Polyaniline emeraldine salt as selective electrochemical sensor for HBr over HCl: a systematic density functional theory study through oligomer approach.

Density functional theory (DFT) calculations are performed to rationalize the experimentally observed sensitivity and selectivity of polyaniline emeraldine for hydrogen bromide over hydrogen chloride. The interaction behaviour is studied at UB3LYP method of density functional theory through oligomer approach. The properties for polymers are obtained extrapolation through second-degree polynomial fit. Optimized of geometries, interaction energies, Mulliken and natural bond orbital charges are analysed to study the sensing behaviour. In the preferred orientation modes, emeraldine salt acts as a hydrogen donor, whereas HBr and HCl are hydrogen acceptors (nucleophile). Basis set superposition error corrected interaction energies are calculated for accurate determination of interaction strength between sensor and analyte. The basis set superposition error is significant for HBr complex compared to HCl. The decrease in energy gap between highest occupied and lowest unoccupied molecular of conducting polymer (sensor) on complex formation with both analytes has been studied. The results of computational study show that polyaniline emeraldine salt shows more selectivity towards HBr as compared to HCl and this outcome is in agreement with reported results based on experimental observations. Graphical abstract.