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 + ( 1 S 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 ground state) ions yielded binding energies of 14.7 and 18.0 kcal/mol for the addition of the first two H 2 ligands. Injection of B + into a cell containing 5 Torr of H 2 near 100 K resulted in a BH 6 + 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 BH 6 + 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 the following mechanism, B + + 3H 2 ⇄ B + (H 2 ) 3 ↬ 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 H 2 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 B + (H 2 ) 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 B + (H 2 ) 2 clusters are reported here and provide quantitative agreement with the measured bond energies.