Rate Coefficients and Product Ion Distributions for Reactions

ion provides the dominant channel of reaction (6): C0 2 • C02+ + CH4 CH4+ 42 C0 2 , (6a) CH4+ + C02 -> (C0 2 -CH 4 +)* , (21) (C02 • CH4+)* -> CO2H+ + CH3 , (22) (C02 • CH4+)* + C02 -> C02 • CH4+ + C 0 2 , (23) C02H+ + 2 C02 -> (C02)2H+ + C0 2 . (24) In this mechanism the formation of CO2 • CH4 + occurs by collisional stabilization of the intermediate reaction complex (C02 • CH4)* rather than via the displacement channel in reaction (6). The reaction sequence (21) and (22) leading to the intermediate product C0 2 H + has been investigated by Harrison and Blair [16] by means of pulsed source mass spectrometry and a rate coefficient k21-22 =1 -2 ( — 9) was obtained. The reaction is so rapid that CH4 ions would not be observed in our experiments. On the other hand, a signal at mass number 45, corresponding to CO2H" has been observed with about 1 % of t he total ion intensity. This magnitude is calculated to be consistent with the outlined mechanism provided the rate coefficient for the association reaction (24) is &24 ä! 1 (— 28). Unless this rate coefficient is demonstrated to be much smaller we are inclined to believe that this second mechanism applies. W a t e r V a p o r (I.P. 12.6 eV) From the exponential decrease of CO2 dimer ion intensity with increasing H 2 0 flow we determine the rate coefficient associated with reaction (3) as &3=1.6(—9). This value may represent a lower limit because of the possibility that after preparing the H 2 0 /C0 2 mixture losses of water occurred on the walls of the mixing vessel or the inlet line leading to the reaction chamber. The products expected from this reaction are either H20+ or CO2 • H 2 0 + , or both. A signal at mass number 18 was not detected. The M 62 signal already present from the water impurity did also not rise significantly when the H2O concentration was increased. The most prominent increase occurred at M 37 corresponding to the ion H 30+ • H2O. This ion certainly is a secondary product. The route to its formation cannot be deduced from this experiment. The rate coefficient £3 can be used to calculate the density of impurity water vapor from the quantity k3n\ = 3.6 • 10 derived earlier from Figure 1. One finds n\ = 2.2 • 10 cm 3 corresponding to a vapor pressure of about 7 • 10 5 torr. The rate coefficient for the reaction C02 with water vapor is then determined from the value for a = kzriilkiy2 which was obtained from the slope of the straight line in Fig. 2 as a = 0.021. The result is k2 = 2.1 ( — 9), a value in good agreement with the recent determination by Karpas, Anicich and Huntress [17] who employed the ion cyclotron resonance technique and found k2 = 2.8 ( — 9). ADO theory predicts k2 = 2.3 ( 9 ) . S u l f u r D i o x i d e (I.P. 12.34 eV) Detailed results for this rapid reaction were reported elsewhere [18]. Both charge transfer and molecular displacement products were found. The data are entered in Table 1 for comparison with the other reactions. O x y g e n (I.P. 12.06 eV) Normalized ion intensities observed upon the addition of oxygen to the reaction chamber are displayed in Figure 6. The rate coefficient for the reaction, obtained from the exponential decay of CO2 • C0 2 + ion intensity as a function of 0 2 partial pressure is &g = 1.85 (— 10). This may be compared with a value of 1.5 (—10) reported by Sieck [2]. The reaction appears to proceed both by charge transfer and by molecular displacement, but the formation of CO2 • 02 by third body association from 0 2 + must also be included: C02 • C0 2 + + 0 2 X 02+ + 2 C02 X C02 • 0 2 + + C02 , (8) 02+ + 2 C02 -> C02 • 0 2 + + C02 . (25) Adams et al. [19] reported k25 = 2.3 ( — 29) from flowing afterglow experiments at 200 K for Helium as carrier gas, whereas Sieck [2] gives 4.6 (—30) at 300 K for CO2 as the third body. If one assumes that 02+ is the only product from reaction (8), a rate coefficient of &25 = 1.4 (— 29) is required to approximately represent the 0 2 + ion intensities in Figure 6. On the other hand, if Sieck's value for k25 is employed the 0 2 + ion intensity is well represented with ^ ^ 8 = 0.6. The observation of 0 2 + in C02 free from oxygen provides an opportunity for an independent determination of £25 • Figure 7 shows intensities for 02 and CO2 • 02~ obtained as a function of CO2 pressure in the absence of additional reactants. The A. B. Rakshit and P. Warneck • Reactions of C02 • C0 2 + Ions with Neutral Molecules 1417 02 PARTIAL PRESSURE (10"3 TORR) Fig. 6. R e a c t a n t a n d product ion intensities for the reaction of CO2 dimer ions with oxygen . Solid lines were calculated as described in the text . T h e dashed line gives the calculated CO2 • 0 2 + ion intensity for the case that this ion does not undergo a subsequent reaction with H 2 0 impurity.