Deuteration of CHn+ (n=3-5) in collisions with HD measured in a low-temperature ion trap

Deuteration of small hydrocarbon ions CHn+ via H-D exchange (n = 3-5) has been studied in a 22 pole ion trap at a nominal temperature of 15 K. Sequential deuteration from CH3+ to CD3+ is very fast if one uses pure HD as the target gas. Rate coefficients have been measured to be 1.65 x 10(-9), 1.59 x 10(-9), and 1.50 x 10(-9) cm(3) s(-1). If, however, CH3+ is relaxed in p-H-2 containing traces of HD, the rate coefficient for isotope enrichment is significantly smaller, (4 +/- 2) x 10(-10) cm(3) s(-1). This important result is most probably due to symmetry selection rules influencing this reaction. The ions CH4+ and CH5+ are not observed, within the experimental uncertainties, to exchange H atoms for D atoms at all. Upper limits for the rate coefficients for forming CH3D+ and CH4D+ are 1 x 10(-12) and 5 x 10(-18) cm(3) s(-1), respectively. Hydrogen or deuterium abstraction in collisions of CH4+ with HD occurs with a sum rate coefficient of 4.5 x 10(-10) cm(3) s(-1). Surprisingly, the more exoenergetic and statistically favored product CH4D+ is formed only in 1/3 of the reactive collisions, while CH5+ dominates with 2/3. The results are discussed on the basis of the formation of long-lived collision intermediates, open- and closed-shell ions, and barriers along the reaction path. All experimental data clearly indicate that conservation of total nuclear spin plays an important role in these low-temperature chemical reactions involving identical nuclei. Implications of this laboratory work to isotopic fractionation in astrophysical environments are discussed.