Theoretical investigation of NLO and spectroscopic properties of halogenated aniline

This study presents an investigation of the spectroscopic and nonlinear optical properties of aniline and its halogenated derivatives. Six derivatives were considered: 2, 5-di-fluoro (DFA), 2, 5-di-chloro (DCA), 2, 5-di-bromo (DBA), para-fluoro (PFA), para-chloro (PCA), and para-bromo (PBA). The finite field approach was employed to determine their first (β) and second (γ) hyperpolarizabilities. Spectroscopic characterizations, including geometrical parameters, vibrational and electronic absorption spectra, molecular orbital analysis, and electronic properties, were reported. Geometrical parameters and vibrational frequencies were studied in a water solvent. The dispersion-corrected B3LYP-D3 method, utilizing a diffused and polarized 6-311 + + G (d, p) basis set, was employed. Aniline and its substituents were found to be the most stable at this level of theory, consistent with experimental data. Despite their insolubility in water, significant changes in structural and vibrational frequencies were observed. Electronic absorption spectra were calculated using time-dependent density functional theory (TD-DFT) at the same level of theory. The wavelength of electronic transition, oscillator strength, and HOMO to LUMO energy gap were determined. Aniline and its substituents exhibited greater responsiveness to the first hyperpolarizability than the second, with variations depending on the method and basis set. DBA, DFA, and PCA showed an enhanced response to the first hyperpolarizability compared to other substituents and aniline. β values obtained from DFT methods exceeded those from HF and MP2 methods.