J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1473-1478 Ultra-low Temperature Kinetics of Neutral-Neutral Reactions :Rate Constants for the Reactions of OH Radicals with Butenes between 295 and 23 K Ian R. Sims* and Ian W. M. Smith School of Chemistry, The University of Birmingham Edgbaston ,Birmingham, UK B 152TT Pascal Bocherel, Andre Defrance, Daniel Travers and Bertrand R. Rowe Departement de Physique Atomique et Moleculaire, U.A. 1203 du C.N.R.S., Campus de Beaulieu, Universite de Rennes I, 35042 Rennes Cedex, France The first experiments on the kinetics of reactions of the OH radical at temperatures below 80 K are reported. They have been carried out by applying the pulsed laser photolysis (PLP), laser-induced fluorescence (LIF) technique for studying the kinetics of free radical reactions in the ultracold environment provided by the gas flow in a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus.This method has yielded rate constants for the reactions of OH with but-1-ene, (Z)-but-2-ene and (€)-but-2-ene at temperatures down to 23 K. The rate constants for all three reactions increase monotonically as the temperature is lowered and this dependence of the rate constants on temperature can be fitted to an empirical expression of the form (k/tO-” cm3 molecule-’ s-’) =a exp[b(T/298K)] with a and b equal to 5.2 and -2.8 for but-1-ene, 4.7 and -2.1 for (Z)-but-2-ene and 5.4 and -2.1 for (€)-but-2-ene. Until very recently, the kinetic data base for elementary reac- tions between electrically neutral species at temperatures below ca.200 K was restricted to a study of the recombi- nation of hydrogen atoms by Ham et al.’ In recent years, the increasing use of cryogenically cooled reaction cells has yielded rate constants, down to ca. 80 K in some cases, for the combination of 0 atoms with O,,, and for a number of reactions of the CN3 and OH4 radicals. In the latter experi- ments, by Smith and co-workers, pulsed laser photolysis (PLP) has been used to generate the free radicals, and the laser-induced fluorescence technique (LIF) has been employed to observe their kinetic decay. Despite the success of the above experiments, methods based on the use of cryogenically cooled cells are limited. First of all, it is difficult to operate below the temperature of liquid N, , although Simpson and co-workers5 have measured the rates of energy transfer processes involving stable neutral species such as H,, D, and 4He in the gas phase down to 35 K, using cells cooled by He or N, vapour.Condensation on the walls of cooled cells is rapid and conse- quently the partial pressure of any species, whose concentra- tion must be accurately known in order to extract precise rate constants, must be significantly below its partial pressure at the temperature of the cell walls. In the past two years, we have successfully devised a method of studying the kinetics of elementary gas-phase reac- tions between neutral species at much lower temperatures than hitherto which avoids the limitations inherent in the use of cryogenically cooled cells.These experiments employ a CRESU apparatus which was designed and constructed in Rennes with kinetic measurements on reactions between neutral species specifically in mind. The CRESU technique was originally devised by Bowe and cozworkers6 in order to determine rate constants for ion-molecule reactions at ultra- low temperatures. The method takes advantage of the flow properties of gaseous expansions from convergent-divergent Laval nodes into a low-pressure chamber. In the original experiments on ion-molecule chemistry in the CRESU apparatus, reactions were initiated by creating ions just beyond the exit of the nozzle using an electron beam and the rates and mechanisms of subsequent reactions were observed by sampling a portion of the flow downstream with a quad- rupole mass spectrometer.More recently, a selected ion source has been incorporated into the CRESU apparatus’ and, by cooling the gas reservoir in liquid N,, rate measure- ments have been made at temperatures as low as 8 K6,’The results obtained have made a substantial contribution to the understanding of molecular synthesis in interstellar clouds.6-8 The CRESU technique provides, via the isentropic expan- sion of gas through