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.