Bifurcated Dihydrogen Bonding in the Uptake of Gas-Phase Diborane on Silica

The interfacial chemistry of diborane (B₂H₆) with hydroxylated silica was investigated via in situ Fourier-transform infrared spectroscopy and temperature-programmed desorption. During exposure of silica to B₂H₆ under ultrahigh vacuum conditions, a decline in infrared band intensity assigned to excitation of the interfacial silanol O-H vibration at 3750 cm^-1 and the associated appearance of a feature at 3687 cm^-1 revealed hydrogen-bonding interactions between B₂H₆ and interfacial silanol groups. The IR spectrum for silica was completely recovered following desorption of the adsorbates, indicating that interactions between B₂H₆ and clean silica are reversible, in contrast to other reports on this system. During temperature-programmed desorption of diborane from silica, B₂H₆ was observed to desorb between 80 and 150 K, evidence for weak interactions between B₂H₆ and the surface. Electronic-structure calculations revealed that these interactions were due to bifurcated dihydrogen bonds between two terminal B-H groups of the adsorbate and interfacial silanol groups.