A sensitivity study of the neutral-neutral reactions C + C3 and C + C5
  in cold dense interstellar clouds
  Chemical networks used for models of interstellar clouds contain many
reactions, some of them with poorly determined rate coefficients and/or
products. In this work, we report a method for improving the predictions of
molecular abundances using sensitivity methods and ab initio calculations.
Based on the chemical network osu.2003, we used two different sensitivity
methods to determine the most important reactions as a function of time for
models of dense cold clouds. Of these reactions, we concentrated on those
between C and C3 and between C and C5, both for their effect on specific
important species such as CO and for their general effect on large numbers of
species. We then used ab initio and kinetic methods to determine an improved
rate coefficient for the former reaction and a new set of products, plus a
slightly changed rate coefficient for the latter. Putting our new results in a
pseudo-time-dependent model of cold dense clouds, we found that the abundances
of many species are altered at early times, based on large changes in the
abundances of CO and atomic C. We compared the effect of these new rate
coefficients/products on the comparison with observed abundances and found that
they shift the best agreement from 3e4 yr to (1-3)e5 yr.
