Remarkable reversal of 13C-NMR assignment in d1, d2 compared to d8, d9 acetylacetonate complexes: analysis and explanation based on solid-state MAS NMR and computations.

摘要: 13C solid-state MAS NMR spectra of a series paramagnetic metal acetylacetonate complexes; [VO(acac)2] (d1, S = ½), [V(acac)3] (d2, 1), [Ni(acac)2(H2O)2] (d8, and [Cu(acac)2] (d9, were assigned using modern shielding calculations. This provided reliable assignment the chemical shifts qualitative insight into hyperfine couplings. Our results show reversal isotropic shifts, δiso(13C), for CH3 CO between d1 d2versus d8 d9 complexes. The change from about -150 ppm (d1,2) to roughly 1000 (d8,9), whereas decrease 800 150 d1,2 d8,9, respectively. was rationalized by comparison total spin-density plots computed contact couplings those corresponding singly occupied molecular orbitals (SOMOs). revealed interplay spin delocalization SOMOs polarization lower-energy MOs, influenced both symmetry d-electron configuration. A large positive shift acting in same direction, their cancellation corresponds small shift. SOMO(s) complexes are σ-like, implying spin-delocalization on groups acac ligand, cancelled only polarization. In contrast, d2 systems π-like CO-shift polarization, which accounts reversed δiso(13C) CO.