Abstract: Cyclic nitroxides with several bulky alkyl substituents adjacent to the nitroxide group are known to demonstrate a much higher stability to bioreduction than their tetramethyl analogues. Among these so-called “sterically shielded” nitroxides, the pyrrolidine derivatives are the most stable. The EPR spectra of some sterically shielded pyrrolidine-1-oxyls were reported to show one or two large additional doublet splittings with a hyperfine coupling (hfc) constant (ca. 0.2–0.4 mT). To determine the origin of these hfc, a series of 2-R-2,5,5-triethyl-3,4-bis(hydroxymethyl)-pyrrolidine-1-oxyls with methylene groups stereospecifically enriched with deuterium were prepared, and their CW EPR spectra were studied. In addition, these nitroxides were investigated using quantum chemical calculations on the UB3LYP/def2-TZVP level and NBO analysis. The apparent constants were assigned to hfc with γ-hydrogen in the side chain, with the contribution of the NBO orbital βπ*(N–O) to the natural localized molecular orbital βσ(C–H) playing the major role. This interaction is efficient if the ethyl substituent is in the pseudoaxial position of the ring and the CH2–CH3 bond is codirected with (parallel to) N–O. The apparent constant aH increases with the Boltzmann population of this conformation.

Cite: Polienko Y.F. , Dobrynin S.A. , Lomanovich K.A. , Brovko A.O. , Bagryanskaya E.G. , Kirilyuk I.A.
Origin of Long-Range Hyperfine Couplings in the EPR Spectra of 2,2,5,5-Tetraethylpyrrolidine-1-oxyls
ACS Omega. 2023. Т.8. №41. С.38723–38732. DOI: 10.1021/acsomega.3c06090 WOS Scopus РИНЦ

Dates:

Submitted:	Aug 17, 2023
Accepted:	Sep 8, 2023
Published online:	Oct 9, 2023

Identifiers:

Web of science	WOS:001082659700001
Scopus	2-s2.0-85176150487
Elibrary	63554578
OpenAlex	W4387461152

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