Motions of molecules in condensed systems. XIV. Polarized infrared spectra of single crystals of diborane

The polarized infrared spectra of single crystals of diborane as well as the spectra of the vapor and liquid are presented and analyzed. Evidence for the existence of three solid phases is discussed and the structure of the low-temperature form partially elucidated. This phase is found to be monoclinic with two or more molecules per primitive unit cell located on sites of symmetry no higher than d. Confirmatory evidence for Lord and Nielsen's assignment of v₂≈50 cm^-1 and Smith and Mills' assignment of v ₁≈850 cm^-1 is presented. It is suggested that the strong electronic absorptions in the not-too-far ultraviolet are of B _1u symmetry. Some indication of a special interaction in the solid state is found.