diones further supports this mechanistic hypothesis. Listed in Table III are the ketones and diones for which we have recently determined the temperature dependence of OH reactivity along with an indication of the type of CH3 and/or CH2 groups responsible for reactivity. Clearly, for both acetone and 2,3-butanedione, where only a components react, a positive activation energy of approximately 1 kcal/mol is observed. For 2-butanone and 3-pentanoné, where ß reactivity (proceeding through formation of an adduct intermediate) accounts for almost 30% of the reaction rate (at room temperature), the activation energies drop to values much closer to zero. Finally, for 2,5-hexanedione, where the majority of the molecule's reactivity involves the j3-CH2 groups, a strong "negative activation energy" is calculated.Still further support for this hypothesis comes from our present kinetic data for the reaction of OH with the cyclic ketones. The formation of such a six-membered ring (adduct) is, of course, not possible for these cyclic ketones and, indeed, we do not find any evidence for an enhanced reactivity of the ß position. Thus, the room temperature reactivity per -CH2group is the same (within experimental error) in cyclobutanone as in cyclobutane, in cyclopentanone as in cyclopentane, and in cyclohexanone as in cyclohexane using the rate constant values recommended by Atkinson11 1for the cyclic alkanes.
emission have been observed by irradiating H2 and D2 in the 680-860 A region with synchrotron radiation from the ACO storage ring at Orsay. Comparison of the observed excitation spectra with the absorption spectrum shows clear evidence for the predissociation of molecular Rydberg states above the dissociation thresholds which are observed for the dissociation reactions: H 2 + hv-.H(I S) +H*(n = 2,3,4, 5). Undispersed visible fluorescence is also observed on excitation in the 750-860 A region which is attributed to molecular emission from excited Rydberg states. The results of these photon impact experiments are compared with those from electron impact processes. FIG. 1. Scheme of the experimental arrangement.
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