Otto Dopfer

Zero‐kinetic‐energy photoelectron spectroscopy of the hydrogen‐bonded phenol‐water complex

Two‐photon, two‐color (1+1’) zero‐kinetic‐energy (ZEKE) photoelectron spectra are presented for the 1:1 phenol‐water complex, a prototype system for hydrogen bonding between an aromatic molecule and a simple solvent. ZEKE spectra via different (intermolecular) vibrational intermediate S1 levels of the fully protonated complex (C6H5OH–H2O, h3) as well as the ZEKE spectrum via the vibrationless S1 state of the threefold deuterated complex (C6H5OD–D2O, d3) have been recorded. The spectra are rich in structure, which is mainly attributable to intermolecular vibrations of the ionic complex. Progressions of the intermolecular stretch vibration (240 cm−1) in combination with different intermolecular and intramolecular vibrational levels are the dominant feature of all ZEKE spectra obtained and indicate a large change in the complex geometry along the hydrogen‐bond coordinate on ionization. Comparison between the spectrum of the d3 complex and the spectra via different intermediate intermolecular levels of the h3...

S1 EXCITATION AND ZERO KINETIC ENERGY SPECTRA OF PARTLY DEUTERATED 1:1 PHENOL-WATER COMPLEXES

Two‐photon, two‐color resonant‐enhanced multiphoton ionization (REMPI) spectra of the S1 state of isotopic 1:1 hydrogen‐bonded phenol–water clusters have been recorded. Up to three deuterium atoms are introduced in the phenolic OH group and/or the water molecule. The intermolecular vibrational structure found is in reasonable agreement with previously reported one‐color REMPI spectra, however, a partly different interpretation of the spectra is presented here. Zero kinetic energy photoelectron (ZEKE) spectra have been obtained via different intermediate S1 levels of the various isotopic complexes. The analysis of both the REMPI and the ZEKE spectra supports the new assignment of several vibrational bands observed in the REMPI spectra of the deuterated complexes where one or two hydrogen atoms are substituted by deuterium. For these deuterated complexes, the reassignment given here is based on the assumption that two different nonequivalent isomeric configurations are responsible for the structure observed...

Infrared Spectra of Isolated Protonated Polycyclic Aromatic Hydrocarbon Molecules

Gas-phase infrared (IR) spectra of larger protonated polycyclic aromatic hydrocarbon molecules, H+PAH, have been recorded for the first time. The ions are generated by electrospray ionization and spectroscopically assayed by IR multiple-photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer using a free electron laser. IRMPD spectra of protonated anthracene, tetracene, pentacene, and coronene are presented and compared to calculated IR spectra. Comparison of the laboratory IR spectra to an astronomical spectrum of the unidentified IR emission (UIR) bands obtained in a highly ionized region of the interstellar medium provides for the first time compelling spectroscopic support for the recent hypothesis that H+PAHs contribute as carriers of the UIR bands.

Infrared spectra of the phenol–Ar and phenol–N2 cations: proton-bound versus π-bound structures

Abstract Infrared spectra of the phenol–Ar/N 2 cations (Ph + –Ar/N 2 ), produced in an electron impact ion source, are analyzed in the vicinity of the O–H stretch fundamental, ν 1 . For Ph + –Ar two isomers are identified by their ν 1 frequency shifts upon complexation: the proton-bound global minimum (Δ ν 1 =−65 cm −1 ) and the π -bound local minimum (Δ ν 1 =+2 cm −1 ). The former isomer represents the first aromatic ion-rare gas (Rg) dimer where the Rg atom does not prefer binding to the aromatic π –electron system. The larger frequency shift of proton-bound Ph + –N 2 (Δ ν 1 =−169 cm −1 ) compared to Ph + –Ar is consistent with a stronger intermolecular bond due to the additional charge–quadrupole interaction. Ab initio calculations support the interpretation of the experimental data for both species.

