Reactivity of a substituted m-benzyne biradical

Fourier transform ion cyclotron resonance mass spectrometry has been employ ed to systematically investigate the intrinsic (solvent-free) reactivity of a 1,3-dehydrobenzene (m-benzyne) with a pyridinium charge site in the 5-po sition. The m-benzyne was generated by using a combination of ion-molecule reactions and photodissociation and isolated prior to examination of its ga s-phase reactions. The ionic reaction products and reaction efficiencies (s econd-order reaction rate constant/collision rate constant) were compared t o those measured for the isomeric o-benzyne and the analogous phenyl monora dical. The m-benzyne yields same of the products formed for the o-benzyne b ut it also reacts via distinct radical pathways characteristic of the corre sponding phenyl radical. These radical pathways are not observed for the o- benzyne. However, the reaction efficiencies measured for the m-benzyne are significantly lower than those measured for the analogous phenyl radical or the isomeric o-benzyne. These findings are partially rationalized by the r elatively strong coupling (about 21 kcal mol(-1)) between the two formally unpaired electrons in the m-benzyne that hinders radical reactions. On the other hand, the greater distance between the reactive sites in the m-benzyn e makes alkyne-type addition reactions sterically and energetically less fa vorable than for the o-benzyne.