Reduction of the effective rotational Hamiltonian for pyramidal XY3 molecules with a large-amplitude inversion motion

Abstract The previously described reduction of the effective rotational Hamiltonian for semirigid molecules of C 3v or D 3 point group symmetry [M. R. Aliev and V. T. Aleksanyan, Opt. Spectrosc. 24, 201–206 (1968)] has been extended to nonrigid molecules with a significant inversion splitting of the energy levels (e.g., NH 3 , H 3 O + , CH 3 − , or SiH 3 molecules). Although for semirigid molecules like PH 3 or AsH 3 , the parameters α and η ′ 3 which appear in the terms α [ J + 3 + J − 3 , J z ] + and η ′ 3 ( J + 6 + J − 6 ) are almost completely correlated, the effects of the inversion splitting and the accidental resonance which can occur between the interacting rotational levels in nonrigid molecules make it possible to determine α and η ′ 3 separately. The results of fitting the experimental data for 14 NH 3 and 15 NH 3 [S. Urban, Romola D'Cunha, K. Narahari Rao, and D. Papousek, Canad. J. Phys. 62, 1775–1791 (1984); Romola D'Cunha, S. Urban, K. Narahari Rao, L. Henry, and A. Valentin, J. Mol. Spectrosc. 111, 352–360 (1985)] are in agreement with this conclusion. The possibility of the determination of the sign of η 3 from a simultaneous analysis of the allowed and Δ k = ±3 forbidden transitions in semirigid XY 3 molecules has been discussed.