Proton motion and proton transfer in the formic acid dimer and in 5,8-dihydroxy-1,4-naphthoquinone: A PAW molecular dynamics study

A molecular dynamics study on proton motion and (double) proton transfer in the formic acid dimer (FAD) and in 5,8-dihydroxy- 1,4-naphthoquinone (DHN) is reported that has been performed with the Projector Augmented Wave method (PAW). PAW trajectories were calculated with a time interval of 0.12 fs, for evolution time periods up to 20 ps, and for temperatures in the range 500-700 K. Two basic situations can be clearly distinguished: normal periods that correspond to normal asymmetric O-H ... O hydrogen bonds, where the proton remains trapped at one oxygen atom, and active periods that correspond to (near-)symmetric O .. H .. O hydrogen bonds, where the proton undergoes large amplitude motions between the two adjacent oxygen atoms. Within the active periods one may distinguish between isolated transitions, where a proton just moves from one to the other oxygen atom, crossing-recrossing events, where a proton moves from one to the other oxygen atom but almost immediately turns back, and shuttling periods, where a proton undergoes several consecutive transitions. Moreover, one may also distinguish between single processes, where only one O-H O group is involved, and double processes, where both O-H O groups are simultaneously involved. It is shown that a reasonable and descriptive understanding of the active processes can be obtained by considering the time evolution of the potential energy that governs the motion of the proton between the two adjacent oxygen atoms. A main difference between FAD and DHN concerns the double proton transfer processes. In the first case these are almost exclusively simultaneous one-step processes, whereas in the second case these are mainly two-step processes, i.e. two successive single transitions. This difference can be attributed to the fact that with DHN a single proton transfer process yields the metastable 4,8-dihydroxy-1,5-naphthoquinone tautomer, whereas with FAD single proton transfer does not result in a metastable intermediate.