STRUCTURAL EFFECTS ON STABILITIES OF IMINOXY RADICALS

The equilibrium acidities for the O-H bonds of seven ketoximes in dime thyl sulfoxide and the oxidation potentials of their conjugate anions have been measured. The combination of the pK(HA) and E(OX)(A(-)) valu es using eq 1 has allowed estimates to be made of the ethalpies for ho molytic 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 usin g eq 1, BDE = 1.37pK(HA) + 23.1E(OX)(A(-)) + 73.3, were found to be wi thin experimental error of those obtained by a direct calorimetric met hod. 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. Th e 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 i ncreasing ground state energies and decreasing BDEs with increasing si ze of the alkyl groups, as the result of progressively larger relief o f steric strain as the O-H bond is broken homolytically. These and oth er 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 keto xime has been measured and compared with that of dimethyl ketoxime.