Gas Phase Production of Complex Hydrocarbons, Cyanopolyynes, and Related Compounds in Dense Interstellar Clouds

The pseudo-time-dependent gas-phase model of Herbst and Leung (1986) for the chemistry of dense interstellar clouds is extended to include hydrocarbons as complex as C9H2 and CH3C6H, and organonitrogen compounds as complex as HC9N and CH3C7N. About 600 reactions are added to the model. Most of the reaction rate coefficients and product branching ratios are estimated based on analogous reactions involving smaller species or on theoretical considerations. Calculated abundances are compared with observations in TMC-1, showing generally good agreement. It is found that the observed C(n)H abundances in TMC-1 can be understood in terms of gas-phase production schemes, although the model contains predictions at early time that are too large. The calculated peak abundances for cyanopolyynes show a small but increasing tendency to be low compared with observation as molecular size increases.