DFT zwitterion model for vibrational and electronic structure of unnatural 3-amino-3-(4-fluorophenyl)propionic acid, aided by IR and Raman spectroscopy

Abstract 3-Amino-3-(4-fluorophenyl)propionic acid - a fluorinated building block in synthesis and a non-proteinogenic amino acid - belongs to the class of β-amino acids. We present here ab initio and DFT computed zwitterionic (ZW) monomer and dimer structures of 3-Amino-3-(4-fluorophenyl)propionic acid. These structures comprise intra- and inter-H-bonds (dimer), namely, N–H⋯O bonds formed between NH3+ and COO¯ moieties and the inter-N−H⋯O bond vibrational modes are correlated to the characteristic absorptions in the experimental IR spectrum. We propose two structures for 3-Amino-3-(4-fluorophenyl)propionic acid: a ZW monomer resulting from the explicit solvation model with four water molecules and a ZW dimer resulting from the implicit solvation model in water. By varying the bond length, rH … O around 1.70 A, we get stable degenerate structures, getting the most stable ZW dimer mentioned above at 1.72 A. Computed vibrational frequencies of NH3+ and COO¯ moieties for the ZW cluster and dimer complement each other in so far as good agreement with experimental IR and Raman spectra. We also have characterized both H-bonds using electronic properties computed from NBO, AIM and NCI methods. The natural bond analysis has shown that in both intra- and inter- N–H⋯O bonding, there is delocalization of electronic charge from n orbital on the O atom to σ* orbital on the N atom. The bond critical points are present between the cationic NH3+ and anionic COO¯ moieties along with atomic bond path confirming the N–H⋯O bond and is shown to be medium strong by NCI.