Calculation of Hypershielding Contribution to Isotropic Nitrogen Shielding in Strong Magnetic Fields

Hypershielding contributions to magnetic shielding of the nitrogen N nucleus have been evaluated for some nitroso (RNO) and isodiazene (R1R2NN) compounds in the presence of an external spatially uniform, time-independent magnetic field, accounting for cubic response via Rayleigh−Schrödinger perturbation theory. Numerical estimates have been obtained at the coupled Hartree−Fock level of accuracy within the conventional common-origin approach. Medium-size basis sets of gaugeless (that is, without gauge-including phase factors) Gaussian functions have been employed in a numerical test to show that the isotropic hypershielding contribution τNB2, τN = 1/2⟨ΣαβγδN⟩, eqs 2−4 in the text, to average nitrogen shielding in PhNO (τN ≈ 1.1 × 10−5 ppm T−2), (CH3)3CNO (τN ≈ 2.3 × 10−5 ppm T−2), and (CH3)2NN (τN ≈ 4.4 × 10−5 ppm T−2) are similar and quite large. For 15N at the highest currently available high-resolution NMR field strength of 22.3 T (ωH/2π = 950 MHz, ω15N/2π = 96.3 MHz) the change due to the additional shielding contribution for these compounds is between ∼0.5 and ∼2 Hz to lower frequency (upfield). Employing modern NMR instrumentation, shielding perturbations of this magnitude are, in principle, within detection limits, although instrumental instabilities and other field-dependent shielding phenomena make unambiguous detection at different field strengths difficult.