the Laval nozzle, a ‘collimated’ flow of ultra-cold gas which is uniform in temperature, density and velocity. The expansion and subsequent cooling are rapid enough that heavily supersaturated conditions may prevail, avoiding the major problem of condensation associated with the use of cryogenically cooled cells. However, in contrast to free jet expansions/molecular beams where the concept of temperature is not really valid, the relatively high gas density (10’6-10’7 molecule cm-3) of the supersonic flow ensures that frequent collisions take place during the expansion and subsequent flow, maintaining thermal equilibrium at all times.The environment is an excellent one in which to measure thermal rate constants for reactions between neutral species by the well established PLP-LIF meth~d.~.~ In our first experiments applying the PLP-LIF method in a CRESU apparatus, we determined rate constants for reactions of the CN radical with O2down to 13 K and with NH, down to 26 K,’ and with the hydrocarbons C&, C2H4 and CZH2 down to 26 K.” A full and detailed description of this new experi- mental method has been given.” The results which have been obtained have already attracted the attention of theoreticians” and astrochemists12 who seek to model the complex chemistry responsible for molecular synthesis in interstellar clouds.In this paper, we report results from the first experiments at ultra-low temperatures on reactions of the OH radical. One limitation of our new experimental technique is that the reaction under investigation must remove the radical species which is being observed with a pseudo-first-order rate con- stant which exceeds ca. lo3 s-’. This requirement arises because the gas downstream from the exit of the Laval nozzle moves at ca.600 m s-’ and good flow conditions only survive for ca. 30 cm. Consequently, the sample of radicals which is generated by the photolysis laser propagating along the axis of the gas flow moves rapidly past the point, located at the downstream end of the uniform flow, at which the rela- tive radical concentrations are observed by LIF. At pulse- probe time delays greater than CQ. 1 ms, there will still be some radicals in the gas in the observation zone, but they will have been created by photolysis of the radical precursor in the nozzle or the gas reservoir. Coupled with the fact that the concentration of the second reagent cannot exceed a few per cent of the total gas density without destroying the integrity of the flow, the requirement that the kinetic decay constant exceeds ca. lo3 s-' means that it is difficult to measure second-order rate constants which are less than ca.lo-', cm3 molecule -'s -'. The above limitations were considered when deciding which reactions to target for our first experiments on the kinetics of the OH radical at ultra-low temperatures. Many reactions of OH have been extensively studied above 200 K, because of their importance in atmospheric1 and comb~stion'~chemistry. In general, the reactions of OH rad- icals with saturated molecules are slower than the corre-sponding reactions of CN, although there are exceptions.15 Furthermore, the reactions of OH with unsaturated hydro- carbons occurs by addition (and the rates for small alkenes and alkynes therefore depend on total pressure) rather than by the pressure-independent replacement of an H atom as in the reactions with CN." For these reasons, we chose to study the reactions of OH with a number of butenes which seemed likely to be rapid and in their high-pressure limit under the conditions generated in the CRESU apparatus.The kinetics of these reactions have been examined in a number of investigations at room temperature.16-22 These studies demonstrate that the rate is independent of pressure down to 1 Torr at room temperature and that the predomi- nant reaction channel is addition to form a hydroxybutyl radical, although there is apparently some disagreement2'S2' as to the level of the small contribution of H-atom abstraction to the total rate.Atkinson and Pitts17 have measured rate constants for the reaction of OH with the four butenes, but-1-ene, isobutene, (Z)-but-2-ene and (E)-but-2- ene, between 295 and 425 K. At 295 K, their values for the rate constants are: 3.5 x lo-", 5.1 x lo-", 5.4 x