonian describing globally all vibrational-rotational energy levels of C2H2 molecule in the ground electronic state has been written up to fourth order in Amat-Nielsen ordering scheme using symmetry arguments. This Hamiltonian contains all resonance interaction terms within polyad scheme discussed by M.E.Kellman (J. Chem.Phys. 95. 8671 (1991)). D.M.Jonas et al (J.Chem.Phys. 99, 7350 (1993)). M.Abbouti Temsamani and M.Herman (J.Chem.Phys. 103. 593 1 (1995)). The reduction of this effective Hamiltonian by means of unitary transformations has been performed.
Within the framework of the linked ordering schemes theory of vibration-rotational interactions in molecules the introduction of Herman-Wallis factors into intensities of multiquantum vibrational transitions has been presented in effective dipole moment operator by sequences of types Meff M1 +M2 + M3 + ..., where n = 1, 2, 3The results of grouptheoretical analysis of tensorial parameters in M1 (n 1 ÷ 3) for symmetric top molecules have been presented.Generalization of these results for multiquantum vibrational transitions is analyzed. The another goal of the contribution was the analysis of the ways of the construction of nonpolynomial models for Herman-Wallis factors on the base of the representation of Meffin forms of infinite sequences on angular momentum operators. Some simple models for HW factors in Pade-approximant form are considered. On the base of developed and presented theory of vibration rotation line intensities in molecular spectra the conception offitting molecular dipole moment transition of quasirigid molecules is discussed for the first time.
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