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.