Figure 4. The simulated rotational spectra of
EQ0# for different J values at a temperature of
10 K.
Apart from the rotational region, the infrared region is used mainly for
investigating the laboratory based interstellar ice compositions. The
matrix-isolation IR experiments are quite challenging for analysing the
gas phase spectral details of neutral molecules. Moreover, the lab-based
IR spectra of gas-phase zwitterionic species can be even more difficult
to observe.66 In such a case, the anharmonic
vibrational spectral data provided in the present study can act as
crucial aid to the laboratory based astrochemical studies. In fact, the
search of zwitterionic type species among the presence of radicals and
anions can help in unveiling a vast network of exotic molecular species
in the ISM.
Moreover, in some astrochemical detections, the molecular species under
investigation is not searched directly but through some related tracer
species. This is particularly in the case of a molecular species having
either quite low or zero dipole moment or having negligible induced
polarization, for which the rotational and vibrational transitions are
too weak to be observed. For example, in the typical case of
N2, its protonated form
N2H+ was successfully rationalized in
the ISM while utilizing the quantum-mechanically computed data, thereby,
acting as a tracer for locating the
N2.67 In a similar manner, the charged
isomeric species (EQ1R1, EQ1R3a,
EQ2R3) being investigated in the present work, which in
fact have a high dipole moment, can act as tracer for the detection of
corresponding conformers of Leucine. These conformers have low dipole
moment, therefore, exhibit only weak rotational and vibrational
transitions.
Furthermore, in the astronomical searches for Glycine, two important
conformers havingcis -(α-COOH)-{(NH2)N—-HO=C} andtrans -(α-COOH)-{(NH2)N—-H–O}
configuration are generally considered in the
observations.68 This is due to an equal probability
for the presence of these conformers irrespective of their relative
thermal stability because of the peculiar environment of ISM. In fact,
it is not prudent to presume that only a particular conformer will be
present in extra-terrestrial environment. Therefore, in the present
study, all the conformers of Leucine involved in its stereoinversion are
investigated so as to assist their detection in the outer space. It
should be noted that except the global minimum EQ0#,
the studies on other conformers of Leucine are considerably lacking.
Besides this, in the present work, the rotational parameters for
transition state species connecting the isomeric species investigated in
this work located along the stereoinversion pathways of Leucine have
also been computed and additionally provided in SI Table S4 of
supporting information. Although the detection of such transient species
is highly unlikely but can be an exception in the exotic environment of
ISM.