Molecular-shape Selectivity by Naphthalimido-modified Silica Stationary Phases: Insight into the Substituents Effect of Naphthalene on Shape Recognition and π-π Interactions Via Electrostatic Potential

This report describes the molecular-shape selectivity of four naphthalimido-modified silica (NIM) stationary phases. To investigate the selectivity, several alkylbenzenes and polycyclic aromatic hydrocarbons (PAHs) were tested using capillary electrochromatography (CEC) with each NIM stationary phase. Results revealed that nitro group-substituted NIM phases had outstanding molecular-shape recognition ability toward PAHs; a strong negative correlation existed between the k' value and L/B ratio that is opposite those observed for C18 phases, in addition, one possessed 10-fold greater molecular-shape recognition ability compared to existing stationary phases. In addition, the position of the nitro group on the naphthalimide moiety modulated the π-stacking interaction due to steric hindrance. Furthermore, structure optimization and electrostatic potential (ESP) surfaces were used to clarify the mechanism of recognition. Based on this high recognition ability, separations of several positional isomers were examined. Results indicated that structure optimization and ESP maps could reveal the effect of functional groups on π-π interactions, which will aid in the design of new NIM stationary phases with shape selectivity, while taking advantage of electrostatic interactions.