IR spectra of anthracene and pyrene derivatives, serving as models for isolated, linear and isolated, compact PAHs, respectively, have been calculated using ab-initio quantum mechanical methods. The separate and combined effects of ionization and multiple dehydrogenation have been studied. This study confirms and refines the trends of our preliminary paper on the smallest possible PAH, naphthalene. If small PAHs are responsible for any UIR bands, they should be ionized and partially dehydrogenated, with a few triple bonds at the periphery of the carbon skeleton. In the appendix are given the complete IR spectra of all the isomers of the derivatives of anthracene and pyrene calculated for the purpose of this study. Tables I are for anthracene and Tables II for pyrene. Positions of the the missing hydrogens in the dehydrogenated species are referred as in Figures 1 and 2 of the original publication.

The hydrogen exchange reaction H+CNH->HCN+H may be a key step in gas-phase interstellar nitrogen chemistry. It is one of the reactions supposed to cause increasing HNC depletion with increasing temperature in dense interstellar clouds. In this paper we report the results of extensive ab-initio calculations on the H+CNH<->H+HCN system that partially confirm this hypothesis. It is shown that both forward and reverse reactions possess activation barriers. However, the activation energy of the H+CNH channel (4.2+/-1.0kcal/mol.) is four times smaller than for the endothermic HCN+H path. Calculations on the rate of the forward reaction show that tunneling under the entrance channel barrier allows a small rate coefficient at the temperatures under 100K and that the rate coefficient increases steadily with increasing temperature for T>100K.

For the HNC/HCN interconversion we show that the push-pull hydrogen exchange reaction H+CNHright harpoon over leftHCNHright harpoon over leftHCN+H is favoured over internal isomerization; the formation of H2CN or CNH2 followed by rearrangement to HCNH and subsequent elimination are more energy demanding processes. Both push-pull forward and reverse reactions present activation barriers. However, the activation energy on the H+CNH entrance channel (4.2±1.0 kcal/mol) is four times smaller than on the HCN+H path. As a consequence, it can be anticipated that there will be a range of temperatures where the H+CNH reaction will be efficient while the reverse HCN+H process is still inhibited. This process, much less endothermic than internal isomerization, should become an important path for HNC/HCN conversion with increasing temperature in star forming regions.