INTRINSIC STRUCTURE-REACTIVITY RELATIONSHIPS IN GAS-PHASE S(N)2 REACTIONS - IDENTITY EXCHANGE OF SUBSTITUTED BENZYL CHLORIDES WITH CHLORIDE-ION

The potential energy surface along the gas-phase S(N)2 reaction coordi nate for a series of substituted benzyl chlorides undergoing identity exchange with chloride ion, Cl- + X-C6H4CH2Cl, has been investigated u sing both experimental and theoretical methods. The rate of identity s ubstitution and the equilibrium ion-molecule complexation energy (chlo ride affinity) for the series X = H, m-CH3, m-OCH3, m-F, m-Cl, and m-C F3 have been measured using Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. Application of RRKM theory to the experimental kinetics yields estimates for the activation energies. AM1 semiempiric al calculations1 were also used to estimate the energetics of the surf ace for comparison with experiment and to extend the results to other systems. We show that although the experimental observables vary over a considerable range, the intrinsic activation energy, as measured fro m the ion-molecule complex, remains constant for the entire series of substituted benzyl chlorides studied.