DL-3-Aminoisobutyric acid: vibrational, NBO and AIM analysis of N-H⋯O bonded-zwitterionic dimer model.

A zwitterionic dimer model constructed of inter-molecular -N-H⋯O bonding has been proposed for the solid sample of DL-3-Aminoisobutyric acid consistent with IR absorption and Raman spectral features measured in the 3500-400/50 cm-1. This zwitterionic dimer model in water as solvent has been computed at B3LYP/6-311++G(d,p) and B3LYP-D3/6-311++G(d,p) levels including Grimme's dispersion correction associated with the -N-H⋯O interaction and SCRF-SMD method. Of the several possible monomer and dimer conformational structures, the most stable dimer constructed of two zwitterion monomer units has produced vibrational modes due to the -NH3 + cation and -CO2‾ anion involved in the -N-H⋯O bonding in fair agreement with the observed broad but composite IR modal features near the 3500-2000 cm-1. Except for the frequency of asymmetric stretching mode of the -NH3 + cation, its symmetric and bending modes agree with the observed values. As for the -CO2‾ anion, the frequencies of all of its modes are in good agreement with the experiment. Natural bond orbital (NBO), molecular electrostatic potential (MEP), atoms-in-molecules (AIM) and non-covalent interaction (NCI) analyses have been used to understand electronic characterization of the -N-H⋯O bonding.

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

The vibrational spectra for l-ß-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-ß-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).