High resolution infrared study of SbHD2: The ground state and the Sb-H stretching bands nu1 and 2nu1

High resolution infrared spectra of 121SbHD2 and 123SbHD2 have been studied in the region of ν 1 , the Sb–H stretching fundamental, from 1780 to 1990 cm−1. The 2 ν 1 stretching overtone band of 123SbHD2, located in the 3640–3790 cm−1 range, has also been investigated. The SbHD2 molecule is an asymmetric rotor of Cs symmetry with the asymmetry parameter κ = 0.61. The ν 1 band is of hybrid type, formed by strong C-type and weak B-type transitions, and almost unperturbed. For 123SbHD2, 2092 transitions have been assigned: 70% of these belong to the C component, the other 30% are of B-type. The assigned transitions have been fitted using a Watson type S-reduced Hamiltonian in the III l representation, with a standard deviation of the fit σ = 0.45 × 10−3 cm−1. In order to determine the ground state parameters all possible ground state combination differences (GSCD) have been generated from the ν 1 transitions. In total, 3942 GSCD up to J ″ = 27, K a ″ = 25, and K c ″ = 20 have been fitted with σ = 0.52 × 10−3 cm−1. Only C-type transitions have been observed in the weak 2 ν 1 overtone band. The 556 assigned transitions have been fitted with σ = 2.6 × 10−3 cm−1 using the same Hamiltonian as for ν 1 . In the ν 1 band of 121SbHD2 771 C-type transitions have been assigned, and the v 1 = 1 spectroscopic constants obtained from a fit with σ = 0.70 × 10−3 cm−1. Using 618 GSCD the ground state spectroscopic constants of 121SbHD2 have been derived with σ = 1.0 × 10−3 cm−1. The molecular parameters for the ground and the v 1 = 1 states of the two isotopologues agree well. The quartic theoretical ab initio force field of SbH3 has been used to predict all relevant spectroscopic parameters for 123SbHD2, 121SbHD2, 123SbH2D, and 121SbH2D. Relations between the harmonic frequencies and between the anharmonicity constants obtained in the expanded local mode theory, for the XH3 → XH2D/XHD2 isotopic substitution, have been compared with those obtained in the present study.