Hydrogen-atom transfer across the hydrogen bonds of benzoic acid dimers has been studied previously by spectroscopic and X-ray methods, but no high-resolution neutron single crystal structures have ever been reported. The system is of interest because the hydrogen-atom dynamics change from classical Arrhenius behaviour to phonon-assisted quantum tunnelling as a function of decreasing temperature. Here we present a series of neutron single-crystal structure refinements at five temperatures ranging from 175 K, where Arrhenius behaviour holds, down to 20 K, where translational tunnelling is dominant. The refinements yield accurate atomic coordinates, thermal parameters and site occupancy factors for all atoms, and show that conversion of the B dimer configuration to the lower energy A dimer configuration in the range 175 to 50 K is primarily enthalpy driven, in good agreement with spectroscopic data. Site occupancies for the hydrogen-bonded hydrogen atom at 20 K do not fit the trend in occupancies observed at the four higher temperatures, and a further series of structure refinements at temperatures below 50 K is planned to investigate this.Benzoic acid, C,H,O, , crystallises as a dimer, hydrogen bonded across a crystallographic centre of symmetry.' Previous crystallographic and spectroscopic evidence '-lo has indicated disordering of the hydrogen atoms which participate in the hydrogen bonds.C-0-H.. .O=Ct,C=O..
.H-O-CThis gives rise to two dimer configurations, A and B, and the crystal structure progressively orders into configuration A as the sample temperature is Single-crystal neutron diffraction enables the atomic coordinates of hydrogen atoms to be determined more accurately than single-crystal X-ray diffraction, and so provides a means by which to carry out detailed examinations of the hydrogenbonding interaction. A series of single-crystal pulsed neutron diffraction refinements on benzoic acid has been carried out in order to follow the hydrogen atom disordering as a function of temperature. By refining the site occupancies of the hydroxy hydrogen, H6, over two positions, the variation in the relative proportion of configurations A and B at each temperature is obtained. Further to our previous report on this work, l o we present here a detailed account of the molecular and crystal structures at five temperatures.7 Supplementary material deposited with the British Library (SUP 57172,47 pp.). Details are available from the Editorial Ofice.
We have applied a combination of spectroscopic and diffraction methods to study the adduct formed between squaric acid and bypridine, which has been postulated to exhibit proton transfer associated with a single-crystal to single-crystal phase transition at ca. 450 K. A combination of X-ray single-crystal and very-high flux powder neutron diffraction data confirmed that a proton does transfer from the acid to the base in the high-temperature form. Powder X-ray diffraction measurements demonstrated that the transition was reversible but that a significant kinetic energy barrier must be overcome to revert to the original structure. Computational modeling is consistent with these results. Modeling also revealed that, while the proton transfer event would be strongly discouraged in the gas phase, it occurs in the solid state due to the increase in charge state of the molecular ions and their arrangement inside the lattice. The color change is attributed to a narrowing of the squaric acid to bipyridine charge-transfer energy gap. Finally, evidence for the possible existence of two further phases at high pressure is also presented.
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