Calculation of NMR paramagnetic shift temperature dependence in actinide methyl borohydrides

The tacit assumption that the Fermi contact shift in lanthanide and actinide complexes is directly proportional to the total spin, [S-z], is investigated. The individual contributions of the 5f(0), 5f(+/-1), 5f(+/-2) and 5f(+/-3) orbitals (z-axis is the C-3-axis) to the total spin were calculated for the tetrahedral molecules M(BH3CH3)(4) (M = Pa, U and Np) at various temperatures. [S-x], [S-y] and [S-z] were all calculated. It was found that the temperature dependencies for the four components were markedly different and the individual components of the total spin were anisotropic, particularly for the 5f(0) and 5f(+/-1) orbitals, even though the total spin is isotropic. It is demonstrated that, if one assumes the four components of the spin contribute differently to the transfer of spin into the ligand, the observed temperature dependencies for the shifts of H-1, B-11 and C-13 in these compounds can easily be explained. (C) 1998 Elsevier Science S.A. All rights reserved.