Rotational and vibrational analysis of astrophysically relevant isomeric species of proteinogenic glutamic acid: A quantum-mechanical computational study

Quantum-mechanical Rotational and Vibrational Signatures of Astrophyscially relevant Gas-Phase Stereo-isomeric Species of Leucine

“…Moreover, in the present work, the anharmonic treatment of large amplitude vibrational motion associated with the intramolecular hydrogen bonding interactions in Leucine has also been carried out through VPT2, similar to the one employed in the case of Glutamic acid and Threonine. 33,59 Note that due to an extended structural framework of Leucine and its isomeric species, all hydrogen bonding interactions span a distance over 1.6Å as evident in Figure 1, which is similar to the case of Glutamic acid. 33 Thus, a closer agreement of anharmonically predicted frequencies with the experimental data for Leucine clearly reveals the efficacy of anharmonic approximation utilized in the present work.…”

“…33,59 Note that due to an extended structural framework of Leucine and its isomeric species, all hydrogen bonding interactions span a distance over 1.6Å as evident in Figure 1, which is similar to the case of Glutamic acid. 33 Thus, a closer agreement of anharmonically predicted frequencies with the experimental data for Leucine clearly reveals the efficacy of anharmonic approximation utilized in the present work. Therefore, the anharmonically calculated vibrational data in the present work for other isomeric species of Leucine, in the mid infrared and far-infrared frequency regions, is highly reliable, which is further provided in supporting information SI Table S3.…”

“…Moreover, the non-zero dipole moment (μ ) in all the three directions (μ α ,μ β , μ ς ) causes even more complexity in the rotational spectra. Notably, the dipole moment of the conformers of Leucine is comparatively lower than that of other proteinogenic amino acids investigated in the previous studies, 33,59 mainly due to non-polar nature of the side chain substituent in Leucine, and also due to the reason that all the conformers relevant for inverting the chirality are oriented in 'cis ' position than the more polarized 'trans ' configuration as has been discussed in the previous work on Leucine. 10 Furthermore, the Ray's asymmetry parameter, κ = (2B-A-C)/(A-C), is an indicative of the asymmetric nature of the species.…”

“…The methodology adopted in the present work to compute the rotational and vibrational data is similar to that utilized for the spectral study of Glutamic acid in our previous such work. 33 The conformers of biologically significant left-handed isomer (L )-Leucine and its isomeric species being investigated in this work are depicted in Figure 1. Note that the global minimum (lowest-energy conformer) of Leucine (EQ0 # ) has been verified through its structure optimization performed at different levels of quantum-mechanical theory (for details, see Ref.…”

“…The procedure adopted for obtaining the scaling factors is similar to the one employed in our previous study on Glutamic acid. 33 In the present work, the ratio of experimental (exp) and computed rotational constants of global conformer EQ0 # act as scaling factors, thus, 0.999 (A exp /A e ), 0.996 (B exp /B e ), 0.994 (C exp /C e ) are the scaling factor used for the rotational constants A, B and C, respectively. The scaled rotational constants calculated for all the species are further listed in Table 3.…”