AplicaÃÃo de mÃtodos correlacionados de estrutura eletrÃnica ao estudo de estados excitados de molÃculas em fase gasosa e em soluÃÃo

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

Ab-initio electronic structure calculations, at correlated level, are applied to the study of excited states of molecules in gas-phase and solution. The electron correlated methods comprise the multiconfigurational SCF (MCSCF), and the multi-reference CI method with single and double excitations (MR-CISD), which is used in combination with MCSCF, then provides the reference configurations (CSFs) from which the excited CSFs are generated, and the total number of CSFs used in the CI expansion are thus created. Size-extensivity corrections for MR-CISD are computed through the use of extended Davidson (MR-CISD+Q) and MR-AQCC (multi-reference averaged quadratic coupled cluster) methods. The aforementioned methods are applied to the study of four problems of reasonable relevance in terms of computational quantum chemistry. The first one corresponds to the study of proton transfer (PT) and the dissociation (proton detachment, PD) of malonaldehyde, one important prototype molecule for studying intramolecular PT. Four excited states are studied, namely, nπ*, ππ*, πσ* e nσ*, where the last two are more important in the study of PD reaction. Geometry optimisations have been performed for ground state and the first two excited states, at both MR-CISD and MR-AQCC levels, and the barriers for PT at the excited states have been computed. The importance of a careful choice of the reference configurations is clearly demonstrated in order to give more accurate results at a lower computational cost. The second application refers to the solvent effects (represented by a dielectric continuum) in the absorption spectra of some organic molecules. Solvatochromisms have been computed for a representative set of n-π* and π-π* states of formaldehyde, acrolein and pyrazine using several solvents ranging from apolar to water. Such solvent effects are taken into account via implementation of the COSMO continuum solvation model into the standard MCSCF/MR-CISD approach used in COLUMBUS program system. Such implementation included equilibrium and non-equilibrium solvation. Agreement with experimental shifts is good within the limits of a continuum solvation model. The third application is concerned to the study of the excited stated of H3O+. The stationary points of the potential energy surface of the first three excited states, with planarity restriction, have been obtained and characterized. The first three dissociation channels have also been studied, and the role of a conical intersection as an important intermediate structure, which connects the stationary points of the excited states to a dissociating structure, is clarified. In the last application, the electronic factor (Δ) for thermal electron transfer between methylene groups in [CH2−(benzeno)n−CH2]−model system is studied. The dependence of Δ2 and β upon the dihedral angle (θ) between methylene and its adjacent benzene unit is studied, identifying θ values at which a exponential dependence of Δ2 with the distance between CH2 groups is obtained. The calculations are performed at RHF, ROHF and CASSCF levels, with several basis sets

ASSUNTO(S)

mÃtodos correlacinais quimica estrutura eletrÃnica estados excitados

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