Structure and bonding in the B2H5 radical and cation

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Abstract
The B2 H5 radical has been generated by the F+B2 H6 reaction, and studied by photoionization mass spectrometry. The photoion yield curve for B2H+5 (B2 H5 ) is extremely weak at the adiabatic threshold (∼6.945 eV), at least three orders of magnitude weaker than at its maximum (∼9.67 eV). This observation provides support for recent ab initio calculations, which predict a singly bridged B2 H5 and a triply bridged B2H+5 as ground states. Evidence is presented for the coexistence of a doubly bridged B2 H5 isomer, ∼3 kcal/mol higher in energy. From the appearance potential of B2H+3 (B2 H5 ), a B2 H5 –H bond energy of ≲102.7 kcal/mol is obtained. From the photoion yield curve of B2H+3 (B2 H5 ), at least one, and perhaps two, excited states of B2H+5 can be inferred. The lower one also branches into parent B2 H5, an apparent violation of QET.