Accurate computations of 77Se NMR chemical shifts with the GIAO-CCSD method

Relative NMR chemical shifts (δ(Se)) have been computed at the GIAO-MBPT(2) and GIAO-CCSD levels for the selenium-containing compounds H2Se, SeCO and CSe2. With a large basis set, 77Se shifts calculated at the GIAO-CCSD level for these molecules are −335, −468 and 281 ppm, respectively, relative to Me2Se. The results for H2Se and CSe2 are in good agreement with gas phase NMR measurements of −345 and 243 ppm, while the value for SeCO is close to a value measured in solution (−447 ppm). Correlation contributions to the shielding are severely overestimated at the GIAO-MBPT(2) level; inclusion of higher-order effects such as those included in GIAO-CCSD is essential for a quantitative description of selenium chemical shifts. The δ(Se) value of 2618 ppm predicted for the transient H2CSe at the GIAO-CCSD level indicates that the selenium nucleus in this molecule is more deshielded than in any other known compound.