Photoionization-Induced Water Migration in the Hydrated trans-Formanilide Cluster Cation Revealed by Gas-Phase Spectroscopy and Ab Initio Molecular Dynamics Simulation

Photoionization-induced water migration in the trans-formanilide-water 1:1 cluster, FA-(H2O)(1), has been investigated by using IR-dip spectroscopy, quantum chemical calculations, and ab initio molecular dynamics simulations. In the S-0 state, FA-(H2O)(1) has two structural isomers, FA(NH)-(H2O)(1) and FA(CO)-(H2O)(1), where a water molecule is hydrogen-bonded (H-bonded) to the NH group and the CO group, respectively. In addition, the S-1-S-0 origin transition of FA(CO)-(H2O)(2), where a water dimer is H-bonded to the CO group, was observed only in the [FA-(H2O)(1)] mass channel, indicating that one of the water molecules evaporates completely in the D-0 state. These results are consistent with a previous report [Robertson, E. G. Chem. Phys. Lett., 2000, 325, 299]. In the D-0 state, however, [FA-(H2O)(1)](+) produced by photoionization via the S-1-S-0 origin transitions of FA(NH)-(H2O)(1) and FA(CO)-(H2O)(1) shows essentially the same IR spectra. Compared with the theoretical (H2O)(1)](+) is produced by photoionization of FA(CO)-(H2O)(1). [FA-(H2O)(1)](+) produced by photoionization of FA(CO)-(H2O)(2) also shows the IR spectrum corresponding to [FA(NH)-(H2O)(1)](+). In this case, the water migration from the CO group to the NH group occurs with the evaporation of a water molecule. Ab initio molecular dynamics simulations revealed the water migration pathway in [FA-(H2O)(1)](+). The calculations of classical electrostatic interactions show that charge-dipole interaction between FA(+) and H2O induces an initial structural change in [FA-(H2O)(1)](+). An exchange repulsion between the lone pairs of the CO group and H2O in [FA-(H2O)(1)](+) also affects the initial direction of the water migration. These two factors play important roles in determining the initial water migration pathway.