AB-INITIO COMPUTATION OF SE-77 NMR CHEMICAL-SHIFTS WITH THE IGLO-SCF,THE GIAO-SCF, AND THE GIAO-MP2 METHODS

Ab initio geometries and Se NMR chemical shifts have been computed for Me(2)Se, H2Se, MeHSe, EtSeH, Se(SiH3)2, SeF4, SeF6, SeOF2, H2CSe, Me( 2)CSe, SeCO, CSe2, and selenophene (C4H4Se). The mean absolute deviati ons from experimental delta(Se-77) data are 97, 76, and 78 ppm at the IGLO-SCF/II/Expt., GIAO-SCF, and GIAO-MP2/962+(d)/MP2/962(d) levels, r espectively, over a range of 2800 ppm. For selenium chemical shift cal culations, diffuse functions have to be included in the basis set in o rder to describe the Se long pairs properly, which provide the largest localized valence MO contributions to each Se shielding. A delta(Se-7 7) value of ca. -40 ppm is predicted for gaseous EtSeH. Both SCF and M P2 methods fail to reproduce the delta(Se-77) chemical shift of Se-4(2 +) which requires a multireference description.