J . Phys. Chem. 1988, 92,4941-4951
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Ion-Molecule Reactions Involving H,O+, H20+, and OH+ at Thermal Energy R. J. Shul, R. Passarella, L. T. DiFazio, Jr., R. G. Keesee, and A. W. Castleman, Jr.* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania I6802 (Received: November 17, 1987)
The rate coefficients for a variety of reactions involving OH', HzO+,and H30+with CSz, H2S, S02, NO, NOz, N20, N,, 02,CH4, COz, CO, NH,, and H2 are obtained with a seiected ion flow tube. The product channels include proton transfer, charge transfer, atom abstraction, and association. Most of the reaction rates are fast and have high reaction efficiencies. Reaction of OH' with NO2 is found to produce both NO2+and NO'; even though both reaction channels are also exothermic with HzO+, only NO2+ is produced. H 3 0 + is found to undergo association reactions with SO2 and N 2 0 and H 2 0 + does so with COz and N20. The reaction of H20+with NzO also involves a second-order process in addition to the association pathway.
Introduction Information on the rates and mechanisms of reactions of H,O+ ions is vital to a variety of Such data find particular application in modeling atmospheric processes.4-6 For example, Viggiano and co-workers' have presented reaction schemes for the formation of H30+, H20+, and OH+ in dense and interstellar clouds. Laboratory studies to measure the thermodynamic and kinetic properties of various reactions involving these species are of significant value to the astrophysical community. Reactions of H,O+ ions frequently involve proton-transfer mechanisms which often have familiar counterparts in solution chemistry. Comparison of gas-phase and condensed-phase chemistry can provide insight into the effects solvents have on the chemical properties and reactivity of the species of interest.* Various experimental techniques are now available which permit the production, chemical manipulation, and detection of solvated ions in the gas phasee9 Furthermore, gas-phase measurements of the rate coefficients and equilibrium constants for protontransfer reactions can be used to evaluate the relative proton affinities of the reactant species.1s13 Reactions involving the ionic species OH', HzO', and H30+ are considered in this study. Thermal energy rate coefficients are measured and reaction pathways are identified. Charge transfer, proton transfer, atom abstraction, and association products are observed. Experimental Section The selected ion flow tube (SIFT) apparatus used in our laboratory has been described previously;14therefore, only the details pertaining to the present study are discussed. The reactant ions (OH+, H20+, and H 3 0 + ) are produced by electron impact on a free-jet expansion of a H e H 2 0 gas mixture into vacuum. Helium (1) Herbst. E.: Klemoerer. W. AsfroDhvs. J. 1973. 185. 505. (2j Mackay, 6. I.; Tanner; S. D.; Hdpkkon, A. C.; Bohme, D. K. Can. J . Chem. 1979, 57, 1518. (3) Smith, D.; Adams, N. G. Astrophys. J . 1977, 217, 741. (4) Black, J. H.; Dalgarno, A. Astrophys. J.,Suppl. Ser. 1977, 34, 405. ( 5 ) Smith. D.:Adams. N. G. Tonics in Current Chemistrv 89. SDrinner. Vehag: Berlin, 1980; Vol. 1, p 1. . (6) Ferguson, E. E.; Fehsenfeld, F. C.; Albritton, D. L. In Gas Phase Ion Chemistry, Bowers, M. T., Ed.; Academic: New York, 1979; Vol. 1, p 45. (7) Viggiano, A. A,; Howorka, F.; Albritton, D. L.; Fehsenfeld, F. C.; Adams, N. G.; Smith, D. Asfrophys. J. 1980, 236, 492. (8) Castleman, Jr., A. W.; Keesee, R. G. Arc. Chem. Res. 1986, 19, 413. (9) Castleman, Jr., A. W.; Keesee, R. G. Chem. Reu. 1986, 86, 589. (10) Bohme, D. K.; Mackay, G. I.; Schiff, H. I. J. Chem. Phys. 1980, 73, 4976. (11) Fennelly, P. F.; Hemsworth, R. S.; Schiff, H. I.; Bohme, D. K. J. Chem. Phys. 1973,59, 6405. (12) Bohme, D. K.; Hemsworth, R. S.; Rundle, H. W.; Schiff, H. I. J. Chem. Phys. 1973, 58, 3504. (13) Hemsworth, R. S.; Rundle, H. W.; Bohme, D. K.; Schiff, H. I.; Dunkin, D. B.; Fehsenfeld, F. C. J. Chem. Phys. 1973, 59, 61. (14) Shul, R. J.; Upschulte, B. L.; Passarella, R.; Keesee, R. G.; Castleman, Jr., A. W. J. Phys. Chem. 1987, 91, 2556.
