Química de vibrações moleculares

Author(s)

Leonardo José Duarte

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Química
• Defense date: 2023-02-02

Examining board members

Roy Edward Bruns; Adalberto Bono Maurizio Sacchi Bassi; Diego Pereira dos Santos; Mauro Carlos Costa Ribeiro; Roberto Luiz Andrade Haiduke

• Advisor: Roy Edward Bruns

Abstract

A charge-charge transfer-dipolar polarization model, based on the Quantum Theory of Atoms in Molecules, successfully estimates theoretical dipole moment derivatives and infrared intensities determined directly from wavefunction calculations within numerical errors. High quality quantum level calculations usually agree with experimental measurements within a few percent. For out--of--plane vibrations the inclusion of atomic dipole moments, i.e. dipolar polarization effects, is very important to obtain accurate electronic density descriptions for small amplitude vibrations. They can be expected to be just as important for large molecular distortions. For out--of--plane bends, the sp2 to sp3 tendency will be increased for larger angular movements from equilibrium. These polarizations have been found to be important for predicting accurate intensities for most molecules. Models containing only atomic charges will tend to underestimate charge transfer owing to the charge transfer – dipolar polarization vibrational relaxation effect. Considering the imaginary normal mode of SN2 transition states, the principal contributions are the charge transfers that occur between the nucleophile and leaving group through the carbon atom. As the atoms vibrate, the changes in the molecular electronic density are consistent with the mechanism of the reaction itself and the direction of CT vectors corresponds to the movement of electrons as described by the classical curved arrows representation of the reaction mechanism. The difference between the charge transfer of the nucleophile and the leaving group indicates the tendency of carbon to receive charge along the reaction coordinate. Dipolar polarizations contributions results from the inversion of the molecular geometry and from the substitution of LG by Nu with different electronegativities. These results show that the charge- charge transfer-dipolar polarization QTAIM formulation provides a much more accurate and detailed description of electronic density changes for small amplitude vibrations than other available models. It can readily be applied to larger molecular distortions as well and may be useful to study chemical reactivity. Molecular vibrations are determined by force constants, which arise from the potential energy second derivatives. Since the energy of a molecular system results from a sum of atomic terms, the force constant can also be divided into atomic contributions. In order to calculate such contributions, a modification of Wilson's FG method was made, in which a new dimension is added to the Hessian matrix. The new dimension contains second derivatives of IQA contributions to the total energy of the system. The method is able to reproduce experimental and theoretical data obtained with analytic methods. The magnitude of the force constants receive major contributions from the Coulomb and intra--atomic potentials. Exchange-correlation contributions, however, appear as determining factor only for the homonuclear diatomic molecules. The increase in bond order is accompanied by the increase in the Coulomb contribution, while exchange-correlation and intra-atomic terms remains almost unchanged. Although the results herein are incipient, the refinement of the methodology can lead to a better understanding of the potential energy surface and can act as an auxiliary tool in designing force fields for molecular mechanics (AU)

• FAPESP's process: 17/22741-3 - Using atomic multipoles and developing machine learning models to investigate transition states
• Grantee: Leonardo José Duarte
• Support Opportunities: Scholarships in Brazil - Doctorate (Direct)