Ion/molecule reactions of CO 2+ with simple aliphatic hydrocarbons at thermal energy

Abstract

A thermal ion-beam apparatus has been used for studying ion/molecule reactions of CO 2 + with CH 4, C 2H n ( n = 2, 4, 6), and C 3H 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 CH 4 with branching ratios of CO 2H + (72 ± 1%) and CH 4 + (28 ± 1%), whereas only charge-transfer channels leading to parent ion and/or fragment ions are found for C 2H n ( n = 2, 4, 6) and C 3H 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 (C 2H 4 and C 3H 6). This is explained as being due to a strong interaction between a vacant orbital of CO 2 + and the highest occupied π C=C orbital of the reagent molecule. The total rate constant for C 2H 2 is 0.56 × 10 −9 cm 3 s −1, while those for CH 4, C 2H n ( n = 4, 6), and C 3H n ( n = 6, 8) are in the range (0.78–0.99) × 10 −9 cm 3 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 C 2H 2 is attributed to the lack of near-resonant ionic states with favorable Franck—Condon factors for ionization.