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.