Microhydration of BH₄^-: Dihydrogen Bonds, Structure, Stability, and Raman Spectra

Hydridic-to-protonic interactions in unconventional dihydrogen bonding influence the structure, reactivity, and selectivity in solution and in the solid state. In this study, the structure, stability, and Raman spectra of BH₄^- hydrated clusters, [BH₄(H₂O)_n]^- (n = 1-8, 10, 12, 14, 16) are systematically investigated using density functional theory (DFT) at the wB97XD/6-311++g(3df,3pd) basis set level. The successive microhydration process is described to illustrate in detail the changes in dihydrogen bonding with increasing hydration cluster size. The results of DFT calculations indicate that seven or eight water molecules hydrate BH₄^- with a total of 12 dihydrogen bonds in the tetrahedral edge or tetrahedral corner forms, and a maximum of six water molecules in the tetrahedral-edge form. Raman spectra of [BH₄(H₂O)_n]^- show a blue shift in the B-H stretching band due to hydration. Car-Parrinello molecular dynamics simulations verify strong BH₄^- water interactions. The hydration number of BH₄^- is 6.7, with a hydration B-O(W) distance of 3.40 Å, and each hydrogen in BH₄^- bonds with 2.66 hydrogen atoms from water.