1A08
Influence of excess energy in the cationic ground state on photoionizationinduced solvent migration in Acetanilide-water 1:1 cluster studied bytime-resolved IR spectroscopy*（Chemical Resources Laboratory, Tokyo Institute of Technology ，Technology Licensing

Organization, Tokyo University of Science**，Graduate School of Science, Kyushu
University***，Institut fuer Optik und Atomare Physik, Technische Universitaet Berlin ****）○Martin Weiler*，Takashi Nakamura*，Saori Matsuyama**，Mitsuhiko Miyazaki*，
Makoto Sakai*，Koichi Tsukiyama**，Hiroshi Sekiya***，Otto Dopfer****，Masaaki Fujii*[Introduction]
The interactions between molecule and solvent are manifold, e.g.
dispersion forces, hydrophobic/hydrophilic interactions or hydrogen bonds can be mentioned.
With regard to the importance in biological processes proteins are of great interest. The
biological environment of proteins is strongly affected by water and the hydration has astrong effect on the structure (e.g. α-helix or β-sheet) and reactivity. One of the fundamental
processes for growth and health of biological systems is the folding motion of proteins. This
reconfiguration requires a rearrangement of the solvent molecules.
Acetanilide (AA) is one of the smallest aromatic molecules containing a peptide
linkage and acts as a model substance of peptide-water interaction for mass- and isomer
selective investigation on a molecular level in the gas phase. In case of the water 1:1 cluster, arearrangement of the solvent molecule induced by photoionization is observed.[1] In the S0state, the water molecule is either hydrogen-bonded to the CO or to the NH site of the peptidebond. In the cationic ground state (D0) only the NH bound isomer is observed. By applying
time resolved infrared (IR) spectroscopy, the migration dynamics was observed in real timeand 5 ps lifetime of the migration, role of intracluster vibrational redistribution (IVR) andexistence of an intermediate have been revealed for the first time.[2]In this work, we investigated theinfluence of excess energy in the D0 state (byreducing stepwise from 9250 cm-1 to 4450cm-1) on the dynamics of the mentioned
migration by applying time resolved ps-IR
spectroscopy in a two color photoionization
experiment.
[Experiment] The principle and setup of theps time resolved UV-UV-IR measurements
are described elsewhere [2] and a scheme isshown in Fig.1. For investigating thedependence on the excess energy, theexperiments were performed via a resonant
two photon ionization (R2PI). The clusters
Figure 1: scheme of the setup
were isomer selectively excited by using afor UV-UV-IR measurements
first UV photon (UVexc) and then they areionized by using a second UV photon(UVion). By varying UVion, the excess energy in D0 can be controlled.

[Results and discussion]
Fig. 2 shows
the ps-R2PI spectra of AA(H2O) recordedby using two different ionization
wavelengths (320 nm in red and 330 nm inblack). The transition at 35697 cm-1 isassigned to the NH-bound isomer while theone at 36050 cm-1 is assigned to the CObound isomer. By reducing the excessenergy in the D0 state the relative
intensities of the clusters change
significantly. For the higher excess energy,
the CO-bound isomer is dominant while
Figure 2: ps-R2PI spectra of AA(H2O)
the NH-bound water cluster dominates atmeasured by using different ionization
lower excess energy. This effect can bewavelength
explained by different Franck-CondonFactors of the isomers.
To obtain information of thedynamics of the solvent migration, the time
evolution of the H-bonded NH stretchingvibration of the product (3185 cm-1) wasinvestigated. Fig. 3 shows the results for
UVion in the range from 277.4 nm to 316
nm. This corresponds to an excess energyof about 9250 cm-1 to 4842 cm-1. By fitting
the time evolution curves, the water
migration time can be estimated. The time
constant  changes from a)  = (5.8±0.3)
ps, b)  = (6.0±0.2) ps, c)  = (6.4±0.5) ps,d)  = (7.0±0.3) ps to e)  = (7.7±0.4) pswith decreasing excess energy. This is ingood agreement with the expectation. The Figure 3: time evolution of H-bonded NH
repulsive effect in the D0 state depends onstretching vibration of migration productthe energy induced by photoionization.
a) 277.4 nm; b) 310 nm; c) 312 nm
Furthermore, we measured the timed) 314 nm and e) 316 nm
resolved IR spectra for different excessenergies. These spectra are similar to the already reported results and verify the reaction
pathway as well as the existence of an intermediate state.
For further analysis, more sophisticated theoretical calculations (including multidimensional
potential energy surface) are required.
As a conclusion, the reducing of the excess energy from about 9250 cm-1 to 4842 cm-1
causes a slowing down of the photoionization induced migration of the water molecule fromthe CO- to the NH-binding site of the AA, but the time resolved measurements of the IR
spectra show that there is no significant change in the reaction pathway. The already reported
existence of an intermediate state [2] during the migration has been confirmed.
References:[1][2]

K. Sakota, S. Harada, Y. Shimazaki, H. Sekiya, J. Phys. Chem. A 2011, 115, 626.
K. Tanabe, M. Schmies, A. Patzer, M. Schuetz, H. Sekiya, M. Sakai, O. Dopfer, M. Fujii,Angew. Chem.-Int. Edit. 2012, 51, 6604.