NOTES Energy dependence of the reaction of CH$ with HCI J. Glosik,‘) W. Freysinger, and W. Lindinger Institut ftir Ionenphysik der Leopold Franzens Universitiit, Technikerstr. 25, A 6020 Innsbruck, Austria (Received 5 October 1990; accepted 30 April 1991) Since the establishment of various drift tube’ and flow- drift tube2’3 techniques, a big variety of data on the energy dependencies of rate coefficients k of ion-molecule reac- tions has been obtained for the energy regime from thermal to a few eV center-of-mass kinetic energy KE,.,.. On the basis of such data the assumption seemed to be safe, that reactions of positive ions which are fast at room tempera- ture rarely show marked changes in k between thermal energy and a few eV (Refs. 3 and 4) KE,.,,. The expres- sion “fast reaction” means that the thermal rate coefficient is close to the Langevin516 or AD07-limiting values, k, or kmo, respectively. Indeed, most of the fast reactions, in- cluding proton transfer, hydrogen abstraction, and charge transfer processes show variations of k within the above- mentioned energy regime being smaller than a factor of 2. A reduction of the rate coefficient by a factor of 3 with increasing KE,,. as observed for the reaction of NHf with NH3 was regarded as a rare example of a large vari- ation of k of a “fast reaction.“4 This situation has led to the widely acknowledged as- sumption that it is quite unnecessary to investigate the energy dependence of a reaction once it is known to have a large thermal rate coefficient (k-k,). Amongst such re- actions are the series of CH$ (n = O-5)8 and NH,$ (n = o-4)9 ions reacting with a variety of neutrals as well as hundreds of other fast reactions, the thermal rate coeffi- cients of which are listed in a recent compilation of Ikezoe et al. lo Reactions of complex hydrocarbon ions with neu- trals are well known to exhibit strong temperature depen- dencies of rate coefficients. ” The title reaction is a special example for the case of “simple” ionic and neutral reac- tants showing a pronounced energy dependence of a rate coefficient close to its collisional value over more than one order of magnitude with KE,,, in a swarm experiment. We have reinvestigated the fast reaction of CHZ with HCl in the Innsbruck SIFDT which has been described in the literature.” The rate coefficients obtained as dependent on KE,.,. are shown in Fig. 1. At near thermal energies the rate coefficients have values of - 1.5 X 10 - 9 cm3 s - ’ de- clining dramatically at elevated energies and reaching val- uesaslowask~lx10-‘“cm3s-1atKE,,,~ -2eV. The reaction channels observed are CH,+ + HCl k 1” CH,+ + Cl (la) kb ~ CHCl+ +-Hz (lb) kk -* CH2Cl + + H. (lc) Channel ( la) dominates above 1 eV ( > 50%) and declines to lO-20% at near thermal energies. Channels (lb) and (lc) are of equal strength and change with increasing K%n. correspondingly. Channels (lb) and (lc) require the breaking and formation of bonds, therefore one does expect a decrease of klb and k,, However, the strong de- cline of kl, (almost two orders of magnitude) is quite startling, because this seemed to be a hydrogen abstraction reaction and this type of process usually is fast at all ener- gies (thermal to a few eV) whenever it is exoergic. The small rate coefficient at high energies indicates that another reaction mechanism dominates the process. In the SIFDT experiment used for the present investi- gation KE,.,. is changed by variation of E/N (electric field strength divided by the density of the buffer gas helium) which causes a change in the ion drift velocity. Thus the variation of KE,,, does not change the internal excitation of the neutral reactant HCl which remains at room tem- perature. The reactant ions CHf , however, change their kinetic energy with variation of E/N and also their rota- tional and vibrational energy, T,, and ret,, respectively, due to collisions with the buffer gas.” rr,,, and r., are determined by the collisions between the CH2+ ions and the He buffer gas and thus in drift experiments at elevated E/N (Ref. 13) (except, when H2 is the neutral reactant*4) always remain considerably lower than the values of =Ln.- We therefore assume, that the extremely strong decrease of k with increasing E/N is by far dominated by the influence of KE,,.. It can be seen from Fig. 1 that above KE,,. = 0.3 eV the values of rate coefficients k fol- low the equation k-(KE,.,) -m, m-1. (2) Further work will be done investigating fast reactions k[cm’/s] lom,[[j FIG. 1. Dependence of the rate coefficient on KE,,,,. 3020 J. Chem. Phys. 95 (4), 15 August 1991 0021-9606/91/043020-02$002.10 0 1991 American Institute of Physics