Rate constants and Arrhenius parameters have been measured for the reactions of tert-butoxyl radicals with alkenes by using laser flash photolysis in benzene-di-tert-butyl peroxide (1 :2 v/v) as solvent. With norbornene and norbornadiene, additions to the double bonds were the only modes of reaction, whereas with cyclopentene and cyclopentadiene, hydrogen abstraction was a competitive process. The measured activation energies were quite low; thus for norbornene, norbornadiene, and cyclopentene, E, (kcal/mol) = 2.36 f 0.40, 1.56 f 0.55, and 2.35 f 0.22, respectively, while the corresponding A factors were log (A/(M-' s-I)) = 7.76 f 0.31, 7.65 f 0.43, and 8.56 f 0.18. For comparison, hydrogen abstraction by tert-butoxyl at cyclopentane proceeds with E , (kcal/mol) = 3.47 f 0.59 and log (A/(M-' s-l)) = 8.47 f 0.45. The reactions of the fert-butoxyl radical with hydrocarbon substrates in solution have been the subject of extensive invest i g a t i~n .~-~~ With alkanes the reaction is a simple hydrogen

CHEONG WONG. Can. J. Chem. 58, 2537 (1980). The half-wave reduction potentials, measured by cyclic voltammetry, and n,x' triplet energies, measured by phosphorescence spectroscopy, were determined for a series of eighteen symmetrically and unsymmetrically substituted benzophenones. Attempts are made to correlate the results with Hammett substituent constants. Better correlations are observed when the data are correlated with a two-parameter function consisting of Hammett substituent constants and a set of substituent parameters describing variations in free radical stability. Significant deviations from "normal" behaviour are observed for benzophenones substituted by both electron-donating and electron-withdrawing substituents. These deviations are attributed to merostabilization of the radical-like species, and an empirical approach designed to evaluate the importance of this effect is developed. Ab initio calculations of molecular orbital energies in meta-and para-substituted benzaldehydes are used to evaluate the substituent effects on E,12red and ETn"* in terms of the effect on the energies of the n-and n:*-orbitals.WILLIAM J. LEIGH, DONALD R. ARNOLD, ROBERT W. R. HUMPHREYS et P o CHEONG WONG. Can. J. Chem. 58, 2537 (1980). On a determine les potentiels de reduction de demi-vague mesurts par voltametrie cyclique et les Cnergies de l'etat triplet, n,n' mesurees par spectroscopic de phosphorescence, pour une sOrie de 18 benzophenones substituees symttriquement et asymktriquement. On a essaye d'bablir une correlation entre les resultats et les constantes de substituant de Hammett. On a observe les meilleures correlations quand les donnees sont relikes a une fonction adeux paramktres se composant des constantes de substituant de Hammett et a un ensemble de paramktres de substituant decrivant les variations de stabilitk du radical libre. On a observe des deviations importantes par rapport au comportement "normal" dans le cas des benzophenones substitutes a la fois avec des substituants donneurs et des substituants accepteurs d'electrons. Ces deviations sont attribuees a une merostabilisation de I'espltce radicalaire et on a developpk une approche empirique pour tvaluer I'importance de cet effet. On a utilise des calculs a b initio d'energies d'orbitales moltculaires dans des benzaldehydes substitues en me'ta et enpara pour &valuer les effects de substituants sur E112rrdetE~nprr* en fonction de l'effet sur les energies des orbitales n et x'.[Traduit par le journal] Introduction stituents were, in fact, influencing the energy of the One of the early successes of molecular orbital 7~'-0rbita1(3). theory was the recognition that the half-wave reIt was also found that there is a correlation beduction of polynuclear aromatic hy-tween the n,nf triplet energy of a series of substidrocarbons correlates with the energy calculated tuted aromatic carbony1 compounds and their for the lowest unoccupied molecular orbital half-wave reduction potentials (3). This was attri-(LUMO; that is, the n*-orbital) of the aromatic buted to ...

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