On the Ultra-violet Absorption Spectrum of Diborane

Infrared and Ultraviolet Spectra of Diborane(6): B2H6 and B2D6

We recorded absorption spectra of diborane(6), B2H6 and B2D6, dispersed in solid neon near 4 K in both mid-infrared and ultraviolet regions. For gaseous B2H6 from 105 to 300 nm, we report quantitative absolute cross sections; for solid B2H6 and for B2H6 dispersed in solid neon, we measured ultraviolet absorbance with relative intensities over a wide range. To assign the mid-infrared spectra to specific isotopic variants, we applied the abundance of (11)B and (10)B in natural proportions; we undertook quantum-chemical calculations of wavenumbers associated with anharmonic vibrational modes and the intensities of the harmonic vibrational modes. To aid an interpretation of the ultraviolet spectra, we calculated the energies of electronically excited singlet and triplet states and oscillator strengths for electronic transitions from the electronic ground state.

Ab Initio and Electron Propagator Theory Study of Boron Hydrides

Boron hydrides (BH3, B2H6, B3H7, B4H10, B5H9, and B5H11) and their cations are studied by the coupled cluster CCSD(T) theory, the second-order Mller-Plesset (MP2) perturbation method, and the electron propagator theory in the partial third-order quasi-particle approximation, using the 6-311G(d,p) basis set. The vertical ionization potential energies are calculated, indicating an excellent agreement with the experimental data from photoelectron spectroscopy. Assignments to the experimental spectra are made on the basis of the present computational analyses. A significant Jahn-Teller effect on BH3+ leads to two states, 2A1 and 2B2, with the split energy of 0.14 eV. The triple and double B-H-B bridges are formed in B2H6+ and b-B3H7+, respectively. A new B-H-B bridge is formed while two B-B bonds are broken in B5H11+. The Jahn-Teller effect lowers the symmetry of B5H9 (C4v) to B5H9+ (C2) but slightly influences the structure of ara-B4H10 (C2v). The calculated properties of geometries, vibrational frequencies, and energies are compared with the experimental data available in the literatures.

The oxidation of diborane

Two experimental studies have been made of the gas phase oxidation of diborane. First, conventional oxidation in furnace-heated vessels, at temperatures between 80 and 200 °C was used to show that degenerately branching chains must be involved. Secondly, some transient intermediates were monitored by kinetic absorption spectroscopy in oxidations initiated by flash photolysis. Since diborane absorbs only weakly above 2000 A, only very pure silica (Spectrosil A) would transmit enough energy to produce dissociation. The spectrum of BH and a pressure change were evidence of this when diborane was flashed alone at about 0*01 atm. A very rapid reaction was produced on flashing diborane with oxygen both at low total pressures, about 0-03 atm, and with the total pressure raised to about 1 atm by the addition of inert gas. In apparatus of conventional silica, a chromophoric sensitizer (nitrogen dioxide) was necessary. An incubation period of about 100 us followed the flash, towards the end of which the spectra of OH and B 02 appeared. With low total pressure, BO and BH also appeared as the temperature began to rise in the developing explosion. The gaseous products from both types of reaction were analysed, and a reaction scheme is proposed which takes account of them and of the observed radicals. The hydroxyl radical is undoubtedly intimately concerned in the principal reactions. Predictions made from this scheme for non-explosive conditions are in fair agreement with observed kinetics of the slow oxidation.