Microwave spectrum and structure of trimethylphosphine selenide

Abstract The microwave spectra of (CH 3 ) 3 P 82 Se, (CH 3 ) 3 P 80 Se, (CH 3 ) 3 P 78 Se and (CH 3 ) 3 P 76 Se from 26.5 to 39.0 GHz have been measured at ambient temperature and the observed transitions have been assigned for the ground vibrational states for each isotope. With an assumed carbon—hydrogen distance of 1.090 A the following r 0 structural parameters have been determined: r (PSe) = 2.091 ± 0.017 A; r (PC) = 1.817 ± 0.006 A; ∠CPSe = 113.8 ± 0.9°; and ∠HCP = 110.1±1.9°. The bond length for the PSe bond lies close to the value expected for a double bond and it is in agreement with the previously reported value for this bond from an electron diffraction study. Ab initio calculations with the STO-3G* basis set have been carried out and compared with the experimental values.