Radicals

The application of IR-spectroscopy supplemented by method of isodesmic (formal) reactions which characterize the relative stabilities of free radicals is demonstrated as a useful tool for getting novel data of the enthalpies of formation for certain types of free radicals. In a result, ∆Hf0 values for 23 XC(•)=CH2 and 17 XC(•)=O novel radicals were obtained and for 8 more drastically corrected from literature νCH values in XCH=O and XCH=CH2 molecules, respectively. On the example of H-CH2C(X)=O molecules it was demonstrated the possibility to estimate the enthalpies of formation of ground state free radicals from νCH values for excited C-H bonds. Using the results on the latter radicals the ∆Hf0 values for 7 novel •CH2C(X)=CH2 radicals were estimated with the aid of isodesmic reactions. In all these calculations the variable ∆νCH value at H → X replacement equivalent to 1 kcal mol-1 in C-H bond dissociation energy was applied: 9 cm-1 for H-CH2R bond and 11 cm-1 for vinylic C-H bond. The strong destabilization effect in both vinylic type XC(•)=O and XC(•)=CH2 free radicals at H → X replacement (X – substituent with lone pair or π-electrons) was found. For XC(•)=O radicals even with such groups as RO or R2N, traditionally treated as strong electron-donating, destabilization effect at H → X replacement was demonstrated. The origin of this effect was suggested in the absence of overlapping of free radical center with lone pair or π-electrons of substituent X. Stabilization (destabilization) of vinylic free radicals is found to be the result of electronegativity/polarizability interplay of a substituent