## 4 Results and Discussion
### Vibrational Analysis
#### 4.4.3 O-H Group Modes (9)  
u\)(O-H) mode of the COOH group was observed in the range 2500-3050 cm\({}^{-1}\)[25; 26; 20]; however, the \(\
u\)(O-H) mode for the OH group(s) attached to the phenyl ring was observed in the range 2900-3660 cm\({}^{-1}\) in different surroundings [17; 22]. Generally, the OH stretching appears with medium to strong intensity and a broad band signature in the IR spectra, while it is weak or even may not be observed in the Raman spectra. The experimental IR spectrum of L-dopa contains a broad intense envelope in the range 2200-3500 cm\({}^{-1}\), which was deconvoluted to yield four bands: 3378, 3216, 3045 and 2770 cm\({}^{-1}\) (Figure 5). The computed vibrational frequencies of the three OH stretching modes of the lowest energy monomer C-I and its three lowest-energy dimers D\({}_{1}\), D\({}_{2}\) and D\({}_{3}\) are given in Table 6, which also includes the corresponding deconvoluted IR bands. A perusal of this Table suggests that considering the lowest-energy monomeric conformer C-I, out of the four deconvoluted bands only two bands could be assigned, whereas when considering the dimers D\({}_{1}\) and D\({}_{2}\) all the four deconvoluted bands could be assigned. However, only one deconvoluted band could be assigned considering the dimer D\({}_{3}\). The dimers D\({}_{2}\) and D\({}_{3}\) have much higher (\(\sim\)1200 and \(\sim\)3500 K) energies than the dimer D\({}_{1}\), suggesting that at room temperature, the L-dopa molecule exists in the dimeric form D\({}_{1}\) only. The \(\
u\)(O\({}_{12}\)-H\({}_{13}\)), \(\
u\)(O\({}_{10}\)-H\({}_{11}\)) and \(\
u\)(O\({}_{20}\)-H\({}_{21}\)) modes were found to have the computed frequencies 3378, 3327 and 2870 cm\({}^{-1}\), whereas for the dimer D\({}_{1}\), the computed frequencies for these modes were found to be 3366/3366, 3048/3052 and 2942/2943 cm\({}^{-1}\), respectively. Considering the monomer C-I, only the two deconvoluted bands 3378 and 2770 cm\({}^{-1}\) could be assigned, whereas with the dimer D\({}_{1}\) the three deconvoluted bands 3378, 3045 and 2770 cm\({}^{-1}\) could be correlated to the three OH stretching modes. The considerably lower observed frequency 2770 cm\({}^{-1}\) for the \(\
u\)(O\({}_{20}\)-H\({}_{21}\)) mode as compared to its computed value 2942 cm\({}^{-1}\) is a result of the strong H\(\ldots\)N intra-molecular interaction. The remaining fourth deconvoluted IR band 3216 cm\({}^{-1}\) could be assigned as the first overtone of the NH\({}_{2}\) scissoring mode observed at 1606 cm\({}^{-1}\). The \(\alpha\)(C-O-H) modes are generally observed in the range 1170-1250 cm\({}^{-1}\)[20]. In the present case, the computed \(\alpha\)(C-O-H) modes for the O\({}_{10}\)H, O\({}_{12}\)H and O\({}_{20}\)H groups were found to have the frequencies 1192, 1222 and 1412 cm\({}^{-1}\) and these were correlated to the observed frequencies 1188(IR), 1202(IR) and 1404(IR)/1406(R) cm\({}^{-1}\), respectively. The computed frequency 1412 cm\({}^{-1}\) for the \(\alpha\)(C\({}_{19}\)-O\({}_{20}\)-H) mode has considerably higher IR intensity as compared to the other two \(\alpha\)(C-O-H) modes. A large enhancement in the magnitude (1412 cm\({}^{-1}\)) of the frequency due to the mode \(\alpha\)(C\({}_{19}\)-O\({}_{20}\)-H) as compared to its frequency (1178 cm\({}^{-1}\)) in Phenol[17] is a result of the strong intra-molecular H-bonding. Similar to the OH stretching mode, the torsional mode of the OH group is strongly affected due to the intra- and inter-molecular H-bonding. The torsional modes of the \begin{table}
\begin{tabular}{c c c c c c} \hline \multicolumn{2}{c}{**Observed**} & \multicolumn{4}{c}{**Computed**} \\ \hline
**IR** & **Raman** & **C-I** & **D\({}_{1}\)** & **D\({}_{2}\)** & **D\({}_{3}\)** \\ \hline
2770 s & - & 2870 & 2942/2943 & 2939/2919 & 2758/2949 \\ \hline
3045 m & & - & 3048/3052 & 3045 & 3174 \\ \hline
3216 & 3206 m & - & 2 \(\times\) 1606 & 3226 & - \\ \hline
3370 m & 3362 vw & 3378/3326 & 3366/3366 & 3326/3326 & 3381/3336/3325 \\ \hline \end{tabular}
\end{table}