Formation of LiBH4 hydrate with dihydrogen bonding

Abstract

Lithium borohydride hydrate (LiBH4·H2O) was formed at room temperature when LiBH4 was exposed to air with about 5% relative humidity. The O–H stretching peak and the H–O–H bending peak that originated in a hydration water molecule were observed in an infrared spectrum. The structure of LiBH4·H2O was found to be monoclinic with the space group P21/c from a powder X-ray measurement using the Rietveld analysis and a density functional theory (DFT) calculation. The H⋯H contacts in the structure calculated by geometry optimization were 1.58–2.02Å, which were much shorter than twice the value of the van der Waals radius of a hydrogen atom (2.4Å). These H⋯H contacts would produce dihydrogen bonds. The MD simulation at 300K confirmed that the structural model is stable, and the BH4− tetrahedron rotates frequently in a time order of a pico-second at 300K.