Vibrational Spectroscopy Study of Abscisic Acid and Proline Based on Density Functional Theory

Based on density functional theory (DFT), this study employed the B3LYP/6-311+G(d,p) basis set to optimize the geometric structures of abscisic acid (ABA) and proline (Pro) molecules, yielding their stable spatial configurations. By investigating the characteristics of molecular orbital energy levels, the HOMO–LUMO energy gaps of ABA and Pro were determined to be 4.39 and 5.92 eV, respectively. Meanwhile, theoretical simulations of the Raman and infrared (IR) spectra for both molecules were completed. Under room temperature conditions, the Raman and IR spectra of ABA and Pro reference standards were experimentally measured. The theoretical results showed excellent consistency with the experimental data, with the fitting coefficient of determination R2 all greater than 0.99, confirming the reliability of the DFT calculation method. With the aid of Gauss View 6.0 visualization software, the assignment of spectral vibration modes was accomplished. It was found that both ABA and Pro exhibit significant Raman and IR activity in the ranges of 500–2000 and 2900–3200, the 2900–3200 cm–1 range is dominated by C–H stretching vibrations, while the 500–2000 cm–1 range features diverse molecular vibration modes and densely distributed spectral peaks. Further analysis of the vibrational frequencies and assignments of characteristic Raman peaks and IR absorption peaks revealed that IR spectroscopy is more sensitive to the vibrations of polar groups, whereas Raman spectroscopy responds significantly to non-polar groups. This confirms the distinct complementarity of the two spectroscopic techniques. This study provides reliable theoretical basis and experimental support for the molecular structure identification and vibrational characteristic analysis of ABA and Pro, realizes the comprehensive analysis of the spectral information of the two molecules.