Experimental and DFT dimer modeling studies of the H-bond induced-vibration modes of l-β-Homoserine

The vibrational spectra for l-β-Homoserine have been measured (IR absorption: 4000-400cm^-1/Raman spectra: 4000-200cm^-1). Characteristic vibrational modes of ammonium (-NH₃⁺), carboxylate (-CO₂^-) and hydroxyl (-OH) groups across the 3700-1400cm^-1 are all identified to have originated in inter-molecular hydrogen bonding involving these functional groups. DFT calculations at B3LYP/6-311++G(d, p) level have yielded a single neutral monomer in the gas phase. Since as a member of the amino acids which are known to possess zwitterionic structure in condensed phase, the neutral monomer of l-β-Homoserine is optimized to a zwitterionic structure in a water medium. Consideration of two dimer structures, one dimer with -NH‧‧‧O bond and another -OH‧‧‧O bond, has given rise to vibrational modes that satisfactorily fit to all the observed absorption and Raman bands. It is found that the dimer with -OH‧‧‧O bond (binding energy, 8.896kcal/mol) is more tightly bound than the dimer with -NH‧‧‧O bond (8.363kcal/mol).