Direct magnetic-field dependence of NMR chemical shift. Issue 16 (14th April 2020)

• Record Type: Journal Article
• Authors: Kantola, Anu M.; Lantto, Perttu; Heinmaa, Ivo; Vaara, Juha; Jokisaari, Jukka
• Is Part Of: Physical chemistry chemical physics. Volume 22:Issue 16(2020)
• Page Start: 8485
• Page End: 8490
• Publication Date: 2020-04-14
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/d0cp01372b
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : It is proven with experiments and computations the significant contribution of the direct field effect on the 59 Co chemical shift in Co(acac)3 dissolved in CDCl3 . Abstract : 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 59 Co 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 eg ← t2g 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 59 Co 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.