3.1.1 Rotational Signatures in the Conformers of Leucine
The different conformers of Leucine can be differentiated via their own unique rotational parameters listed in Table 2 and hence, having distinct rotational features. All the conformers except EQ0II, because of relatively high value of dipole moment component µb , can be characterised by theirb -type transitions corresponding to selection rule:ΔK −1 = ±1,±3,±5,…, andΔK +1= ±1, ±3, ±5,…, whereK −1 and K +1 are pseudo rotational quantum numbers defining the prolate and oblate type projections.58 The conformer EQ0II, however, possesses higher dipole moment values along a - andc -axis, thus, can be differentiated by a -type transitions corresponding to ΔK −1 = 0, ±2, ±4,…, and ΔK +1 = ±1,±3, ±5,…, along withc -type transitions following ΔK −1 = ±1,±3,±5,…and ΔK +1 = 0, ±2, ±4,…, selection rules.
However, the value of principal rotational constant ‘A’ for conformers EQ0II and EQ0IV are nearly the same, thus, differentiating their rotational transitions can be quite strenuous. To resolve this complexity, two signatures can be followed. First is to locate their respective characteristic transitions, and second is the distinctly different values for nuclear quadrupole constants (χaa, χbb, χcc). In fact, the marked differences in the values of the nuclear quadrupole constants, arising from the presence of nitrogen (with nuclear spin I=1/2), is mainly due to differing electronic environment resulting from differently oriented substituents around the nitrogen in the two conformers. These distinct values result into different hyperfine splitting pattern which additionally acts as the characteristic rotational feature.
Another challenge is encountered in resolving the conformers EQ0I and EQ0III. The orientation of nitrogen is same in both besides a similar electronic environment due to the same surrounding atoms. However, EQ0I can be differentiated by locating c -type transitions which are absent in EQ0III because of nearly zero value of dipole moment along the c -axis. But note that the value of dipole moment in EQ0I is also quite low, therefore, c -type transitions appear too weak. To additionally assist in differentiating these conformers, the associated isomeric species need to be followed as explored along the pathways discussed in stereoinversion of Leucine.10 From the pathways, it is quite apparent that the isomeric species, EQ1R1 and EQ1R2b, can act as tracer for conformer EQ0I whereas EQ1R3a and EQ2R3 are related to conformer EQ0III.