3.1.2 Rotational Signatures in the Isomeric Species of Leucine
All the isomeric species located along the stereoinversion pathways of Leucine exhibit higher values of dipole moment and hence, exhibit more intense rotational transitions than the conformers of Leucine. Besides this, there is a significant variation in the value of rotational constants among these species, therefore, these can be uniquely identified. The zwitterionic ammonium ylide species, EQ1R1, has the highest value of dipole moment followed by zwitterionic EQ1R3a and EQ2R3 which can be characterised by stronger b -type transitions in EQ1R1 and a- type transitions in EQ1R3a and EQ2R3. The isomers EQ1R3a and EQ2R3 , despite being structurally the same, have entirely different rotational parameters, and hence can be distinctly characterised by their unique (B+C) transition line patterns associated with a -type transitions.58
Similarly, the imine-diol-type species, EQ1R2a , EQ2R2a , EQ1R4, exhibit their own characteristic a -type transitions of quite high intensity. To the contrary, the en-diol species EQ1R2b exhibits strongerb -type transitions.58 The spectral line database of significant ‘R’-branch rotational transitions for all the species under study was further created using PGOPHER and is provided in SI Table S2 of supporting information. Note that in the present work, rotational transitions were considered only in the frequency range of 10000 MHz – 45000 MHz which are limited to the rotational quantum number, J ≤ 30. This frequency region actually belongs to higher intensity rotational transition lines shown in Figure 4, and is astrophysical quite important as discussed in the next section. Also note that the line-data in the higher frequency range of 34000 MHz to 45000 MHz, which though is still lower than the typical astronomical observations, has also been provided in accordance to the highest frequency band ‘one’ of ALMA.