Molecular electronic properties and inter-molecular interactions in various polymers



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Investigations of molecular electronic properties and inter-molecular interactions have been carried out using various high-level ab initio methods and density functional theory calculations. This dissertation focus on comparing these results with the experimental findings and the reported highly accurate calculations to find alternative ways to simulate and improve on computational times. One of the main areas addressed is the inter-molecular π -π stacking interactions of conjugated π systems for their many important roles in the nature. Sandwich (AA type) and slipped parallel (AB type ) dimer structures of quasi one-dimensional acenes and two-dimensional sheets containing pyrene to coronene encircled with layers of benzene rings have been studied in this regard. The optimized structures using the standard Møller-Plesset theory with spin scaling (SOS-MP2) shows significant biconcave deviation from planarity of the inter-sheet CC distances. The DFT variants studied retained a quasi-planar structure in contrast to the afore-mentioned SOS-MP2 results. The extrapolated binding energies (N   ) of –74.3 meV/atom agrees well with the experimental defoliation energy of –52 meV/atom (–67 meV/carbon atom). The results showed that standard computationally cheaper ab initio methods can be used in lieu with the high-level methods such as the CCST(T) which is the gold standard in computational calculations. This study was further extended in an attempt to find better stabilizer molecules in graphene exfoliation process that can prevent the re-association of graphene sheets by means non covalent interactions such as π -π stacking interactions. Functionalized pyrene derivatives by varying the number of functional groups and the electronegativity have been analyzed to better understand the role of these functional groups play on the inter-molecular interactions. It was apparent the charge and the number of functional groups played an important role in binding interactions. The NPA and MEP analysis showed the polarization of neutral graphene sheets in the presence of functional groups the these lead to electrostatic attractions between the sheets. Solvent effects were computed by means of global solvation methods (COSMO) and explicit solvation methods to understand the nature of their role. The stabilization of the functional groups by the solvent molecules led to an decrease of the binding energies relative to the isolated systems. The microhydration effects on two polymer electrolyte membrate materials, Nafion and PFSI were investigated by means normal modes of of vibration. Peak broadening and mechanical coupling of the C-F vibrational modes (specifically skeletal C-F modes) with the polar head groups makes the correct interpretation of bands difficult. However, this issue as addressed by fixing the backborne atoms and an attempt was made to assign the vibrational modes in a more precise and accurate way. A strong effect of microhydration was observed by a large red shift of the O-H stretching mode of the SO3H polar head group from 3642 cm-1 to 2600-2800 cm-1for Nafion. In addition that our frequency calculations show how different types of OH stretching vibrations contribute to the overall high frequency vibrational spectra. The proton transfer from the ionomer material to the water molecules was observed early on for the PFSI variant. The geometry studies and the analysis of hydrogen bonding explains this observation well. The study of charge transfer in a carotene-porphyrin-fullerene based molecular triad system was performed in an attempt to identify better computational methods to study long range interactions. There is a great interest on studying these systems due to their many applications in electronics and photo voltaic industry . It is not possible to use high level of theory in such large systems due to practical limitations. Therefore, there is a pressing need to find alternative methods such as long range corrected DFT functionals and reasonable ab-initio methods to study such long range phenomena. In an effort to find such methods the system was studied using ADC(2) method and the range separated DFT B97XD and CAM-B3LYP approaches. ADC(2) method results were in well agreement with the experimental excitation energies and the alternative DFT methods showed promising agreement by stabilizing the CT states upon solvent equilibration for the CT states. However, CAM-B3LYP method energies are more consistent with the ADC(2) method and is more suitable for CT analysis .



Graphene sheets, ππ stacking interactions, Quantum chemical calculations, Polycyclic aromatic hydrocarbons (PAHs), Nafion, Polymer electrolyte membranes, Triad, DFT, ADC(2)