C-H Bond Dissociation Energy of Malononitrile

The C H bond dissociation energies of closed-shell molecules decrease with increasing stability of the resulting radicals. From the electron affinity of the dicyanomethyl radical, (CH)-C-center dot(CN)(2), EA[(CH)-C-center dot(CN)(2)] = 2.88 +/- 0.01 eV, measured by photoelectron imaging of the CH(CN)(2)(-) anion, and the acidity/electron affinity, thermodynamic cycle, we obtained the C-H bond dissociation enthalpy of malononitrile; CH2(CN)(2), DH298[H-CH(CN)(2)] = 87 +/- 2 kcal/mol. This result is compared, to the corresponding value for acetonitrile, DH298(H-CH2CN) = 93 +/- 2 kcal/mol, determined from a similar measurement of EA((CH2CN)-C-center dot) = 1.53 +/- 0.01 eV. The relative weakness of the C-H bonds in malononitrile and acetonitrile, compared to most closed-shell neutral organic molecules, is attributed to pi-resonance stabilization of the unpaired electrons in (CH)-C-center dot(CN)(2) and (CH2CN)-C-center dot.