Low-Temperature Single-Crystal Raman and Neutron-Diffraction Study of the Hydrogenous Ammonium Copper(II) Tutton Salt and the Deuterated Analogue in the Metastable State

Low-temperature (15 K) single-crystal neutron-diffraction structures and Raman spectra of the salts (NX4)2[Cu(OX2)6](SO4)2, where X=H or D, are reported. This study is concerned with the origin of the structural phase change that is known to occur upon deuteration. Data for the deuterated salt were measured in the metastable state, achieved by application of 500 bar of hydrostatic pressure at approximately 303 K followed by cooling to 281 K and the subsequent release of pressure. This allows for the direct comparison between the hydrogenous and deuterated salts, in the same modification, at ambient pressure and low temperature. The Raman spectra provide no intimation of any significant change in the intermolecular bonding. Furthermore, structural differences are few, the largest being for the long Cu-O bond, which is 2.2834(5) and 2.2802(4) A for the hydrogenous and the deuterated salts, respectively. Calorimetric data for the deuterated salt are also presented, providing an estimate of 0.17(2) kJ/mol for the enthalpy difference between the two structural forms at 295.8(5) K. The structural data suggest that substitution of hydrogen for deuterium gives rise to changes in the hydrogen-bonding interactions that result in a slightly reduced force field about the copper(II) center. The small structural differences suggest different relative stabilities for the hydrogenous and deuterated salts, which may be sufficient to stabilize the hydrogenous salt in the anomalous structural form.

What we can learn about fast chemical processes from slow diffraction experiments

The potential energy surface is an important determinant of a chemical reaction. Three ways of deducing non-trivial properties of such surfaces from the results of crystal structure analyses are discussed and illustrated with examples. (1) The mapping approach brings together structures of the same molecular fragment from different environments to outline reaction coordinates and vibrations. (2) Correlations between molecular structures and activation energies for a given reaction type reveal general and quantitative relations between seemingly independent entities such as ground state structure, force constants, reaction path length, activation energy and catalysis. (3) The evolution of atomic mean square amplitudes (displacement parameters) with temperature uncovers frequencies and atomic displacement patterns of large-amplitude vibrations in molecular crystals. Examples include the vibrations of molecular zeolite building blocks, the crankshaft motion of stilbenes, the dynamic coupling between pyramidal deformation of the amide NH2 group and hydrogen bonding, the bowl inversion of corannulenes and nucleophilic addition/elimination reactions.