3.2 Vibrational Signatures
Leucine, a branched chain amino acid, and its isomeric species, all with 22 atoms, can exhibit sixty fundamental modes of vibrations in the mid- and far-infrared regions. In the present work, each mode has been characterised using a potential energy distribution (PED) analysis provided in SI Table S3. As discussed in the previous section on computational details, the DFT/B3LYP-D3(BJ)/cc-pVTZ method was found to be the optimum level for computing the vibrational data. To further examine the reliability of this quantum-mechanical method, the computed vibrational modes for global minimum EQ0# are compared with the experimentally available gas-phase vibrational data,30,31 as listed in Table 4. The predicted gas-phase harmonic spectrum for EQ0# is further depicted in Figure 3.
Note that Table 4 compares the vibrational modes computed for the gas-phase non-zwitterionic form of the global conformer of Leucine (EQ0#) with those from the experimental matrix-isolation IR and fast thermal heating vibrational studies which are also associated with the neutral form of Leucine. The anharmonic calculations of the present work predict an intense –OH stretching vibration to appear at 3565 cm-1, which is in excellent agreement with the experimentally observed frequency listed in Table 4. The computed ‘harmonic’ frequencies though seem to be unreliable. The asymmetric and symmetric –N-H stretching frequencies observed experimentally are also closely predicted by anharmonically computed frequencies at 3387 cm-1 and 3339 cm-1, respectively. Besides these, as listed in Table 4, the anharmonic –C-H stretching frequencies corresponding to the modes, υ4, υ5 and υ12 , are also computed within a reasonable agreement with the experimental values.
The intense characteristic –C=O stretch in the global conformer was experimentally observed at 1768 cm-1 in a study by Stepanian et al., and at 1773 cm-1 by Linder et al., whereas the calculated harmonic frequency is at 1813 cm-1, however, its computed anharmonic frequency at 1783 cm-1 is in closer agreement with the experimental observations. Besides this, the bending vibrations including –NH2 bending, –OH and –CH2 bending or even the torsional vibrations are also successfully predicted by the anharmonic calculations at B3LYP-D3(BJ)/cc-pVTZ level of theory as can be clearly seen in Table 4. However, the vibrational transitions in the far-infrared region are too complex to be characterized simply by observation. To resolve this complexity, potential energy distribution is a significant method to identify each mode successfully as has been additionally provided for global minimum EQ0# in Table 4.