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3455are due to propyl torsions, features also noted in the spectra of n-propylbenzene and 4-n-propyltoluene, The observation of only three origins also substantiates the conclusion derived from the data on the ethylbenzenes that T~ = 90'.These results show (a) supersonic molecular jet spectroscopy is capable of observing specific molecular conformations which have very low barriers to conformational interchange, (b) aromatic ring methyl groups in the p-and m-xylenes can be considered as independent, nearly free rotors in So and SI, (c) the aromatic ring methyl groups in o-xylene are considerably more hindered in So and SI, and for SI, cross kinetic and potential terms must be introduced, (d) the barriers for rotation are greater in SI than So, (e) the dynamic nature of the motion of the ring methyl groups is contrasted by a more static, locked-in character of the aromatic ethyl and n-propyl substituents, (0 aromatic n-alkyl substituents have their first C-C torsion perpendicular to the plane of the aromatic ring, and (8) aromatic n-propyl substituents exist in anti and gauche conformations, as observed in these TOFMS/DE experiments. Acknowledgment.We thank A. Kassman, W. Kuhn, B. LaRoy, and C. Lilly for the encouragement and support of this work.

Oxidation of propylene to acrolein over bismuth molybdate catalysts was studied in a flow reactor at atmospheric pressure and at temperatures between 400°and 460°C. The kinetics were determined as first order in propylene and zero order in oxygen. Over this temperature range, the activation energy was 29 kcal per mole. The side products.• acetaldehyde, formaldehyde, ethylene, carbon monoxide, and carbon dioxide, were formed almost exclusively in a consecutive pathway via acrolein or its surface species precursor when excess oxygen was present, A parallel pathway from propylene for the formation of carbon dioxide became important when there was an oxygen deficiency. A gas-phase, homogeneous oxidation of acrolein also became important when the postcatalytic volume of the reactor was increased.

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