Ab initio study of proton transfer between protonated formohydroxamic acid and water molecules

Proton transfer between protonated formohydroxamic acid (FAH)⁺ and water molecules (H₂O-FAH-H₂O)⁺ is studied theoretically. In a proton-relay mechanism, the carbonyl oxygen in formohydroxamic acid (HCONHOH, FA) accepts a proton from the hydronium ion (H₃O⁺) and releases a proton either from the amine group (N-H) or the N-hydroxyl (N-OH) of formohydroxamic acid to another H₂O molecule. The movement of the two protons is not simultaneous but through a stepwise process. The catalytic effect of the H₂O molecule in reducing the proton:transfer barrier compared to the catalytic effect of pure formohydroxamic acid through intramolecular proton transfer is substantial. The transfer barriers of the two protons in a relay process are asymmetric. The proton on either the amine group (N-H) or the N-hydroxyl (N-OH) of FAH⁺ will be transferred to the second H₂O molecule, and the proton transfer is determined by the lengths R_N⋯O and R_O⋯O, which are related to the distance between FAH⁺ and the second H₂O molecule. At the same R_N⋯O and R_O⋯O, the proton on amino group (N-H) has a transfer energy barrier lower than that of the proton on the N-hydroxyl site by about 4.1 kcal/mol. If we elongate the distance between the two water molecules and let protonated FA shuttle freely as a proton carrier between the two water molecules, then the energy barrier for the movement of protonated FA increases slowly with the increase of the distance between two H₂O's separated by no more than 10 Å. However, the proton-transfer barrier between protonated FA and water is independent of the separated distance of the two water molecules.