The rovibrational spectrum of BeH, MgH and CaH at high temperatures in the X 2Σ+X\,{}^2\Sigma^+ state: a theoretical study

Accurate line lists for three molecules, BeH, MgH and CaH, in their ground electronic states are presented. These line lists are suitable for temperatures relevant to exoplanetary atmospheres and cool stars (up to 2000K). A combination of empirical and \textit{ab initio} methods is used. The rovibrational energy levels of BeH, MgH and CaH are computed using the programs Level and DPotFit in conjunction with `spectroscopic' potential energy curves (PECs). The PEC of BeH is taken from the literature, while the PECs of CaH and MgH are generated by fitting to the experimental transition energy levels. Both spin-rotation interactions (except for BeH, for which it is negligible) and non-adiabatic corrections are explicitly taken into account. Accurate line intensities are generated using newly computed \textit{ab initio} dipole moment curves for each molecule using high levels of theory. Full line lists of rotation-vibration transitions for $^9$BeH, $^{24}$MgH, $^{25}$MgH, $^{26}$MgH and $^{40}$CaH are made available in an electronic form as supplementary data to this article and at \url{www.exomol.com}.Comment: MNRAS (in press

A line confusion limited millimeter survey of Orion KL. II: Silicon-bearing species

We present a study of the Silicon-bearing species detected in a line confusion limited survey towards Orion KL performed with the IRAM 30-m telescope, which ranges from 80 to 280 GHz. Our aim is to derive physical and chemical conditions for each family taking into account all observed lines from all isotopologues of each species. We have modeled the lines of the detected molecules using a radiative transfer code, which permit us to choose between LVG and LTE approximations depending on the physical conditions of the gas. We have used appropriate collisional rates for the LVG calculations. For the v=1 state of SiO we have detected the J=2-1 line and, for the first time in this source, emission in the J=4-3 transition, both of them showing strong masering effect. For SiO v=0, we have detected 28SiO, 29SiO, and 30SiO; in addition, we have mapped the J = 5-4 SiO line. For SiS, we have detected the main species, 29SiS, and SiS v=1. Unlikely other species detected in Orion KL (IRc2), the emission peak of SiS appears at a velocity of ~15.5 km s-1; a study of the 5-4 SiO line around IRc2 shows this feature as an extended component that probably arises from the interaction of the outflow with the ambient cloud. We derive a SiO/SiS column density ratio of ~13 in the plateau component. Besides, we provide upper limits to the column density of several non-detected Silicon-bearing species. The results of our chemical models show that while it is possible to reproduce SiO in the gas phase (as well as on the grains), SiS is a product of surface reactions, most likely involving direct reactions of S with Si.Comment: Astronomy and Astrophysics, in pres

On the electron affinity of SiN and spectroscopic constants of Si N

Accurate spectroscopic constants and energetics were calculated for the two lowest-lying states of SiN and Si N- employing the coupled cluster methodology and very large basis sets (up to doubly augmented sextuple- ζ quality) accounting also for corevalence correlation, one-electron Douglas-Kroll-Hess relativistic effects, and atomic spin-orbit couplings. Our best estimate for the adiabatic electron affinity of SiN is 3.002 eV, in very good agreement with the recent, experimentally determined value of 2.949 (8) eV. However, the calculated bond length of the Si N- X Σ+1 state at the same level, re =1.5904 Å, is smaller than the indirectly extracted experimental value of 1.604 (5) Å, pointing out that the latter value is either a bit overestimated or not as accurate as the ±0.005 Å error bar indicates. For the neutral SiN, all calculated data are in excellent agreement with previous accurate experimental results. © 2005 American Institute of Physics

Electronic structure of scandium and titanium carbide cations, ScC+ and TiC+. Ground and low-lying states

The monopositive scandium and titanium carbides, ScC+ and TiC+, have been studied by multireference methods coupled with quantitative basis sets. We have constructed potential energy curves for the ground and twelve low-lying excited states for both species, focusing on dissociation energies and bonding mechanisms. We have found that the ground states of ScC+ and TiC+ are of 3Π and 2Σ+ symmetries, with binding energies (D0) of 68.8 (71.5) and 85.2 (86.7) kcal/mol at the MRCI (MRCI+Davidson correction) level of theory. These values are compared favorably with the corresponding experimental findings, D0(ScC+) = 77.0 ±1.4 and D0(TiC+) = 93.4 ± 5.5 kcal/mol. © 2000 American Chemical Society

