In this thesis quantum-classical dynamics is applied to the study of quantum condensed phase processes. This approach is based on the quantum-classical Liouville equation where the dynamics of a small subset of the degrees of freedom are treated quantum mechanically while the remaining degrees of freedom are treated by classical mechanics to a good approximation. We use this approach as it is computationally tractable, and the resulting equation of motion accurately accounts for the quantum and classical dynamics, as well as the coupling between these two components of the system. By recasting the quantum-classical Liouville equation into the form of a generalized master equation we investigate connections to surface-hopping. The link between these approaches is decoherence arising from interaction of the subsystem with the environment. We derive an evolution equation for the subsystem which contains terms accounting for the effects of the environment. One of these terms involves a memory kernel that accounts for the coherent dynamics. If this term decays rapidly, a Markovian approximation can be made. By lifting the resulting subsystem master equation into the full phase space, we obtain a Markovian master equation that prescribes surface-hopping-like dynamics. Our analysis outlines the conditions under which such a description is valid. Next, we consider the calculation of the rate constant for a quantum mechanical barrier crossing process. Starting from the reactive-flux autocorrelation function, we derive a quantum-classical expression for the rate kernel. This expression involves quantum-classical evolution of a species operator averaged over the initial quantum equilibrium structure of the system making it possible to compute the rate constant via computer simulation. Using a simple model for a proton transfer reaction we compare the results of the rate calculation obtained by quantum-classical Liouville dynamics with that of master equation dynamics. The master equation provides a good approximation to the full quantum-classical Liouville calculation for our model and a more stable algorithm results due to the elimination of oscillating phase factors in the simulation. Finally, we make use of the theoretical framework established in this thesis to analyze some aspects of decoherence used in popular surface-hopping techniques.

STUDIES OF INTERACTION OF SMALL MOLECULES WITH WATER CONDENSED MEDIA<br /><br /> The present work reports experimental and theoretical studies of the intermolecular interactions in condensed water media. The chemical objects comprise pristine ice and polar organic substances: acetone, acetaldehyde, methanol and chloroform and bi-component water-organic deposits. The experimental part of the studies includes the Fourier Transform Infrared Reflection Absorption spectral (FTIR RAS) examination of the processes of film growth by vapor deposition on cold metal substrate and subsequent annealing. The theoretical studies include <em>ab initio</em> (<em>MP2</em>) and semi-empirical (<em>B3LYP</em>) calculations on the small water and water-organic clusters and classical molecular dynamics simulations of the adsorption of inert guests (Xe/Rn) on the ice surface. The FTIR RA spectral studies reveal that depending on the deposition conditions condensed water media exist in two principal structural forms: noncrystalline and polycrystalline. The former is characterized by porous structure while the latter exists as a non-porous medium with smooth external interface. On annealing, characteristic spectral changes indicate on a rapid crystallization occurring at a certain temperature range. The initial adsorption of organic molecules is accompanied by the hydrogen-bonded coordination between the functional group of organic species and non-coordinated hydroxyl group of the ice surface, the topology of which depends on the electronic properties of the functional group. The computational studies of small water-organic clusters reveal, in particular, two major coordination minima for carbonyl group: a single hydrogen-bonded in-plane complex and a double hydrogen-bonded in-plane complex. The classical molecular dynamics of Xe/Rn species on the ice interface is consistent with two distinctly different surface adsorption sites: one that delocalized over the entire surface and one that confined to small opening in the top ice layer, disrupted by the thermal molecular motion. The penetration barrier is associated with van der Walls repulsion of guest species from the ordered water hexagonal arrangement. A thermo-disruption of latter leads to a rapid diffusion of guest species inside ice medium.