IR spectrum of the benzene-water cation : direct evidence for a hydrogen-bonded charge-dipole complex

The infrared spectrum of the benzene–water cation, C6H6+–H2O, was recorded in the O–H stretch region to obtain the first experimental information about its geometry and interaction strength. The spectrum is consistent with a charge–dipole structure in which the oxygen atom of H2O approaches the C6H6+ cation in the aromatic plane. The dissociation energy estimated from the spectral shifts is D0≈14±3 kcal/mol. The interaction in the C6H6+–H2O cation is rather different from the one in the neutral dimer, demonstrating the dramatic ionization-induced changes of the interaction between an aromatic hydrocarbon and a polar ligand.

A new approach to vibrational spectroscopy of ion clusters: the “zero kinetic energy (ZEKE)” photoelectron spectrum of the phenol—water complex

Abstract The vibrational spectrum of the phenol—H2O cation complex is obtained by two-colour resonant pump—probe “zero kinetic energy (ZEKE)” spectroscopy. The ZEKE electron signal only appears when the total photon energy exactly matches a transition to a cluster ion state. The results are: an accurate value of the adiabatic ionisation energy, and a well resolved vibrational spectrum assigned to intermolecular modes. The ZEKE method provides a resolution (0.4 cm− close to laser bandwidths. Thus, vibrational frequencies of the cluster ion are obtained to a precision not available with any other method.

Protonation of aromatic molecules: competition between ring and oxygen protonation of phenol (Ph) revealed by IR spectra of PhH+–Arn

Abstract The protonation sites of phenol are investigated by IR photodissociation spectroscopy of protonated phenol complexed with one and two Ar ligands, PhH + –Ar n ( n =1,2), in the vicinity of the O–H stretch fundamentals. The complexes are produced in a supersonic expansion of Ph, H 2 , and Ar combined with electron impact ionization. The vibrational analysis of the IR spectra unambiguously reveals the presence of at least two PhH + isomers in the expansion: protonation occurs at oxygen and at the aromatic ring (in para and/or ortho position). This observation represents the first spectroscopic evidence for the existence of several PhH + isomers in the gas phase. The Ar ligands in PhH + –Ar n prefer hydrogen bonding over other binding sites. Quantum chemical calculations support the interpretation of the experimental data and provide further insight into the relative stability of various PhH + isomers and their complexes with Ar.

Effect of protonation on the electronic structure of aromatic molecules: naphthaleneH+.

Protonated naphthalene, the smallest protonated polycyclic aromatic hydrocarbon cation, absorbs in the visible, around 500 nm, which corresponds to an unusually large red shift with respect to the neutral naphthalene counterpart.

Watching water migration around a peptide bond.

