Electronic excitation spectra of the S(1)← S(0) transition obtained by resonance-enhanced two-photon ionization (REMPI) are analysed for phenol-Ar(n) (PhOH-Ar(n)) clusters with n≤ 4. An additivity rule has been established for the S(1) origin shifts upon sequential complexation at various π binding sites, which has allowed for the identification of two less stable isomers not recognized previously, namely the (2/0) isomer for n = 2 and the (2/1) isomer for n = 3. Infrared (IR) spectra of neutral PhOH-Ar(n) and cationic PhOH(+)-Ar(n) clusters are recorded in the vicinity of the OH and CH stretch fundamentals (ν(OH), ν(CH)) in their S(0) and D(0) ground electronic states using IR ion dip spectroscopy. The small monotonic spectral redshifts Δν(OH) of about -1 cm(-1) per Ar atom observed for neutral PhOH-Ar(n) are consistent with π-bonded ligands. In contrast, the IR spectra of the PhOH(+)-Ar(n) cations generated by resonant photoionization of the neutral precursor display the signature of H-bonded isomers, suggesting that ionization triggers an isomerization reaction, in which one of the π-bonded Ar ligands moves to the more attractive OH site. The dynamics of this isomerization reaction is probed for PhOH(+)-Ar(3) by picosecond time-resolved IR spectroscopy. Ionization of the (3/0) isomer of PhOH(+)-Ar(3)(3π) with three π-bonded Ar ligands on the same side of the aromatic ring induces a π→ H switching reaction toward the PhOH(+)-Ar(3)(H/2π) isomer with a time constant faster than 3 ps. Fast intracluster vibrational energy redistribution prevents any H →π back reaction.

Infrared photodissociation (IRPD) spectra of mass-selected 4-aminobenzonitrile-(water)n cluster cations, ABN(+)-(H2O)n with n ≤ 4, recorded in the N-H and O-H stretch ranges are analyzed by quantum chemical calculations at the M06-2X/aug-cc-pVTZ level to determine the evolution of the initial microhydration process of this bifunctional aromatic cation in its ground electronic state. IRPD spectra of cold clusters tagged with Ar and N2 display higher resolution and allow for a clear-cut structural assignment. The clusters are generated in an electron impact source, which generates predominantly the most stable isomers. The IRPD spectra are assigned to single isomers for n = 1-3. The preferred cluster growth begins with sequential hydration of the two acidic NH protons of the amino group (n = 1-2), which is followed by attachment of secondary H2O ligands hydrogen-bonded to the first-shell ligands (n = 3-4). These symmetric and branched structures are more stable than those with a cyclic H-bonded solvent network. Moreover, in the size range n ≤ 4 the formation of a solvent network stabilized by strong cooperative effects is favored over interior ion hydration which is destabilized by noncooperative effects. The potential of the ABN(+)-H2O dimer is characterized in detail and supports the cluster growth derived from the IRPD spectra. Although the N-H bonds are destabilized by stepwise microhydration, which is accompanied by increasing charge transfer from ABN(+) to the solvent cluster, no proton transfer to the solvent is observed for n ≤ 4.

The structures, binding energies, and vibrational and electronic spectra of various isomers of neutral and ionic phenol-Ar n clusters with n r 4, PhOH (+) -Ar n , are characterized by quantum chemical calculations. The properties in the neutral and ionic ground electronic states (S 0 , D 0 ) are determined at the M06-2X/aug-cc-pVTZ level, whereas the S 1 excited state of the neutral species is investigated at the CC2/aug-cc-pVDZ level. The Ar complexation shifts calculated for the S 1 origin and the adiabatic ionisation potential, DS 1 and DIP, sensitively depend on the Ar positions and thus the sequence of filling the first Ar solvation shell. The calculated shifts confirm empirical additivity rules for DS 1 established recently from experimental spectra and enable thus a firm assignment of various S 1 origins to their respective isomers. A similar additivity model is newly developed for DIP using the M06-2X data. The isomer assignment is further confirmed by Franck-Condon simulations of the intermolecular vibrational structure of the S 1 ' S 0 transitions. In neutral PhOH-Ar n , dispersion dominates the attraction and p-bonding is more stable than H-bonding. The solvation sequence of the most stable isomers is derived as (10), (11), (30), and (31) for n r 4, where (km) denotes isomers with k and m Ar ligands binding above and below the aromatic plane, respectively. The p interaction is somewhat stronger in the S 1 state due to enhanced dispersion forces. Similarly, the H-bond strength increases in S 1 due to the enhanced acidity of the OH proton. In the PhOH + -Ar n cations, H-bonds are significantly stronger than p-bonds due to additional induction forces. Consequently, one favourable solvation sequence is derived as (H00), (H10), (H20), and (H30) for n r 4, where (Hkm) denotes isomers with one H-bound ligand and k and m p-bonded Ar ligands above and below the aromatic plane, respectively. Another low-energy solvation motif for n = 2 is denoted (11) H and involves nonlinear bifurcated H-bonding to both equivalent Ar atoms in a C 2v structure in which the OH group points toward the midpoint of an Ar 2 dimer in a T-shaped fashion. This dimer core can also be further solvated by p-bonded ligands leading to the solvation sequence (H00),

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. [1] Proteins are one of the most important biological supramolecules and offer at the CO and NH sites of the -CONH-linkages of the peptide chain attractive hydrogen-bonding sites, in which H 2 O 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. [2] 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. [2h,l,m] 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. [2a,e-g,i,k,n] 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 H 2 O 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. [2a,f-h,l-o] 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, [3] 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. [1] IR spectroscopy is particularly sensitive to structura...

Auf verschlungenem Pfad: Die Bewegung eines einzelnen Wasserliganden um eine Peptidbindung in Acetanilid wurde mit zeitaufgelöster IR‐Spektroskopie in Echtzeit untersucht. Ausgelöst durch Photoionisation wird der Wasserligand von der CO‐Seite der Peptidbindung freigegeben und an der NH‐Seite derselben Peptidbindung nach einer Migrationsphase von 5 ps eingefangen (siehe Bild).