Molecular dynamics simulation, DFT calculations and vibrational spectroscopic study of N[sbnd]H•••O bound dimer models for DL-β-phenylalanine and 3-amino-3-(4-chlorophenyl)propionic acid

Abstract

Zwitterionic NH•••O bonded dimer structures have been proposed one each for DL-β-Phenylalanine and 3-Amino-3-(4-chlorophenyl)propionic acid. Stable zwitterionic monomer possessing NH3+ and COO¯ moieties in implicit aqueous solvation were computed at B3LYP/6-311++G** level and used for classical MD simulations. MD simulations yielded, among many NH•••O bonded transient dimer and trimer species, the two dimer structures with longest lifetime τ in water medium (τ = 111.67 ps for DL-β-Phenylalanine and 111.53 ps for 3-Amino-3-(4-chlorophenyl)propionic acid). The first peaks in radial distribution functions (RDF) confirmed the formation of NH•••O bonded dimer structures, one each for DL-β-Phenylalanine and 3-Amino-3-(4-chlorophenyl)propionic acid; the first minima determined the cut-off values for the inter-molecular H•••O distances. Further, the first minimum-solvation shell also indicated one zwitterionic dimer structure with 10 water molecules as solvent medium. Experimental IR frequencies due to the NH3+ and COO¯ moieties agree within 5% of the computed values for the dimer structures. Low-frequency Raman bands near 100–50 cm−1 are assigned to the torsional vibration modes of zwitterionic NH•••O bonded dimer structures. On the role of Chlorine with respect to all the molecular properties of 3-Amino-3-(4-chlorophenyl)propionic acid vs DL-β-Phenylalanine computed from NBO, AIM and NCI calculations, its influence is not substantial but discernible enough for gaining insights into the two NH•••O bonded zwitterionic dimer species.