CHEMICAL-REACTIONS IN MIXED MOLECULAR CLUSTERS, STUDIED BY RESONANT 2-PHOTON IONIZATION

After a short overview on the different types of intracluster reactions, observed so far, this survey focuses on nucleophilic ipso-substitution (S(N)) reactions in clusters consisting of one (1,n)-chlorofluorobenzene cation with one (1:1) of two (1:2) ammonia molecules. The reaction efficiencies are compared with that in bimolecular ion-molecule reactions as studied by Tholmann and Grutzmacher. Two types of (S(N)2)-reactions are observed, 1. abstraction of a chlorine radical, 2. abstraction of HX (X = Cl, F). The latter reaction is only observed in clusters but not in an ion-molecule gas phase collision. For both channels different mechanisms are proposed. The Cl-production strongly depends on the dipole moment of the neutral chromophore. It is rationalized in the framework of the theory of Shaik and Pross on polar reactions by an electron transfer as rate determining step. The HX formation channel, on the other hand is explained by a transition state with a concerted electron and proton transfer. It is strongly promoted by additional solvent molecules, probably due to bifunctional catalysis.