A theoretical NMR study of selected benzazoles: Comparison of GIPAW and GIAO‐PCM (DMSO) calculations

Abstract

This paper compares the absolute shieldings obtained by gauge-including-projected-augmented-wave (GIPAW) to those obtained by gauge-invariant atomic orbital/Becke, 3-parameter, Lee-Yang-Parr (GIAO/B3LYP)/6-311++G(d,p)-polarizable continuum model (PCM, dimethyl sulfoxide) for nine benzazoles (benzimidazoles, indazoles, and benzotriazoles) recorded in the solid-state. Three nuclei were explored, C, N, and F, and the gauge-including-projected-augmented-wave approach only proved better for N MAS NMR.

A theoretical NMR study of polymorphism in crystal structures of azoles and benzazoles

“…15 N‐ and 13 C‐NMR parameters were computed by using the CASTEP package. The calculated absolute shieldings ( σ ) were transformed to the corresponding chemical shifts ( δ , ppm) by using the Equations and . These values were compared with those experimentally reported previously, if they are available and with those determined in the present work for 2 and 2′ .…”

“…Then, following our conclusion that, in some cases, CASTEP was not much better than GIAO if all the hydrogen atoms were optimized keeping all the non‐hydrogen atoms in the positions determined by X‐ray crystallography, we calculated the 2 and 2′ trimers at the GIAO/B3LYP/6‐311++G(d,p) level. Excluding the C4‐Br signal, the results are the following: …”

“…The fact that, in some cases, less‐expensive GIAO/B3LYP/6‐311++G(d,p) calculations yields comparable and even slightly better results that GIPAW calculations, a fact that we have previously observed, needs more studies before reaching a definitive conclusion. In all probabilities, what approach is the best will depend on individual cases.…”

“…Recently, we have reported a theoretical NMR study of a set of benzazole crystal structures by using GIPAW calculations . We concluded that this method reproduces with high accuracy the experimental 13 C‐NMR and 15 N‐NMR measurements in the solid state.…”

A theoretical and spectroscopic (NMR and IR) study of indirubin in solution and in the solid state

“…Finally, chemical shifts for crystal structure INDRUB were simulated by using the GIPAW approach. Computed absolute shielding values were transformed into chemical shifts, according to linear equations established previously for the azole‐family compounds in solid state, and reported in Table as solid state (SS, see below). The comparison of experimental CPMAS values with these calculated ones provided a linear equation with an R 2 of 1.000 (Equation ).…”