Condensation reactions between 1,3-butadiene radical cation and acetylene in the gas phase

The present paper reports the first experimental and theoretical results co ncerning the reaction of [1,3-butadiene](+.) radical cation, I, with neutra l acetylene C2H2, 2. Experiments conducted in the gas phase and under low p ressure in an FT-ICR mass spectrometer show that the reaction leads to C6H7 + ions. Complete analysis of the experimental data reveals that a mixture o f three distinct C6H7+ species is produced: it consists of benzenium ions, a (approximate to 55%), and the two most stable protonated forms of fulvene (i.e., a-protonated fulvene, b. approximate to 25%, and bicyclo[3,1,0]-hex enyl, c, approximate to 20%). A mechanism, supported by ab initio molecular orbital calculations at the UMP2/6-311+G(d,p)//UMP2/6-31G(d) + ZPE level, is proposed to account for the observations. A Diels-Alder type reaction in volving stepwise bond formation is at the origin of ionized 1,4-cyclohexadi ene, 5, the first precursor of the benzenium ion a. Starting from 5, hydrog en-atom migrations and ring-contraction processes generate the precursors o f the two other products, b and c.