Este trabalho trata do estudo teórico atomístico das conformações da molécula de estireno sobre a superfície de Si(100)(2xl):H. Estudamos a molécula fisissorvida e quimissorvida sobre esta superfície. Os cálculos foram realizados através de Dinâmica Molecular Clássica. Nós reparametrizamos o Universal Force Field (UFF) com base em cálculos ab initio para sistemas modelo, e comparação a dados experimentais. Nossos resultados indicam que no processo de fississorção a região de vale da superfície é preferencial, e ocorre formação de agregados de moléculas antes do contato com a superfície. Quando passamos à análise da quimissorção de uma molécula, a região de vale permanece sendo o sítio preferencial para a posição do grupo vinil. Para as conformações de linhas de estireno, a estrutura mais estável tem todas as moléculas dispostas sobre a região de vale (ordenamento tipo \"pilha-1r\"); identificamos também outra estrutura, quase degenerada em energia, na qual o grupo vinileno se dispõe em conformação tipo \"espinha de peixe\", alternando entre a região de vale e sobre o dímero. / In this work we present a theoretical study of ,the conformation of styrene molecules on the Si(100)(2xl):H surface. We studied the conformations for styrene molecules physisorbed and chemisorbed on this surface. The study was conducted by Classical Molecular Dynamics. We performed a re-parametrization of the Universal Force Field (UFF), based on ab initio calculations for model structures, and comparison to experimental data. Our results show that for the physisorbed situations, the styrene molecules preferentially interact with the surface valley. We also identified that the molecules aggregate in clusters before reaching the surface. The valley is again the region of lowest energy for the vinylene position, for chemisorption of a single styrene molecule on the monohydride surface. For systems where we have a styrene layer chemisorbed on the dimer row, the most stable structure has ali the molecules arranged on the valley region o f the surface ( 1r -stack); we find another structure, almost degenerate in energy, in which molecules arrange in a herringbone- like configuration, with the vinylene group alternating between the valley and dimer regions.

STUDIES OF INTERACTION OF SMALL MOLECULES WITH WATER CONDENSED MEDIA<br /><br /> The present work reports experimental and theoretical studies of the intermolecular interactions in condensed water media. The chemical objects comprise pristine ice and polar organic substances: acetone, acetaldehyde, methanol and chloroform and bi-component water-organic deposits. The experimental part of the studies includes the Fourier Transform Infrared Reflection Absorption spectral (FTIR RAS) examination of the processes of film growth by vapor deposition on cold metal substrate and subsequent annealing. The theoretical studies include <em>ab initio</em> (<em>MP2</em>) and semi-empirical (<em>B3LYP</em>) calculations on the small water and water-organic clusters and classical molecular dynamics simulations of the adsorption of inert guests (Xe/Rn) on the ice surface. The FTIR RA spectral studies reveal that depending on the deposition conditions condensed water media exist in two principal structural forms: noncrystalline and polycrystalline. The former is characterized by porous structure while the latter exists as a non-porous medium with smooth external interface. On annealing, characteristic spectral changes indicate on a rapid crystallization occurring at a certain temperature range. The initial adsorption of organic molecules is accompanied by the hydrogen-bonded coordination between the functional group of organic species and non-coordinated hydroxyl group of the ice surface, the topology of which depends on the electronic properties of the functional group. The computational studies of small water-organic clusters reveal, in particular, two major coordination minima for carbonyl group: a single hydrogen-bonded in-plane complex and a double hydrogen-bonded in-plane complex. The classical molecular dynamics of Xe/Rn species on the ice interface is consistent with two distinctly different surface adsorption sites: one that delocalized over the entire surface and one that confined to small opening in the top ice layer, disrupted by the thermal molecular motion. The penetration barrier is associated with van der Walls repulsion of guest species from the ordered water hexagonal arrangement. A thermo-disruption of latter leads to a rapid diffusion of guest species inside ice medium.

