Density functional calculations of NMR shielding tensors for paramagnetic systems with arbitrary spin multiplicity:: Validation on 3d metallocenes

The calculation of nuclear shieldings for paramagnetic molecules has been implemented in the ReSpect program, which allows the use of modern density functional methods with accurate treatments of spin-orbit effects for all relevant terms up to order O(alpha(4)) in the fine structure constant. Compared to previous implementations, the methodology has been extended to compounds of arbitrary spin multiplicity. Effects of zero-field splittings in high-spin systems are approximately accounted for. Validation of the new implementation is carried out for the C-13 and H-1 NMR signal shifts of the 3d metallocenes (VCp2)-V-4, (CrCp2)-Cr-3, (MnCp2)-Mn-2, (MnCp2)-Mn-6, (CoCp2)-Co-2, and (NiCp2)-Ni-3. Zero-field splitting effects on isotropic shifts tend to be small or negligible. Agreement with experimental isotropic shifts is already good with the BP86 gradient-corrected functional and is further improved by admixture of Hartree-Fock exchange in hybrid functionals. Decomposition of the shieldings confirms the dominant importance of the Fermi-contact shifts, but contributions from spin-orbit dependent terms are frequently also non-negligible. Agreement with C-13 NMR shift tensors from solid-state experiments is of similar quality as for isotropic shifts. (c) 2007 American Institute of Physics.