Triad νn(A1), νt(E), νt′(E) in C3v molecules: Energy and intensity formulation (computer programs)

Abstract The lower frequency IR absorption of mono- and trideuterated hydrides X H 3 D and X D 3 H ( X = C, Si, Ge, Sn) is due to the three bending modes ν n ( A 1 ), ν t ( E ), v t ′ ( E ), forming a triad with rather strong Coriolis couplings. Formulation and computer programs suitable for analyzing such a triad are presented here. Wavenumbers and absolute line strengths are calculated according to the strict selection rules on J and symmetry species, without any restriction on K , so that the perturbation allowed transitions are accounted for in a straightforward manner. Programs have been developed allowing us either to fit the energy parameters in the ground state (via GS differences) and the triad upper states v n = 1, v t = 1, v t ′ = 1, or to predict frequencies and intensities for the whole absorption covered by the triad ν n , ν t , ν t ′ . The application to the analysis of ν 3 , ν 5 , ν 6 of 12 CH 3 D is presented elsewhere.