Double H-bridged and single H-bridged diboryl radicals

The structures of B₂H₅·, B₂H₅CO·, and B₂H₅N₂· radicals are investigated using the 6–31G^* basis set. Both double H-bridged and single H-bridged isomers are found to be local minima on the potential energy surface. The effects of electron correlation are taken into account using single point MP4/6–31G^* calculations and, for the diboryl radicals, complete MP3/6–31G^* optimizations. In all cases the single H-bridged isomers are found to be more stable than the corresponding double H-bridged isomers.The transition state for the double H-bridged to single H-bridged B₂H₅· isomerization reaction is calculated to be 2.54 kcal mol^–1 above the double H-bridged radical at the MP4SDTQ/6-31G^*//UHF/ 6–31 G^* level when corrected for zero point energy. Barrier tunneling increased the reaction rate by a factor of 2.5–3.0, strongly suggesting the system is fluxional at this temperature.The addition of CO and N₂ to the diboryl radicals leads to relocation of the unpaired electron and rehybridization of the C and N atoms adjacent to the boron atoms. The isomers of B₂H₅CO· and B₂H₅N₂· are different and should be distinguishable experimentally. While the CO moiety is bound to the diboryl radicals isomers by over 19 kcal mol^–1, no binding energy is evident for N₂.