Computational (59)Co NMR Spectroscopy:  Beyond Static Molecules

GIAO-B3LYP computations of (59)Co NMR chemical shifts are reported for CoH(CO)4, Co(CO)4(-), CoCp(C2H4)2, Co(CN)6(3)(-), Co(NH3)3(CN)3, Co(NH3)6(3+), Co(NH3)4(CO3)(+), Co(acac)3, and Co(H2O)6(3+), employing both static calculations for equilibrium geometries as well as methods which include zero-point and classical thermal effects. The zero-point effects were computed by applying a perturbational approach, and the classical thermal effects were evaluated using Car-Parrinello molecular dynamics simulations. Both methods lead to a downfield shift of δ((59)Co) with respect to the equilibrium values, which can be attributed to a large extent to cobalt-ligand bond elongation. In some cases the zero-point and classical thermal corrections improve the agreement between computed and experimental values, but especially for complexes where the experimental NMR data were obtained in aqueous solution, the error increases somewhat. Mean absolute deviations between averaged and experimental δ((59)Co) values are on the order of 500-760 ppm over a chemical shift range of almost 20 000 ppm. The computed structures and properties of three Co2(CO)8 tautomers reproduce the experimental data very well. Two transition states for interconversion of these tautormers were located:  low barriers are obtained, consistent with the observed fluxionality on the NMR time scale. Two model cobaloximes were taken as test cases to study the change of δ((59)Co) upon deuteration three bonds away from the metal. The sizable downfield shift of δ((59)Co) observed on going from H to D is attributed to a changed vibrational wave function, which causes a noticeable cobalt-ligand bond elongation.