Spectroscopic investigaion of some organic molecules by using density functional theory

One of the most significant aromatic amines is aniline, a primary aromatic amine one hydrogen atom of a benzene molecule substituted with an amino group (NH2). This study contains an experimental and theoretical investigation of 2,3-, 2,4-, 2,5-, and 3,4-difluoroaniline molecules (2,3-, 2,4-, 2,5- and 3,4-DFA). The experimental results were obtained using fourier transform infrared and Raman (FT-IR and FT-Raman), 1H and 13C nuclear magnetic resonance (NMR), ultraviolet-visible (UV–vis) spectral thecniques. The results were supported theoretically by density functional theory (DFT) calculations. The vibrational (FT-IR and Raman) spectra of the molecules were recorded in the region 4000–400 cm–1 and 3500–10 cm–1, respectively. The 1H and 13C NMR chemical shift values of the molecules were obtained in choloroform (CDCl3) solution. The UV-Vis electronic absorption spectra of the molecules were recorded in ethanol and water in the range of 190–800 nm at the room temperature. The calculations related to geometric structures and vibrational spectra of the molecules were performed by DFT/B3LYP-6-311++G(d,p) basis set. Fundamental vibrational assigments were made on the basis of the potential energy distribution (PED) by using VEDA 4 (Vibrational Energy Distribution Analysis) program. The NMR chemical shift values (1H and 13C) were achieved theoretically by using the gauge-invariant atomic orbital (GIAO) method. Additionally, the electronic properties, such as excitation energies, oscillator strength, excitation wavelengths, frontier molecular orbitals energies were calculated by time-dependent density functional theory (TD-DFT) method. The molecular electrostatic potential surfaces (MEPs), and density of state (DOS) diagrams were prepared and evaluated. The nonlinear optical properties (NLO) and thermodynamic features were aslo carried out theoretically. All results, both experimental and theoretical, helped to detailed description of the structural and physicochemical properties of the studied molecules. In addition, this work contributes to a deep understanding of the structural and spectroscopic features of aniline derivatives.