## 4 Results and Discussion
### Vibrational Analysis
#### 4.4.2. Phenyl Ring Moiety Modes (30)  
The phenyl ring has six ring stretching modes (1, 8a, 8b, 14, 19a and 19b), three planar ring deformation modes (6a, 6b and 12) and three non-planar ring deformation modes (4, 16a and 16b). In benzene derivatives, the assignments of the modes 8a, 8b, 19a and 19b are well established [16; 23; 24]. The two modes 8a and 8b were found to be localized and were observed slightly above and below 1600 cm\({}^{-1}\). In tri-light substituted benzenes, the mode 19a was observed around 1500 cm\({}^{-1}\), while the component 19b was observed in the range 1400 \(\pm\) 50 cm\({}^{-1}\)[20]. The frequency corresponding to the ring breathing mode-1 varies considerably and appears with varying IR intensity and good intensity in the Raman spectrum. [PERSON] [20] has assigned this mode based on the concept of the light (mass \(<\) 25 amu) and heavy (mass \(>\) 25 amu) substituents and suggested different frequency ranges for this mode depending upon the number, nature and positions of the substituents. The L-dopa molecule is of the 1,2,4-thi-light substituted benzenes category, for which the frequency range for the ring breathing mode as suggested by [PERSON] [20] is 630-740 cm\({}^{-1}\). Presently, the ring breathing mode was observed at 777 cm\({}^{-1}\), a frequency slightly higher than the upper limit of the proposed range for the 1, 2, 4-thi light substituted benzenes. The observed frequency 777 cm\({}^{-1}\) of L-dopa has medium IR and strong Raman intensities, which is also supported by the computed intensity pattern for this mode. The Kekule ring stretching mode-14 was presently observed at 1362 (sh, IR) cm\({}^{-1}\) in L-dopa, which is in good agreement with the assignment of this mode in substituted benzenes proposed by the earlier works [20; 23; 24]. Out of the three planar ring deformation modes 12, 6a and 6b, the mode 12 showed considerable variation in the form and frequency of substituted benzenes. However, the modes 6a and 6b retained their forms in benzene derivatives and appeared with variable frequencies. The computed frequencies for the modes 12, 6b and 6a were found to be 723, 597 and 426 cm\({}^{-1}\), corresponding to the benzene frequencies 1010, 606 and 606 cm\({}^{-1}\), respectively. The reduction of the magnitude of the trigonal angle bending mode 12 is a result of the loading of both the triangles formed by the alternate C-atoms of the ring. Similarly, the loading of the pair of para positions C\({}_{3}\) and C\({}_{6}\) of the phenyl ring is responsible for the lowering of the frequency of the mode 6a. During the vibration in the mode 6b, the ring atoms C\({}_{3}\) and C\({}_{6}\) are practically stationary and the atoms C\({}_{1}\), C\({}_{2}\), C\({}_{4}\) and C\({}_{5}\) participate equally. Hence, the loading of the OH group at the site C\({}_{1}\) has little effect on the vibrational frequency corresponding to the mode 6b. The assignment of the mode 6b is corroborated the earlier assignment for this mode by [PERSON] et al. [12]. The three non-planar ring deformation modes 4, 16a and 16b retain their forms of vibration in the substituted benzenes. The mode 4 involves motion of the alternative C-atoms in opposite directions and showed small frequency variation in the benzene derivatives. Contrary to the above, the modes 16a and 16b showed considerable variation in the magnitudes of the frequencies in the benzene derivatives. The computed frequencies for the modes 4, 16a and 16b were found to be 735, 653 and 449 cm\({}^{-1}\). The modes 4 and 16b show small variation in the magnitudes of the computed frequencies for the L-dopa as compared to the corresponding mode of benzene. However, the mode 16a is considerably enhanced as compared to the corresponding mode of benzene (404 cm\({}^{-1}\)). For the L-dopa molecule, the three C-H stretching modes were computed to be 3043, 3043 and 3062 cm\({}^{-1}\) with the corresponding observed frequencies 3046, 3046 and 3072 cm\({}^{-1}\), which could be, respectively, correlated to the modes 20a, 20b and 2 of benzene.