STRUCTURE OF A DIHYDRATED, STRONGLY HYDROGEN-BONDED CRYSTAL

The C-l carbon of the dihydrated sodium salt of hydrogen bis(4-nitroph enoxide) gives rise to a single, unsplit, unbroadened signal in the so lid-state, magic-angle spinning C-13 NMR spectrum of a crystalline pow der even at 12 K. The results of rotational-echo, double-resonance C-1 3 NMR experiments with H-2 dephasing for the single C-13-1 resonance, when combined with the observation of a large isotropic J coupling for this carbon and other information, led to the conclusion that the bri dging hydrogen (or deuterium) oscillates rapidly between the two basic oxygen sites. The average lifetime of a proton in one of these sites must be <10(-4) s even at 12 K. The bridge hydrogen therefore occupies a low-barrier, double-well potential with a ground vibrational level below the central maximum. These conclusions are consistent with the c rystal structure, in which the two phenoxide units are related by a ro tational axis of symmetry, and the oxygen-oxygen distance is only 2.45 2 Angstrom. The conclusions are also consistent with the unusual isotr opic chemical shift of the bridge deuterium of 16.8 ppm relative to ex ternal tetramethylsilane. An apparent isotopic fractionation factor of 0.63 has been determined for the bridge hydrogen in the dihydrated so dium salt. This value appears to be too high in view of the other char acteristics of the bridge hydrogen in the crystalline solid, which sug gests that the solid was not in isotopic equilibrium with the solvent from which it was precipitated.