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

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• 作者：Jonathan Boyd ; Gabriel I. Pagola ; Maria C. Caputo ; Marta B. Ferraro ; Paolo Lazzeretti
• 刊名：Journal of Chemical Theory and Computation
• 出版年：2009
• 出版时间：May 12, 2009
• 年：2009
• 卷：5
• 期：5
• 页码：1343-1349
• 全文大小：118K
• 年卷期：v.5,no.5(May 12, 2009)
• ISSN：1549-9626

文摘

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−Schrdinger 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^NB², τ^N = 1/2Σ_αβγδ^N, eqs 2−4 in the text, to average nitrogen shielding in PhNO (τ^N ≈ 1.1 × 10⁻⁵ ppm T⁻²), (CH₃)₃CNO (τ^N ≈ 2.3 × 10⁻⁵ ppm T⁻²), and (CH₃)₂NN (τ^N ≈ 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 (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.