THE PROTON POTENTIAL FUNCTION AND DYNAMICS IN SODIUM-HYDROGEN BIS(4-NITROPHENOXIDE) DIHYDRATE

The structure of crystalline sodium hydrogen bis(4-nitrophenoxide) dih ydrate (1) has been studied by X-ray diffraction at 105, 173, and 297 K; and by neutron diffraction at 105 K. Vibrational spectra of 1 have been obtained by infrared absorption at temperatures from 14 K to room temperature, and also by inelastic neutron scattering at 15 K. 1 is a Speakman-Hadzi compound, with an O,O distance of 243 pm (neutron diff raction) or 246 pm (X-ray diffraction) at 105 K. This distance increas es to 248 pm (X-ray diffraction) at 297 K. The neutron structure sugge sts that the bridging hydrogen is distributed between two positions, s eparated by 35 pm, at the lowest temperature. The asymmetric O,H stret ching frequency was found by inelastic neutron scattering, using isoto pic substitution, at 605 cm(-1). A number of LR fingerprint bands whic h are separate below 200 K merge between 200 and 250 K, simplifying th e appearance of the spectra. Prominent examples occur around 1600 cm(- 1) (a) and around 1450 cm(-1) (b). The (a) pair is separated by about 28 cm(-1) and the (b) pair about 20 cm(-1). At low temperature 1 appea rs to be a mixture of degenerate tautomers. At higher temperature eith er the proton shift accelerates, adn band averaging occurs, or else th e proton is centralized. If the acceleration model is adopted the taut omer lifetime is about 0.5 ps at the convergence temperature, 225 K. P artial or complete centralization may also be responsible for the chan ges in the IR spectrum. Thermal excitation of one or more low frequenc y vibrations causes the acceleration and/or centralization. The O,H st retching frequency is too high to be substantially excited at the coal escence temperature, so this cannot be the relevant vibration. The reg ular increase in the O,O distance with temperature makes it seem unlik ely that centralization is the sole cause of the spectral changes.