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.