STRUCTURAL EFFECTS ON STABILITIES OF IMINOXY RADICALS

The equilibrium acidities for the O-H bonds of seven ketoximes in dimethyl sulfoxide and the oxidation potentials of their conjugate anions have been measured. The combination of the pK(HA) and E(OX)(A(-)) values using eq 1 has allowed estimates to be made of the ethalpies for homolytic cleavage of the O-H bonds (BDEs). The BDEs estimated for the O-H bonds in t-Bu(2)C=NOK-and t-Bu(1-Ad)C=NOH by our simple method using eq 1, BDE = 1.37pK(HA) + 23.1E(OX)(A(-)) + 73.3, were found to be within experimental error of those obtained by a direct calorimetric method. The BDEs of the O-H bonds become progressively weaker as the size of the alkyl groups R and R' in the RR'C=NOH oximes become larger. The BDEs cover a range of 14 kcal/mol from that of 95.8 kcal/mol for the BDE of the O-H bond in Me(2)C=NOH to 81.7 kcal/mol for the BDE of the O-H bond in t-Bu(Ad)C=NOH. The effects are rationalized in terms of increasing ground state energies and decreasing BDEs with increasing size of the alkyl groups, as the result of progressively larger relief of steric strain as the O-H bond is broken homolytically. These and other effects of structural changes in oximes on BDEs of their O-H bonds are consistent with delocalization of the odd electron in a p orbital on oxygen to carbon. The X-ray crystal structure of di-tert-butyl ketoxime has been measured and compared with that of dimethyl ketoxime.