## 5 Conclusions  
The L-dopa molecule was found to exist in pairs of 108 stable conformers. All the conformers were found to have energies higher than 300 K with respect to the lowest-energy conformer C-I. The broad intense envelop in the range 2200-3500 cm\({}^{-1}\) of the IR spectrum necessitates the presence of L-dopa in the dimeric form. The complete observed IR and Raman spectra could be explained in terms of the computed spectra of the lowest energy dimer (D\({}_{1}\)) of C-I monomer. A large number of amino acids were found to exist in the Zwitterionic form; however, the intense IR band peaked around 2800 cm\({}^{-1}\), suggesting the existence of the L-dopa molecule in the OH form. Out of the 69 normal modes of L-dopa, 65 modes were observed experimentally. In going from the C-I monomer to its dimer D\({}_{1}\), the C-O bond lengths were found to be reduced; on the other hand, the O\({}_{10}\)-H\({}_{11}\) and O\({}_{12}\)-H\({}_{13}\) bond lengths were considerably enhanced due to inter-molecular hydrogen bonding. The phenyl ring OH groups were found to have nearly two times the barrier heights of the OH group in phenol. However, the OH group of the carboxylic group had nearly five times the barrier height of the phenol OH group due to strong N...H bonding. The MEP plot of the L-dopa molecule suggests that it has multiple active sites. In going from the C-I monomer to the D\({}_{1}\) dimer, the number of active sites increases. From the HOMO-LUMO analysis, the L-dopa molecule was found to be chemically soft in nature. Out of the six bioactive scores \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Conformers**} & \multicolumn{2}{c}{**Energy in Unit (eV)**} & \multicolumn{2}{c}{**Electronegativity**} & \multirow{2}{*}{**Chemical**} \\ \cline{2-2} \cline{4-5}  & **HOMO** & & & & **LMO** \\ \hline C-I & \(-\)5.8276 & \(-\)0.9630 & 4.8646 & 3.3953 & 2.4323 \\ \hline D1 & \(-\)5.7966 & \(-\)0.9388 & 4.8578 & 3.3677 & 2.4289 \\ \hline D2 & \(-\)6.2115 & \(-\)0.7170 & 5.4945 & 3.4643 & 2.7473 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: HOMO-LUMO energies and related quantities for L-dopa. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Absorption Bands**} & \multicolumn{2}{c}{**Excitation**} & \multicolumn{2}{c}{**Oscillator**} \\ \hline \(\mathbf{\lambda_{\text{exp}}}\) (nm) & \(\mathbf{\lambda_{\text{cal}}}\) (nm) & **Energies (eV)** & **Strength (f)** & \multicolumn{1}{c}{*} **Contributions**} \\ \hline
271.6 & 259.2 & 4.7837 & 0.0636 & H-I\(\rightarrow\)L+3 11\% \\  & & & & & H-I\(\rightarrow\)L+2 86\% \\ \hline
235 [FOOTNOTE:]Footnote *: H stands for HOMO, L stands for LUMO;[ENDFOOTNOTE] & 246.7 & 5.0252 & 0.0088 & H-I\(\rightarrow\)L+2 38\% \\ \hline
225–215 [FOOTNOTE:]Footnote *: H stands for HOMO, L stands for LUMO;[ENDFOOTNOTE] & 241.6 & 5.1327 & 0.0108 & H-I\(\rightarrow\)L+3 7\% \\  & & & & & H-I\(\rightarrow\)L+3 7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 9: UV-vis absorption bands and the corresponding transitions for L-dopa. of the L-dopa molecule, two scores were found to be positive, suggesting that the it is a molecule with strong bioactive properties. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app132413336/s1](https://www.mdpi.com/article/10.3390/app132413336/s1)]([https://www.mdpi.com/article/10.3390/app132413336/s1](https://www.mdpi.com/article/10.3390/app132413336/s1)), Figure S1: Total energy vs. dihedral angle plots of the different tops of L-dopa; Figure S2: The optimized structures of the first six lower energy conformers; Figure S3: Computed and observed UV-vis spectra of L-dopa; Table S1: Some important parameters of L-dopa; Table S2: Geometrical parameters of the lowest-energy conformer C-I and dimer D1 of L-dopa; Table S3: Normal mode distributions of L-dopa; Table S4: Computed vibrational frequencies and their assignment of L-dopa; Table S5: Geometrical parameters of the first six lower energy conformers of L-dopa. Conceptualization: R.K.Y.