In this thesis quantum-classical dynamics is applied to the study of quantum condensed phase processes. This approach is based on the quantum-classical Liouville equation where the dynamics of a small subset of the degrees of freedom are treated quantum mechanically while the remaining degrees of freedom are treated by classical mechanics to a good approximation. We use this approach as it is computationally tractable, and the resulting equation of motion accurately accounts for the quantum and classical dynamics, as well as the coupling between these two components of the system. By recasting the quantum-classical Liouville equation into the form of a generalized master equation we investigate connections to surface-hopping. The link between these approaches is decoherence arising from interaction of the subsystem with the environment. We derive an evolution equation for the subsystem which contains terms accounting for the effects of the environment. One of these terms involves a memory kernel that accounts for the coherent dynamics. If this term decays rapidly, a Markovian approximation can be made. By lifting the resulting subsystem master equation into the full phase space, we obtain a Markovian master equation that prescribes surface-hopping-like dynamics. Our analysis outlines the conditions under which such a description is valid. Next, we consider the calculation of the rate constant for a quantum mechanical barrier crossing process. Starting from the reactive-flux autocorrelation function, we derive a quantum-classical expression for the rate kernel. This expression involves quantum-classical evolution of a species operator averaged over the initial quantum equilibrium structure of the system making it possible to compute the rate constant via computer simulation. Using a simple model for a proton transfer reaction we compare the results of the rate calculation obtained by quantum-classical Liouville dynamics with that of master equation dynamics. The master equation provides a good approximation to the full quantum-classical Liouville calculation for our model and a more stable algorithm results due to the elimination of oscillating phase factors in the simulation. Finally, we make use of the theoretical framework established in this thesis to analyze some aspects of decoherence used in popular surface-hopping techniques.

Este trabalho trata do estudo teórico atomístico das conformações da molécula de estireno sobre a superfície de Si(100)(2xl):H. Estudamos a molécula fisissorvida e quimissorvida sobre esta superfície. Os cálculos foram realizados através de Dinâmica Molecular Clássica. Nós reparametrizamos o Universal Force Field (UFF) com base em cálculos ab initio para sistemas modelo, e comparação a dados experimentais. Nossos resultados indicam que no processo de fississorção a região de vale da superfície é preferencial, e ocorre formação de agregados de moléculas antes do contato com a superfície. Quando passamos à análise da quimissorção de uma molécula, a região de vale permanece sendo o sítio preferencial para a posição do grupo vinil. Para as conformações de linhas de estireno, a estrutura mais estável tem todas as moléculas dispostas sobre a região de vale (ordenamento tipo \"pilha-1r\"); identificamos também outra estrutura, quase degenerada em energia, na qual o grupo vinileno se dispõe em conformação tipo \"espinha de peixe\", alternando entre a região de vale e sobre o dímero. / In this work we present a theoretical study of ,the conformation of styrene molecules on the Si(100)(2xl):H surface. We studied the conformations for styrene molecules physisorbed and chemisorbed on this surface. The study was conducted by Classical Molecular Dynamics. We performed a re-parametrization of the Universal Force Field (UFF), based on ab initio calculations for model structures, and comparison to experimental data. Our results show that for the physisorbed situations, the styrene molecules preferentially interact with the surface valley. We also identified that the molecules aggregate in clusters before reaching the surface. The valley is again the region of lowest energy for the vinylene position, for chemisorption of a single styrene molecule on the monohydride surface. For systems where we have a styrene layer chemisorbed on the dimer row, the most stable structure has ali the molecules arranged on the valley region o f the surface ( 1r -stack); we find another structure, almost degenerate in energy, in which molecules arrange in a herringbone- like configuration, with the vinylene group alternating between the valley and dimer regions.

