Absolute rate constants for S(N)1-like ionization reactions in zeolites: Determination of zeolite ionizing power

A kinetic probe molecule has been developed to investigate the absolute ion izing power of cation-exchanged (LiY, NaY, KY, RbY, and CsY) Y-zeolites. Th e probe, the 2-chloro-1-(4-methoxyphenyl)ethyl radical, was generated withi n the supercages of MY upon laser photolysis of 2-chloro-1-(4-methoxyphenyl )ethyl acetate under both dry and hydrated conditions. The ionizing power o f the cation exchanged Y-zeolites depends on the nature of the counterion a nd increases upon going from CsY to LIY. In addition, the ionizing powers o f the dry zeolites are weaker than that of neat water and are more similar to water/methanol mixtures containing from 30% to 70% water. A significant reduction in the absolute ionizing ability of the zeolite was found when th e zeolite was hydrated by the coadsorption of water.