Nitrogen-14 and oxygen-17 hyperfine interactions in perturbed nitroxides

We have measured the isotropic and anisotropic hyperfine splittings for di-tert-butyl nitroxide (DTBN) and 2,2,6,6-tetramethylpiperidine N-oxyl (TMPN) perturbed by molecular complex formation, by hydrogen bonding, and by solvent effects in aprotic solvents. We use these measurements to obtain the σ-π interaction parameters for a Karplus-Fraenkel equation (a₁ = Q₁₁ ⁱρi^π + Q_jj ⁱρ_j ^π) which relates the ¹⁴N and ¹⁷O isotropic hyperfine splittings to the spin density in the nitroxide N-O π-electron system; Q_NN ^N = 23.9 ± 1.0 G, Q_OO ^N = 3.6 ± 0.4 G, Q_OO ^O = 31.7 ± 0.7 G, and Q_NN ^O = 4.7 ± 0.5 G. In addition for protonated nitroxides Q_OO ^N = 13.2 ± 0.3 G. The values of Q_NN ^N for TMPN and DTBN are identical and discussion of the angle dependence of the parameter suggests that the radicals in solution do not differ with respect to planarity at nitrogen.