CLUSTER-ASSISTED THERMAL-ENERGY ACTIVATION OF THE H-H SIGMA-BOND IN H-2 BY GROUND-STATE B-1(0)) IONS - OVERCOMING A 77 KCAL((S)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 equili brium methods. The dissociation energies at 0 K were determined to be 3.8 and 3.0 kcal/mol, respectively. Similar measurements on inserted H BH+(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. Inject ion 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 f ound to peak near 100 K, rapidly decreasing at higher and lower temper atures. This highly unusual behavior was successfully quantitatively m odeled by assuming;he following mechanism, B+ + 3H(2) F reversible arr ow B+(H-2)(3) -->(o) HBH(H-2)(2)(+), where the third uninserted cluste r could rearrange with a 0.52 +/- 0.5 kcal/mol barrier to form the muc h lower energy inserted ion. ;High-level ab initio calculations (ref 1 7) found a barrier of 77 kcal/mol for this-insertion process when grou nd-state B+ reacts with a single H2 molecule. Our experiments show tha t addition of two weakly bound H-2 ligands reduces the barrier to near zero. To confirm this result, large basis set DFT calculations were d one to explore the reaction pathway. These calculations do, in fact, p redict a near-zero barrier for insertion upon adding a third H-2 to gr ound-state Bf(H2)2 ions. This DFT result has recently been confirmed b y high-level ab initio calculations published elsewhere (refs 29 and 3 0). Additional high-level ab initio calculations on the Bf(H2)2 cluste rs are reported here and provide quantitative agreement with the measu red bond energies.