Considerations for reliable calculation of 77Se chemical shifts

The theoretical chemical shifts of a large series of selenium compounds have been calculated using GIAO-MP2 and -DFT methods in several basis sets. Reliable chemical shifts are calculated for many compounds, especially with the mPW1PW91 exchange-correlation functional and either a triple-zeta basis set (tzvp: 13% mean absolute error) or a limited RECP set chosen for practical applications on complex molecules (BSL 11.8% mean absolute error). Molecules with three-center-four-electron bonding or low-lying n ->pi* states require additional diffuse functions and nonperturbative methods, respectively, but terminal selenium anions cannot be calculated reliably in the gas phase due to the neglect of solvation. When these cases are excluded, the mean absolute error decreases from 16.5% to 8.9% in GIAO-MP2/BSL but only slightly for DFT methods.