Experimental appraisal of solvent effects on internal rotation barriers and stabilization energies of disubstituted methyl radicals

The EPR spectra of aminopropynyl and aminocyanomethyl radicals were recorded in a variety of solvents with different dielectric constants. For the former radical the spectra showed exchange broadening due to restricted rotation about the H₂NC˙ bond. Arrhenius parameters for the bond rotation, i.e. for the interconversion of the two rotamers, were obtained by computer simulation of the exchange‐broadened spectra. The rotational barrier height was found to change only marginally in solvents with dielectric constants ranging from 2.1 to 37.5. For both radicals the temperature dependences of the α(NH₂) values also indicated little change in their H₂NC˙ rotation barriers in solvents of differing polarity. Because rotation barriers are directly related to the stabilization energies (SE) of the radicals, it was concluded that solvent polarity has little effect on the SE of a capto‐datively substituted radical.