As seen in Fig. (a) -4 (c), the penetration of the N…H bond (PN…H) jumps from 77%, 81% and 74% to 92%, 95% and 95% and the penetration index (PH…O) of the H…O bond falls from 92%, 91% and 94% to 79%, 70% and 73% at P=3 GPa, 5 GPa and 15 GPa for the adducts of pyridine…formic acid, pyridazine…formic acid and pyrazine…formic acid crystals respectively. Such an increase in the penetration index of PN…H and simultaneous decrease in PH…O at the critical pressure, Pc unambiguously confirms the formation of the covalent N-H bond in the NH + …O -ion-pair. This is commensurate with the co-crystal → salt conversion under pressure. In contrast, no such sharp increase in PO…H is observed for the furan…formic acid adduct till 30 GPa in harmony with the absence of the salt phase (See Supp. Info. Fig. ). Complete proton-transfer from formic acid to the bases is observed. On the otherhand, the predicted crystal for the 1:1 furan…formic acid does not reveal any salt formation (See Supp. Info. Fig. ). As reported earlier at such high pressures furan itself gets polymerized into nano-threads. The trade-off between the formation of co-crystal vs. salt for the adducts under pressure can understood on the basis of the dissection of the interaction energies into the various energy components namely, Pauli repulsion, orbital energies, electrostatic energy and dispersion energies. As seen from Fig. (a) -6 (f), comparing the interaction between the neutral H-bonded fragment for the co-crystal and the ionic fragment for the salt, the electrostatic interactions overwhelmingly stabilize the ionic structures under pressure. At ambient pressure, the interaction energies arising from the hydrogen-bonding, ΔEint co-crystal (kcal/mol) = -18.4, -20.4 and -16.6 are one-order magnitude lower than the freeenergies for proton-transfer, ΔGPT (in kcal/mol) 68 = +120.1, +125.5 and +134.6 between formic acid and pyridine/ pyridazine/pyrazine respectively. As a consequence, salt formation is prohibited at ambient conditions. However, under pressure (P ≥ Pc), the interaction energies with the exalted electrostatic interactions, ΔEint salt (kcal/mol) = -137.4, -130.0 and -136.7