INTERSTELLAR SILICON-NITROGEN CHEMISTRY .2. SPECTRAL SIGNATURES OF THE SINH2+ MOLECULAR ION

Interest for the gas phase interstellar silicon chemistry has been rec ently renewed by the detection of SST in the outer envelope of the IRC +10216 carbon star. In this contribution we present a theoretical stud y of the SiNH2+ molecular ion which can be seen as a precursor of sili con-nitrogen products. The radio, infrared, and electronic signatures, computed by high-level ab initio treatments using at least a 6-311++G * atomic expansion are reported. The geometry and corresponding rotat ional constants have been determined at the Moller-Plesset (MPn, n = 2 , 3, 4), complete active space self-consistent field (CASSCF), and cou pled cluster (CCSDT) levels of theory using a scaling procedure that r equired the evaluation of the same quantities, at the same levels of c alculations, for the HNSi molecule whose rotational spectrum has been obtained experimentally. Special attention has been given to the dipol e moment whose best estimated value of 0.5 +/- 0.1 D has been obtained in a series of converging treatments, including up to second-order co nfiguration interaction. The IR spectrum, calculated at the MP2, MP3, MP4, CCSDT, and CASSCF levels, shows intense bands in the 3350 and 650 cm(-1) regions. Vibrational frequencies have been corrected using sca ling factors derived from a previous study on the HNSi, HSiN, HSiNH2, and H2SiNH molecules. Rotational constants and vibrational frequencies are also provided in this report for a part of the isotopomers that c an be formed upon deuteration or upon substitution by either Si-29 or Si-30 in order to facilitate future experimental interpretations or as trophysical searches. Finally, the electronic spectrum of SiNH2+. has been obtained using a coupled multiconfiguration SCF-perturbation stra tegy (MC/P) with an extended basis built by adding low-exponent functi ons to the original set: such a procedure has been shown previously to give accurate predictions for the electronic spectra of the ethylene, formaldehyde, and vinylidene molecules. In the present case, the spec trum is characterized by three intense features which should make this species observable even in low-abundance conditions.