Revealing Single Molecular Solvent Reorientation Dynamics by Complementary Use of Picosecond Time Resolved IR Spectroscopy and MD Simulation

Dynamics of solvent molecules around a solute molecule plays a crucial role in chemical and biological processes, such as chemical reactivity, biological recognition, and hydrophobic interaction. Though extensive studies on the solvation dynamics have been carried out, the single molecular level information about the dynamics is hard to obtain in the condensed phase suffered by averaging effects over solvent molecules in various environments. In this study, gas phase hydrated clusters, for which size and orientation of hydration can be specifically defined, are utilized as a model system to elucidate the solvation dynamics in a molecular specific fashion by complementary use of picosecond time resolved IR spectroscopy and on-the-fly DFT MD simulation. An ionization induced CO → NH water reorientation in the CO bound acetanilide–water cluster was investigated as the first example of solvent reorientation. The time resolved IR spectra revealed that the reaction has an intermediate and takes ca. 6 ps to finish the reorientation. The MD simulation showed that the reaction is composed of two different channels; one is a fast channel in which the water molecule travels around the CH 3 group and the other is a slow channel in which water molecule once stays above the molecular plane. This detailed information about the water reorientation dynamics is first obtained by introducing a new dimen-sion, i.e. time, into the established method of determining static cluster structures, IR spectroscopy + quantum chemical calculations. This concept would open a new stage to study dynamic processes in the molecular level using gas phase solvated clusters.