INTERSTELLAR SILICON-NITROGEN CHEMISTRY .1. THE MICROWAVE AND THE INFRARED SIGNATURES OF THE HSIN, HNSI, HSINH2, HNSIH2, AND HSINH+ SPECIES

The experimental and the theoretical interests for the silicon chemist ry have been renewed by the recent detection of SiN in space. In this contribution a theoretical study of the HSiN, HNSi, HSiNH2 and HNSiH2 molecular systems is presented that aims to help in the interpretation of available experimental results as well as in the attribution of ne w interstellar lines. The main goal of this report remains, however, t he calibration of ab initio calculations on still-unknown silicon-nitr ogen systems: the infrared and the microwave signatures of the HSiNHcation are reported as a direct application. The signatures of the fiv e molecules under investigation have been computed at increasing level s of post-Hartree-Fock theories, using up to a 6-311++G* atomic orbit al expansion. Accurate geometries and B-e rotational constants have be en determined at the Moller-Plesset MPn (n = 2, 3, 4), CASSCF and CCSD (T) theoretical plateaus for HNSi The comparison with experimental dat a allows then to derive the scaling factors needed to obtain accurate rotational constants for related species: they are applied as such on the crude constants determined for HSiN, HSiNH2, HNSiH2, and finally H SiNH2 in its floppy linear singlet ground state and in its lowest cis- bent a(3)A' state as well. Dipole moments are reported in order to ass ess the feasability for these species to be detected owing to their ro tational signatures either in the laboratory or in space using millime tric radioastronomy techniques. Infrared (IR) signatures are computed at the same levels of theory and compared to the recent matrix isolati on experiments devoted to HSiN, HNSi, HSiNH2 and HNSiH2. The calculati ons unambiguously confirm that all these species have been effectively produced and observed. They also lead to the determination of accurat e IR scaling factors that are significantly larger than the usual ones . Such an approach allows then to quantitatively predict the IR spectr a of the still-unknown HSiNH+ entity. The study of the IR spectra furt hermore points out the failure of single-reference correlation methods to obtain predictive IR signatures in some cases, as is unambiguously illustrated in the case of the HSiN species.