Synthesis, Characterization, and Reactivity of the m-Xylylene Anion in the Gas Phase. The Enthalpy of Formation of m-Xylylene

The enthalpy of formation of m -quinodimethane ( m -xylylene) has been determined using two different gas-phase approaches. The first involves combining the electron affinity of the biradical (0.919 ± 0.008 eV, Wenthold, P. G.; Kim, J. B.; Lineberger, W. C. J. Am. Chem. Soc. 1997, 119, 1354) with the acidity of the 3-methylbenzyl radical. The acidity of the 3-methylbenzyl radical was determined by bracketing the proton affinity of m -xylylene ion, prepared from the reaction of m -xylene and atomic oxygen ions in a flowing afterglow triple quadrupole apparatus. Deuterium labeling and reactivity studies show that 75% of the H 2 + abstraction product formed in the reaction is the m -xylylene ion, with the rest being 3-methyl-α, n -dehydrotoluene ions resulting from [α, ring] hydrogen abstraction. The m -xylylene negative ion underwent reactions similar to those observed for 3-methylbenzyl anion and for other open-shell ions. The m -xylylene ion also reacted with mesitylene by H 2 + transfer. The acidity of the 3-methylbenzyl radical was 382.5 ± 2.1 kcal/mol, which leads to an enthalpy of formation of m -xylylene of 80.1 ± 3.8 kcal/mol. The enthalpy of formation of m -xylylene was also determined to be 81.2 ± 3.0 kcal/mol using collision-induced dissociation (CID) threshold energy measurements with the 3-(chloromethyl)benzyl ion. The measured enthalpies of formation indicate a second C−H bond dissociation energy in m -xylene of 90.7 ± 2.9 kcal/mol, only slightly higher than the first (90.1 ± 1.7 kcal/mol). The fact that the strength of the second C−H bond in the 3-methylbenzyl radical is essentially the same as that in m -xylene indicates that the interaction between the two unpaired electrons is negligible, as would be expected for a triplet biradical.