INTERSTELLAR SILICON-NITROGEN CHEMISTRY .4. WHICH REACTION PATHS TO HSIN AND HNSI - AN EXTENSIVE AB-INITIO INVESTIGATION WITH CRUCIAL CONSEQUENCES FOR MOLECULAR ASTROPHYSICS

In order to provide a possible explanation for the lack of detection o f both HSiN and HNSi in the interstellar medium an ab initio study of the Si++NH3 reaction is presented: it includes accurate energetic cons iderations and sketches dynamics discussions as well. It is unambiguou sly concluded that the X(1)A(1) ground state of the SiNH2+ cation is t he only exit channel of this reaction assuming interstellar conditions . The rotational and vibrational constants of this species are reporte d to stimulate its experimental and astrophysical searches. Upon disso ciative recombination, it is likely that SiNH2+ can evolve toward HNSi : unfortunately, the dramatic weakness of the dipole moment of the lat ter species (0.05 D) makes it an unlikely candidate for today's radiot elescopes. At variance with HNSi the high dipole moment value of HSiN (4.5 D) would make it a much more attractive candidate for astrophysic al searches, but under interstellar conditions, we show that it can de rive neither from the unimolecular HNSi<->HSiN equilibration nor from the Si++NH3, N+SiH3+ or N++SiH3 reactions as sometimes incorrectly sta ted in the astrophysical models that deduce interstellar silicon chemi stry from that of carbon. Throughout this study, the very hazardous ch aracter of conclusions deduced from isoelectronic considerations shoul d be considered as the leading feature: the finishing stroke to such i soelectronic analogies is given by our study of the H++HNSi<->HSiN+Hreactions which leads to the conclusion that HSiN might be unlikely to survive interstellar hydrogenation processes.