Direct magnetic-field dependence of NMR chemical shift.

Nuclear shielding and chemical shift are considered independent of the magnetic-field strength. Ramsey proposed on theoretical grounds in 1970 that this may not be valid for heavy nuclei. Here we present experimental evidence for the direct field dependence of shielding, using Co-59 shielding in Co(acac)(3) [tris(acetylacetonate)cobalt(iii)] as an example. We carry out NMR experiments in four field strengths for this low-spin diamagnetic Co(iii) complex, which features a very large and negative nuclear shielding constant of the central Co nucleus. This is due to a magnetically accessible, low-energy e(g) <- t(2g) orbital excitation of the d(6) system. The experiments result in temperature-dependent magnetic-field dependence of -5.7 to -5.2 ppb T-2 of the Co-59 shielding constant, arising from the direct modification of the electron cloud of the complex by the field. First-principles multiconfigurational non-linear response theory calculations verify the sign and order of magnitude of the experimental results.