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evidence was described previously1,3 and is noted above, but a more conclusive argument can be made from the positions and fwhm (6) Fleyfel, F.; Devlin, J. P. Manuscript in preparation.of the y2 and bands of isotopically decoupled 13C02. Recently published infrared spectra of crystalline and amorphous thin films of C02 at 77 K show that the crystal-phase decoupled bands are at 2282.5 and 638.9 cm"1 with a fwhm of ~2.5 cm"1 34while the amorphous-phase bands are at 2280.3 and 643.0 cm"1 with a fwhm greater than 5 cm"1.7 Clearly, the cluster values noted above, 2282 and 638 cm'1 and fwhm of ~3 cm"1, identify the cluster structure with the crystalline rather than the amorphous phase.Acknowledgment. This research was funded by the National Science Foundation under Grant CHE-8719998. The assistance of Matthew S. Devlin in sampling, and the O.S.U. Physics Instrument Shop (Mike Lucas, manager, and Wes Cash, builder) in fashioning the cluster cell, is also acknowledged.

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Adsorption equilibria have been gravimetrically measured at 10, 20, 30, 50, and 70°C for benzene on pure crystalline silicalite. Isotherms change from type I to type IV with decreasing temperature, indicating heterogeneous behavior. The transition temperature is determined to be 38 t 1 "C by isobaric experiments. Silicalite is energetically homogeneous since there are no cations present. The observed structural heterogeneity is due to the comparable sizes of benzene molecules and the silicalite pore system. Silicalite is modeled as two types of homogeneous pores where a van der Waals model describes local equilibrium. Model parameters are calculated via nonlinear regressions with prescribed pore distribution determined from crystallographic considerations. Data and model predictions are in good agreement with highly significant parameters.

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