Ionization dynamics of trans-formanilide-H2O complexes:: A direct ab initio dynamics study

Ionization dynamics of trans-formanilide-water 1:1 complexes FA(H2O) have been investigated by means of direct ab initio trajectory method. From the static ab initio calculations, three conformers of the FA(H2O) complexes were obtained as stable structures: namely, these are the N-H site, the C=O carbonyl site and the bridge site, which are different in the positions of H2O around FA. In the N-H and C=O sites, a water molecule binds to the hydrogen and oxygen atoms of the peptide (-NH-CO), respectively. In the bridge site, the hydrogen and oxygen atoms of H2O bind to the C=O carbonyl and a hydrogen of benzene ring (o-position) of FA, respectively. The trajectories from the vertical ionization points of these three structures were calculated by means of full dimensional direct ab initio trajectory method. It was found that the H2O molecule in the N-H site is still remained in its site after ionization, i.e., the strong complex cation where H2O binds to the N-H site of FA(+) is directly formed. On the other hand, in the cases of the ionization from both CO and bridge sites, the water molecule was moved easily around both the benzene ring and the C=O carbonyl group. The mechanism of the ionization of FA(H2O) was discussed on theoretical results.