lo-" and 7.0 x lo-'' cm3 molecule-' s-', respectively, and they found the rates to decrease as the temperature was raised. In the light of these previous results, the reactions of OH with butenes seemed promising candidates for our first experiments on reactions of OH radicals in the CRESU apparatus. Here, we report rate constants for the reactions of OH with but-1-ene, (q-but-2-ene and (E)-but-2-ene at tem- peratures between 295 and 23 K.OH radicals were generated by pulsed laser photolysis of H,O, at 266 nm and their kinetic decays observed by exciting LIF in lines of the (1, 0) band of the OH(A ,E+-X'll) system at ca. 282 nm.4 Experimental The apparatus and procedures used in the present series of experiments have been described in detail elsewhere.9b In par- ticular, interested readers are referred to the schematic of the apparatus given as Fig. 1 of ref. 9(b). Here, we shall give a relatively brief description of the method, emphasising those aspects of the experiments which are peculiar to measure- ments on reactions of the OH radical. The heart of the CRESU apparatus is an axisymmetric Laval nozzle which is mounted on a reservoir fitted with a perforated Teflon disc to ensure laminar flow and good mixing of the gas streams entering the reservoir.Although the gas reservoir is jacketed, permitting cryogenic cooling, this was not made use of in the present experiments. All the tem- peratures in the gas flows were achieved by isentropic expan- sion of the gas mixture prepared in the reservoir through the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 nozzle and into the main chamber. This expansion generates a supersonic flow of gas in which the Mach number, the tem- perature, the density of the gas, and the mole fraction of the reagent in excess (here, a butene) are constant along the axis of the flow. Several nozzles were employed, each providing a particular temperature and density for the selected carrier gas.The integrity of the gas flow, and therefore the design of a particular nozzle, could be checked in two ways. All of the nozzles were characterised by impact-pressure measurements. In addition, the temperatures provided by some nozzles were determined by performing spectroscopic measurements on the (0, 0) band of the (B2E+-X2E+) system of CN.9b The temperature determined from the relative intensities of the rotational lines confirmed the values inferred from the impact-pressure measurements. OH radicals were generated by the photolysis of H,O, at 266 nm using the pulsed output from a frequency-quadrupled Nd: YAG laser (Spectron Lasers). 85% H,O, (Solvay Interox) was placed in a glass/stainless-steel vessel and H,O, vapour entrained into the main carrier gas flow, by bubbling a controlled flow of He through the liquid H202 and intro- ducing this small precursor flow directly into the reservoir by means of PTFE tubing.Detection of OH radicals was achieved by LIF, exciting the (1,O) band of the OH (A ,Z+-X,lI)system at ca. 282 nm, and detecting off-resonance fluorescence from the (1, 1) band, and any in the (0, 0) band resulting from vibrational relax- ation in the electronically excited state, at ca. 310 nm. Probe laser radiation was provided by a Nd: YAG-pumped dye laser coupled to an autotracking frequency doubler unit (Spectra Physics). Kinetic data were gathered using the strongest available rotational line, usually the Ql(l) line at 282 nm.The photolysis and probe laser beams were combined on a dichroic mirror and directed along the axis of the supersonic flow. They entered the CRESU apparatus through a Brewster angle window, passed through another such window mounted on the back of the reservoir and co-propagated out through the throat of the Laval nozzle and along the axis of the flow, before leaving the vacuum chamber via a third Brewster angle window. LIF was gathered at a known dis- tance downstream of the Laval nozzle (usually 10-30 cm)by a UV-enhanced, optically fast telescope-mirror combination mounted inside the main vacuum chamber, focused through a slit to reduce scattered light and directed onto the photo- cathode of a UV-sensitive photomultiplier tube (EMI) after passing through a narrowband interference filter centred at 310 nm (bandpass 10 nm FWHM; Corion).The signals