Also note that in the previous study by Stepanian et al, only the scaled harmonic frequencies computed using B3LYP/aug-cc-pVTZ method were used to support the analysis from matrix-isolation IR stduy.³⁰ However, the present study, employing the dispersion and anharmonic corrections at B3LYP/cc-pVTZ level of theory, successfully predicts the vibrational signatures without the use of any adhoc scaling factor (generally employed to harmonic values) as well as without using computationally demanding long range diffuse basis functions (as in aug-cc-pVTZ basis set). 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 1.6 Å 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.