Electronic Structure Calculations of Weak Intermolecular Interactions

Abstract

This thesis focuses on a novel the application of Symmetry-Adapted Perturbation Theory (SAPT), Functional-group SAPT (F-SAPT), and F-SAPT difference analysis to chiral self-recognition. In addition to these, a variety of highly accurate ab initio electronic structure approaches were applied on the ethylene-oxygen complex to investigate the weak intermolecular interactions in the ground electronic state and several low-lying excited electronic states. SAPT decomposes the fundamental components of noncovalent interactions between two molecules into electrostatics, exchange, induction, and dispersion. These components can be split even more by a method called Functional-group SAPT, which provides the aforementioned separation of the total interaction energy into the contributions of pairs of functional groups. F-SAPT difference analysis transforms the effects of pairs of functional groups to the contributions of substituted functional groups.

We have applied SAPT, F-SAPT, and F-SAPT difference analysis methods to chiral complexes in order to elucidate the origin of chiral self-recognition and investigate the chirodiastaltic (chiral discrimination) energy, the energetic difference between homochiral and heterochiral diastereomers of a complex. For this matter, we chose propylene oxide (PO) as the simplest chiral molecule containing an epoxide ring and glycidol as one of chemical derivatives of PO. 12 possible dimer structures of PO and 14 complexes of glycidol were reoptimized and their interaction energies were computed at different levels of electronic structure theory and basis set up to the complete basis set (CBS) limit of the coupled-cluster approach with single, double, and perturbative triple excitations (CCSD(T)). Then, a variety of symmetry-adapted perturbation theory analyses were applied to both PO dimer structures and glycidol dimers including conventional SAPT, F-SAPT, and F-SAPT difference analysis. Our results showed that the largest diastereomeric energetic effects come from the electrostatic and dispersion SAPT contributions for PO and induction for glycidol in addition to electrostatic and dispersion contributions. To complement our findings, frequency computations were carried out to distinguish the effect of chiral interactions on the vibrational frequencies of an isolated PO molecule and glycidol. In another study, we picked an ethylene molecule and an oxygen molecule as a complex and studied their interaction energy at optimized geometry, both in ground state and a few low-lying excited states. In addition, the spin splitting and spin inversion for this complex is going to be performed which has important effects in biology.