Proton motion and proton transfer in the formamidine-formic acid complex: an Ab initio projector augmented wave...

An ab initio molecular dynamics study performed with the projector augmented wave method (PAW) on proton motion and (double) proton transfer in the formamidine-formic acid complex is reported. The PAW trajectories were calculated with a time interval of 0.12 fs, for a total evolution time period of 36ps, and for temperatures in the range 500-600 K. All proton-transfer processes start with a proton transition at the O-H...N group, and are followed by a second proton transition, either at the same group ("single crossing-recrossing transitions") or at the other group, namely the N-H...O group ("double proton transfers"). According to the delay between the two transitions (more or less than 15 fs), one may distinguish between "concerted" (42%) or "successive" (16%) single crossing-recrossing transitions, and between "simultaneous" (7%) or "successive" (35%) double proton transfers. Successive processes take place via a zwitterionic intermediate, which remains stable for up to approximately 120 fs ("ionic regions"). The findings are in excellent agreement with the results of ab initio (HF, MP2) 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. Furthermore, it is shown that the optimized geometries of stationary points (ground state, transition state, and zwitterion) comply well with corresponding average data obtained from the PAW trajectories for normal periods, crossover points, and ionic regions.