Acid-Base Chemistry of Short Hydrogen Bonds: A Tale of Schrödinger's Cat in Glutamine-Derived Crystals.

Short hydrogen bonds, defined by donor-acceptor distances of less than 2.5 Å, 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 a short hydrogen bond featuring a double-well potential indicative of an activated proton transfer. Here, using path-integral ab initio molecular dynamics, we show that nuclear quantum effects completely eliminate the classical barrier leading to a symmetrization of the proton along the hydrogen bond. Classically, proton transfer is strongly coupled to the rocking motion of a neighboring ammonium ion; under quantum effects, this coupling is significantly reduced. Furthermore, examination of the electronic structure through Wannier centers reveals a quantum-driven redistribution of bonding electrons, blurring the distinction between hydrogen bonding and covalency. Taken together, our findings indicate that nuclear quantum effects in this organic crystal create a regime in which the donor and acceptor simultaneously act as the acid and base.