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(CN)2, EA[•CH(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) = 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 π-resonance stabilization of the unpaired electrons in •CH(CN)2 and •CH2CN.
ASJC Scopus subject areas
- Materials Science(all)
- Physical and Theoretical Chemistry