Imaging proton migration from X-rays and neutrons

Abstract

The short hydrogen bond in the urea-phosphoric acid system has been studied by multiple-temperature X-ray single crystal diffraction. The hydrogen atom is imaged using difference Fourier methods from these data and also from previous neutron diffraction data. The migration of the hydrogen atom is clearly observed using the X-ray difference Fourier maps. The hydrogen atom positions determined from these maps are more reliable than from X-ray refinements of this atom. A greater apparent shift of the proton is observed from X-ray than from neutron diffraction, as might be expected.

Polarization-induced σ-holes and hydrogen bonding

“…This effect has long been recognized [22,23]; it is the reason why standard crystallographic methods, which use buildups of electronic density to assign nuclear positions, typically underestimate hydrogen bond lengths by roughly 0.1 Å [24,25]. However, we are not aware that its relevance for hydrogen bonding, or as the dominant polarization mechanism of molecules with A-H bonds, has been pointed out.…”

The strong OHO hydrogen bond. How much covalency?

“…These two methods, combined with analysis of the Fourier maps, may deliver a complete description of the strong OHO hydrogen bond in the phosphoric acid-urea 1:1 complex [6].…”

The evolution of hydrogen atom parameters under changing external conditions by time-of-flight single crystal neutron diffraction

“…To discover if it was possible to study this migration using X-rays, a variable temperature X-ray study was carried out and compared to previously collected neutron data at the same temperatures [133]. In order to image the short strong hydrogen bond within the structure, the hydrogen atom in the short strong hydrogen bond was removed from the refinements and Fourier Difference maps calculated for both X-ray and neutron data at all the temperatures.…”

“…This refined well, and the hydrogen atom density found progressively to order into configuration A as the temperature is lowered, confirming A as the lower energy configuration. A similar experiment was carried out on another benzoic acid derivative, p-chlorobenzoic acid [81], at eight temperatures (20,40,80,100,133,200, 250 and 296 K), in an experiment requiring just 5 days of beam time. The structural evolution is shown in figure 10, again showing the characteristic elongation of the thermal ellipsoid of the hydrogen-bonded H atom as the temperature increases.…”