Ion–molecule reactions of ArN+2 with simple aliphatic hydrocarbons at thermal energy

Abstract

The product ion distributions and rate constants are determined for ion–molecule reactions of ArN+2 with C2Hn (n=2,4,6) and C3Hn (n=6,8) by using a thermal ion–beam apparatus. Although charge-transfer channels leading to parent ions and/or fragment ions are found, no displacement reaction leading to ArCmH+n and N2CmH+n is detected. A comparison of the product ion distributions with breakdown patterns of the parent ions suggests that fragment ions, formed through cleavage of C–H and/or C–C bonds, are produced via near-resonant ionic states in the 13.1–13.4 eV range. The branching ratios of parent ions for C2H4 (68%) and C3H6 (20%) are larger than those for C2H6 (5%) and C3H8 (5%). The large branching ratios of the parent ions for the unsaturated hydrocarbons are explained as due to a strong interaction of a vacant orbital of ArN+2 with the highest occupied πC=C orbital of the unsaturated hydrocarbons which induces nonresonant charge transfer. The total rate constant for C2H2 is 6.8×10−10 cm3 s−1, while those for C2Hn (n=4,6) and C3Hn (n=6,8) are in the range (8.5–9.8)×10−10 cm3 s−1. The former and the latter values correspond to 69% and 77%–90% of the calculated values from Langevin or average dipole orientation (ADO) theory. The smaller kobs/kcalc ratio for C2H2 is attributed to the lack of near-resonant ionic states with favorable Franck–Condon factors for ionization.