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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) / Theoretical Study of the Gas-Phase SN2 Reactions: RCl + OH- ROH + Cl- (R=Methyl, Ethyl, n-Propyl, i-Propyl, n-Butyl, s-Butyl e t-Butyl).Souza, Ana Carolina Bello de 03 October 2012 (has links)
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Previous issue date: 2012-10-03 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior - CAPES / 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. / Este trabalho trata do estudo te?rico das rea??es de substitui??o nucleof?lica de segunda
ordem, CH3Cl + OH- CH3OH + Cl-, em fase gasosa, visando estudar a superf?cie de
energia potencial, obter as constantes de velocidade e ainda verificando o efeito do
aumento da cadeia lateral (trocando o radical CH3 na rea??o descrita acima por radicais
etil, n-propil, i-propil, n-butil, s-butil e t-butil). Primeiramente, c?lculos te?ricos para
otimiza??es de geometria e frequ?ncia foram realizados em n?vel MP2/6-31+G(d) para
a rea??o CH3Cl + OH- CH3OH + Cl- e, em seguida, c?lculos single-point em n?vel
CCSD(T)/6-31+G(d) foram realizados para corrigir os valores da energia eletr?nica dos
pontos estacion?rios obtidos no caminho de rea??o. Entretanto, os valores obtidos para
as energias relativas em n?veis MP2 e CCSD(T) foram muito pr?ximos, n?o sendo
estritamente necess?rio refinar,atrav?s de c?lculos single-point em n?vel CCSD(T)/6-
31+G(d),os valores de energia de todos os pontos obtidos na superf?cie de energia
potencial. O caminho de rea??o menor energia foi descrito pela coordenada de rea??o
intr?nseca, calculada por otimiza??es de geometrias de uma sequ?ncia de configura??es
ao redor do ponto de sela em n?vel MP2/6-31+G(d). A diferen?a de entalpia a 298K
calculada para a rea??o foi de -49,5 kcal/mol, em bom acordo com o dado da literatura,
-50,5 kcal/mol. A constante de velocidade da rea??o obtida foi de 1,41 x 10-9
cm3.mol?cula-1.s-1, a 298,15K, em excelente acordo com o dado experimental: 1,3 ? 1,6
x 10-9 cm3.mol?cula-1.s-1. Al?m disso, as constantes de velocidade globais apresentam
comportamento n?o-Arrhenius, diminuindo conforme a temperatura aumenta, em um
perfil consistente com a observa??o experimental. Dessa forma, a aplica??o da teoria de
estado de transi??o se mostra satisfat?ria para essa rea??o. A partir do aumento da
cadeia lateral, outros canais de rea??o foram observados, em prov?vel competi??o ?
substitui??o nucleof?lica de ordem 2: a elimina??o de segunda ordem, E2. O ataque do
nucle?filo pela frente da cadeia tamb?m foi obtido e investigado. Para as rea??es dos
cloretos de alquila com cadeia lateral de n carbonos (1 < n ? 4), o n?vel B3LYP/6-
31+G(d,p) foi adotado para c?lculos de otimiza??es e frequ?ncias. Posteriormente,
c?lculos em n?vel CCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p) foram realizados.
Comparando os canais de rea??o de substitui??o nucleof?lica back e de elimina??o, o
canal cineticamente favorecido foi o de elimina??o, por?m os produtos
termodinamicamente mais est?veis s?o os de substitui??o nucleof?lica. Como esperado,
observa-se uma barreira de potencial muito alta para as rea??es substitui??o pela frente,
sendo esses canais desfavorecidos em todos os casos.Em geral, a diferen?a de energia
dos pontos de sela em rela??o aos reagentes isolados mostra pequena depend?ncia com
o aumento da cadeia lateral linear
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