Theoretical investigation of nonlinear optical and spectroscopic properties of some aromatic molecules containing hetereoatoms

The aim of this thesis is to investigate nonlinear optical and spectroscopic properties of some aromatic molecules containing heteroatoms in their ground states using Hartree-Fock (HF) and Becke-3-Lee-Yang-Parr (B3LYP) methods with different basis sets. Two different molecular structures containing 1,3,4-oxadiazole are used to investigate these properties. The reason for choosing these molecules is that they are efficient materials in various fields, especially in pharmaceutical and optoelectronic technologies. Due to these technological applications, a number of studies have been performed on different molecules containing 1,3,4-oxadiazole in the literature.To investigate nonlinear optical properties, 18 molecular structures (5a-f, 8a-f ve 9a-f) which have heterocyclic 1,3,4-oxadiazole structure substituted by various electron donor and acceptor groups have been used. Having obtained optimized structures and structural parameters, induced dipole moment, polarizability, hyperpolarizability, HOMO and LUMO energy values have been investigated. Besides, the hardness and electronegativity parameters have been obtained from these energies. How these properties change with effects of different electron donor and acceptor groups have also been investigated. In this situation, the interaction of charge transfer (CT) in the ionic molecules (9a-f) is more strength than the other neutral molecules (5a-f and 8a-f). More explicitly, ?? 1/ ? has been obtained.To investigate spectroscopic properties, 2-aryl-1,3,4-oxadiazole derivatives (5a-g) represented by seven different molecular structures have been studied. The vibrational frequencies, scaling-factors and chemical shifts using different models for 2-aryl-1,3,4-oxadiazole derivatives (5a-g) have been calculated, and results have been discussed. As a result, the accuracy of computational models has been supported with experimental data.