were accumulated, processed and analysed by the same procedures as before." The flows of reagent butene gases [Union Carbide: but-1- ene 99% ;(Z)-but-2-ene 95% ;(E)-but-Zene 95%] and carrier gases (He, Ar or N,; Air Liquide, U-grade) were taken directly from the cylinders and regulated by means of mass flow controllers (Tylan). Knowledge of the total gas density from aerodynamic (Pitot) measurements and the individual gas flows enabled the calculation of the butene concentration in the supersonic flow, essential for the kinetic measurements. Results and Discussion In our experiments on reactions of CN,'*l' the radicals were produced by photolysis of NCNO at 583 nm, a wavelength only just below the threshold for photodissociation of this molecule to CN + NO.23 Consequently, the CN radicals were produced exclusively in the lowest vibrational level, tr = 0, and with very little rotational excitation: any relax- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ation necessary to bring the radicals into thermal equilibrium with the bath gases would certainly be very rapid. In contrast to the situation just described, in the photolysis of H202 at 266 nm, 242 kJ mol-' of energy is available for redistribution among the relative translational and internal motions of the two OH radicals which are produced. Studies of the photodissociation dynamics at this wavelength24 show that almost all of the large excess energy appears as trans- lational recoil energy of the OH fragments, with the remain- der (ca. 10%) going into the rotations of the two fragments.Production of OH in vibrational levels above u = 0 could not be detected. Although the fractional yield of rotational excitation is quite low, the initial rotational energy is nevertheless well in excess of its thermal value, even that at room temperature, and the OH radicals are distributed quite widely over the rotational energy levels. An initial rotational temperature of 1500-1700 K has been determined.24 Furthermore, because the rotational energy levels of OH are quite widely spaced, rotational relaxation is comparatively slow.Fig. 1 displays two LIF spectra of OH recorded under identical conditions in the CRESU apparatus, but with differ- ent time delays between the photolysis and probe laser pulses. The first spectrum reflects the 'hot' rotational dis- tribution resulting from the photolysis of H202, the second the thermal distribution after time has been allowed for com- plete rotational relaxation. The second spectrum confirms that, at the lowest temperatures attainable in our experi-ments, about 90% of OH radicals occupy the lowest rovib- ronic energy level. In kinetics experiments, it was essential to allow time for complete rotational relaxation before fitting the LIF signals to a single exponential decay curve in order to find a pseudo-first-order rate constant (kist) for reaction under the conditions of that experiment, An example of a trace of the LIF signal decay from OH of the kind used to extract values of klsI is displayed in Fig.2(a) (a) I 281.2 281.4 281.6 281.8 282.0 282.2 282.4 excitation wavelength/nrn 281.2 281.4 281.6 281.8 282.0 282.2 282.4 excitation wavelength/nrn Fig. 1 (a) LIF spectrum of OH recorded at 23 K in He with a delay of 100 ns between the pulses from the photolysis and probe lasers. (b)LIF spectrum from the same gas mixture at the same temperature but with a delay of 40 ps between the pulses from the photolysis and probe lasers. 1.o it 0.5$ 0.0 E -0.5 -1.o I I I I I 7 h 8 I ' I I I I I I cv).- 56 4 -z4 C CJ,'Z 2 !A J 0 0 20 40 60 80 100 120 delay t ime/ps 18 16 r I4 12 "0 10F 28 6 4 2 t 1 0 0 1013 2x1013 3x1013 [(E)-but-2-ene] /molecu les cm -Fig.2 (a) First-order decay of LIF signal from OH in the presence of 2.6 x lOI3 molecules of (E)-but-Zene at 23 K in He, fitted to a single exponential decay, with residuals shown above. (b) First-order decay constants for OH at 23 K in He plotted against the concentration of (E)-but-2-ene. and its quality is typical of those obtained in the present experiments. These curves were fitted to a single exponential decay function using a non-linear least-squares fitting program employing the Levenburg-Marquardt algorithm. Examination of the residuals provided by the computer program confirmed