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.