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 –NH(3)(+) cation and –CO(2)‾ 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 –NH(3)(+) cation, its symmetric and bending modes agree with the observed values. As for the –CO(2)‾ 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.