University of Oulu

Andersen, A., Pyykkönen, A., Jensen, H., McKee, V., Vaara, J., Nielsen, U.; 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. Phys. Chem. Chem. Phys., 2020,22, 8048-8059.

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

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Author: Andersen, Anders B. A.1; Pyykkönen, Ari2; Jensen, Hans Jørgen Aa.1;
Organizations: 1Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej 55, DK-5230 Odense, Denmark
2NMR Research Unit, University of Oulu, FI-90014, Finland
3School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland
Format: article
Version: accepted version
Access: open
Online Access: PDF Full Text (PDF, 3.7 MB)
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Language: English
Published: Royal Society of Chemistry, 2020
Publish Date: 2021-03-23


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

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Series: PCCP. Physical chemistry chemical physics
ISSN: 1463-9076
ISSN-E: 1463-9084
ISSN-L: 1463-9076
Volume: 22
Pages: 8048 - 8059
DOI: 10.1039/D0CP00980F
Type of Publication: A1 Journal article – refereed
Field of Science: 114 Physical sciences
116 Chemical sciences
Funding: We are grateful to the Carlsberg Foundation (grant CF15-0675) for funding towards the X-ray diffractometer. This work was funded by The Danish Council for Independent Research Science and Universe (grant DFF-7014-00198), the Academy of Finland (grant 296292) and University of Oulu (Kvantum Institute).
Academy of Finland Grant Number: 296292
Detailed Information: 296292 (Academy of Finland Funding decision)
Copyright information: © the Owner Societies 2020. The final authenticated version is available online at