The rotational spectrum of methylarsine

Abstract The rotational spectrum of an organoarsenic compound, methylarsine, CH 3 AsH 2 , was studied in the frequency range 10–80 GHz using the Stark-modulated spectrometer in Oslo, and in the frequency range 150–660 GHz using the Lille spectrometer. The experimental work was augmented by high-level ab initio calculations. The analysis of large amplitude torsional motion in CH 3 AsH 2 was performed using the rho axis method and the RAM36 code modified to take into account the nuclear quadrupole hyperfine structure. The final fit used 48 parameters (including 3 parameters of quadrupole coupling) to give an overall unitless root-mean-square deviation of 0.92 for the dataset consisting of 2753, 991, and 414 transitions belonging, respectively, to the ground, first, and second excited torsional state. Compact grouping of rotational transitions due to the small asymmetry ( κ = − 0.9986 ), torsional splittings due to internal rotation of the CH 3 group, and relatively large hyperfine splittings due to a rather large value of the arsenic nuclear quadrupole moment ( χ a a = 35.4 MHz, χ c c = − 117.2 MHz, χ a c = 115.7 MHz) represent an interesting case of interplay of different splittings in a molecule with a rather strong coupling between internal and overall rotations ( ρ = 0.413 ).