This dissertation describes the development and testing of modified embedded atom method (MEAM) interatomic potentials for Al, Si, Mg, Cu, Fe, and their alloys, with primary concentration on Mg-Al system. We performed the density functional theory (DFT) based ab initio calculations to determine the structural and elastic properties of element pairs that are impractical to obtain from experimental measurements. Specifically, we estimated the cohesive energy, equilibrium atomic volume, bulk modulus, and elastic moduli of every element pair in the NaCl reference structure. Based on the results of DFT calculations, MEAM parameters for each element pair were constructed. We extensively tested the new MEAM potential for Mg-Al alloy system. The new Mg-Al MEAM potential was compared with DFT calculations, previously published semi-empirical interatomic potentials, and experiments. Applicability of the new MEAM potential to atomistic modeling was demonstrated by calculating stress-strain responses from molecular dynamics (MD) simulations of Mg and Al systems in a variety of configurations. The effects of alloying, porosity, and strain rate conditions on the stress-strain response were quantified. The underlying mechanisms for tension-compression asymmetry observed in the macroscale experiments of Mg alloys were investigated at the nanoscale. This work presents a contribution to the task of bridging quantum-mechanical and classical atomistic scale simulations. Information from ab initio electronic structure calculations was used to construct parameters of semi-empirical MEAM potentials for large-scale atomistic simulations of alloys. The results of the new MEAM models compare extremely well to those from other published interatomic potentials. The applicability of the new MEAM potential to investigate nanoscale mechanisms of the deformation and fracture for Al, Mg and Mg-Al alloys was demonstrated. It has been shown that the MEAM provides a single universal formalism for classical atomistic simulations of a wide range of elements and their alloys.

A constante emissão de CO2 na atmosfera devido a fontes antropogênica despertou uma preocupação crescente em função da sua atuação no efeito estufa. Um número crescente de metodologias para redução da concentração deste gás na atmosfera vem sendo proposta e uma alternativa atrativa é a da inserção do CO2 em anéis epóxidos para formação de carbonatos cíclicos. Apesar de já existirem inúmeros trabalhos a respeito destas reações, o mecanismo pelo qual elas ocorrem não está completamente esclarecido. Neste trabalho é apresentado o estudo mecanístico da cicloadição de CO2 em diferentes epóxidos catalisada por líquidos iônicos haletos de alquil-imidazólios, CnMIm X (n = 1, 2, 3, 4 e X = Cl, Br, I), através da Teoria do Funcional da Densidade empregando-se o funcional !B97X-D e o conjunto de base 6-31G(d,p) e LanL2DZ (somente para o I). Propuseram-se duas rotas distintas para o mecanismo, uma composta por três etapas e outra composta por somente duas etapas. Ambas as rotas tiveram a primeira etapa como sendo a mais energética e correspondendo à abertura do anel epóxido e mostraram ser competitivas entre si energeticamente. Esta etapa mostrou depender de ambos cátion e ânion do catalisador para ocorrer, onde uma ligação de hidrogênio não clássica com o H2 do imidazólio aparece no estado de transição. Verificou-se que no geral a energia de ativação da reação diminui com o aumento da cadeia alquílica do cátion imidazólio assim como diminui com o aumento do caráter nucleofílico do haleto (Cl > Br > I). O substituinte do anel epóxido também exerce influência sobre a energia de ativação da reação, porém não há uma tendência bem definida. Constatou-se que o sítio mais favorável para ataque nucleofílico é o carbono não substituído do anel epóxido tanto pela diferença de energia quanto por análise de índices de reatividade de Fukui e de interações não covalentes. Uma análise de 14 funcionais da densidade e do método perturbativo de segunda ordem MP2 em comparação ao método composto G4MP2 revelaram a forte dependência das energias de ativação com o método empregado. Através