Lucas Paquin CHEM 166A April 30 th , 2018 Modeling an S N 2 Reaction with Quantum Chemistry Introduction For this experiment, the software program Jaguar from Schrödinger, Inc. was used on a computer. Quantum calculations were run to optimize the molecular models, and calculate the energies as well as the vibrational frequencies for the following reaction: Figure 1: S N 2 reaction of CH 3 F + F - For this reaction, it can be assumed that in the reactant/product state, the two molecules CH 3 F and F - do not have any interaction. Therefore, the total energy of the reactant/product state is the sum of individual molecules’ energies. There could be two intermediate states, which are complexes of the reactants and products respectively. In addition, a transition state (TS) should bridge the two intermediate states. Given the difficulty of solving the exact solution of the Schrödinger equation, theories for quantum calculations introduce approximations as a matter of practice. The Hartree-Fock Self Consistent Field (HF) method often assumes that the exact N-body wave function of the system can be approximated by a single Slater determinant of N spin-orbitals. Using the variational method, one can derive a set of N-coupled equations for the N spin orbitals. A solution of these equations yields the Hartree–Fock wave function and energy of the system. The solution is expected to be improved in multiple HF calculations. Based on the perturbation theory, the Møller–Plesset perturbation theory (LMP2) is one of the post-Hartree–Fock methods for quantum chemistry. The LMP2 method improves on the Hartree–Fock method by adding second order correction. Finally, most quantum chemistry calculations require a basis set, which contains a set of functions to approximate the “real” wave function. In short, before a quantum
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