The electrophilic additions of hydroperoxyl (HO2.), alkylperoxyl (RO2.), an
d halogenated alkylperoxyl radicals to ethylene were studied using the AM1
and PM3 semiempirical MO methods at the SCF/UHF level. Reactantlike transit
ion states were predicted for the title additions. The AM1 activation entha
lpies (Delta H-f*)were found to be increased in the order HO2. < CH3O2. < C
2H5O2. < i-C3H7O2.. The reactivity of an alkylperoxyl radical toward ethyle
ne was found to be increased as the degree of halogen substitution on the a
lkyl group increased. A good correlation was established between Delta H-f*
and the Taft polar substituent constants, sigma*. The Evans-Polanyi correl
ation between Delta H-f* and Delta H(r)degrees was justified and the validi
ty of the Hammond postulate was indicated. The calculated results were comp
ared with the available experimental findings. (C) 1999 John Wiley & Sons,
Inc.