Relation between the Infrared Spectrum of Water and Decarboxylation Kinetics in Cetyltrimethylammonium Bromide in Dichloromethane

The structure of water in cetyltrimethylammonium bromide (CTABr) reverse micelles in dichloromethane has been studied as a function ofW) [H2O]/[surfactant] by using the IR absorption due to O-Hstretching modes in the 3800-3000 cm-1 range. We restricted our analysis to the region of small amounts of water (0<W<20) where the properties of the system depend strongly uponW. The IR spectra can be represented as a sum of contributions from interfacial and bulklike water, with a minor contribution from water in dichloromethane. The fractions of water in the two “regions” within the reverse micelle were evaluated as a function of W. Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion in the reverse micelles is strongly inhibited by an increase in W, and first-order rate constants level off at W > 5. The rate effects can be ascribed to an increase in polarity and hydration of the reaction region as water is added and are related quantitatively to changes in the IR spectrum.