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

Carla I Nieto; Pilar Cabildo; M Ángeles García; Rosa M Claramunt; Ibon Alkorta; José Elguero

doi:10.3762/bjoc.10.168

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

Beilstein J Org Chem. 2014 Jul 16:10:1620-9. doi: 10.3762/bjoc.10.168. eCollection 2014.

Authors

Carla I Nieto¹, Pilar Cabildo¹, M Ángeles García¹, Rosa M Claramunt¹, Ibon Alkorta², José Elguero²

Affiliations

¹ Departamento de Química Orgánica y Bio-Orgánica, Facultad de Ciencias, UNED Senda del Rey 9, E-28040 Madrid, Spain.
² Instituto de Química Médica, CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain.

Abstract

This paper reports the (1)H, (13)C and (15)N NMR experimental study of five benzimidazoles in solution and in the solid state ((13)C and (15)N 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.

Keywords: CPMAS; DNMR; GIAO; proton transfer; tautomerism.