Tunable Liquid Chromatographic Separation of H/D Isotopologues Enabled by Aromatic π Interactions

We report hydrogen/deuterium (H/D) isotope effects based on weak intermolecular interactions with polar functional groups and aromatic rings in liquid chromatography (LC). Various LC experiments with different aromatic analytes, separation media, and nonpolar mobile phases were conducted under normal phase LC conditions, where the hydrophobic interaction was completely suppressed. The separation media that had polar functional groups, such as silanol groups, allowed for higher separation efficiencies for the pairs of aromatic H/D isotopologues. In comparing the ¹³C NMR spectra of protiated and deuterated aromatic analytes, the electron density of the deuterated analyte was found to be slightly higher than that of the protiated analytes. In the case of silanol functional groups, aromatic rings of the analyte acted as donors through the OH-π interaction to hydrogen atoms in the silanol groups. Thus, the deuterated analytes were able to be greatly retained by the stronger OH-π interactions. Furthermore, a C₇₀-fullerene bonded monolithic column (C70 column), which effectively provides CH-π interactions, allowed the opposite isotope effect. Briefly, an electrostatic attraction based on the dipole-(induced) dipole interaction dominated in the CH-π interactions, according to a van't Hoff analysis. Hence, the bonding lengths of the C-H or D bonds were sensitively affected, such that we were able to conclude that the CH-π interaction depended on the geometric effect. Applying these opposing H/D isotope effects, we were able to finally demonstrate effective H/D isotopologue separations by utilizing the complementary action of the OH-π and CH-π interactions.