In order to provide a possible explanation for the lack of detection of both HSiN and HNSi in the interstellar medium, an ab initio study of the Si+ + NH3 reaction is presented: it includes accurate energetic considerations and sketches dynamics discussions as well. It is unambiguously concluded that the X1A1 ground state of the SiNH2+ cation is the only exit channel of this reaction assuming interstellar conditions. The rotational and vibrational constants of this species are reported to stimulate its experimental and astrophysical searches. Upon dissociative recombination, it is likely that SiNH2+ can evolve toward HNSi: unfortunately, the dramatic weakness of the dipole moment of the latter species (0.05 D) makes it an unlikely candidate for today's radiotelescopes. At variance with HNSi, the high dipole moment value of HSiN (4.5 D) would make it a much more attractive candidate for astrophysical searches, but under interstellar conditions, we show that it can derive neither from the unimolecular HNSi ↔ HSiN equilibration nor from the Si+ + NH3, N + SiH3+ or N+ + SiH3 reactions as sometimes incorrectly stated in the astrophysical models that deduce interstellar silicon chemistry from that of carbon. Throughout this study, the very hazardous character of conclusions deduced from isoelectronic considerations should be considered as the leading feature: the finishing stroke to such isoelectronic analogies is given by our study of the H+ + HNSi ↔ HSiN + H+ reactions which leads to the conclusion that HSiN might be unlikely to survive interstellar hydrogenation processes.

Although density functional theory (DFT) is more and more commonly used as a very efficient tool for the study of molecules and bulk materials, its applications to weakly bonded systems remain rather sparse in the literature, except studies that consider hydrogen bonding. It is, however, of essential interest to be able to correctly describe weaker van der Waals complexes. This prompted us to investigate more precisely the reliability of several widely-used functionals. The equilibrium geometries and the binding energies of C₆H₆···X (X = O₂, N₂, or CO) complexes are determined within the standard Kohn−Sham approach of DFT using different exchange−correlation functionals and at the MP2 level of theory for comparison. It is comprehensively concluded that extreme care must be taken in the choice of the functional since only those that behave properly at large and intermediate values of the reduced density gradient s give relevant results. The PW91 exchange functional, the enhancement factor of which does not diverge at increasing s, appears as the most reliable for the studied systems. It is furthermore demonstrated that the quality of the DFT results is determined by the exchange energy component of the total energy functional.

In this second paper, the philosophy of coupling multiconfigurational variational wave functions to perturbation treatments (MC/P methodology) is extended to the calculation of electronic spectra. The corresponding methodology is presented with emphasis on its flexibility and an overview of other available approaches is given. The contracted MC/P scheme is then applied to ethylene H2C=CH2, formaldehyde H2C=O vinylidene H2C=C. It is shown that combining well-designed averaged zeroth-order MCSCF wave functions to a barycentric Møller-Plesset (BMP) partition of the electronic Hamiltonian provides accurate spectra, contrary to Epstein-Nesbet partitions. The MC/BMP transition energies compare with experimental data within a few hundreds of cm−1. These results have been obtained using a polarized double-zeta quality basis set augmented by a set of semi-diffuse functions (6–31 + G*) and by an extra set of diffuse orbitals to account for Rydberg states. Since non-dynamic correlations effects that are important for a proper description of the manifold of the excited states of interest are included in the MCSCF zeroth-order space will all remaining correlation effects (non-dynamic and dynamic) are treated at the perturbation level, the present study lets anticipate applications of the MC/P methodology to medium size systems without much computational trouble.

The recent detection of SiN in the outer envelope of the IRC+10216 carbon star has renewed the interest for the gas phase interstellar silicon chemistry. In this contribution, we present a theoretical study of the H2SiN⁺ molecular ion, the silicon hydrogenated counterpart of the previously studied SiNH^ + ₂. On many points, the differences relative to the SiNH^ + ₂ isomer have been found to be dramatic. As an example, the dipole moment is computed to be 3.8 D while being only 0.5 D in SiNH^ + ₂. The radio, infrared and electronic signatures have been evaluated at a quantitative level. The rotational constants and vibrational frequencies have been determined using Möller–Plesset MPn (n=2,3,4), coupled cluster (CCSDT) and complete active space self-consistent field (CASSCF) methods for H2SiN⁺ and some of its isotopomers. These quantities have been corrected using a scaling procedure derived from previous studies on the HNSi, HSiN, HSiNH₂, H₂SiNH, and SiNH^ + ₂ species in order to provide quantitative results. The failure of single-reference perturbation theories to predict a relevant infrared spectrum is discussed. Intense bands around 550, 950, and 2300 cm^–1 are predicted. The electronic spectrum has been obtained using a coupled multiconfiguration SCF–perturbation treatment (MC/P): It is characterized by a large number of excited states, none of them having a strong transition moment. The lowest excited state is predicted to lie 0.54 eV above the ground state, but the first allowed transition having a nonnegligible oscillator strength has to be searched at 6.44 eV

The experimental and the theoretical interests for the silicon chemistry 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 new interstellar lines. The main goal of this report remains, however, the calibration of ab initio calculations on still-unknown silicon-nitrogen systems: the infrared and the microwave signatures of the HSiNH+ cation are reported as a direct application. The signatures of the five molecules under investagation have been computed at increasing levels of post-Hartree-Fock theories, using up to a 6–311 + + G** atomic orbital expansion. Accurate geometries and Be rotational constants have been determined at the Möller-Plesset MPn(n = 2, 3, 4), CASSCF and CCSD(T) theoretical plateaus for HNSi. The comparison with experimental data 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 HSiNH2 in its floppy linear singlet ground state and in its lowest cis-bent a3A′ state as well. Dipole moments are reported in order to assess the feasability for these species to be detected owing to their rotational signatures either in the laboratory or in space using millimetric radioastronomy techniques. Infrared (IR) signatures are computed at the same levels of theory and compared to the recent matrix isolation experiments devoted to HSiN, HNSi, HSiNH2 and HNSiH2. The calculations unambiguosly confirm that all these species have been effectively produced and observed. They also lead to the determination of accurate IR scaling factors that are significantly larger than the usual ones. Such an approach allows then to quantitatively predict the IR spectra of the still-unknown HSiNH+ entity. The study of the IR spectra furthermore points out the failure of single-reference correlation methods to obtain predictive IR signatures in some cases, as is unambigously illustrated in the case of the HSiN species.

The electroaffinity of the O2 molecule is revisited using density functional theory (DFT) and perturbation treatments built on a MCSCF wavefunction that includes most of the non-dynamic correlation effects (MC/P approach). Using a standard 6–31 + G* basis set, DFT treatments based on BLYP or B3LYP functionals provide electroaffinities of the order of +0.6 eV that compare favorably to experiment. Coupled MCSCF/perturbation treatments using an Epstein-Nesbet partition of the molecular Hamiltonian give a more accurate value of +0.492 eV in excellent agreement with the most recent experimental data (+0.431 eV) as well as with highest-level purely variational ab initio treatments which are far less tractable for larger systems. The analysis of the results in terms of differential correlation effects made it possible to identify the failure of the previous MCSCF-limited treatments as arising from the dynamic correlation of the electron pair describing the σO---:O bond.