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.