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 (R 1 R 2 NN) compounds in the presence of an external spatially uniform, time-independent magnetic field, accounting for cubic response via Rayleigh-Schrödinger pert...
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| Autores principales: | , , , , |
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| Formato: | JOUR |
| Acceso en línea: | http://hdl.handle.net/20.500.12110/paper_15499618_v5_n5_p1343_Boyd |
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| Sumario: | Hypershielding contributions to magnetic shielding of the nitrogen N nucleus have been evaluated for some nitroso (RNO) and isodiazene (R 1 R 2 NN) 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 τ N B N , τ N = 1/2〈Σ N αβγδ 〉, eqs 2-4 in the text, to average nitrogen shielding in PhNO (τ N ≈ 1.1 × 10 -5 ppm T -2 ), (CH 3 ) 3 CNO (τ N ≈ 2.3 × 10 -5 ppm T -2 ), and (CH 3 ) 2 NN (τ N ≈ 4.4 × 10 -5 ppm T -2 ) are similar and quite large. For 15 N at the highest currently available high-resolution NMR field strength of 22.3 T (ω H /2π = 950 MHz, ω 15N / 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. © 2009 American Chemical Society. |
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