de cálculos de dinâmica molecular clássica foi possível estudar dinamicamente o sistema brometo de 1-butil-3-metil imidazólio, o óxido de estireno e o CO2 e notou-se a formação de duas fases com a presença de uma interface. Adicionalmente, observou-se que a probabilidade da reação ocorrer no bulk do líquido iônico é maior, pois a proporção catalisador/substrato é maior nesta região. Por meio da dinâmica molecular de Born-Oppenheimer constatou-se que o anel epóxido também pode ser ativado através de interações com os hidrogênios H4 e H5 do anel imidazólio. / The constant emission of CO2 into the atmosphere due to anthropogenic sources has generated a growing concern regarding the greenhouse effect. Many methodologies to reduce the atmospheric CO2 concentration have been proposed and an alternative is the insertion of CO2 into epoxides to form cyclic carbonates. Although there are a lot of studies in this area, the reaction mechanism by which they occur is still unclear. In this work the cycloaddition mechanism of CO2 into different epoxides catalyzed by alkyl-imidazolium halide ionic liquids, CnMIm X (n = 1, 2, 3, 4 e X = Cl, Br, I), is presented. Density Functional Theory in conjunction with the functional !B97X-D and 6-31G(d,p) and LanL2DZ (for I atoms) basis sets were employed. Two distinct routes were proposed for the mechanism: one composed of three steps and another composed by only two steps. Both routes showed that the first step regarding the epoxide ring opening is the determined one and they are energetically competitive with each other. This step depends on both cation and anion from the catalyst to proceed through a non-classical hydrogen bond in the transition state. It was found that the activation energy decreases with the chain length of the alkyl group from the imidazolium ring as well as with the nucleophilic character of the halide (Cl > Br > I). The epoxide ring substituent also exerts influence on the activation energy of this reaction, but there is no well defined behaviour. The most favourable site for nucleophilic attack is the non-substituted epoxide ring carbon as was shown by the reaction energy difference and through reactive Fukui index and non-covalent interaction (NCI) analysis. 14 exchange-correlation density functionals were investigated and compared to the well established second order perturbation theory (MP2) method and G4MP2 composite method. One found out that the activation energies strongly depends on the chosen method. Through classical molecular dynamics it was possible to study the system 1-butyl-3-methyl-imidazolium bromide together with styrene oxide and CO2 e the formation of two phases with the presence of an interface was observed. Additionally, it was shown that the probability of the reaction to occur in the ionic liquid bulk is bigger because the catalyst/substrate proportion is bigger in this region. Born-Oppenheimer molecular dynamics was used to prove that the H4 and H5 hydrogen atoms from the imidazolium ring may interact with the oxygen atom from the epoxide and activate the C–O bond for the reaction to proceed.

A constante emissão de CO2 na atmosfera devido a fontes antropogênica despertou uma preocupação crescente em função da sua atuação no efeito estufa. Um número crescente de metodologias para redução da concentração deste gás na atmosfera vem sendo proposta e uma alternativa atrativa é a da inserção do CO2 em anéis epóxidos para formação de carbonatos cíclicos. Apesar de já existirem inúmeros trabalhos a respeito destas reações, o mecanismo pelo qual elas ocorrem não está completamente esclarecido. Neste trabalho é apresentado o estudo mecanístico da cicloadição de CO2 em diferentes epóxidos catalisada por líquidos iônicos haletos de alquil-imidazólios, CnMIm X (n = 1, 2, 3, 4 e X = Cl, Br, I), através da Teoria do Funcional da Densidade empregando-se o funcional !B97X-D e o conjunto de base 6-31G(d,p) e LanL2DZ (somente para o I). Propuseram-se duas rotas distintas para o mecanismo, uma composta por três etapas e outra composta por somente duas etapas. Ambas as rotas tiveram a primeira etapa como sendo a mais energética e correspondendo