that the decays were truly exponential.The values of kist obtained at a particlar temperature and for different concentrations of a given butene were plotted against the concentration of the butene as shown in Fig. 2(b). The gradients of these plots yielded the second-order rate constants which are listed in Tables 1-3. As the errors quoted for the second-order rate constants comprise the standard error resulting from an unweighted least-squares analysis of the klsI us. [butene] plots, multiplied by the Student's t-factor appropriate for the 95% confidence interval and the number of degrees of freedom. In the CRESU apparatus rate constants can be determined at ambient temperature (295 K) by increasing the pressure in the main chamber, causing a shock front to form, thus ensur- ing the recovery of the original reservoir temperature within the subsequent flow.The rate constants at room temperature obtained in this manner are compared in Table 4 with those for the reactions of OH with but-1-ene, (Z)-but-2-ene and (15)-but-2-ene measured in previous direct experiments.' 6-' The agreement is entirely satisfactory. The rate constants for all three reactions which we have studied increase monotonically as the temperature is lowered from 295 to 23 K. Although there is no theoretical justifica- tion, the temperature dependence of the rate constant, k, can, in each case but especially for the reactions with (2)-but-2- 1476 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Rate constants for the reaction of OH with but-1-ene obtained in the CREW apparatus at temperatures between 295 and 23 K carrier number of total density but-1-ene] rate constant T/K gas measurements molecules cm-3 /loi4 molecules /lo-" cm3 molecules-' s-' ~ 23 He 8 4.68 0.2-0.7 42.7 f5.6" 44 Ar 7 2.87 0.2-1.0 31.5 f4.0 75 N2 7 1.67 0.6-1.4 27.3 f1.6 170 N* 7 0.57 0.3-1.3 7.71 f1.06 295 Ar 7 45.6 5.9-23.4 3.49 f0.11 295 Ar 7 76.2 4.2-29.2 3.30 2 0.12 ~~ " Errors quoted are fta statistical error where t is the appropriate value of the Student's t-distribution for the 95% point. Table 2 Rate constants for the reaction of OH with (Z)-but-2-ene obtained in the CRESU apparatus at temperatures between 295 and 23 K carrier number of total density [(Z)-but-Zene] rate constant T/K gas measurements molecules cm-3 /loi4 molecules cm-3 /lo-" cm3 molecules-' s-I 23 He 9 4.73 0.04-0.3 38.9 f2.3" 44 Ar 6 2.90 0.1-0.3 32.8 f3.3 75 N2 7 1.67 0.4-1.3 30.2 f 1.4 170 N2 7 0.57 0.3-1.1 13.0 _+ 1.3 295 Ar 6 45.6 5.7-22.7 6.18 f0.57 " Errors quoted are fta statistical error where t is the appropriate value of the Student's t-distribution for the 95% point.Table 3 Rate constants for the reaction of OH with (E)-but-Zene obtained in the CRESU apparatus at temperatures between 295 and 23 K ~~~~ carrier number of total density [( E)-bu t-2-enel rate constant TIK gas measurements molecules /loi4 molecules cm-3 /lo-" cm3 molecules-' s-' 23 He 9 4.73 0.03-0.3 45.2 f3.2" 44 Ar 8 2.90 0.2-0.8 40.3 f4.4 75 N2 11 1.67 0.9-2.9 31.7 f2.4 170 N2 6 0.57 0.3-1.1 16.9 _+ 0.83 295 Ar 7 45.6 12.3-48.9 6.83 & 0.22 " Errors quoted are _+ ta statistical error where t is the appropriate value of the Student's t-distribution for the 95% point, ene and (E)-but-2-ene, be fitted to expressions of the form: stants do increase as the temperature is lowered and second that the rate constants for all three reactions appear to(k/10-" cm3 molecule-' s-l) = a exp[b(T/298K)] approach a common limiting value at the lowest tem-The values of a and b are: peratures which we have studied.a = 5.2 k0.4; b = -2.80 & 0.15 for but-1-ene, Both these features of the kinetics are also observed in the a = 4.7 f0.2; b = -2.06 f0.18 for (Z)-but-2-ene, and reactions of CN with C,H, and C,H," and the values of the a = 5.4 & 0.1; b = -2.12 k0.06 for (E)-but-2-ene, where the errors quoted correspond to a single standard devi- ation as estimated by the non-linear least-squares fitting pro- cedure.To facilitate comparison with the temperature dependence reportedg." for the rate constants for other reactions at ultra-low temperature, the rate constants for the reactions of OH with but-1-ene, (Z)-but-2-ene and (E)-but-Zene are dis- played in Fig. 3-5 on log-log plots. These diagrams illustrate two aspects of the results: