The FTIR and Raman spectra of dopamine were also recorded in the spectral region 20 - 4000 cm^-1 and 400 - 4000 cm^-1 respectively. The optimized molecular geometry and fundamental vibration frequencies are interpreted using structural optimizations based on the DFT method. The B3LYP / 6-311G (d, p) was used to determine the calculations of the chemical descriptor. The dopamine (DA) molecule can acquire a maximum charge of 0.513 eV from its environment. To understand the molecular interactions of Dopamine, the MEP is a crucial tool. Nonlinear optical descriptors (NLO) and the lengths, bond angles are also determined by DFT and MP2 methods. The DFT method was used to determine the chemical shifts of ¹H and ¹³C of the product. These constants have been calculated by employing the GIAO (Gauge-Including-Atomic-Orbit), CSGT (Continuous Set of Gauge Transformations) and IGAIM (Individual Gauges for Atoms In Molecules) methods at the B3LYP/6-311G (d, p). The comparison of the theoretical chemical shifts with the experimental ones shows that the CSGT method is the best.

The FT-IR spectrum of salicylic acid (SA) was studied in the region 10-4000 cm-1. The optimized molecular geometry and fundamental vibration frequencies are interpreted using structural optimizations based on the Functional Density Theory (DFT) method. Additionally, we used B3LYP / 6-311G (d, p) to determine the chemical descriptor, the ionization potential (I), the electron affinity (A), the chemical potential (μ), the chemical hardness (η). Nonlinear optical descriptors (NLO) such as dipole moment (μ), polarizability (α), first hyperpolarizability (β) and second hyperpolarizability (γ), 3D maps of HOMO and LUMO orbitals, lengths and Bond angles of salicylic acid isomers are also determined by both DFT and MP2 (The Møller-Plesset theory of order 2 perturbation) methods. The overall descriptors and the non-linear optical properties confirmed that salicylic acid is the reactive molecule and para-hydroxybenzoic acid is the stable molecule. An electrostatic molecular potential (MEP) study was also performed to determine the reactivity of molecules.