Thermochemical Relationships and Potential Energy Surfaces

Note that Equation 7.8 uses bond-dissociation energies of neutral species (Table A.3), to which the organic chemist s intuition and knowledge of mechanistic principles should be directly applicable. For two competing reactions of the same molecular ion (and thus the same E M) values). Equation 7.8 predicts that AEq is determined by Alt of the respective radicals (Stevenson s Rule) and AD of the respective bonds. Further, a structural change usually affects / (AB ) much more than 0(AB—CD) or /E(ABCD), which is why product ion stability is usually the most influential factor determining product ion abundance (Section 8.2). 

Potenlial-cncrgy surfaces resulting in (a) higher or (b) lower similarity for 

Quite a different situation is indicated for the decomposition of p-amino-1,2-diphenylethane. The presence of the p-amino group should lower the ionization energy of the benzyl radical, lowering E , and thus lower the kinetic shift. This is not observed, however, suggesting a substantial reduction in the looseness of the activated complex. The presence of the amino group will increase the elec- 

C6H5CH2CH2CfiH5 ions in the energy region required for this metastable decomposition. 

Bowers 1979, 1984 Futrell 1986 Ghosh 1983 Holmes 1986 Levsen 1978 Lossing and Holmes 1984 Mark 1985 Morrison 1986 Robinson and Holbrook 

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