Acid Base Chemistry of Short Hydrogen Bonds: A Tale of Schr\"odinger's Cat in Glutamine-Derived Crystals

TL;DR

This study uses advanced simulations to show that nuclear quantum effects in short hydrogen bonds within glutamine-derived crystals eliminate classical barriers, leading to proton symmetrization and a quantum-shared bonding character.

Contribution

It demonstrates how nuclear quantum effects fundamentally alter the understanding of short hydrogen bonds, revealing proton symmetrization and electron redistribution in organic crystals.

Findings

NQEs eliminate classical barriers in SHBs

Proton becomes symmetrized along the hydrogen bond

Bonding electrons are redistributed, blurring hydrogen bond and covalent distinctions

Abstract

Short hydrogen bonds (SHBs), defined by donor-acceptor distances below 2.5 Angstrom, represent a distinct regime in acid-base chemistry where conventional models of hydrogen bonding break down. In an organic crystal formed via a temperature-induced chemical transformation of L-glutamine, we previously identified an SHB that is permissive to proton transfer, leading to a characteristic double-well potential implying an activated process. Herein, using path-integral ab initio molecular dynamics (PI-AIMD), we show that nuclear quantum effects (NQEs) completely eliminate the classical barrier, leading to a symmetrization of the proton along the hydrogen bond. While in the classical case the proton transfer is strongly coupled to the rocking modes of the ammonium ion in the crystal, NQEs significantly reduce these correlations. Examining the electronic structure through Wannier centers (WCs) further reveals a quantum-driven redistribution of bonding electrons, blurring the distinction between hydrogen bonding and covalency. Taken together, our findings indicate that NQEs in this organic crystal lead to a situation where both the donor and acceptor can simultaneously act as both an acid and a base.