Estudo te?rico das rea??es de SN2 em fase gasosa: RCI+OH??ROH+CI? (R = Metil, Etil, n-Propil, i-Propil, n- Butil, s-Butil e t-Butil)

• Ano de defesa: 2012
• Autor(a) principal: Souza, Ana Carolina Bello de
• Orientador(a): Souza, Ana Carolina Bello de
• Banca de defesa: Sant'Anna, Carlos Maur?cio Rabello de, Martins, Eduardo Monteiro
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal Rural do Rio de Janeiro
• Programa de Pós-Graduação: Programa de P?s-Gradua??o em Qu?mica
• Departamento: Instituto de Ci?ncias Exatas
• País: Brasil
• Palavras-chave em Português: Substitui??o Nucleof?lica. .; Elimina??o; Teoria do Estado de Transi??o
• Palavras-chave em Inglês: Nucleophilic Substitution; Elimination; Transition State Theory
• Área do conhecimento CNPq: Qu?mica
• Link de acesso: https://tede.ufrrj.br/jspui/handle/jspui/1547

In this work, the theoretical study of the gas-phase bimolecular nucleophilic substitution reaction, CH3Cl + OH- CH3OH + Cl-, is introduced, aiming the description of the potential energy surface, the calculation of rate constants and the investigation of the effect of increasing the side chain (changing the CH3 radical in the reaction cited above by the radicals ethyl, n-propyl, i-propyl, n-butyl, s-butyl e t-butyl). The theoretical calculations have been first performed at the MP2/6-31+G(d) level for the geometry optimizations and vibrational frequencies calculations. Single point calculations at the CCSD(T)/6-31+G(d) level have also been performed in order to improve the total energies for the stationary points. However, the relative energies of these stationary points at both MP2 and CCSD(T) level shown close results, so that the single point calculations at the CCSD(T) level have not been proved strictly necessary and have therefore not been performed for all the points along the potential energy surface. The minimum energy path has been described by the intrinsic reaction coordinate method, calculated at the MP2/6-31+G(d) by performing sequential geometry optimizations starting from the saddle point. The calculated enthalpy difference at 298K for the reaction has been determined as -49.5 kcal/mol, in good agreement with the literature value: -50.5 kcal/mol. The calculated rate constant has been obtained as 1.41 x 10-9 cm3.molecule-1.s-1, at 298,15K, in excellent agreement with the experimental data: 1.3 ? 1.6 x 10-9 cm3.molecule-1.s-1.Moreover, the rate constants show non-Arrhenius behavior, decreasing as the temperature increases, which is consistent with the experimental expectation. In this way, the performance of the variational transition state theory for this reaction can be considered satisfactory. By increasing the side chain of the reactant, other reaction channels have been observed: the bimolecular elimination E2 channel and the attack of the nucleophile from the same plane of the exit group (the front-SN2).For these reactions of the alkyl chlorides on n carbon atoms (1 < n ? 4), the B3LYP/6-31+G(d,p) level has been adopted for geometry optimizations and vibrational frequencies. Then, single point calculations at the CCSD(T)/6-31+G(d,p)//B3LYP/6- 31+G(d,p) level have been performed. A comparison of the reaction channels, back- SN2 and E2, shows that the E2 channel is kinetically favored, whereas the SN2 products are thermodynamically more stable. As expected, high values for the potential height have been observed for the front-SN2, being these channels disfavored in all cases. In general, the energy of the saddle points in respect to the isolated reactants slightly depend upon the size of the side chain.