Cycle formation from acetylene addition on C4H3 radicals

Abstract

Various reactions following acetylene addition on i- and n-C₄H₃ radicals are studied using a modified G3B3 method. For the addition of C₂H₂ on n-C₄H₃, the commonly admitted phenyl radical decomposition path of Madden and coworkers has been considered. In addition, several energetically competitive cyclisation steps are described for the first time. Data concerning addition to form non-cyclisable compounds are also provided. Addition on i-C₄H₃ is extensively described including new cyclisation steps and hydrogen elimination reactions. The lowest energy path from n- or i-C₄H₃ to a cyclic system goes through dehydro-fulvene radicals formation, which in the case of n-C₄H₃ + C₂H₂ is energetically competitive to the formation of the phenyl radical. Considering the degradation of the latter, the formation of 6-dehydrofulvene is energetically competitive as compared to the decomposition to ortho-benzyne + H, which is the assumed main degradation channel of the phenyl radical. Rate constants for all the steps considered in this work are obtained through the conventional transition state theory and provided at the end of the paper.