Electronic structure of vanadium and chromium carbide cations, VC + and CrC+. Ground and low-lying states

The ground and low-lying states of the monopositive vanadium and chromium carbides, VC+ and CrC+ have been studied by multireference methods and quantitative basis sets. Potential energy curves for 17 (VC +) and 19 (CrC+) states have been fully calculated. A variety of binding modes is revealed in the low-lying spectrum of the two molecular cations, often accompanied with an electronic charge transfer from the metal cation towards carbon. Two states compete for the ground state identity in both systems. One state comprises two π and 1/2σ bonds (similarly to ScC+ and TiC+), while the other state forms a genuine triple bond. After a rather intricate analysis including core electron effects, scalar relativity and curve shifts, the formal ground states of VC+ and CrC+ are found to be of 3Δ and 2Δ symmetry, with estimated energy differences from the competing 1∑+ and 4∑- states of 1-3 and 3-7 kcal/mol, respectively. At the highest level of theory including core/valence correlation and one-electron relativistic effects, the calculated ground-state binding energies are in satisfactory agreement with available experimental values

Electronic structure and bonding nature of the ground state monocarbide cations, ScC+, TiC+, VC+, and CrC+

The ground states of the transition-metal diatomic carbide cations, MC+ (M = Sc, Ti, V, and Cr), are studied using multireference configuration interaction (MRCI) methods in conjunction with quantitative basis sets. Full potential energy curves are calculated for all four systems. When 3s23p6 core/valence correlation contributions and scalar relativistic effects are taken into account, our best estimates for the zero-point-corrected dissociation energies of the MC+ series are in good agreement with relevant experimental results. For TiC+, the recent correlation-consistent-type basis sets for Ti of Bauschlicher are also exploited to extract complete basis set limits of selected properties. The ground states of VC+(X 3Δ) and CrC+(X 2Δ) are reported for the first time in the literature. For CrC+ an interesting competition is revealed between the 2Δ and 4∑- states; although 4∑- is formally the ground state at the MRCI level of theory, when core/valence and/or relativistic effects are included, the ground state of CrC+ becomes of 2Δ symmetry, with a calculated energy separation (a 4∑- ← X 2Δ) of 2.3 kcal/mol

A multireference coupled-cluster potential energy surface of diazomethane, CH2N2

The intrinsically multireference dissociation of the C-N bond in ground-state diazomethane (CH2N2) at different angles has been studied with the multireference Brillouin-Wigner coupled-cluster singles and doubles (MRBWCCSD) method. The morphology of the calculated potential energy surface (PES) in Cs symmetry is similar to a multireference perturbational (CASPT3) PES. The MRBWCCSD/cc-pVTZ H2C-N2 dissociation energy with respect to the asymptotic CH2(ā 1A1) + N2(X1∑g +) products is De = 35.9 kcal/mol, or a zero-point corrected D 0 = 21.4 kcal/mol with respect to the ground-state CH 2(X̃3B1) + N2(X 1∑g+) fragments. © 2005 American Chemical Society

Structure and energetics of gaseous HZnCl

The HZnCl molecule has recently been observed both in the gas phase and in argon matrixes, and the complexity of its spectrum has created big discrepancies between experimental and theoretically predicted Zn-H bond distances. In an attempt to resolve the questionable H-ZnCl bond length issue, as well as to study its stability relative to bond breakage, we have investigated at a very high ab initio level its geometric and energetic parameters. At the coupled cluster level of theory [CCSD(T)] with the new correlation-consistent basis sets for Zn by Balabanov and Peterson, and with the inclusion of core/valence and one-electron Douglas-Kroll-Hess relativistic effects, the basis set extrapolated H-ZnCl and HZn-Cl bond lengths have been calculated to be 1.499 and 2.079 Å, compared with the experimental estimates of 1.596-1.789 and 2.08346 Å, respectively. With the same procedure, the experimentally unknown bond length of ground-state ZnCl has been found to be equal to 2.122 Å. As in the free diatomics ZnCl and ZnH, the HZn-Cl bond is stronger than the H-ZnCl bond by about 30 kcal/mol. © 2006 American Chemical Society