Quantum-chemical Investigation on the Potential Energy Surface of the Reaction of BH⁺₂ with C₂H₂

The potential energy surface of the ion-molecular reaction of BH⁺₂ with C₂H₂ in gas-phase has been investigated theoretically by using the B3LYP/6-311G(d, p) and the high-level electron-correlation CCSD (T)/6-311G (2df, p) single-point levels. Four isomers H₂B⁺ • C₂H₂ (a), HBCHCH⁺₂ (b), H₂BCCH⁺₂ (c) and H₂ • BHCCH⁺ (d) are found, among which isomer (b) is the most stable both thermodynamically and kinetically while isomers (a), (c) and (d) are kinetically unstable. The mechanism for the reaction BH⁺₂ + C₂H₂ → HBCCH⁺ + H₂ is discussed based on the potential energy surface. It is shown that BH⁺₂ may carry out an electrophilic addition towards C₂H₂ followed by multi-step H-shifts and final H₂-elimination to form product HBCCH⁺ + H₂, the total reaction is barrier-free and largely exothermic. The calculated results presented in this paper may be helpful for understanding the chemical behavior of electron-deficient borohydride cations such as BH⁺₂.