Rotational barriers in bisadducts of 1-cyano-1-methylethyl radicals with nitrones and nitroso compounds

The temperature-dependent ¹H NMR spectra have been measured for the bisadducts 2a and 3 of 1-cyano-1-methylethyl radicals with α-phenyl-N-benzylnitrone [N-(benzylidene)benzylamine N-oxide] and nitrosobenzene, respectively. A free energy of activation of ΔG^≠ = 62.8 ～ 64.4 kJ mol^?1 (15.0 ～ 15.4 kcal/mol) at 24 ～ 45 °C has been obtained for 2a by applying the Eyring equation to the rate constants at the coalescence points of the methyl signals. The line shapes due to four methyl signals exchanging between two sites of equal population have been simulated by the theoretical calculation to give an activation enthalpy of 52.3 kJ mol^?1 (12.5 kcal/mol) and the corresponding entropy of ?39.3 J K^?1 mol^?1 for 3. As the inversion barriers at pyramidal nitrogens of hydroxylamine derivatives should be lowered in N-phenyl derivatives because of conjugation, the rate process which is responsible for the observed temperature dependent NMR spectra can be assigned to restricted rotation around one of the skeletal C? N? O? C bonds. The relevance of the results to the conformations of nitroxides derived from the spin trapping method using α-phenyl-N-t-butylnitrone [N-(benzylidene)-t-butylamine N-oxide] and nitroso spin traps is discussed.