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.