An experimental and theoretical NMR study of NH-benzimidazoles in solution and in the solid state: proton transfer and tautomerism

Abstract

SummaryThis paper reports the 1H, 13C and 15N NMR experimental study of five benzimidazoles in solution and in the solid state (13C and 15N CPMAS NMR) as well as the theoretically calculated (GIAO/DFT) chemical shifts. We have assigned unambiguously the "tautomeric positions" (C3a/C7a, C4/C7 and C5/C6) of NH-benzimidazoles that, in some solvents and in the solid state, appear different (blocked tautomerism). In the case of 1H-benzimidazole itself we have measured the prototropic rate in HMPA-d 18.

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

“…On the other hand, for 15 N solid‐state NMR, GIPAW is much better (compare Equations 9 and 10). This is probably related to the absence of hydrogen bonds involving N–H and N atoms in the GIAO calculations (monomers, except 9 ), reason why calculations of the central molecule in model hydrogen‐bonded trimers improves considerably the results …”

Benzimidazolium Lead Halide Perovskites: Effects of Anion Substitution and Dimensionality on the Bandgap

“…The protonation of benzimidazole becomes also obvious in the observed upfield shifts for C 4 and C 9 as compared to neutral benzimidazole. [18] Notably, C 2 is downfield shifted on going from the iodide to the bromide to the chloride, which is consis- Figure 5.…”

“…[18] The assignment of the signals in the 13 C and 15 N spectra is displayed in detail in the Supporting Information S6.…”

Separation and identification of the mixture of 2‐(3,4‐dimethoxyphenyl)‐1‐n‐propyl or (4‐chlorobenzyl)‐5 and (6)‐1H‐benzimidazolecarbonitriles

“…Thus, 1 H ‐benzimidazole‐5‐carbonitrile is a tautomer of 1 H ‐benzimidazole‐6‐carbonitrile, and both structures represent the same compound as 5(6). The NMR spectra of benzimidazoles show the presence of two tautomeric forms, which are rapidly equilibrating on the NMR timescale, and the relations between the tautomer and nontautomer type were reported in different deuterated solvents . Figure shows the 1 H and 13 C NMR spectra of compound 3 under normal conditions.…”