Energetics of Br-H-Br- formation from HBr dimer anion: An ab initio study

Experimental studies have shown that the dissociative attachment of electro ns to HBr monomer; viz., HBr + e(-) --> H-. + Br- is an endothermic process , which requires electrons of energy near the endothermicity (Delta E = +38 kJ/mol). In contrast the attachment to the dimer via the reaction: HBr . H Br + e(-) reversible arrow HBr . HBr.- --> H-. + BrHBr- proceeds readily wi th electrons of thermal (i.e., near zero) energy. The energetics of the rea ctions with both the monomer and the dimer have been studied by ab initio m ethods. In each case the structure of the neutral and anion species have be en computed using large basis sets and diffuse orbitals at the MP2 level. E nergies have been obtained at this level and at the CCSD(T) level. The init ial interaction of the free electron with the molecule produces a dipole co ntinuum state (DCS) of the anion. The DCS subsequently undergoes a transiti on to a bound state via an avoided crossing. On the basis of the CCSD(T) en ergies, the H-Br.-.. HBr interaction lowers the energy for dissociation of the H-Br.- bond to the thermal energy range. Also the decomposition of HBr . HBr.- to H-. and BrHBr- is 47 kJ/mol exergonic and the BrHBr- fragment is expected to be in a vibrationally excited state. In both the monomer and d imer anions the transition from DCS to the bound valence state is very sudd en, and is marked by enormous (10-fold) changes in the dimensions of the or bitals occupied by the electron.