2A13

Isomer selective investigation of solvent migration induced byphotoionization around peptide linkage in Acetanilide-(methanol)
1:1 cluster(Tokyo Institute of Technology) Martin Weiler, Mitsuhiko Miyazaki, 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 theimportance in biological processes proteins are of great interest. Because of their peptide linkage,these biological supramolecules can either act as proton donor (by NH-group) or as proton acceptor(by CO-group). The biological environment of proteins is affected by water and the hydration has astrong effect to the structure (e.g. α-helix or β-sheet) and reactivity. The dynamics of solvent
molecules becomes important, e.g. in case of folding motion, because here the solvent molecule has tomigrate and a rearrangement of the system is necessary.
Acetanilide (AA) is one of the smallest aromatic molecules containing a peptide linkage andacts as a model substance for investigation on a molecular level. In case of the water 1:1 cluster, arearrangement of the solvent molecule induced by photoionization is observed.[1] The solvent migrates
from the CO-binding site to the NH-binding site. By applying time resolved IR spectroscopy, themigration dynamics was observed in real time and 5ps lifetime of the migration, role of intracluster
vibrational redistribution (IVR) and existence of an intermediate have been revealed.[2]In this work, we will investigate the clusters of acetanilide with methanol (AA-(MeOH)) andwill discuss the effect of the solvent. We measured the R2PI and IR-UV hole burning spectra for twoisomers (see Fig. 1). By applying combined IR/UV techniques a migration of the solvent is observed
after ionizing the CO-binding isomer like in the AA-(H2O). Furthermore, we will compare our resultswith DFT calculations (M06-2X/cc-pVDZ) and to the former results of the AA-(H2O) clusters.

Fig. 1: R2PI spectrum of AA-MeOH in the region from 35500-36800 cm-1 and the structures of the assigned
NH-bound and CO-bound isomers and their relative stabilities a). The IR-UV hole burning spectrum of the NHisomer is shown in b) and the one of the CO-isomers in c)

[Results] Fig. 1a shows the S1-S0 R2PI spectrum of AA-MeOH. From the analogy to AA-H2O, theband systems at around 35692 cm-1 and 36064 cm-1, are tentatively assigned to NH-bound and CObound isomers of AA-MeOH cluster, respectively. An experimental proof is given by measuring theIR-UV hole burning spectra of both isomers. The IR laser is fixed at 3494 cm-1 (H-bound OH
stretching vibration of CO-isomer) and the UV laser is scanned. The resulting UV excitation spectrumof the NH-isomer is shown in Fig. 1b. The UV excitation spectrum of the CO-isomer is measured in
the same way by fixing the IR laser to 3395 cm-1 (H-bound NH stretching vibration of NH-isomer).
The structures are also shown in Fig. 1. The observed relative intensities of the species are confirmedby the calculations. The AA(CO)-MeOH is 2.19 kJ/mol less stable than the AA(NH)-MeOH species.
Furthermore, the assignments are immediately confirmed by the IR dip spectroscopy that gives the IR
transitions in the S0 state.

Fig. 2: IR/R2PI spectra of AA(NH)-MeOH (left) and AA(CO)-MeOH (right) in the S0- (lower traces) and D0state (upper traces) and the related calculated stick spectra. For the D0 state of the AA(CO)-MeOH isomer, thestick spectrum of the assigned solvent migrated AA(NH)-MeOH is shown in black and the stick spectrum of thelast optimized structure for the AA(CO)-MeOH in red. The free NH stretching vibration of the monomer in theS0 state is observed at 3472 cm-1.

The IR spectrum of the NH-bound isomer in the S0 state is shown in Fig. 2 (left side). Thetransition at 3395 cm-1 is assigned to the H-bonded NH stretching vibration, the transition at 3680 cm1
to the free OH stretching vibration of the MeOH. The corresponding transitions in the AA(NH)-H2O
are observed at 3417, 3644 and 3743 cm-1. The IR spectrum of the CO-isomer in the electronic groundstate is shown in Fig. 2 (right side). The transition at 3494 cm-1 is assigned to the H-bonded OH
stretching vibration. The same vibration in AA(CO)-H2O is observed at 3492 cm-1. A very weaktransition is observed at 3473 cm-1 and can be assigned to the free NH stretching vibration (3472 cm-1
in the AA monomer). From both measured IR spectra, the assignments of NH- and CO-bound isomers
are confirmed.
The IR spectra in the D0 state for both ionized species are shown in Fig. 2 in the upper traces.
No significant differences are observed. The transitions at 2960 cm-1 are assigned to the H-bonded NH
stretching vibration of the AA(NH)-MeOH isomer and the transition at 3661 cm-1 to the free OH
stretching vibration of the MeOH. This assignment is also valid for the AA(CO)-MeOH isomer. The
existence of the same structure in the D0 state can be explained by isomerization, the solvent molecule
migrates from CO to NH binding site after ionization.
In the presentation, we will discuss the IR spectra for both isomers after electronic excitationand photoionization with regard to the structural dynamics induced by the photoionization and willcompare the results for methanol with water.

[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.