Cluster-assisted thermal energy activation of the H-H σ bond in H2 by ground state B+(1S0) ions:: Overcoming a 77 kcal/mol barrier

Binding energies for the sequential addition of two dihydrogen ligands to ground-state B+(S-1(0)) ions have been measured with use of equilibrium methods. The dissociation energies at 0 K were determined to be 3.8 and 3.0 kcal/mol, respectively. Similar measurements on inserted HBH+(1 Sigma(g)(+) ground state) ions yielded binding energies of 14.7 and 18.0 kcal/mol for the addition of the first two Hz ligands. Injection of B+ into a cell containing 5 Torr of H-2 near 100 K resulted in a BH6+ terminal ion that was not in equilibrium with the lower mass B+, B+(H-2), and B+(H-2)(2) species. The rate constant for;formation of this BH6+ terminal ion was measured as a function of temperature and found to peak near 100 K, rapidly decreasing at higher and lower temperatures. This highly unusual behavior was successfully quantitatively modeled by assuming;he following mechanism, B+ + 3H(2) F reversible arrow B+(H-2)(3) -->(o) HBH(H-2)(2)(+), where the third uninserted cluster could rearrange with a 0.52 +/- 0.5 kcal/mol barrier to form the much lower energy inserted ion. ;High-level ab initio calculations (ref 17) found a barrier of 77 kcal/mol for this-insertion process when ground-state B+ reacts with a single H2 molecule. Our experiments show that addition of two weakly bound H-2 ligands reduces the barrier to near zero. To confirm this result, large basis set DFT calculations were done to explore the reaction pathway. These calculations do, in fact, predict a near-zero barrier for insertion upon adding a third H-2 to ground-state Bf(H2)2 ions. This DFT result has recently been confirmed by high-level ab initio calculations published elsewhere (refs 29 and 30). Additional high-level ab initio calculations on the Bf(H2)2 clusters are reported here and provide quantitative agreement with the measured bond energies.