A Study of the Nitrogen NMR Spectra of Azoles and their Solvent Dependence

Abstract

Ab initio calculated values of the nuclear magnetic shielding constants and nuclear quadrupole coupling constants of nitrogen atoms in azoles are compared with experimental data. The calculations are performed within the multiconfigurational SCF response approach. The solvent dependence of the molecular properties is accounted for, and it is described using solvent response theory. The experimental data for 14 N resonance half-height width are reported for a number of different solvents. For both NMR parameters studied, we find a fair agreement of the measured and calculated values. The analysis of the solvent dependence of the shielding constants and quadrupole coupling constants for all the nitrogen atoms shows that the main experimental trends are well reproduced in the calculations.

14,15N NMR Shielding Constants from Density Functional Theory

“…A radical improvement in the accuracy of the calculations is obtained only through the use of multiconfigurational methods. 42 A fairly good linear regression and correlation analysis are attained for the pyrrole-type nitrogen, indicating that the substituent-induced shifts are reproduced quite well by the calculation. Meanwhile, a much more scattered pattern is observed for the pyridine-type 14,15 N shieldings.…”

Shielding in and around Oxazole, Imidazole, and Thiazole: How Does the Second Heteroatom Affect Aromaticity and Bonding?

“…Isotropic shieldings for all nuclei in oxazole, imidazole and thiazole, selected nuclei in 1-methylimidazole, and NICS(0), NICS(0.5) and NICS(1) values (in ppm), calculated at the HF-GIAO/6-311++G(d,p) and MP2-GIAO/6-311++G(d,p) levels of theory.data [33][34][35]. The difference between the N1 and and N3 shieldings in 1-methylimidazole, measured in cyclohexane by Witanowski et al,33 amounts to 113.52 ppm.…”

“…Surprisingly, the gas-phase HF-GIAO/6-31++G(d,p)//HF/6-31++G(d,p) estimate of this difference, 122.5 ppm, 34 is better than our HF-GIAO difference of 137.4 ppm, obtained using a larger basis, 6-311++G(d,p), and for a geometry, optimized at a higher level of theory, MP2(FC)/aug-cc-pVTZ. A more accurate gas-phase RASSCF-GIAO/HII//MP2/6-311G(d,p) calculation (using a restricted active space self-consistent field wavefunction with the Huzinaga HII basis set) places the σ iso (N1) − σ iso (N3) difference at 111.64 ppm 35. Despite the fact that our HF-GIAO and MP2-GIAO calculations overestimate and underestimate the difference between the experimentally measured N1 and N3 shieldings at 137.4 ppm and 99.7 ppm, respectively, it is possible to use these numbers in order to obtain an improved theoretical value for this difference: As the correlation corrections to isotropic shieldings obtained at the MP2-GIAO level of theory are often too large, Chesnut has suggested 36 an approximate infinite-order perturbation theory prescription for estimating…”

Computational NMR Spectroscopy

“…Only in a few papers [14,35,36], we carried out GIAO/B3LYP/6-311 þþ G(d,p) calculations on bromopyrazoles in DMSO using the PCM model. On the other hand, many authors have used continuum models to calculate shielding constants [173][174][175][176], especially important are those of Ruud and coworkers [177,178]. A last paper by the same group reports solvent effects on nitrogen NMR shieldings in 1-methyltriazoles [179].…”