DETERMINATION OF REACTIVITY BY MO THEORY .86. THEORETICAL-STUDIES OF SUBSTITUENT EFFECTS ON S(N)1 REACTIVITIES OF BENZYLDRYL AND BENZHYDRYL SYSTEMS

AM1-MO theoretical calculations are carried out to investigate effects of substituents on the S(N)1 reactivities of cationic benzyl, I , mono-substituted benzhydryl, II, and di-substituted benzhydryl, III , systems with neutral leaving group of FH. The effects of para-substituents are markedly greater than those of meta-substituents in all systems due to a strong positive charge delocalization of electron donating substituents at para position. The enthalpies of activation, DELTAH(not-equal), are linearly correlated with the enthalpies of reaction, DELTAH0, with slopes of 0.48 for I , and 0.39 for II and III , indicating that the transition state (TS) is reached at about half point along the reaction coordinate for I, but somewhat earlier for II and III. The linear correlations between DELTAH(not-equal) and C(alpha)-leaving group bond length (d(c-LG)) and between DELTAH(not-equal) and stretching of d(c-LG) in the TS (DELTAd(not-equal)(C-LG)) manifest that all reaction series conform to the Kirby's rule and the Bell-Evans-Polanyi principle, respectively. Satisfactory Hammett type plots, DELTAH(not-equal) vs sigma(p)+, are obtained for the para substituents with the corresponding rho+ values of -3.0 , -1.3 and -1.2 for the I , II and III series. For the meta substituted series, the difference in the substituent effects is small since the effects are solely due to polar effect.