Calculation of hypershielding contribution to isotropic nitrogen shielding in strong magnetic fields

J. BOYD; G. I. PAGOLA; M. C. CAPUTO; M. B. FERRARO; P. LAZZERETTI

JOURNAL OF CHEMICAL THEORY AND COMPUTATION

AMER CHEMICAL SOC

Año: 2009 vol. 5 p. 1343 - 1349

1549-9618

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 = ½ <ΣNabγδ>, 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.