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The present work reports a combined experimental and theoretical study of the 9-methylguanine (9MG) nucleobase and its monohydrate in the gas phase. Laser spectroscopy provides optical evidence for the presence of only one tautomer of 9MG, among the three forms expected from an energetic basis. Comparison with the calculated vibrational frequencies enables us to assign with confidence this form to the anti rotamer of the 9MG enol tautomer. The same approach allows us to conclude that the monomer conformation is retained in the monohydrate, with the water molecule bridging the OH proton donor and the N7 acceptor sites of 9MG. A complete comparison between all IR absorption data known for the enol tautomers of guanine and related compounds in the gas phase and the present calculations enables us to propose a consistent assignment for all of these enol tautomers in terms of rotamers. Finally, the fact that one does not observe either the most stable (and biologically relevant) keto form or the other enol rotamer (very close in energy) raises the question of the efficiency of UV spectroscopy to detect properly these missing species.

A microscopic approach to the in vacuo energetics of the binding of a water molecule to the side chain of tryptophan, modeled by an indole molecule, is presented. Two binding sites have been studied, the most bound one, which corresponds to the conventional NH−OH2 hydrogen bond and a slightly less bound, so-called π-type hydrogen bond in which the hydrogen atoms interact with the π aromatic ring of indole. The structure of these two complexation sites as well as the potential energy surface has been obtained by a semiempirical model coupled with efficient procedures for the exploration of the surface. The H-bonded complex was observed in the supersonic expansion, and its binding energy (4.84 ± 0.23 kcal/mol) was measured using a laser two-color photofragmentation technique. The nonstandard H-bonded complex, not observed with indole, was observed with 1-methylindole, a substituted indole, in which the formation of the conventional bond is hindered. Its binding energy, measured with a similar accuracy (4.10...

The solvent effect on the appearance of a red-shifted twisted intramolecular charge-transfer (TICT) emission is studied in molecular clusters. Using a supersonic expansion, dimethylaminobenzomethyl ester−acetonitrile (DMABME−(CH3CN)n) clusters are studied by monitoring at the same time mass spectra and dispersed fluorescence spectra as well as by lifetime measurements. For DMABME−(CH3CN)n clusters, a clear red-shifted fluorescence is observed readily when the cluster contains one solvent molecule and the fluorescence decay becomes biexponential. Short-time evolution of the system has been monitored using a femtosecond pump/probe technique and picosecond photoelectron spectroscopy. The femtosecond dynamics is solvent dependent and is interpreted as a fast decay from the locally excited state to the TICT state reaching an equilibrium between these two states. The equilibrium shifts to the TICT states as the cluster size increases. The role of the triplet state in this process and in the biexponential charac...

The present study combines both experiment and molecular dynamics simulations in order to document the ionization behavior of the C6H6−H2O and C6H6−D2O complexes close to the ionization threshold, in particular its nonadiabatic character. Using the two-color two-photon resonant ionization laser technique, the ionization thresholds of these species have been measured together with the threshold for dissociative ionization. A binding energy has been deduced for the neutral species: D0(C6H6−H2O) = 106 ± 4 meV and D0(C6H6−D2O) = 116 ± 5 meV, which significantly increases the precision compared to literature. Using a semiempirical potential model, the minimum energy structures of the neutral and ionic species have been determined, and the potential energy surfaces have been analyzed using a two-dimensional approach. As a result, the formation of a stable C6H6+−H2O complex close to the threshold is found to be controlled by a pure quantum effect and is ascribed to the classically forbidden region of the neutra...

The present study combines both experiment and molecular modeling to describe the photoionization behavior of the gas-phase hydrogen-bonded complexes of phenol with water, methanol, and dimethyl ether, in particular the occurrence of fragmentation following ionization. Using the two-color two-photon resonant ionization laser technique, the threshold for dissociative ionization of these species has been measured. For the first time, precise binding energies have been deduced for the neutral species: D0(phenol−H2O) = 243 ± 5 meV and D0(phenol−CH3OH) = 265 ± 8 meV. Using a semiempirical potential model, the minimum energy structures of both neutral and ionic species have been determined. This theoretical study has emphasized the role of the dispersive interactions in the geometry of these neutral complexes, in particular the interactions between the alkyl group of the solvent molecule (CH3 in the case of methanol or dimethyl ether) and the π-cloud of the aromatic molecule. In addition, the comparison between...

Two-color two-photon femtosecond ionization experiments have been performed on NaI. The wave packet evolution of the A excited state has been followed by detecting photoions and photoelectrons. The results indicate that the Na+ ions are formed when the wave packet is located at the outer turning point of the excited state. Surprisingly, the NaI+ ions are also observed to be in phase with the Na+ signal. Photoelectron spectra show that high kinetic energy electrons are produced when ionizing around the outer turning point, in agreement with the NaI+ formation. The absence of signal corresponding to ionization from the covalent part of the excited state potential can only be understood if the absolute ionization cross section is much smaller in the covalent region of the A state (where the molecule can be considered as a van der Waals complex) than in the ionic Na+···I- part of the A state potential (where the interatomic distance is such that the ionization process may be considered as a photodetachment of...

The chemical reactivity of benzene cations in medium-sized (n ∼ 25) (H2O)n water clusters has been investigated. After formation in a supersonic jet, the neutral Bz-(H2O)n species are ionized by resonance-enhanced two-photon ionization. The existence of magic numbers in the mass spectra of these cluster ions has been revealed by reflectron time-of-flight mass spectrometry. These magic numbers are assigned to size-dependent evaporation processes following the ionization. The similarities observed for the present magic numbers compared to those of protonated water clusters allow us to assign them to the formation of a protonated water cluster without ejection of the benzenoid product from the cluster ion. With this diagnosis, we present evidence for the existence of a size-dependent chemical reactivity of benzene cations: a reaction is observed in the medium size range (n ∼ 20−30), whereas small clusters are known not to react. This result is compared to the chemical reactivity of toluene+ in water cluster...