The proton potential fuction and dynamics in sodium hydrogen bis(4-nitrophenoxide) dihydrate

The structure of crystalline sodium hydrogen bis(4-mtrophenoxide) dihydrate (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-Hadži compound, with an O,O distance of 243 pm (neutron diffraction) or 246 pm (X-ray diffraction) at 105 K. This distance increases to 248 pm (X-ray diffraction) at 297 K. The neutron structure suggests that the bridging hydrogen is distributed between two positions, separated by 35 pm, at the lowest temperature. The asymmetric O,H stretching frequency was found by inelastic neutron scattering, using isotopic substitution, al 605 cm^-1. A number of IR fingerprint bands which are separate below 200 K merge between 200 and 250 K, simplifying the 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 appears to be a mixture of degenerate tautorners. At higher temperature either the proton shift accelerates, adn band averaging occurs, or else the proton is centralized. If the acceleration model is adopted the tautomer lifetime is about 0.5 ps at the convergence temperature, 225 K. Partial or complete centralisation may also be responsible for the changes in the IR spectrum. Thermal excitation of one or more low frequency vibrations causes the acceleration and/or centralization. The O,H stretching frequency is mo high to be substantially excited at the coalescence temperature, so this cannot be the relevant vibration. The regular increase in the O,O distance with temperature makes it seem unlikely that centralization is the sole cause of the spectral changes.