4. ASTROPHYSICAL IMPORTANCE
It is well-known from the physical and chemical traits of ISM that there is a continuous circulation of matter and energy at different stages of cosmic cycle in the ISM. Thus, there is a variation in the molecular density and temperature within the different regions of ISM. The low-temperature rotational line data computed in the present study can be utilized for searching the species in the cold regions of ISM while the computed vibrational spectroscopic data is quite reliable for search in the warmer star-forming regions of the ISM. In fact, these are the probable regions for observing various species being investigated: the neutral conformers of Leucine, zwitterionic ammonium ylide (EQ1R1, EQ1R3a, EQ2R3), imine-diol (EQ1R2a , EQ2R2a, EQ1R4), and ene-diol (EQ1R2b), similar to the case of Glutamic acid.33
Also note that in the present work, the rotational database for all the species is purposely created in lower frequency region than the frequencies utilized for the search of the simplest amino acid Glycine in the ISM. In all the astronomical searches so far for Glycine, for example, in the star forming regions SgrB2, Orion, and low mass protostar IRAS, the frequency region searched is greater than 80 MHz and up to few GHz.60–62 However, a high moment of inertia associated with comparatively larger Leucine molecule and its isomeric species results into a comparatively lower value of rotational constants.45 Therefore, it is the lower frequency region where more intense rotational transitions for Leucine and its isomeric species can be observed. The lower frequency region becomes more important for search in dense cold molecular clouds where the temperature can be as low as 10 K. To assist this, the rotational spectra simulated for EQ0# with different values of rotational quantum number J (20-80) at temperature of 10K is further provided in Figure 4. It can be seen that the frequency range of 10000 MHz to 45000 MHz is the region of high intensity rotational transitions at 10 K. The higher rotational levels though will also be get populated as in higher temperature zone but a large number of transitions from these will be too complex to resolve as can be concluded using values of rotational quantum numbers:K a and K c (with -JK a or K cJ ). Therefore, in the present study, the rotational transitions have been mainly analysed in the frequency range of 10000 MHz to 45000 MHz (with highest rotational state of J = 30). Nevertheless, the lower frequency region of this range can act as important aid towards the laboratory assignment of the transitions whereas the higher range can directly assist the astronomical observations, for example, through Very Large Array Sky Survey (VLASS), futuristic lower frequency ALMA Band, and Square Kilometer Array (SKA).63–65