The thermal chemistry of trideuterioethylene on Pt(111) surfaces was characterized by reflection−absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The vibrational data indicate that thermal activation of a saturated layer of adsorbed ethylene by heating the surface to 350 K produces surface ethylidyne species with an isotopic composition of about 45% perdeuterioethylidyne and 55% dideuterioethylidyne. The hydrogen TPD data confirm that result, and also highlight the fact that the proportion of hydrogen-to-deuterium elimination from the original ethylene molecules changes with initial coverage. To explain these observations a previously proposed three-step mechanism is used where ethylene first isomerizes to ethylidene and ethylidene then either isomerizes back to ethylene or dehydrogenates to ethylidyne. The changes in hydrogen isotope removal probability with coverage are explained by changes in the relative rates between the latter two steps because of the fact that the outgoing hydrogen in the dehydrogenation step requires an empty surface site that may be blocked by ethylene molecules at high coverages.

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Samples of a bacterial polyester, poly@-hydroxybutyrate), have been degraded thermally as a thin film on a thermocouple-controlled filament at 350 "C. In one series of these thermal experiments a short pyrolysis time (5 s) was used in order to ensure only partial pyrolysis, while in another series of thermal experiments the pyrolysis was allowed to proceed until all the sample had decomposed to volatiles. Samples from the same batch were also degraded in solution by methanolysis under acid conditions, while under reflux at 84 "C. In all cases, the yields and molecular weights of the products were assessed by gas chromatography. The number and weight distributions of the oligomeric products observed have been compared with those predicted statistically on the basis of random scissions. The results show that total thermolysis can be interpreted exclusively in terms of a random chain scission mechanism. However, the partial pyrolysis results are not consistent with random scission statistics; these results imply that there are some kinetically favored scissions occurring near the ends of the molecules. The product distributions from methanolysis are also somewhat inconsistent with those expected on the basis of random scission and may suggest that dimers hydrolyze more readily than higher molecular weight species.

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