1. INTRODUCTION
Spectroscopy is the only viable route to probe exotic species in the challenging environment of interstellar medium (ISM).1,2 In fact, numerous anomalous species are being reported in the ISM,3,4 for example, the molecular ions of noble gases such as ArH+ and HeH+, the equal abundance of HCN and its terrestrially less stable isomer HNC, significant abundance of branched chain isopropyl cyanide compared to its linear chain isomer, to name a few.5–7 To interpret and resolve the complex spectroscopic information received from the molecular species present in the ISM, the insights from quantum-mechanical computations have been becoming quite valuable,8 which in fact has played quite a significant role in updating the interstellar census of molecules.9 In our recent study,10the stereoinversion of Leucine, under gas-phase conditions akin to ISM, was revealed to proceed through exotic molecular species which exhibits diverse chemistry (see later).
The rotational spectroscopy has remained the key method for remote observation of interstellar and circumstellar species. The spectroscopy in millimetre(mm) and sub-mm regions of wavelength corresponds to rotational transitions. In fact, it is the rotational spectroscopy which has led to the detection of majority of the species identified in the outer space.11,12 The structure specific characteristics of rotational transitions can be used to differentiate the molecular species irrespective of having the same mass. Moreover, this has also been responsible for driving the advancement in the futuristic telescopes in mm/sub-mm region.13 However, the laboratory based studies on interstellar ices are generally carried out in the infrared (IR) region with wavelengths lower than sub-mm.14 Now-a-days, the IR spectral predictions via quantum computations are widely assisting such laboratory studies. In fact, the confirmation of molecular species such ascyclo -C3H2, HOCO+, HSS etc.,15–20 in the ISM are exemplary towards such assistance by quantum computational spectroscopy to the laboratory IR spectra and hence the astronomical observations. Therefore, the spectroscopic data computed in the rotational and infrared regions is quite valuable in sorting the complex spectroscopic information received from the outer space as well as from the laboratories mimicking the outer space environment.21 Not only this, the interstellar spectroscopic studies are providing unique opportunities to broaden up our understanding of physical and chemical characteristics of the universe.22,23 The various regions of ISM are speculated to be associated with all the core functional groups of organic prominence, thereby, acting as significant players in the extra-terrestrial formation of bio-molecules. At present, it is quite clear that ISM is home to branched chain molecules and chiral molecules but the specificity in their handedness is still debated.24–25
In the present work, the rotational and vibrational signatures are quantum-mechanically computed for the important conformers of Leucine and its isomers responsible for its gas-phase stereoinversion under the conditions of ISM as revealed in our previous study.10Leucine is the most abundant amino acid in the proteins, therefore, it is reasonable to advocate prioritizing its search in the ISM relative to other proteinogenic amino acids. Moreover, the branched chain amino acids found in meteoritic composition dominate over the straight chain isomers, which additionally acts as a proponent for the search of Leucine in outer space.27 The ISM is a region of isolated molecules on account of ultra-low pressure and density.28 Thus, its molecular characterization involves the gas-phase spectral analysis without solvent consideration. In the existing literature, a gas-phase rotational study has been performed by Cocinero et al.,29 to identify the conformers of Leucine through Fourier transform laser-ablation microwave experiments. Besides this, two different experimental studies are also available on the gas-phase vibrational analysis of Leucine for characterizing its conformers. The recent of the two studies is based on low-temperature matrix-isolation IR experiments by Stepanian et al.30 However, the second study probing the IR spectra is based on a high temperature fast thermal heating experiments by Linder et al.31 Additionally, the latter analysis also signifies the stability of amino acids under high temperature of up to 570 K which supports the presence of Leucine in the hot cores of the protostars in the ISM. In fact, this region of ISM has been proposed as a suitable region for observing stereoinversion in Leucine.10 Apart from the experimental studies, anab-initio investigation of rotational and vibrational spectra of gas-phase conformers of Leucine has also been reported by Dokmaisrijan et al.32
In the present work, the rotational and vibrational parameters have been computed for the conformers of Leucine as well as its stereo-isomeric species (depicted in Figure 1) using the methodology described in the next section. The species studied include global minimum conformer of Leucine (EQ0#), its conformers (EQ0I, EQ0II, EQ0III, EQ0IV) besides the isomeric species explored along the gas-phase stereoinversion pathways of Leucine.10 Notably, the isomeric species investigated have diverse chemistry involving zwitterionic ammonium ylides (EQ1R1, EQ1R3a, EQ2R3), imine-diol (EQ1R2a, EQ2R2a, EQ1R4), and ene-diol (EQ1R2b) besides having a branched carbon-chain skeleton. In fact, ammonium ylides are known to be involved in the synthesis of amino acids. The computed spectroscopic parameters of the molecular species investigated in this work are further compared with available experimental data besides emphasizing the astrophysical importance of the data as discussed in the subsequent sections.