[en] We present a variational calculation of the first comprehensive T = 300 K rovibrational line list for thioformaldehyde, H₂CS. It covers 41 809 rovibrational levels for states up to J_max = 30 with vibrational band origins up to 5000 cm⁻¹ and provides the energies and line intensities for 547 926 transitions from the ground vibrational state to these levels. It is based on our previously reported accurate ab initio potential energy surface and a newly calculated ab initio dipole moment surface. Minor empirical adjustments are made to the ab initio equilibrium geometry to reduce systematic errors in the predicted intra-band rotational energy levels. The rovibrational energy levels and transition intensities are computed variationally by using the methods implemented in the computer program TROVE. Transition wavelengths and intensities are found to be in excellent agreement with the available experimental data. The present calculations correctly reproduce the observed resonance effects, such as intensity borrowing, thus reflecting the high accuracy of the underlying ab initio surfaces. We report a detailed analysis of several vibrational bands, especially those complicated by strong Coriolis coupling, to facilitate future laboratory assignments

[en] We present the first variational calculation of the isotropic hyperfine coupling constant of the carbon-13 atom in the CH_3 radical for temperatures T = 0, 96, and 300 K. It is based on a newly calculated high level ab initio potential energy surface and hyperfine coupling constant surface of CH_3 in the ground electronic state. The ro-vibrational energy levels, expectation values for the coupling constant, and its temperature dependence were calculated variationally by using the methods implemented in the computer program TROVE. Vibrational energies and vibrational and temperature effects for coupling constant are found to be in very good agreement with the available experimental data. We found, in agreement with previous studies, that the vibrational effects constitute about 44% of the constant’s equilibrium value, originating mainly from the large amplitude out-of-plane bending motion and that the temperature effects play a minor role

[en] For the stibine isotopologue ¹²¹SbH₃, we report improved theoretical calculations of the vibrational energies below 8000 cm^-1 and simulations of the rovibrational spectrum in the 0-8000 cm^-1 region. The calculations are based on a refined ab initio potential energy surface and on a new dipole moment surface obtained at the coupled cluster CCSD(T) level. The theoretical results are compared with the available experimental data in order to validate the ab initio surfaces and the TROVE computational method [Yurchenko SN, Thiel W, Jensen P. J Mol Spectrosc 2007;245:126-40] for calculating rovibrational energies and simulating rovibrational spectra of arbitrary molecules in isolated electronic states. A number of predicted vibrational energies of ¹²¹SbH₃ are provided in order to stimulate new experimental investigations of stibine. The local-mode character of the vibrations in stibine is demonstrated through an analysis of the results in terms of local-mode theory.

[en] A new nine-dimensional potential energy surface (PES) and dipole moment surface (DMS) for silane have been generated using high-level ab initio theory. The PES, CBS-F12"H"L, reproduces all four fundamental term values for "2"8SiH_4 with sub-wavenumber accuracy, resulting in an overall root-mean-square error of 0.63 cm"−"1. The PES is based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set limit, and incorporates a range of higher-level additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, and scalar relativistic effects. Systematic errors in computed intra-band rotational energy levels are reduced by empirically refining the equilibrium geometry. The resultant Si–H bond length is in excellent agreement with previous experimental and theoretical values. Vibrational transition moments, absolute line intensities of the ν_3 band, and the infrared spectrum for "2"8SiH_4 including states up to J = 20 and vibrational band origins up to 5000 cm"−"1 are calculated and compared with available experimental results. The DMS tends to marginally overestimate the strength of line intensities. Despite this, band shape and structure across the spectrum are well reproduced and show good agreement with experiment. We thus recommend the PES and DMS for future use

[en] Two new nine-dimensional potential energy surfaces (PESs) have been generated using high-level ab initio theory for the two main isotopologues of methyl chloride, CH_3"3"5Cl and CH_3"3"7Cl. The respective PESs, CBS-35" "H"L, and CBS-37" "H"L, are based on explicitly correlated coupled cluster calculations with extrapolation to the complete basis set (CBS) limit, and incorporate a range of higher-level (HL) additive energy corrections to account for core-valence electron correlation, higher-order coupled cluster terms, scalar relativistic effects, and diagonal Born-Oppenheimer corrections. Variational calculations of the vibrational energy levels were performed using the computer program TROVE, whose functionality has been extended to handle molecules of the form XY _3Z. Fully converged energies were obtained by means of a complete vibrational basis set extrapolation. The CBS-35" "H"L and CBS-37" "H"L PESs reproduce the fundamental term values with root-mean-square errors of 0.75 and 1.00 cm"−"1, respectively. An analysis of the combined effect of the HL corrections and CBS extrapolation on the vibrational wavenumbers indicates that both are needed to compute accurate theoretical results for methyl chloride. We believe that it would be extremely challenging to go beyond the accuracy currently achieved for CH_3Cl without empirical refinement of the respective PESs

[en] Highlights: • First laboratory detection of electric quadrupole rovibrational transitions in CO₂. • High sensitivity optical-feedback-cavity enhanced absorption spectroscopy (OFCEAS) in the 3.3 µm transparency window of CO₂. • The ν₂+ν₃ band of ¹²C¹⁶O₂ involves both electric-quadrupole (E2) and magnetic-dipole (M1) contributions. • Good agreement with ab initio predictions of the electric-quadrupole line intensities. • E2 and M1 transitions are missing in spectroscopic databases while their intensities can be above the electric-dipole line intensity cut-off. The recent detections of electric-quadrupole (E2) transitions in water vapor and magnetic-dipole (M1) transitions in carbon dioxide have opened a new field in molecular spectroscopy. While in their present status, the spectroscopic databases provide only electric-dipole (E1) transitions for polyatomic molecules (H₂O, CO₂, N₂O, CH₄, O₃…), the possible impact of weak E2 and M1 bands to the modeling of the Earth and planetary atmospheres has to be addressed. This is especially important in the case of carbon dioxide for which E2 and M1 bands may be located in spectral windows of weak E1 absorption. In the present work, a high sensitivity absorption spectrum of CO₂ is recorded by Optical-Feedback-Cavity Enhanced Absorption Spectroscopy (OFCEAS) in the 3.3 µm transparency window of carbon dioxide. The studied spectral interval corresponds to the region where M1 transitions of the ν₂+ν₃ band of carbon dioxide were recently identified in the spectrum of the Martian atmosphere. Here, both M1 and E2 transitions of the ν₂+ν₃ band are detected by OFCEAS. Using recent ab initio calculations of the E2 spectrum of ¹²C¹⁶O₂, intensity measurements of five M1 lines and three E2 lines allow us to disentangle the M1 and E2 contributions. Indeed, E2 intensity values (on the order of a few 10^–29 cm/molecule) are found in reasonable agreement with ab initio calculations while the intensity of the M1 lines (including an E2 contribution) agree very well with recent very long path measurements by Fourier Transform spectroscopy. We thus conclude that both E2 and M1 transitions should be systematically incorporated in the CO₂ line list provided by spectroscopic databases.