NMR study of CHN hydrogen bond and proton transfer in 1,1-dinitroethane complex with 2,4,6-trimethylpyridine

The intermolecular complex with a CHN hydrogen bond formed by 1,1-dinitroethane (DNE) and 2,4,6-trimethylpyridine (collidine) dissolved in CD2Cl2 was studied experimentally by 1H NMR spectroscopy at 180–300 K. Equilibrium between the molecular CH···N form and the zwitterionic C–/HN+ form was detected in the slow exchange regime in the NMR time scale. No sign of a direct C–···HN+ bond was observed; the ion pair is likely to be held by Coulomb interactions. Moreover, there are indications that the protonated base is involved in the formation of homoconjugated (NHN)+ collidine–collidinium hydrogen bonded complexes. The reaction pathway of proton transfer in the DNE–pyiridine complex in a vacuum was studied computationally at the B3LYP/6-31++G(d,p) level of theory. NMR chemical shifts and coupling constants were calculated for a series of snapshots along the proton transfer coordinate. While the central carbon atom has a pyramidal (sp3) configuration in DNE, it is flat (sp2) in the DNE carbanion. As a result, the most indicative computed NMR parameter reflecting hybridization of a carbon atom appeared to be 1JCC, which starts to change rapidly as soon as a structure with a quasi-symmetric C··H··N bond is reached. Couplings within the hydrogen bridge, 1JCH, 1hJHN, and 2JCN, can serve as good indicators of the degree of proton transfer.