文摘
The electronic structures, intramolecular proton transfers, and rotational barriers around C–N, O–H, and N–H bonds in the biuret isomers have been investigated at B3LYP/6-311+G(2d,p) level of theory. The results showed that biuret is nearly planar and the diketo form 1 involving hydrogen in the central nitrogen N6 and in a trans-conformation of amino group and carbonyl group is the most stable isomer. Furthermore, in the next step, conversion of five most stable isomers to each other was also investigated. The results indicated that intramolecular proton-transfer step is the rate-determining step in all paths. Study of the relative stabilities and mechanism of conversion of isomers have been performed with polarizable continuum model, and it was found that with going to polar solvents the order of stabilities did not change. The heights of barrier for PT mechanism assisted by one-water molecule was also studied and calculated results showed that the water molecule assisting PT has much lower energy barrier than analogous barrier in the gas phase. Moreover, NBO calculation is carried out for tautomers to obtain donor–acceptor interactions.