| More than 120 gas-phase molecules, predominantly organic in nature and ranging up to 13 atoms in size, have been observed in interstellar space to date. Their formation is attributed to a large variety of gas-phase and some grain-surface reactions. A few specific gas-phase processes pertaining to interstellar clouds are studied and incorporated into large chemical networks that aim to reproduce the observed molecular abundances.;The effects of variations in the gas-phase carbon-to-oxygen elemental abundance ratio and the absolute gas-phase carbon and oxygen elemental abundances on calculated molecular concentrations are studied for three gas-phase chemical models of dense interstellar clouds. Both the C and O elemental abundances are varied from their "low metal" values, in which C/ O = 0.42. These variations have a large and time-dependent effect on calculated molecular concentrations for all three models. An order-of-magnitude agreement with observed abundances for up to 90% of the species observed in two typical dark clouds, namely TMC-1 and L134N, is achieved.;A statistical calculation of the rate coefficients for radiative attachment of an electron to small linear carbon clusters containing 4 to 9 atoms is presented. We conclude that for molecules with 6 or more C atoms, the attachment occurs on every collision at the low temperatures of diffuse interstellar clouds.;The attempt to understand the temperature dependence of the HNC/HCN abundance ratio in interstellar clouds has been long-standing and indecisive. We report studies of the products and interstellar importance of the reactions between C and NH2 and between N and CH2. We find that although these reactions do lead initially to the products suggested by astronomers, there is so much excess energy available in both reactions that the HCN and HNC products are able to undergo efficient isomerization reactions after production.;The possibility of nitrogen isotopic fractionation due to ion-molecule exchange reactions involving the most abundant N-containing species in dense interstellar clouds has been explored. We find that exchange reactions between N-atoms and N-containing ions influence the fractionation the most although the extent of fractionation is currently too small to be readily detectable. |