à abertura do anel epóxido e mostraram ser competitivas entre si energeticamente. Esta etapa mostrou depender de ambos cátion e ânion do catalisador para ocorrer, onde uma ligação de hidrogênio não clássica com o H2 do imidazólio aparece no estado de transição. Verificou-se que no geral a energia de ativação da reação diminui com o aumento da cadeia alquílica do cátion imidazólio assim como diminui com o aumento do caráter nucleofílico do haleto (Cl > Br > I). O substituinte do anel epóxido também exerce influência sobre a energia de ativação da reação, porém não há uma tendência bem definida. Constatou-se que o sítio mais favorável para ataque nucleofílico é o carbono não substituído do anel epóxido tanto pela diferença de energia quanto por análise de índices de reatividade de Fukui e de interações não covalentes. Uma análise de 14 funcionais da densidade e do método perturbativo de segunda ordem MP2 em comparação ao método composto G4MP2 revelaram a forte dependência das energias de ativação com o método empregado. Através de cálculos de dinâmica molecular clássica foi possível estudar dinamicamente o sistema brometo de 1-butil-3-metil imidazólio, o óxido de estireno e o CO2 e notou-se a formação de duas fases com a presença de uma interface. Adicionalmente, observou-se que a probabilidade da reação ocorrer no bulk do líquido iônico é maior, pois a proporção catalisador/substrato é maior nesta região. Por meio da dinâmica molecular de Born-Oppenheimer constatou-se que o anel epóxido também pode ser ativado através de interações com os hidrogênios H4 e H5 do anel imidazólio. / The constant emission of CO2 into the atmosphere due to anthropogenic sources has generated a growing concern regarding the greenhouse effect. Many methodologies to reduce the atmospheric CO2 concentration have been proposed and an alternative is the insertion of CO2 into epoxides to form cyclic carbonates. Although there are a lot of studies in this area, the reaction mechanism by which they occur is still unclear. In this work the cycloaddition mechanism of CO2 into different epoxides catalyzed by alkyl-imidazolium halide ionic liquids, CnMIm X (n = 1, 2, 3, 4 e X = Cl, Br, I), is presented. Density Functional Theory in conjunction with the functional !B97X-D and 6-31G(d,p) and LanL2DZ (for I atoms) basis sets were employed. Two distinct routes were proposed for the mechanism: one composed of three steps and another composed by only two steps. Both routes showed that the first step regarding the epoxide ring opening is the determined one and they are energetically competitive with each other. This step depends on both cation and anion from the catalyst to proceed through a non-classical hydrogen bond in the transition state. It was found that the activation energy decreases with the chain length of the alkyl group from the imidazolium ring as well as with the nucleophilic character of the halide (Cl > Br > I). The epoxide ring substituent also exerts influence on the activation energy of this reaction, but there is no well defined behaviour. The most favourable site for nucleophilic attack is the non-substituted epoxide ring carbon as was shown by the reaction energy difference and through reactive Fukui index and non-covalent interaction (NCI) analysis. 14 exchange-correlation density functionals were investigated and compared to the well established second order perturbation theory (MP2) method and G4MP2 composite method. One found out that the activation energies strongly depends on the chosen method. Through classical molecular dynamics it was possible to study the system 1-butyl-3-methyl-imidazolium bromide together with styrene oxide and CO2 e the formation of two phases with the presence of an interface was observed. Additionally, it was shown that the probability of the reaction to occur in the ionic liquid bulk is bigger because the catalyst/substrate proportion is bigger in this region. Born-Oppenheimer molecular dynamics was used to prove that the H4 and H5 hydrogen atoms from the imidazolium ring may interact with the oxygen atom from the epoxide and activate the C–O bond for the reaction to proceed.

