A comparative MP2 study between water- and acid-assisted proton transfer: allophanic acid as a case of study

Electronic structures of allophanic acid were studied using MP2/6-311++G(d,p) level of theory. Five most stable tautomers were selected and stability of them was studied in detail. Obtained data showed that tautomer 1 having hydrogen atom in the central nitrogen N3 and also in a trans conformation of carbonyl and amino functional groups becomes the most stable one. Then, interconversion of these tautomers to each other was investigated step by step through internal rotation and proton transfer routes. Results indicated that movement of protons determines rate-determining step of all paths A–E. Effects of different solvents were carefully surveyed for each tautomer, and among investigated solvents, water made slight stabilization. Activation barrier for proton exchange assisted by one water molecule and one formic acid molecule was also studied, separately. Both molecules had a great influence on lowering barrier especially for proton transfer routes. Comparative MP2 study showed that the barrier will be lowered much more when acid-assisted proton exchange takes place.