Inverse Intermolecular Kinetic Isotope Effects in Unimolecular Reactions. Differential Quantal Effects in the Non-Equilibrium Behavior

Abstract

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An Experimental Generalization of Quantum Statistical Weight Effects in Nonequilibrium Unimolecular Reactions1

“…where Rf is the total amount of CH3(CH2)"-c-C3H6* formed, and similarly for Rf. Division of ( 2) by (3) with rearrangement yields…”

Intramolecular Kinetic Carbon Isotope Effect in the Gas Phase Decomposition of Deuteriooxalic Acid

“…In an attempt to explain this unusual phenomenon, we showed that: the size of the temperature dependence of kn/kv could be accounted for by combination of kinetic, equilibrium {e.g., between cyclic and noncyclic structures arising through intramolecular intercarboxyl hydrogen bonding), and tunneling isotope effects; the observed magnitude of (kn/ki> -1) is reasonable if there were impressed on the above combination a virtually temperature-independent inverse isotope effect such as that described and observed by Rabinovitch and his co-workers. [3][4][5][6][7] Since the Rabinovitch effect should appear only in the low-pressure region of a gaseous unimolecular decomposition, and since we have no information on the rate of the oxalic acid pyrolysis as a function of pressure, it seemed possible that study of the temperature dependence of C13 kinetic isotope effects might reveal additional information about the reaction mechanism. Under the experimental conditions accessible to us,26 only the intramolecular carbon isotope effect (that arising in the bifunctionality of the reagent and determined from measurements on the isotopic constitution of the products) could be studied conveniently.…”

Carbon-13 kinetic isotope effect in the structural isomerization of cyclopropane. Temperature and pressure dependence

“…The statistical effect of isotopy at but one of the three positions in the cyclopropane ring complicates any interpretation of the C13 isotope effect and must be taken correctly into account in any theoretical calculations and comparisons. It is worthwhile to attempt a statistical correction by the method introduced by Weston.6 Consider the possible reactions of the isotopic cyclopropanes with skeletons C123 and C122C13 C123H6 C12H3C12H=C12H2 12*j, If it is assumed that C12-C12 bond rupture is equally probable in these two isotopic cyclopropanes and if it is further assumed that the C13 isotope effect arises principally in C-C bond rupture, as compared with hydrogen transfer, then *2 = *3 = fc13 (12) These assumptions lead to the expression for the statistically corrected isotopic rate constant ratio in the isomerization of cyclopropane hese are large C13 isotope effects, consistent with a transition state involving considerable ring relaxation.…”