Ab initio calculation of 77Se NMR shielding constants in H_2Se and SeF_6 and other second-order properties of H_2Se

Ab initio random phase approximation first-order polarization propagator calculations using medium size polarized bases yield a 77Se isotropic NMR shielding constant αasSe of 2163 ppm for H2Se at its equilibrium geometry. After relativistic correction is made to the diamagnetic shielding, the resulting calculated shielding of not, vert, similar2426 ppm is in fortuitously good agreement with the recently determined experimental value of 2401 ppm. The calculated shielding decreases rapidly with increasing Se---H distance but is almost independent of H---Se---H angle. The calculated anisotropy in αSe is 665.9 ppm, compared to an experimental value of 375.2 ppm in H2Se(s) at 77 K. The calculated static electric dipolar polarizability, α, is 27.35 a03 compared to an experimental value of 30.86 a03 in the gas phase. Values are also reported for frequency-dependent polarizabilities and the calculated dispersion is related to the experimental UV spectral properties of H2Se. A calculation on SeF6 using a less flexible Se basis gives a deshielding of 598 ppm vs H2Se, considerably smaller than the 965 ppm deshielding observed experimentally.