Pressure-Driven Cooperative Intermolecular-Covalent Bond Relaxation in a Hydrogen-Bonded Organic Framework

Ordered hydrogen-bond networks provide crystalline organic materials with structural adaptability, yet how compression is distributed between intermolecular hydrogen bonds and intramolecular donor covalent bonds remains unclear. Here, we combine in situ high-pressure Raman spectroscopy and first-principles calculations to resolve the bond-specific pressure response of a melamine-cyanuric acid hydrogen-bonded organic framework (M-CA HOF). Between 1.04 and 8.35 GPa, the low-frequency mode predominantly associated with the N-H···N motif hardens, whereas the N-H···O-associated mode shows little net change; two N-H stretching modes shift to lower wavenumber. Calculations relate these opposite responses to concurrent shortening and bond-order gain of the H···N/H···O and elongation/bond-order loss of the connected N-H bonds, accompanied by hydrogen-bond angular adjustment and local electronic redistribution. These results support a cooperative intermolecular-covalent relaxation mechanism through which an ordered hydrogen-bond network accommodates compression by correlated local-bond adjustments.