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 (1)H 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 (sp(3)) configuration in DNE, it is flat (sp(2)) in the DNE carbanion. As a result, the most indicative computed NMR parameter reflecting hybridization of a carbon atom appeared to be (1)JCC, 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, (1)JCH, (1h)JHN, and (2)JCN, can serve as good indicators of the degree of proton transfer.