Successive mechanism of double-proton transfer in formic acid dimer: A classical study

The dynamics of double-proton transfer reaction in formic acid dimer is inv estigated by performing ab initio molecular dynamics simulations. From the viewpoint of optimized energetics alone, the synchronous (simultaneous) pro ton transfer is more favorable than the successive one. However, a full-dim ensional classical dynamics shows that there is a certain time lag, about 8 fs in average, between two proton transfers. When a proton undergoes the f irst transfer, it moves from an oxygen with higher electron density to the counterpart having the lower one. The proton thus needs an energy sufficien t enough to break the chemical bond, resulting in a clime of a potential ba rrier. On the other hand, the second proton moves from the lower electron-d ensity oxygen atom to the higher one. Hence, the second proton is shifted p redominantly by the thus-formed electronic field. Not only due to the time lag observed but mainly because of the difference in the mechanism of trans fer, therefore, the present double-proton transfer is identified as success ive. A detailed study on dynamics shows that the vibrational modes of the O -C-O skeletons dominate the second proton transfer. (C) 2001 American Insti tute of Physics.