To gauge the influence of external pressure on the ground-state structures of these four adducts, CSP were performed with increasing pressure. Pressure leads to significant alterations in the hydrogen-bonding patterns in the co-crystals as shown in Fig. ). Under pressure, the N…H hydrogen-bond shortens while the O-H covalent bond lengthens resulting in a co-crystal → salt transformation. Specifically, the pyridine…formic acid co-crystal generates a salt at 3 GPa while the pyridazine…formic acid co-crystal forms salt at 5 GPa. Pyrazine…formic acid co-crystal requires a rather high external pressure of 15 GPa to generate the salt configuration. This is in harmony with the lowest pKa (weakest base) of pyrazine amongst the three nitrogenous bases. Interestingly, in the case of the furan…formic acid adduct (Fig. ), no significant changes in the O…H and O-H is observed in the O-H…O hydrogen bonding and it remains a co-crystal even at P=30 GPa.