AN EVALUATION OF ADDITIVITY SCHEMES FOR THE ESTIMATION OF HEATS OF FORMATION OF DISTONIC RADICAL CATIONS

The validity of additivity schemes for the determination of the heats of formation of distonic radical cations, CH2.(CH2)(n-1)OH2+ (n = 1-4) , has been examined using high level [G2(MP2,SVP) and G2(MP2)] nb init io calculations. The additivity schemes in essence assume that either (a) the dissociation energy of a C-H bond in a protonated molecule (e. g., CH3OH2+) is the same as that of the corresponding unprotonated mol ecule (e.g., CH3OH), or that (b) the proton affinity of a radical (e.g ., (CH2OH)-O-.) is the same as that of the corresponding molecule (e.g ., CH3OH). These assumptions an found not to hold well for alpha-disto nic radical cations (e.g., (CH2OH2+)-O-.), i.e., species for which the charge and radical sites are formally located on adjacent atoms (n = 1). However, they lead to reasonable estimates for the heats of format ion of larger distonic radical cations (n > 1). A rationalization of t hese observations in terms of orbital interactions is presented.