NMR spectra of nitrogen-containing compounds. Correlations between experimental and GIAO calculated data

Correlations between experimentally determined chemical shifts (15N, 13C, 1H in CDCl3 and DMSO solutions) and GIAO-calculated isotropic shielding constants, δexpt = a + bσcalcd, are reported that were obtained from a series of nitrogen-containing heterocycles (5-, 6-, 5+6-, 6+6- and 6+6+6-membered rings). Based on HF, MP2 and B3LYP optimized geometries [6–31G(d,p) basis set], GIAO calculations were performed at the HF, BLYP, and B3LYP levels of theory [6–311++G(d,p) basis set]. The performance of theoretical NMR calculations and the resulting eligibility for routine practical use were assessed from correlation coefficients and from standard deviations of the theoretically predicted shifts. Depending on the experimental conditions and on computational levels, linear regressions between experimental and theoretical data resulted in standard deviations of about 6–12 ppm for all nitrogens, 5–8 ppm for aromatic nitrogens, 1–2 ppm for aromatic carbons and 0.1–0.15 ppm for aromatic hydrogens. Several points that influence the accuracy of theoretically predicted chemical shifts are discussed.