Dihydrogen Bonds in Aqueous NaBD 4 Solution by Neutron and X-ray Diffraction.

Neutron diffraction, X-ray diffraction, and empirical potential structure refinement modeling were employed to study the structure of alkaline aqueous NaBD4 solutions at different NaBD4 concentrations and temperatures. In 1.0 mol.dm(-3) NaBD4 aqueous solutions, about 5.6 +/- 1.6 water molecules bond to BD4- via tetrahedral edges or tetrahedral corners without a very specific hydration geometry; that is, each hydrogen atom of BD4- bonds to 2.2 +/- 1.0 water molecules through dihydrogen bonds with the D(B)center dot center dot center dot D(W) distance of 1.95 angstrom. The number of dihydrogen bonds decreases with increasing concentration and increases with temperature. Dihydrogen bonding is a predominantly electrostatic interaction which shows relatively lower directionality and saturability in comparison with the regular hydrogen bonds between water molecules. The water orientation around BD4- shows that the proportion of tetrahedral-edge dihydrogen bonds increases with temperature and decreases with concentration.