ChemInform Abstract: ENERGY PARTITIONING BY MASS SPECTROMETRY, CHLOROALKANES AND CHLOROALKENES

Abstract

HCl elimination from vibrationally exicted C2–C4 chloroalkane and chloroalkene ions has been studied by ion kinetic energy spectrometry. The kinetic energy release during unimolecular decomposition was measured from the width of the metastable peak and used, together with other data, to infer the reaction mechanism. An average kinetic energy release of 438–614 meV was associated with 1,2‐elimination except in the chloroethane molecular ion. A much smaller kinetic energy release (∼ 30 meV) was associated with the 1,3‐elimination mechanism. In a few cases, composite metastable peaks were observed due to HCl elimination via two competitive reaction channels. In one such instance, involving the ion C4H7Cl+·, several different methods of generating the ion were used. This had a considerable effect on the relative weights of the two pathways but only a small effect upon the kinetic energy releases. The thermochemistry of many of the 1,2‐elimination reactions has been determined from ionization and appearance potential measurements on the chloroalkanes and chloroalkenes and their dehydrochlorination products. In combination with the ion kinetic energy measurements, this has yielded direct information on the energy partitioning pattern in these reactions. Data for the four‐centered elimination reaction in a large number of chloroalkane molecular ions show that (i) a large fraction of the kinetic energy release originates from the reverse activation energy, (ii) approximately 25% of the reverse activation energy is funneled into the translational energy of separation of the products, and (iii) differences in the energy partitioning behavior caused by modification of the reactant are small provided that such modification does not alter the reaction mechanism. In studying the 1,2‐elimination reaction RRKM calculations were performed in order to provide an estimate of the nonfixed energy of the activated complex and its contribution to the kinetic energy release. Complete thermochemical characterization of the 1,3‐dehydrochlorination has not been possible. It is suggested, however, that the reverse activation energy is small and that a substantial fraction of the kinetic energy release arises from the nonfixed energy of the activated complex.