THE SN2 IDENTITY EXCHANGE-REACTION CLCH2CN + CL--]CL- + CLCH2CN - EXPERIMENT AND THEORY

The rate of substitution for the title reaction has been measured in the gas phase using Fourier transform ion cyclotron resonance (FT-ICR) spectrometry. The value of the observed rate coefficient is found to be 3.3 +/- 1.0 X 10(-10) cm3 s-1 at 350 K, a reaction rate approximately one-tenth of the electrostatic ion-molecule collision rate. The experimental complexation energy for the formation of the intermediate ion-molecule complex, [ClCH2CN.Cl]-, has also been measured using FT-ICR spectrometry by bracketing the associated equilibrium constant relative to values for compounds with known chloride affinities. Hence, DELTAH-degrees-350 = -19.4 kcal mol-1 is obtained for the enthalpy of complexation at 350 K. By applying RRKM theory with the microcanonical variational transition state (muVTS) criterion to model the reaction, it is deduced that the observed experimental efficiency corresponds to an S(N)2 transition state 5.9 kcal mol-1 below the separated reactants, or 13.5 kcal mol-1 above the intermediate ion-molecule complex. To further elucidate the potential surface for the title reaction and to provide data for the RRKM analysis, high-level ab initio quantum chemical results have been determined. In particular, the geometric structures, relative energies, and vibrational frequencies of four salient stationary points on the potential surface (separated reactants, two ion-dipole complex structures, and the S(N)2 transition state) have been investigated at various levels of theory ranging from 3-21G RHF to 6-31+G(d,p) MP4 to TZ3P+(2fd)+diffuse MP2. The ab initio results predict the ion-dipole complex and the S(N)2 transition state to lie 18.4 and 6.9 kcal mol-1, respectively, below the separated reactants, in excellent agreement with experiment. For comparison, improved theoretical predictions are obtained for the CH3CI/Cl- system, viz. a complexation energy of 10.6 kcal mol-1 and an S(N)2 barrier 1.8 kcal mol-1 above the separated reactants. Qualitative bonding models describing the effect of alpha substitution on the stability of S(N)2 transition states are subsequently analyzed using the ab initio results for the CH3Cl/Cl- and ClCH2CN/Cl- systems. The alpha effect in these systems appears not to arise from "resonance' effects in the transition state.