Density Functional Theory Investigations on Vibrational Spectra, Molecular Structure, and Properties of the L‐Serine, L‐Cysteine, and L‐Aspartic Acid Molecules

In this paper, we present a thorough investigation of the conformational space to characterize all possible gas‐phase structures of the neutral L‐serine, L‐cysteine, and L‐aspartic acid molecules. A total of 120 trial structures were generated for L‐aspartic acid and 96 trial structures for L‐serine and L‐cysteine by combining all internal single‐bond rotamers. Various combinations of the Hartree–Fock and density functional theory/B3LYP methods with different bases were used to optimize all possible trial structures. The theoretical studies on the structure, harmonic vibrational spectra, and molecular properties of these amino acids are presented. The assignments of the calculated wave numbers resulting from potential energy distributions were performed using the VEDA 4 program to allow a good interpretation of the theoretical vibrational spectra of the title compounds. The fundamental harmonic frequencies were found to be in good agreement with data in the literature. A natural bond orbital analysis was performed to investigate the charge delocalization throughout the molecules for the three test compounds. Moreover, an extensive discussion of the highest occupied molecular orbital–lowest unoccupied molecular orbital energy gap as well as other related molecular properties are reported. A thorough conformational space investigation is carried out on three neutral amino acids. A complete spectral interpretation is obtained on the basis of information provided by quantum chemical calculations (Hartree–Fock and density functional theory). The calculated vibrational spectra show good agreement with those found in the literature. An extensive analysis of the natural bond orbital, highest occupied molecular orbital, and lowest unoccupied molecular orbital is performed. Other related molecular properties are also reported.