Photolysis of Diborane at 1849 Å

Abstract

The photolysis of diborane at 1849 Å has been studied in a specially constructed, internal-type mercury-vapor lamp. The products have been found to be H2, B4H10, B5H11, and, at low pressures, a —BH— polymer. Reaction orders at 4°C have been obtained from linear plots of reaction products vs time for a range of diborane pressures from 0.08 to 80 cm, and at two light intensities. Linear relations between products and time existed only at very low conversions (∼1%), which required the development of a low-temperature separation method for manipulating and analyzing the traces of B4H10 and B5H11. Because of the reactivity of these compounds, a detailed conditioning procedure was employed for the glass system. A mechanism consistant with the kinetic data and suggested by the kinetic results of thermal and photosensitized decomposition of diborane is postulated: the B5H11 is assumed to be formed from a dissociation of B2H6 into BH3's, the latter arising from an excited molecule. The B4H10 and polymer are assumed to be formed from a dissociation of B2H6 into B2H5 and H, followed by radical recombination. There is a significant difference between the kinetics of thermal and photochemical B5H11 formation, a result which may be due to the considerable energy excess of the 1849 quantum over that needed for dissociation (∼125-kcal excess). These kinetic results raise a number of interesting questions, questions which can only be resolved through further investigations of effects due to light intensity, added inert gases, and temperature. The primary quantum yield of the step forming B2H5 and H is about 10 times higher than that of the one forming BH3's. A rather rough estimate suggests that the former is of the order of magnitude of unity.