Life is believed to have its origin in aqueous environments, and 70% of our body consists of water. The essential components of biological systems have to interact in aqueous solutions with water molecules by intermolecular forces, such as hydrogen bonds, dispersion forces, and hydrophilic/hydrophobic interactions. Proteins are one of the most important biological supramolecules and offer at the CO and NH sites of the -CONHlinkages of the peptide chain attractive hydrogen-bonding sites, in which H2O can act either as a proton donor or a proton acceptor, respectively. The solvation of a protein has a strong effect on its molecular shape, and as a consequence the fluctuations of the water network on the surface have important influence on its folding properties and catalytic function. Most fundamentally, when a protein starts its folding motion, the water network hydrogen-bonded to the protein has to rearrange and thus affects the dynamics. Therefore, up-to-date quantum chemical simulations on protein folding and its functions include water molecules explicitly. A deeper understanding of these phenomena at the molecular level requires the characterization of the dynamical processes of individual water molecules interacting with the protein. However, most experiments yield only indirect dynamical information averaged over water molecules in the first hydration layer and thus only a tentative and often controversial interpretation of the underlying mechanisms. Measurements visualizing the motion of a specific water molecule in a real biological environment are challenging, and so far no experimental data have been reported yet. Such dynamical experiments need to distinguish between each single water molecule, which can bind to numerous different binding sites of the protein and readily exchange their role with other H2O molecules in the same or higher hydration solvation layers. This inherent complexity of the hydrated protein has so far prevented measurements of the migration of individual water molecules in solution, and therefore nearly all information about such processes relies on theoretical approaches. Although quantum chemical simulations for such complex systems have substantially progressed in recent years because of rapid computer developments, their accuracy is still rather limited and experimental benchmark data for model systems are highly requested for calibration purposes. To this end, we have developed in the past decade an experimental strategy for the investigation of dynamical intermolecular processes, which typically occur on the picosecond (ps) time scale. This approach involves the generation of molecular clusters isolated in supersonic beams and the characterization of their dynamics using ps time-resolved IR spectroscopy. The fruitful combination of spectroscopy and quantum chemistry currently provides the most direct and most detailed access to intermolecular interactions. IR spectroscopy is particularly sensitive to structural motifs. In initial benchmark experiments, we developed a three-color UV-UV-IR tunable picosecond pump–probe laser spectrometer and measured the ionization-induced p!H site switching dynamics of rare gas ligands attached to phenol. 5] In this case, the position of the ligand was monitored by the structure-sensitive frequency of the phenolic OH stretching vibration. Although for a very limited number of water complexes with aromatic molecules the laser-induced migration of the water ligand has recently been inferred from “static” spectroscopy using nanosecond lasers, 6] no time-resolved studies about the dynamics of this fundamental process have been reported yet. We have applied our ps pump–probe approach to the trans-acetanilide–H2O (AA-H2O) cluster to monitor the water migration dynamics around a peptide linkage by the time evolution of the IR spectra measured at a delay time Dt after the ionization event (Figure 1). AA is one of the smallest aromatic molecules with a -CONHpeptide bond (Figure 1) and thus serves as a suitable model for proteins and related biomolecules. It can form two different types of hydrogen bonds with water, resulting in NHand CO-bound isomers, which can readily be distinguished by their different IR and electronic spectra 7] (see the Supporting Information). The “static” IR spectrum of the CO-bound isomer (reactand R) in the neutral ground state (S0) measured by nanosecond lasers (Dt = 50 ns, Figure 2) reveals a sharp band at 3473 cm 1 and a broader one at 3496 cm 1 assigned to the free NH stretching vibration (nNH ) and the hydrogen-bonded OH stretching mode (nOH ) of the water ligand, respectively. The corresponding picosecond IR spectra before photoionization [*] K. Tanabe, Dr. M. Miyazaki, Prof. Dr. M. Sakai, Prof. Dr. M. Fujii Chemical Resources Laboratory, Tokyo Institute of Technology Yokohama 226-8503 (Japan) E-mail: mfujii@res.titech.ac.jp

IR signature of the photoionization-induced hydrophobic→hydrophilic site switching in phenol-Arn clusters

IR spectra of phenol-Arn (PhOH-Arn) clusters with n=1 and 2 were measured in the neutral and cationic electronic ground states in order to determine the preferential intermolecular ligand binding motifs, hydrogen bonding (hydrophilic interaction) versus pi bonding (hydrophobic interaction). Analysis of the vibrational frequencies of the OH stretching motion, nuOH, observed in nanosecond IR spectra demonstrates that neutral PhOH-Ar and PhOH-Ar2 as well as cationic PhOH+-Ar have a pi-bound structure, in which the Ar atoms bind to the aromatic ring. In contrast, the PhOH+-Ar2 cluster cation is concluded to have a H-bound structure, in which one Ar atom is hydrogen-bonded to the OH group. This pi-->H binding site switching induced by ionization was directly monitored in real time by picosecond time-resolved IR spectroscopy. The pi-bound nuOH band is observed just after the ionization and disappears simultaneously with the appearance of the H-bound nuOH band. The analysis of the picosecond IR spectra demonstrates that (i) the pi-->H site switching is an elementary reaction with a time constant of approximately 7 ps, which is roughly independent of the available internal vibrational energy, (ii) the barrier for the isomerization reaction is rather low(<100 cm(-1)), (iii) both the position and the width of the H-bound nuOH band change with the delay time, and the time evolution of these spectral changes can be rationalized by intracluster vibrational energy redistribution occurring after the site switching. The observation of the ionization-induced switch from pi bonding to H bonding in the PhOH+-Ar2 cation corresponds to the first manifestation of an intermolecular isomerization reaction in a charged aggregate.