Direct Observation of the Condensed Phase Development Processes by Infrared Spectroscopy of Giant Clusters

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We have studied development of hydrogen bond networks in protic solvents such as water and methanol by using molecular clusters as microscopic model systems of their condensed phases, and following results have been obtained.1.Infrared spectroscopy of size-selected protonated water H^+(H_2O)_n was carried out up to n=100. It showed that a single excess proton totally changes the hydrogen bond structure of surrounding 100 water molecules.2.Infrared spectroscopy of protonated methanol H^+(MeOH)_n was also performed up to n=15. We demonstrated that the hydrogen bond network structure of protonated methanol is very different from that of protonated water, and it has much simpler structure, of which geometrical development is terminated by formation of the bicyclic structures.3.Infrared spectra of size-selected methanol-water mixed protonated clusters H^+(MeOH)_2(H_2O)_n were observed. In the two extreme cases, nm and n<<m, were studied, and the following features of the hydrogen bond network were shown. a)In the case of m<<n, a few methanol molecules are compatible with water in the hydrogen bond network, and the same three-dimensional cage structure as that in the protonated water cluster is formed. b)When a small number of water molecules are added to the hydrogen bond network of protonated methanol, structural changes of the network occur, and the proton is transferred from the methanol site to the water site.4.We also performed infrared spectroscopy of benzene cation clusters solvated by protic solvents such as water, methanol, and ammonia. We observed the solvation structure changes correlated to the proton affinity of the solvent molecule, and we found an intermolecular covalent bond formation between the benzene cation moiety and solvent molecule moiety, which can be regarded as an intermediate in the nucleophilic substitution reaction.