Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004apj...617..685a&link_type=abstract
The Astrophysical Journal, Volume 617, Issue 1, pp. 685-692.
Astronomy and Astrophysics
Astronomy
12
Astrochemistry, Ism: Molecules, Methods: Laboratory
Scientific paper
Deuteration of small hydrocarbon ions CH+n 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 CH+3 to CD+3 is very fast if one uses pure HD as the target gas. Rate coefficients have been measured to be 1.65×10-9, 1.59×10-9, and 1.50×10-9 cm3 s-1. If, however, CH+3 is relaxed in p-H2 containing traces of HD, the rate coefficient for isotope enrichment is significantly smaller, (4+/-2)×10-10 cm3 s-1. This important result is most probably due to symmetry selection rules influencing this reaction. The ions CH+4 and CH+5 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 1×10-12 and 5×10-18 cm3 s-1, respectively. Hydrogen or deuterium abstraction in collisions of CH+4 with HD occurs with a sum rate coefficient of 4.5×10-10 cm3 s-1. Surprisingly, the more exoenergetic and statistically favored product CH4D+ is formed only in 1/3 of the reactive collisions, while CH+5 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.
Asvany Oskar
Gerlich Dieter
Schlemmer Stefan
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