ION-MOLECULE REACTIONS OF CO2+ WITH SIMPLE ALIPHATIC-HYDROCARBONS AT THERMAL-ENERGY

A thermal ion-beam apparatus has been used for studying ion/molecule reactions of CO2+ with CH4, C2H(n) (n = 2,4,6), and C3H(n) (n = 6,8). The product ion distributions and rate constants are determined. Both hydrogen-atom transfer and charge transfer are observed as product channels for CH4 with branching ratios of CO2H+ (72 +/- 1%) and CH4+ (28 +/- 1%), whereas only charge-transfer channels leading to parent ion and/or fragment ions are found for C2H(n) (n = 2,4,6) and C3H(n) (n = 6,8). A comparison of the product ion distributions with breakdown patterns of the parent ions leads us to conclude that charge-transfer products are predominantly formed through near-resonant (pre)dissociative states, which are 0.2-0.5 eV below the resonant states. The formation of a small amount of parent ion due to non-resonant charge transfer is found for unsaturated hydrocarbons (C2H4 and C3H6). This is explained as being due to a strong interaction between a vacant orbital of CO2+ and the highest occupied pi(C=C orbital of the reagent molecule. The total rate constant for C2H2 is 0.56 x 10(-9) cm3 s-1, while those CH4, C2H(n) (n = 4,6), and C3H(n) (n = 6,8) are in the range (0.78-0.99) x 10(-9) cm3 s-1. The former and the latter values correspond to 51% and 60-87% respectively of the calculated values from Langevin theory. The smaller k(obs)/k(calc) ratio for C2H2 is attributed to the lack of near-resonant ionic states with favorable Franck-Condon factors for ionization.