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

The present paper reports the first experimental and theoretical results concerning the reaction of [1,3-butadiene](+.) radical cation, I, with neutral acetylene C2H2, 2. Experiments conducted in the gas phase and under low pressure in an FT-ICR mass spectrometer show that the reaction leads to C6H7+ ions. Complete analysis of the experimental data reveals that a mixture of 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]-hexenyl, 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 involving stepwise bond formation is at the origin of ionized 1,4-cyclohexadiene, 5, the first precursor of the benzenium ion a. Starting from 5, hydrogen-atom migrations and ring-contraction processes generate the precursors of the two other products, b and c.