HYDROGEN-BONDING AND PROTON TRANSFERS OF THE AMIDE GROUP

Ab initio methods are used to probe the proton-bound complex involving a water molecule and an amide, modeled by formamide or acetamide. A polarized basis set was applied in conjunction with MP2 treatment of electron correlation. This approach affords a good reproduction of experimental proton affinities of the species involved. The O atom of the amide is the preferred site of protonation or complexation with the water, with acetamide binding most strongly to the water. The proton-transfer potential of each complex contains a single minimum corresponding to H2NHCOH+...OH2, due to the more basic character of the amide oxygen. A second minimum, wherein the proton is bound to the water, occurs when the two molecules are further apart than their equilibrium separation. The energy barrier for proton transfer between the two minima grows rapidly as the two molecules are further removed from one another. The high barriers lead to very slow removal of the proton from an amide at room temperature.