0022-3654/88/2092-4947$01.50/0
is bubbled through a glass cell containing deionized H 2 0and the gas is expanded at 1-2 atm through a 75-r-diameter orifice. The ions are focused into the SIFT quadrupole mass spectrometer, whereupon the desired species is mass selected from the distribution of ions and injected into the flow tube. The injected ions are thermalized by collisions with the carrier gas and, thereafter, are allowed to interact with the neutral reactant gas which is added at an appropriate downstream 10cation.l~ The rate coefficients are obtained in the conventional manner by monitoring, with a quadrupole mass spectrometer, the intensity of the reactant ion of interest as a function of the concentration of the neutral reactant gas added. Operation of the SIFT quadrupole under high resolution is necessary to mass filter the three reactant ions produced in the source. However, such settings were found to reduce the injection efficiency of the desired ionic species to a point where reliable kinetic data could not be obtained. Therefore, the resolution was somewhat reduced, resulting in the simultaneous injection of OH', H 2 0 + , and H30+ into the flow tube. Product channels are identified by setting the mass resolution of the SIFT quadrupole mass filter to maximize the transmission of the reactant ion of interest. Residual amounts of one or both of the other ions usually persisted, thereby preventing reliable determination of branching ratios and establishing larger error limits for the measured rate coefficients. The gases used in these studies are commercial grade Union Carbide prepurified (99.9% minimum) with no further purification. The vapors of distilled CS2, approximately 400 Torr at room temperature, are used for the reactions with OH+ and H30+. The majority of the reactions studied possess reaction rates which are fast; Le., they proceed at near collision rate. In these cases, mixtures of the neutral reactant gas with the carrier gas, He, are employed to achieve a convenient rate of decrease of the reactant ion intensity with the flow rate through the reactant gas inlet. Residual impurities are condensed out of the He carrier gas by passing it through a sequence of three traps containing molecular sieves that are placed in liquid nitrogen.
Results and Discussion Rate coefficients are determined from the slope of a semilogarithmic plot of the reactant ion intensity versus the flow rate of the neutral reactant gas14 as shown in Figure 1 for the reactions of OH', H20+,and H 3 0 + with H2S. The reproducibility of the data obtained in this manner has been determined to be within 3 5 % under constant experimental conditions. The rate ccefficients measured here are acquired under optimum operating conditions as determined by extensive diagnostic studies of the a p p a r a t ~ s . ' ~ * ' ~ (15) Upschulte, B. L.; Shul, R. J.; Passarella, R.; Keesee, R. G.; Castleman, Jr., A. W. Int. J. Mass Spectrom. Ion Processes 1987, 75, 27. (16) Upschulte, B. L.; Shul, R. J.; Passarella, R.; Keesee, R. G.; Castleman, Jr., A. W. Int. J. Mass Specrrom. Ion Processes, submitted for publication.
0 1988 American Chemical Society
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Shul et al.
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
TABLE I: Rate Constants and Reaction Efficiencies for the Thermal Energy Reactions Involving OH', H,O', and HqO+ reaction -AH, k , cm3/s efficiency kcalimol techniqueu 1.5 x 10-9 0.83 50.8, 66.9 SIFT 1. OH' + CS2 HCS2+, CS2' 3.6 X lo-'' 0.6 0.20 HCS2' + H 2 0 2. H 3 0 + + CS2 SIFT 1.6 X 0.89 53.9, 57.6 SIFT 3. OH' + H2S H3S', H2S' 2.1 x 10-9 H3S' (40%), H2S+ (60%) SIFT 2.82 x 10-9 H3S' (39%), H2S' (61%) ICR 1.5 x 10-9 4. H2O' + H2S H$', H2S' 0.88 28.2, 48.4 SIFT 1.8 x 10-9 H3S' (43%), H2S' (54%), H30' (3%) SIFT 2.18 x 10-9 H3S' (27%), H2S' (31%), H30' (42%) ICR 1.4 x 10-9 0.82 3.7 SIFT 5. H30' + H2S H3S' + H 2 0 1.9 x 10-9 FA H3S+ + H 2 0 1.9 x 10-9 H3S' H20 FA 2.1 x 10-9 HS02', SO2' 1.O 45.3, 16.1 6. OH' + SO2 SIFT 2.0 x 10-9 1 .o SIFT 7. H20' + SO2 HSO,', SO2' 19.6, 6.9 4.3 x 10-28 cm3 SIFT 8. H30' + SO2 (H3O-SO2)' 8.0 x 10-10 HNO', NO' 0.86 9.OH+ + NO 10.7, 86.2 SIFT 6.7 X lo-'' NO' + OH SIFT 9.7 x 10-'0 SIFT HNO' (63%), NO' (37%) 1.3 x 10-9 HNO' (
