Size-dependent development of the hydrogen bond network structure in large sized clusters of protonated water, H+(H2O)n (n = 4 to 27), was probed by infrared spectroscopy of OH stretches. Spectral changes with cluster size demonstrate that the chain structures at small sizes (n less, similar 10) develop into two-dimensional net structures (approximately 10 < n < 21), and then into nanometer-scaled cages (n >/= 21).

Lifetimes of the first electronic excited state (S(1)) of fluorine and methyl (o-, m-, and p-) substituted phenols and their complexes with one ammonia molecule have been measured for the 0(0) transition and for the intermolecular stretching σ(1) levels in complexes using picosecond pump-probe spectroscopy. Excitation energies to the S(1) (ππ*) and S(2) (πσ*) states are obtained by quantum chemical calculations at the MP2 and CC2 level using the aug-cc-pVDZ basis set for the ground-state and the S(1) optimized geometries. The observed lifetimes and the energy gaps between the ππ* and πσ* states show a good correlation, the lifetime being shorter for a smaller energy gap. This propensity suggests that the major dynamics in the excited state concerns an excited state hydrogen detachment or transfer (ESHD/T) promoted directly by a S(1)/S(2) conical intersection, rather than via internal conversion to the ground-state. A specific shortening of lifetime is found in the o-fluorophenol-ammonia complex and explained in terms of the vibronic coupling between the ππ* and πσ* states occurring through the out-of-plane distortion of the C-F bond.

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 (IR) spectra of benzene-(water) n cluster cations (Bz-W n ) + (n ¼ 1-6) in the OH and CH stretching vibrational region were observed in order to investigate their structure and reactivity. The cluster cations were prepared by two different production methods: one is due to collision between bare benzene cations and water clusters; and the other utilizes resonance enhanced multiphoton ionization (REMPI) of neutral clusters. The former method prefers the production of the most stable isomer cluster cations, while the latter would reflect the Franck-Condon restriction in the ionization process. The structures of the n ¼ 1 and n ¼ 2 clusters were determined on the basis of the comparison between the IR spectra and density functional theory (DFT) calculations. In the n ¼ 1 cluster cation, the oxygen atom of the water molecule is located in the benzene ring plane and coordinates to the benzene moiety by two identical CH-O hydrogen bonds. The IR spectra of the n ¼ 2 cluster cation showed absorption bands arising from two different types of isomers: one has a hydrogenbonded water dimer interacting with the benzene cation; in the other isomer two water molecules are independently bound to the benzene cation. The production ratio between the isomers was found to strongly depend on the cluster ion preparation methods. Except for the case of the n ¼ 2 cluster, the cluster cations prepared by the two different methods gave identical IR spectra. This means that quite extensive rearrangements of the cluster structure occur upon ionization of the neutral clusters, leading to the most stable form of the cluster cations. The spectral features of the n ¼ 3 cluster cation are very similar to the n ¼ 2 cluster, suggesting similar structures among these clusters. Higher clusters larger than the n ¼ 3 cluster showed quite different IR spectra from those of the n 3 clusters, but their spectral features are very similar to those of hydrated clusters of protonated species, X-H + -(H 2 O) n , indicating that proton transfer reactions from the benzene cation to the water moiety occur in the larger clusters than those with n ¼ 3.

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