## 4 Results and Discussion
### Vibrational Analysis  
L-dopa is a 25 atomic molecule with 69 normal modes of vibration (Table S3). The computed IR and Raman frequencies along with their relative intensities, depolarization ratios of the Raman bands, PEDs and the corresponding observed IR and Raman frequencies and their intensities and the normal mode assignments are given in Table 5. The computed vibrational frequencies for the lowest-energy monomeric conformer C-I and its lowest-energy dimeric conformer D\({}_{1}\) are compared in Table S4. The observed IR and Raman spectra with the corresponding computed spectra are shown in Figure 4. The complete observed IR spectrum of L-dopa could be explained in terms of the computed spectra of the lowest-energy dimer (D\({}_{1}\)) of the lowest-energy monomer C-I. The experimental IR spectrum of L-dopa contains a broad intense envelope in the range 2200-3500 cm\({}^{-1}\), which could not be explained in terms of the computed spectral data of the C-I monomer. In order to explain the origin of this envelope, the related parameters for the dimers of the C-I monomer were computed. In terms of the dimeric spectral data, the complete observed IR and Raman spectra could be explained satisfactorily. The lowest-energy monomeric conformer C-I forms five dimers (D\({}_{1}\)-D\({}_{5}\)), out of which three dimers (D\({}_{1}\), D\({}_{2}\), D\({}_{3}\)) involve two inter-molecular H-bonds and two dimers (D\({}_{4}\), D\({}_{5}\)) involve only one inter-molecular H-bond. The dimers involving two inter-molecular H-bonds have much lower energies as compared to those with one inter-molecular H-bond. As the dimers D\({}_{2}\) and D\({}_{3}\) have much higher (\(\sim\)1200 and \(\sim\)3500 K) energies relative to the dimer D\({}_{1}\), the L-dopa molecule would exist in the dimeric form D\({}_{1}\) only. Vibrational assignments can be discussed under the following three sub-sections: (i) the 2-aminopropenoic acid (CH\({}_{2}\)-CH-NH\({}_{2}\)CO\({}_{2}\)(H)) moiety modes (30), (ii) the phenyl ring moiety modes (30) and (iii) the OH group modes (9). \begin{table}
\begin{tabular}{c c c c} \hline
**Atomic Site** & **C-I** & **D1** & \\ \hline C\({}_{1}\) & 0.616 & 0.502 & 0.502 \\ C\({}_{2}\) & \(-\)0.141 & \(-\)0.100 & \(-\)0.100 \\ C\({}_{3}\) & \(-\)0.021 & \(-\)0.044 & \(-\)0.044 \\ C\({}_{4}\) & \(-\)0.110 & \(-\)0.099 & \(-\)0.099 \\ C\({}_{5}\) & \(-\)0.074 & \(-\)0.050 & \(-\)0.050 \\ C\({}_{6}\) & 0.657 & 0.485 & 0.485 \\ H\({}_{7}\) & 0.077 & 0.061 & 0.061 \\ H\({}_{8}\) & 0.052 & 0.027 & 0.027 \\ H\({}_{9}\) & 0.068 & 0.048 & 0.048 \\ O\({}_{10}\) & \(-\)0.981 & \(-\)0.762 & \(-\)0.762 \\ H\({}_{11}\) & 0.400 & 0.332 & 0.332 \\ O\({}_{12}\) & \(-\)0.966 & \(-\)0.930 & \(-\)0.930 \\ H\({}_{13}\) & 0.380 & 0.583 & 0.583 \\ C\({}_{14}\) & 0.156 & 0.139 & 0.139 \\ H\({}_{15}\) & \(-\)0.017 & 0.001 & 0.001 \\ H\({}_{16}\) & \(-\)0.033 & \(-\)0.034 & \(-\)0.034 \\ C\({}_{17}\) & 0.274 & 0.192 & 0.192 \\ H\({}_{18}\) & \(-\)0.021 & \(-\)0.026 & \(-\)0.026 \\ C\({}_{19}\) & 1.419 & 1.106 & 1.106 \\ O\({}_{20}\) & \(-\)1.021 & \(-\)0.737 & \(-\)0.737 \\ O\({}_{21}\) & \(-\)1.019 & \(-\)0.938 & \(-\)0.938 \\ N\({}_{22}\) & \(-\)0.697 & \(-\)0.530 & \(-\)0.530 \\ H\({}_{23}\) & 0.247 & 0.195 & 0.195 \\ H\({}_{24}\) & 0.554 & 0.429 & 0.429 \\ H\({}_{25}\) & 0.202 & 0.149 & 0.149 \\ \hline \end{tabular}
\end{table} 
Table 4: APT charges (in e unit) at different atomic sites of L-dopa.