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Various reactions following acetylene addition on i- and n-C(4)H(3) radicals are studied using a modified G3B3 method. For the addition of C(2)H(2) on n-C(4)H(3), 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-C(4)H(3) is extensively described including new cyclisation steps and hydrogen elimination reactions. The lowest energy path from n- or i-C(4)H(3) to a cyclic system goes through dehydro-fulvene radicals formation, which in the case of n-C(4)H(3) + C(2)H(2) 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.

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In this work, the isomerization mechanisms of dehydrofulvene radicals to the phenyl radical are described. This study shows, for 6-dehydrofulvene, a lowenergy isomerization path going through a resonantly stabilized bicyclic system. In a second part, the different established mechanisms are applied to the closing of a second and third aromatic ring. This application highlights the possibility of forming new products, such as 2-naphthyl radical, and the effect of aromatic cycles on the new mechanisms.

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