A constante emissão de CO2 na atmosfera devido a fontes antropogênica despertou uma preocupação crescente em função da sua atuação no efeito estufa. Um número crescente de metodologias para redução da concentração deste gás na atmosfera vem sendo proposta e uma alternativa atrativa é a da inserção do CO2 em anéis epóxidos para formação de carbonatos cíclicos. Apesar de já existirem inúmeros trabalhos a respeito destas reações, o mecanismo pelo qual elas ocorrem não está completamente esclarecido. Neste trabalho é apresentado o estudo mecanístico da cicloadição de CO2 em diferentes epóxidos catalisada por líquidos iônicos haletos de alquil-imidazólios, CnMIm X (n = 1, 2, 3, 4 e X = Cl, Br, I), através da Teoria do Funcional da Densidade empregando-se o funcional !B97X-D e o conjunto de base 6-31G(d,p) e LanL2DZ (somente para o I). Propuseram-se duas rotas distintas para o mecanismo, uma composta por três etapas e outra composta por somente duas etapas. Ambas as rotas tiveram a primeira etapa como sendo a mais energética e correspondendo à abertura do anel epóxido e mostraram ser competitivas entre si energeticamente. Esta etapa mostrou depender de ambos cátion e ânion do catalisador para ocorrer, onde uma ligação de hidrogênio não clássica com o H2 do imidazólio aparece no estado de transição. Verificou-se que no geral a energia de ativação da reação diminui com o aumento da cadeia alquílica do cátion imidazólio assim como diminui com o aumento do caráter nucleofílico do haleto (Cl > Br > I). O substituinte do anel epóxido também exerce influência sobre a energia de ativação da reação, porém não há uma tendência bem definida. Constatou-se que o sítio mais favorável para ataque nucleofílico é o carbono não substituído do anel epóxido tanto pela diferença de energia quanto por análise de índices de reatividade de Fukui e de interações não covalentes. Uma análise de 14 funcionais da densidade e do método perturbativo de segunda ordem MP2 em comparação ao método composto G4MP2 revelaram a forte dependência das energias de ativação com o método empregado. Através de cálculos de dinâmica molecular clássica foi possível estudar dinamicamente o sistema brometo de 1-butil-3-metil imidazólio, o óxido de estireno e o CO2 e notou-se a formação de duas fases com a presença de uma interface. Adicionalmente, observou-se que a probabilidade da reação ocorrer no bulk do líquido iônico é maior, pois a proporção catalisador/substrato é maior nesta região. Por meio da dinâmica molecular de Born-Oppenheimer constatou-se que o anel epóxido também pode ser ativado através de interações com os hidrogênios H4 e H5 do anel imidazólio. / The constant emission of CO2 into the atmosphere due to anthropogenic sources has generated a growing concern regarding the greenhouse effect. Many methodologies to reduce the atmospheric CO2 concentration have been proposed and an alternative is the insertion of CO2 into epoxides to form cyclic carbonates. Although there are a lot of studies in this area, the reaction mechanism by which they occur is still unclear. In this work the cycloaddition mechanism of CO2 into different epoxides catalyzed by alkyl-imidazolium halide ionic liquids, CnMIm X (n = 1, 2, 3, 4 e X = Cl, Br, I), is presented. Density Functional Theory in conjunction with the functional !B97X-D and 6-31G(d,p) and LanL2DZ (for I atoms) basis sets were employed. Two distinct routes were proposed for the mechanism: one composed of three steps and another composed by only two steps. Both routes showed that the first step regarding the epoxide ring opening is the determined one and they are energetically competitive with each other. This step depends on both cation and anion from the catalyst to proceed through a non-classical hydrogen bond in the transition state. It was found that the activation energy decreases with the chain length of the alkyl group from the imidazolium ring as well as with the nucleophilic character of the halide (Cl > Br > I). The epoxide ring substituent also exerts influence on the activation energy of this reaction, but there is no well defined behaviour. The most favourable site for nucleophilic attack is the non-substituted epoxide ring carbon as was shown by the reaction energy difference and through reactive Fukui index and non-covalent interaction (NCI) analysis. 14 exchange-correlation density functionals were investigated and compared to the well established second order perturbation theory (MP2) method and G4MP2 composite method. One found out that the activation energies strongly depends on the chosen method. Through classical molecular dynamics it was possible to study the system 1-butyl-3-methyl-imidazolium bromide together with styrene oxide and CO2 e the formation of two phases with the presence of an interface was observed. Additionally, it was shown that the probability of the reaction to occur in the ionic liquid bulk is bigger because the catalyst/substrate proportion is bigger in this region. Born-Oppenheimer molecular dynamics was used to prove that the H4 and H5 hydrogen atoms from the imidazolium ring may interact with the oxygen atom from the epoxide and activate the C–O bond for the reaction to proceed.