Effects of aromatic substitutions on the photoreactions in Mg center dot+(C6HnF2X4-n) (X = F, CH3) complexes: Formation and decomposition of benzyne radical cations

Photoinduced reactions have been systematically studied on mass-selected complexes of Mg.+ with tri- and tetrasubstituted benzene by F and/or CH3. The complexes fall into two groups. The Group I complexes involve bidentate coordination of the o-F atoms to Mg.+, whereas the Group II complexes feature the linkage of Mg.+ to only one of the F atoms. The o-benzyne radical cations were found to be the predominant photolysis products from the Group I complexes, but the yields of the benzyne radical cation products were low for Group II. For each group of complexes, different substituents. including CH3 (medium sigma-electron donor) and F (strong pi-electron acceptor and weak pi-electron donor), affect the branching ratios significantly. For example, they result in different decomposition thresholds of the o-benzyne radical cations. Through the studies of aromatic substitution, the pathways for the formation and decomposition of o-benzyne radical cations have been brought to light. With the help of quantum mechanics calculations, the observed substitutent effects on the photoformation and decomposition of the o-benzyne radical cations are explained mainly from the energy point of view. We found that although o-benzyne radical cations are formed by the direct abstraction of the F atoms, the formation of m-benzyne radical cations is stepwise, involving a pi-+MgF intermediate.