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Laboratory studies of isotope fractionation in the reactions of C/+/ and HCO/+/ with CO - Interstellar implications
Smith, D.; Adams, N. G.
AA(Birmingham, University, Birmingham, England), AB(Birmingham, University, Birmingham, England)
Astrophysical Journal, Part 1, vol. 242, Nov. 15, 1980, p. 424-431. Research supported by the Science Research Council. (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
Carbon Monoxide, Chemical Fractionation, Interstellar Gas, Ionic Reactions, Molecular Ions, Carbon 13, Gas Temperature, Molecular Clouds, Oxygen 18, Reaction Kinetics
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The fractionation of C and O isotopes in reactions of C(+) and HCO(+) with CO is investigated in relation to the importance of isotope fractionation in interstellar molecular clouds. The SIFT technique was used to determine rates and enthalpies for the reactions (C-13)(+) + (C-12)O yields (C-12)(+) + (C-13)O, HCO(+) + (C-13)O yields H(C-13)O(+) + CO and HCO(+) + C(O-18) yields HC(O-18)(+) + CO at temperatures from 80 to 510 K. The rates of the forward reactions are found to exceed those of the backward reactions at the lowest temperatures, and estimates of the exoergicities from the ratio of the rate constants reveal that they are in agreement with the zero point energy differences between reactants and products. Extrapolation of the data to the temperatures of dark and dense interstellar clouds indicates that C-13 and O-18 isotope fractionation can account well for the observed enhancement of (C-13)O abundance and to some extent for that of H(C-13)O(+).

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