Molecular structure and spectroscopic (FT-IR, FT-Raman, 13C, 1H NMR and UV) studies of 3,4-dihydroxy-L-phenylalanine using density functional theory

Fourier transform infrared (FT-IR) and Fourier transform Raman (FT-Raman) spectra of 3,4-dihydroxy-L-phenylalanine (3,4-DPA) in solid phase were recorded and analysed in this research. Along with this, the IR spectra in CHCl3 and the use of acetone as solvents of 3,4-DPA were also recorded. The equilibrium geometry, bonding features and harmonic vibrational frequencies were investigated with the help of density functional theory (DFT) method. The H-1 and C-13 nuclear magnetic resonance chemical shifts of the molecule were calculated using the gauge including atomic orbital method and compared with experimental results. Stability of the molecule arising from hyperconjugative interactions and charge delocalisation was analysed using natural bond orbital analysis. The results show that charge in electron density (E-D) in the sigma* and pi* antibonding orbitals and second-order delocalisation energies E(2) confirms the occurrence of intramolecular charge transfer within the molecule. UV-vis spectrum of the compound was recorded and the electronic properties, such as HOMO and LUMO energies, were analysed using the time-dependent (TD)-DFT approach. Finally, the calculation results were applied to simulate infrared and Raman spectra of the title compound, which showed good agreement with the observed spectra.