first, the fact that the rate con- Table 4 Comparison of rate constants (k/lO-'' an3 molecule-' s-') for the reactions of OH radicals with butenes at 295 K I I IIIIII 1 I I L IIIref.but-1-ene isobutene (a-but-2-ene (E)-but-2-ene 10-1' 1 I0 100 1000this work 3.30 & 0.12 -6.2 f0.6 6.8 _+ 0.2 16 4.1 6.5 6.1 7.1 T/K 17 3.5 & 0.4 5.1 & 0.5 5.4 f 0.5 7.0 f0.7 Fig. 3 Rate constants for the reaction of OH with but-1-ene at dif- 18 2.96 & 0.19" -4.32 f0.41" -ferent temperatures. The filled circles show the results of the present 18 2.94 & 0.14' -4.26 _+ 0.25b -measurements in the CRESU apparatus and the line represents the 19 3.3 & 0.25 ---exponential fit described in the text, while the open circles show the results of Atkinson and Pitts" at temperatures between 298 and 424 'In 3 Torr He. In 20 Torr He. K. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 i I0 100 1000 T/KFig. 4 Rate constants for the reaction of OH with (Z)-but-2-ene at different temperatures. The filled circles show the results of the present measurements in the CRESU apparatus and the line rep- resents the exponential fit described in the text, while the open circles show the results of Atkinson and PittsI7 at temperatures between 298 and 425 K. rate constants for those reactions at the lowest temperatures studied (26 K) are very similar to the ones we find here at 23 K for the reactions of OH with but-1-ene, (Z)-but-2-ene and (E)-but-Zene, although the latter reactions are appreciably slower at room temperature. The magnitude of these low- temperature rate constants, and the negative dependence of the rate constants on temperature, provide strong evidence for the absence of any maximum of electronic potential energy along the minimum-energy path leading from separat- ed reagents to the radical adduct.At the lowest temperatures, it appears as if the only factor limiting the rate of reaction is the ability of the long-range attractive potential to bring the reagents together. ‘Capture’ theories which treat reactions occurring over attractive potentials have been developed by Clary and co-workers’ for ion-molecule and radical- radical reactions. It appears that the reactions of unsaturated hydrocarbons with strongly electronegative free radicals like OH and CN comprise another case where capture theories could be applied. Two factors may cause the rate constants of these reactions to fall below simple capture values at higher energies.First, 1 I I I 1 Ill 1 I I I I I11lo-” ’ 1 I0 100 1000 T/K Fig. 5 Rate constants for the reaction of OH with (E)-but-Zene at different temperatures. The filled circles show the results of the present measurements in the CRESU apparatus and the line rep- resents the exponential fit described in the text, while the open circles show the results of Atkinson and Pitts17 at temperatures between 298 and 424 K. at higher reagent energies adiabatic potentials develop maxima at shorter reagent separations where the chemical contribution to the intermolecular potential can no longer be neglected. Based on this idea, Klippenstein and Kimllb have provided a satisfactory explanation of the negative tem- perature dependence of the rate constants for the reaction between CN radicals and O2 above 50 K.Alternatively, at low temperatures, reaction may be aided by the formation of weakly bound, energised, van der Waals complexes which survive sufficiently long for the system to explore the potential-energy surface and find the correct configuration for the reaction. At higher temperatures, on the other hand, kinetic energies increase, collisions become direct, and reac- tion only occurs for those collisions with a favourable orien- tation at impact. Summary This paper reports the first kinetic measurements on elemen- tary reactions of the OH radical at ultra-low temperatures (<80 K).The experiments use the PLP-LIF method in the ultra-cold environment provided by expansion through a Lava1 nozzle in a CRESU apparatus. Rate constants are reported for the reactions of OH with but-1-ene, (Z)-but-2- ene and (E)-but-2-ene at temperatures down to 23 K. The rate constants for all three reactions increase monotonically as the temperature is lowered reaching very similar values at 23 K. It is suggested that, at the lowest temperatures of