NMR Studyof CHN Hydrogen Bond and Proton Transferin 1,1-Dinitroethane Complex with 2,4,6-Trimethylpyridine.

The intermolecular complex with aCHN hydrogen bond formed by 1,1-dinitroethane(DNE) and 2,4,6-trimethylpyridine (collidine) dissolved in CD2Cl2was studied experimentally by 1HNMR spectroscopy at 180–300 K. Equilibrium between the molecularCH···N form and the zwitterionic C–/HN+form was detected in the slow exchange regime inthe NMR time scale. No sign of a direct C–···HN+bond was observed; the ion pair is likely to be held by Coulombinteractions. Moreover, there are indications that the protonatedbase is involved in the formation of homoconjugated (NHN)+collidine–collidinium hydrogen bonded complexes. The reactionpathway of proton transfer in the DNE–pyiridine complex ina vacuum was studied computationally at the B3LYP/6-31++G(d,p) levelof theory. NMR chemical shifts and coupling constants were calculatedfor a series of snapshots along the proton transfer coordinate. Whilethe central carbon atom has a pyramidal (sp3) configurationin DNE, it is flat (sp2) in the DNE carbanion. As a result,the most indicative computed NMR parameter reflecting hybridizationof a carbon atom appeared to be 1JCC, which starts to change rapidly as soon as a structure witha quasi-symmetric C··H··N bond is reached. Couplingswithin the hydrogen bridge, 1JCH, 1hJHN, and 2JCN, can serve as good indicators of the degreeof proton transfer. [ABSTRACT FROM AUTHOR]