Trajectory studies of S(N)2 nucleophilic substitution .5. Semiempirical direct dynamics of Cl-CH3Br unimolecular decomposition

Direct dynamics simulations of the dynamics of the Cl----CH3Br complex are performed for 25 ps or until either Cl- + CH3Br or ClCH3 + Br- are formed. Two different potential energy surfaces, AM1-SRP1 and AM1-SRP2, are investigated in the simulations by using the AM1 semiempirical model with two different sets of specific reaction parameters (SRPs). The AM1-SRP surfaces give non-RRKM unimolecular dynamics for Cl----CH3Br as found in a previous simulation based on an analytic potential energy surface, PES1(Br), derived by fitting HF/SV4PP/6-31G* ab initio calculations and experimental data. However, detailed aspects of the Cl----CH3Br intramolecular and unimolecular dynamics are different for the two AMI-SRP surfaces and in some cases strikingly different from those found for the PES1(Br) surface. Global potential energy surface properties, not only those of stationary points and along the reaction path, are expected to influence the Cl----CH3Br nonstatistical dynamics. Of the three surfaces, only PES1(Br) gives a relative translation energy distribution for the ClCH3 + Br- dissociation products which agrees with experiment. The average product translational energy is approximately a factor of 3 too large for each of the AM1-SRP surfaces. A definitive determination of all the dynamics and kinetics for Cl- + CH3Br --> ClCH3 + Br- S(N)2 nucleophilic substitution may require dynamical calculations based on a potential energy surface derived from high-level ab initio calculations.