New H and H2 reactions with small hydrocarbon ions and their roles in benzene synthesis in dense interstellar clouds

This paper summarizes the most recent knowledge concerning normal and three-body association ion-molecule reactions between small hydrocarbon ions (CmHn+; in = 1-6, n = 0-9) and the neutral reactants H and H-2. Selected radiative association reaction rate coefficients have been calculated and estimated based on three-body laboratory results. Some of the normal and radiative association rate coefficients have been incorporated into our "new standard" model of dense cloud interstellar chemistry. We find that atomic H provides an insufficiently appreciated mechanism to increase the level of saturation of CmHn+ ions, with m > 3 in dense interstellar clouds despite the fact that the H/H-2 abundance ratio is only 10(-4). New model results show in particular that a significant abundance of the basic aromatic cyclic molecule benzene (c-C6H6) can now be produced in dense clouds.