ORGANIC MASS SPECTROMETRY, VOL. 28, 1262-1269 (1993) Mechanism, Energetics, Kinetics and Dynamics of the Reaction C2H:' + C2H4+' + H2 Svein Magne Briten, Trygve Helgaker and Einar Uggerud Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway Tore Vulpius Department of Chemistry, H. C. Orsted Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark The distributions of relative translational energy released during (i) loss of H, from metastable CH,CHi' ions, (ii) loss of HD from metastable CH,CDl' ions and (iii) loss of D, from metastable CD,CDl' ions ?.ere measured. The relevant parts of the potential energy surface of the reaction C,H:' + C,Hf ' + H, (including isotopic variants) were investigated using various theoretical methods (high-level ab initio quantum chemistry, RRKM calculations, dynamic reaction trajectory calculations). A consistent reaction model is presented which reproduces available experimental data. Quantum mechanical barrier tunnelling is found to be important, leading to an activa- tion energy (on the microsecond timescale) which is approximately 0.1 eV below the critical energy. INTRODUCTION Dehydrogenation and hydrogenation reactions play a central role in organic chemistry.' Ethylene, which is important intermediate for the production of ethanol and polyethylene, is produced industrially by the gas- phase pyrolysis of ethane., The reaction CH,CH, -+ CH,CH, + H, (1) is known not to occur via direct unimolecular elimi- nation of H, . Instead, a multi-step radical mechanism is followed. This mechanism involves initial homolytic cleavage of the C-C bond of ethane. The direct pathway (1,Zelimination) is avoid,d as the result of an extraordinarily high energy barrier. Quantum chemical calculations by Gordon et aL3 indicated a critical energy of 512 kJ mol-'. The bond dissociation energy of the C-C bond in ethane is known to be 367 kJ mol - To make the unimolecular 1,2-elimination kinetically more feasible, a catalyst may be used. Com- monly used catalysts for hydrogenation and dehydroge- nation include transition metals bonded in various forms. Several transition metal ions are known to promote direct H, elimination. This has been demonstrated in mass spectrometric experiment^.^ Among factors that seem to be essential for the activity of a metal is the availability of empty 4s orbitals for electron accep- tance.,.' An interesting situation occurs when one elec- tron has been removed from ethane, as in CH,CHl'. The activation energy for dehydrogenation of the ethane cation is 56 kJ mol-' (see below), only 11% of the corresponding quantity of reaction (1). Using electron impact ionization, the appearance energy for the formation of C,H:' from C,H, was mea- sured as early as in 1938 by Hipple6 to be 12.2 eV [ionization enegy ZE(ethane) = 11.52 eV].' In 1946, Hipple et d.* demonstrated that metastable C 2 H i s ions lose H, during flight through a magnet sector mass spectrometer. This observation gave support to the idea that an intact H, molecule rather than two H atoms are lost. By performing a series of charge-exchange reac- tions with ethane, Lindholm'" and von Kochgbshowed in 1965 that at the energetic threshold HD (and not H, or D,) is exclusively lost from CH,CD:'. This is a strong indication of a concerted 1,2,-fdihydrogen elimination mechanism. Photoionization" and photoelectron-photoion coincidence (PEPICO) meas- urements" have been reported. From these meas- urements a value for the activation energy of E, = 56 kJ mol-' and of the reverse activation energy of Ear = 25 kJ mol-' are obtained."' A TPEPICO value of E, = 64 kJ mol-' has recently been reported.'Id The first measurement of the translational energy release was made by Taubert', in 1964, giving T = 0.45 eV. Improved measurements by Lifshitz and Sternberg13 gave To,' = 0.20 eV and To.5 = 0.10 eV for reactions (2) and (4), respectively. CH,CH:' -+ CH,CHl' + HH CH,CD:' -+ CH,CD:' + HD CD,CD:' -+ CD,CD:' + DD (2) (3) (4) In 1974, Williams and Hvistendahl reported To.5 = 0.19 eV.14 Williams and Hvistendahl suggested that the reac- tion is symmetry forbidden (Woodward-Hoffmann rules) owing to the large fraction of the reverse activa- tion energy (TIE,') which is released in the form of rela- tive translation of the products. A symmetric transition state was inferred from their analysis. MIND0/3 calcu- lations by Dewar and Rzepa" indicated a 'skewed' transition state. This means that the H, elimination is concerted, but that one of the hydrogens approaches one hydrogen of the other carbon so that the H, entity formed departs in an unsymmetrical fashion. 0030-493)3/93/101262-08 $09.00 0 1993 by John Wiley & Sons, Ltd. Received 25 May 1993 Revised manuscript received 29 June 1993 Accepted 30 June 1993