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The synergy between high-resolution rotational spectroscopy and quantum-chemical calculations is essential for exploring future detection of molecules, especially when spectroscopy parameters are not available yet. By using highly correlated ab initio quartic force fields (QFFs) from explicitly correlated coupled cluster theory, a complete set of rotational constants and centrifugal distortion constants for $ {D2CS} $ D 2 CS and cis/trans-DCSD isomers have been produced. Comparing our new ab initio results for $ {D_2CS} $ D 2 CS with new rotational spectroscopy laboratory data for the same species, the accuracy of the computed B and C rotational constants is within 0.1% while the A constant is only slightly higher. Additionally, quantum chemical vibrational frequencies are also provided, and these spectral reference data and new experimental rotational lines will provide additional references for potential observation of these deuterated sulfur species with either ground-based radio telescopes or space-based infrared observatories. [ABSTRACT FROM AUTHOR]

Accurate binding energies of molecules to water clusters are relevant for understanding intermolecular interactions and various chemical applications. They enter models of interstellar chemical processes, as binding to icy grains influences surface reactions and thus affects calculated gas-phase abundances. Unfortunately, many astrochemical molecules (especially radicals and ions) are incompletely characterised in these models. To address this, we report computational searches for optimal structures and benchmark binding and condensation energies for sets of neutral, radical, cationic, and anionic molecules of astrochemical relevance with clusters of $ N=1{\rm -}4 $ N = 1 − 4 water molecules. These calculations utilised reliable density functionals for geometry optimisation, and coupled cluster (CCSD(T)) single point calculations with large basis sets. Four energetic binding motifs (weak, intermediate, strong or covalently bonded) were observed depending on the chemical nature of the guest molecule. Neutral closed and open-shell molecules with strong dipoles and a greater potential for hydrogen bonding are more tightly bound to water clusters compared to non-polar ones. For closed-shell cationic and anionic species, barrier-less reactions with water clusters occur, which reveals radical-free routes to molecular processing in the gas phase and on amorphous ice surfaces. [ABSTRACT FROM AUTHOR]

To facilitate the data analysis of current and future high-resolution space telescope missions, we adopt 'Best Theory + Reliable High-resolution Experiment' (BTRHE) strategy to develop highly accurate infrared line lists for nitrous oxide (N2O). The 'Ames-1' potential energy surface (PES) is a CCSD(T)/aug-cc-pVQZ PES refined using selected HITRAN experimental data, with σrms = 0.02–0.03 cm-1 for five isotopologues. The 'Ames-1' dipole moment surface (DMS) is fitted from CCSD(T)/aug-cc-pV(T,Q,5)Z dipoles extrapolated to one electron basis set limit. Using the Ames-1 PES and DMS, Ames-296K line lists are computed in the full range of 0–15,000 cm-1 for 12 N2O isotopologues, with S296K ≥ 10−31 cm-1/molecule.cm-2. The reliability and consistency of Ames-296K intensity predictions (SAmes) are demonstrated through comparisons with HITRAN (SHITRAN), NOSL-296 (SNOSL), recent observed intensities (Sobs) and Effective Dipole Model (EDM) intensities (SEDM). Agreements and discrepancies are discussed, along with preliminary uncertainty estimate for SAmes. The SAmes provides a good constraint to prevent substantial errors in intensity predictions (e.g. for weak bands and minor isotopologues) and can be further improved. Ames-296K and NOSL-296 may complement each other to provide improved input for future database updates, combining the strengths of EH/EDM and BTRHE approaches. Data available at and https://doi.org/10.48667/9kmk-0334. [ABSTRACT FROM AUTHOR]

Recent work on cyclopropenylidene, c-C 3 H 2 , sought to improve on past ab initio results with a complete basis set (CBS) quartic force field method capable of predicting fundamental bands to within 5 cm−1 accuracy of gas-phase spectroscopy experiments for semirigid molecules. It was suggested that a problematic coupling between n-body correlation and 1-particle basis set effects prevented a proper application of this method for the out-of-plane bending vibrations of c-C 3 H 2 . We establish here that the actual source of this anomalous behaviour is numerical instability in the out-of-plane bending quartic force constants. It is found that the use of the normal equations to solve for the quartic force field, as well as a bending coordinate grid which is too compact, leads to errors of ca. 20 cm−1 in ν 5 and ν 9 . Our revised analysis shows ν 5 and ν 9 to be 980 cm−1 and 777 cm−1, respectively. [ABSTRACT FROM AUTHOR]

From large basis set coupled cluster calculations and a minor empirical adjustment, an anharmonic force field for silane has been derived that is consistently of spectroscopic quality (1 cm-1 on vibrational fundamentals) for all isotopomers of silane studied. Inner-shell polarization functions have an appreciable effect on computed properties and even on anharmonic corrections. From large basis set coupled cluster calculations and extrapolations to the infinite-basis set limit, we obtain TAE0 = 303:80 0:18 kcal mol- 1, which includes an anharmonic zero-point energy (19.59 kcal mol- 1), inner-shell correlation (-0.36 kcal mol-1), scalar relativistic corrections (-0.70 kcal mol- 1) and atomic spin-orbit corrections (- 0.43 kcal mol- 1). In combination with the recently revised H f [sub 0][Si(g)], we obtain H f ;0[SiH4(g)] = 9:9 0:4 kcal mol- 1 in between the two established experimental values. [ABSTRACT FROM AUTHOR]

The singles and doubles coupled cluster method that includes a per turbational estimate of the effects of connected triple excitations CCSD T together with a triple zeta double polarized TZ2P one particle basis set is used to determine the geometries harmonic frequencies infrared intensities and dipole moments of HOF F O HOOF FOOF ClOOF Agreement with experiment is very good with the exception that the currently accepted experimental assignment of the symmetric and antisymmetric O F stretches in FOOF is shown to be reversed and to be consistent with an earlier experimental study Very accurate heats of formation of HOOF FOOF and ClOOF are also computed using the CCSD T method in conjunction with large atomic natural orbital basis sets The F O bond distances quadratic force constants bond energies and fluorine and oxygen atomic charges from the above five molecules and six previously studied molecules FONO trans FONO cis FONO FOCl FOBr and FON are compared and used to deduce a simple model of F O bonding The unusual relationship between the F O bond distance and quadratic force constant shows that F O bonding is a function of at least three effects which are degree of covalent character degree of ionic character and extent of lone electron pair repulsions All of the data are qualitatively consistent with this simple model The bonding in cis FONO is even more complicated involving also dispersion interactions between fluorine and the terminal oxygen It is suggested that the general importance of lone pair repulsions in F O bonding and the additional importance of intra molecular dispersion interactions explains why many density functionals have difficulty in describing the geometry of cis FONO. [ABSTRACT FROM AUTHOR]

Acknowledges the contributors of the February 20, 1999 issue of the periodical 'Molecular Physics.' Recognition of the death of quantum chemist Bowen Liu; Bowen's contributions to quantum physics.