these studies, reaction occurs whenever the long-range attractive potential between the reagents brings them together, possibly because the formation of an energised van der Waals complex allows the reagents to find a favourable orientation for creation of the hydroxyalkyl radical which is the product of these reactions.We acknowledge funding from the CEC under the Science Plan (Contract No. SC*CT89-0261), as well as from the GDRs ‘Physicochimie des Molecules Interstellaires ’ and ‘Dynamique des Reactions Moleculaires’ programmes. The lasers were borrowed from the SERC Laser Loan Pool at the Rutherford-Appleton Laboratory, for which we express thanks. We are grateful to Solvay Interox of Germany for donating the 85% H20, used in these experiments. References D. 0. Ham, D. W. Trainor and F. Kaufman, J. Chem. Phys., 1970,53,4395. (a) W. T. Rawlins, G. E. Calendonia and R. A. Armstrong, J. Chem. Phys., 1987, 87, 5209; (b) H. Hippler, J. Rahn and J. Troe, J. Chem. Phys., 1990,93,6560. (a) I. R. Sims and I. W. M.Smith, Chem. Phys. Lett., 1988, 151, 481; (b) I. R. Sims and I. W. M. Smith, J. Chem. SOC., Farday Trans., 1993,89, 1. (a)M. J. Frost, P. Sharkey and 1. W. M. Smith, Faraduy Discuss. Chem. SOC., 1991,91, 305; (b)P. Sharkey and I. W. M. Smith, J. Chem. SOC., Faraday Trans., 1993, 89, 631; (c) M. J. Frost, P. Sharkey and I. W. M. Smith, J. Phys. Chem., 1993,89,12254. (a) J. J. Andrew, D. C. McDermott, S. P. Mills and C. J. S. M. Simpson, Chem. Phys., 1991, 153, 247; (b) G. J. Wilson, M. L. Turnidge, A. S. Solodukhin and C. J. S. M. Simpson, Chem. Phys. Lett., 1993,207, 521. (a)B. R. Rowe, G. Dupeyrat, J. B. Marquette and P. Gaucherel, J. Chem. Phys., 1984, 80, 4915; (b) B. R. Rowe and J. B. Mar-quette, Int. J. Mass Spectrom. Ion Processes, 1987,80,239.C. Rebrion, J. B. Marquette and B. R. Rowe, J. Chem.Phys., 1989,91,6142. (a) B. R. Rowe, J. B. Marquette and C. Rebrion, J. Chem. Soc., Faraday Trans. 2, 1989,85, 1631; (b) B. R. Rowe in Rate Coefl- cients in Astrochemistry, ed. T. J. Millar and D. A. Williams, Kluwer, Dordrecht, 1988, p. 135. 1478 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 9 (a) I. R. Sims, J-L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, B. R. Rowe and I. W. M. Smith, J. Chem. Phys., 1992, 97, 8798; (b) I. R. Sims, J-L. Queffelec, A. Defrance, C. Rebrion- Rowe, D. Travers, P. Bocherel, B. R. Rowe and I. W. M. Smith, 18 19 A. R. Ravishankara, S. Wagner, S. Fischer, G. Smith, R. Schiff, R. T. Watson, G. Tesi and D. D. Davis, Int. J.Chem. Kinet., 1978, 10, 783. W. S. Nip and G. Paraskevopoulos, J. Chem. Phys., 1979, 71, 10 11 12 J. Chem. Phys., 1994,100,4229. 1. R. Sims, J-L. Queffelec, D. Travers, B. R. Rowe, L. B. Herbert, J. Karthauser and I. W. M. Smith, Chem. Phys. Lett., 1993, 211, 461. (a) D. C. Clary, T. S. Stoecklin and A. G. Wickham, J. Chem. Soc., Faruday Trans., 1993, 89, 2185; (b) S. J. Klippenstein and Y-W. Kim, J. Chem. Phys., 1993,99,5790. E. Herbst, H-H. Lin, D. A. Howe and T. J. Millar, Mon. Not. R. 20 21 22 23 2 170. H. W.Biermann, G. W. Harris and J. N. Pitts Jr., J. Phys. Chem., 1982,86,2958. K. Hoyermann and R. Sievert, Ber. Bunsenges. Phys. Chem., 1983,87,1027. R. Atkinson, Chem. Rev.,1986,86,69. I. Nadler, M. Noble, H.Reisler and C. Wittig, J. Chem. Phys., 1985,82,2608. 13 14 15 Astron. Sue., 1994, in the press. R. Atkinson, D. L. Baulch, R. A. Cox, R. F. Hampson Jr., J. A. Kerr and J. Troe, J. Phys. Chem. Ref. Data, 1992,21, 1125. D. L. Baulch, C. J. Cobos, R. A. Cox, C. Esser, P. Frank, Th. Just, J. A. Kerr, M. J. Pilling, J. Troe, R. W. Walker and J. Warnatz, J.Phys. Chem. Ref. Data, 1992,21,411. I. R. Sims and I. W. M. Smith, J. Chem. Soc., Faraday Trans. 2, 24 25 (a)S. Klee, K-H. Gericke and F. J. Comes, J. Chem. Phys., 1986, 85,40; (b)K-H. Gericke, S. Klee, F. J. Comes and R. N. Dixon, J. Chem. Phys., 1986, 85, 4463; (c) G. J. Germann and J. J. Valentini, Chem. Phys. Lett., 1989,157, 51. (a) D. C. Clary in Rate Coeflcients in Astrochemistry, ed. T. J. Millar and D. A. Williams, Kluwer, Dordrecht, 1988, p. 1; (b) D. C. Clary, Annu. Rev. Phys. Chem., 1990,41,61. 1989,85,915. 16 17 E. D. Morris, Jr. and H. Niki, J. Phys. Chem., 1971,75,3640. R. Atkinson and J. N. Pitts Jr., J. Chem. Phys., 1975,63,3591. Paper 4/00806E; Received 9th February, 1994