Hydrogen-bond dynamics, proton transfer, and water mobility in crystalline guanidinium carbonates for direct air capture

Abstract Direct air capture requires sorbents that bind CO 2 selectively yet release it with low energy. Bis-iminoguanidines capture CO 2 by crystallizing as hydrated carbonate salts, but the molecular events linking water loss, proton transfer, and CO 2 release remain unresolved. Here we combine variable-temperature single-crystal neutron and X-ray diffraction with ab initio molecular dynamics to determine how two crystalline carbonate phases follow different regeneration pathways. We show that lattice water acts as a molecular gate for carbonate protonation. In a metastable phase, water is isolated from carbonate, creating a softer hydrogen-bond network that promotes surface proton transfer and a transient carbonic-acid configuration. In the stable phase, water hydrogen-bonds directly to carbonate, stabilizing the lattice and suppressing proton transfer until water rearrangement and partial desorption disrupt the hydration environment. Controlling hydration topology together with targeted hydrogen-bond flexibility offers a route to direct-air-capture sorbents that retain selective CO 2 binding while lowering regeneration energy.