Proton motion and proton transfer in the formamidine-formic acid complex: An ab initio projector augmented wave molecular dynamics study

An ah initio molecular dynamics study performed with the projector augmente d wave method (PAW) on proton motion and (double proton transfer in the for mamidine-formic acid complex is reported, The PAW trajectories were calcula ted with a time interval of 0.12 fs. for a total evolution time period of 3 6 ps, and for temperatures in the range 500-600 K. All proton-transfer proc esses start with a proton transition at the O-H . . .N group, and are folIo n ed by a second proton transition, either at the same group ("single cross ing-recrossing transitions") or at the other groupl namely the N-H . . .O g roup (''double proton take: place via zwitterionic intermediate. which rema ins stable for up to transfers"). According to the delay between the two tr ansitions (more or less than 15 fs) one may distinguish between ''concerted " (42%) Or successive" (16%) single crossing-recrossing transitions, and be tween "simultaneous" (7%) or "successive" (35%) double proton transfers. Su ccessive processes take place via a zwitterionic intermediate, which remain s stable for up to more, it is shown that the optimized geometries of stati onary points (ground state, transition state, and zwitterion) approximately 120 fs ("ionic regions"). The findings are in excellent agreement with the results of nb initio (HF, MP?) and density functional theory (DFT: B3LYP. B3P86) calculations. according to which the zwitterionic intermediate that results from the first proton transition is a true local minimum. Furthermo re, it is shown that the optimized comply well with corresponding average d ata obtained from the PAW trajectories for normal periods, crossover points and ionic regions.