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

JONATHAN 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 extbf{N}nucleus have been evaluated for some nitroso (R extbf{N}O) and isodiazene(R$_{1}$R$_{2}$N extbf{N}) compounds in the presence of an external spatiallyuniform, time-independent magnetic field, accounting for cubic response viaRayleigh-Schr{"o}dinger perturbation theory.Numerical estimates have been obtained at the coupled Hartree-Fock level of accuracywithin the conventional common-origin approach.Medium-size basis sets of gaugeless (that is, without gauge-including phasefactors) Gaussian functions have been employed in a numerical test, to show thatthe isotropic hypershielding contribution$ au^N B^2$, $ au^N=1/2leftlangleSigma_{alphaetagammadelta}^N ight angle$,eqs. (2)-(4) in the text,to average nitrogen shielding in Ph extbf{N}O ($ au^N approx1.1 imes$ 10$^{-5}$ppm T$^{-2}$),(CH$_{3})_{3}$C extbf{N}O ($ au^N approx2.3 imes$ 10$^{-5}$ ppm T$^{-2}$)and (CH$_{3})_{2}$N extbf{N} ($ au^N approx4.4 imes$ 10$^{-5}$ppm T$^{-2}$)are similar and quite large.For $^{15}$N at the highest currently available highresolution NMR field strength of $22.3$T (${omega^{H}/{2pi}}=950$ MHz,${omega^{^{15}N}/{2pi}}=96.3$ MHz) the change due tothe additional shielding contribution for these compounds is between $approx$0.5 and$approx$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 makeunambiguous detection at different field strengths difficult.