Study of vibrational spectra of zwitterionic 3-Aminobutanoic acid, as supported by DFT calculations

Abstract

3-Aminobutanoic acid is an unnatural amino acid and by analogy with other amino acids, is represented in its zwitterionic form by R-CH(COO¯)NH­3+ (where side group R = aryl, alkyl or any other group) in solid phase. Experimental IR and Raman modes near 3029, 2871, 1577, 1288 cm-1 measured for 3ABA are strongly in favor of the zwitterionic dimer structure formed of –N-H∙∙∙O–C bonding between –NH­3+ and anion – CO2¯ groups. Zwitterionic monomer and dimer structures at B3LYP/6-311++G(d,p) level are computed. It has been found that the stable zwitterionic monomer and dimer structures are possible only in a water solvent medium computed using SCRF SMD solvation model. The zwitterionic dimer vibrational structure is in good agreement with the experiment and both zwitterionic monomer and dimer models have identically predicted the frequencies of the modes that are localized and unperturbed except those that characterize the H-bonding due to the –N-H∙∙∙O bonding. This is supported by the appearance of two characteristic medium strong Raman modes at 900 and 843 cm-1 that suggest unperturbed C-C skeletal structure and localized modes due to CH, CH2 and CH3 groups bound to it.