pKa Matching Enables Quantum Proton Delocalization in Acid-1-Methylimidazole Binary Mixtures.

Abstract

Short hydrogen bonds (SHBs), characterized by donor-acceptor heteroatom separations below 2.7 Å, are prevalent in condensed-phase systems. Recently, we identified SHBs in nonaqueous binary mixtures of acetic acid and 1-methylimidazole (MIm), where electronic and nuclear quantum effects facilitate extensive proton delocalization. In this work, we explore the conditions favoring SHB formation in binary acid-base mixtures and propose that the difference in pK_a values between the acid and base, measured in a nonaqueous, aprotic solvent like DMSO, is a key determinant. Using MIm as a model base, we perform electronic structure calculations to systematically analyze pK_a matching across 97 acid-MIm pairs in DMSO solutions. Through a combination of first-principles simulations and infrared spectroscopy, we confirm the formation of SHBs and the delocalization of protons in benzoic acid-MIm and salicylic acid-MIm binary mixtures. Our results demonstrate that pK_a matching can significantly alter proton behavior in nonaqueous systems, transforming acid-base interactions from conventional proton transfer to quantum mechanical proton delocalization. This work establishes DMSO as a valuable alternative to water for assessing pK_a matching and highlights the importance of hydrogen bond networks in modulating these conditions. By elucidating the impact of electronic and nuclear quantum effects, our results provides insights for designing organic mixtures that leverage SHBs for advanced material applications.