Further, as evident in Table 2, all the species being investigated in
this work possess a net non-zero dipole moment, thus, all are
rotationally active. Moreover, the non-zero dipole moment (µ ) in
all the three directions (µa ,µb , µc ) causes even more
complexity in the rotational spectra. Notably, the dipole moment of the
conformers of Leucine is comparatively lower than that of other
proteinogenic amino acids investigated in the previous
studies,33,59 mainly due to non-polar nature of the
side chain substituent in Leucine, and also due to the reason that all
the conformers relevant for inverting the chirality are oriented in
‘cis ’ position than the more polarized ‘trans ’
configuration as has been discussed in the previous work on
Leucine.10
Furthermore, the Ray’s asymmetry parameter, κ = (2B-A-C)/(A-C), is an
indicative of the asymmetric nature of the species. All the species are
in prolate range, either near-prolate symmetric top or prolate
asymmetric top, with moment of inertia (I) related as
Ia<Ib~Icalong the three principal axis (a,b,c ). The species with κ value
closer to -1 corresponds to the near-prolate symmetric top with extended
configuration as in the case of conformers EQ0#,
EQ0I, EQ0III as well as isomeric species
EQ2R2a, EQ1R3a, EQ2R3,
EQ1R4. The greater the deviation from κ value of -1,
more is the asymmetric nature of the species. Besides this, the lowest
value κ = −0.41 for the conformer EQ0IV signifies its
most asymmetric nature and folded configuration due to the
intermolecular hydrogen bonding interactions (as shown in Figure 1). The
diverse values of the asymmetric parameter κ observed for the conformers
and stereo-isomeric species of Leucine, as listed in Table 2, can be
attributed to the branched form of side chain which causes variable
extensions of overall molecular framework.