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 a
H increases
with the Boltzmann population of this conformation.