Spectral Signatures of Hydrogen Thioperoxide (HOSH) and Hydrogen Persulfide (HSSH): Possible Molecular Sulfur Sinks in the Dense ISM

Abstract

For decades, sulfur has remained underdetected in molecular form within the dense interstellar medium (ISM), and somewhere a molecular sulfur sink exists where it may be hiding. With the discovery of hydrogen peroxide (HOOH) in the ISM in 2011, a natural starting point may be found in sulfur-bearing analogs that are chemically similar to HOOH: hydrogen thioperoxide (HOSH) and hydrogen persulfide (HSSH). The present theoretical study couples the accuracy in the anharmonic fundamental vibrational frequencies from the explicitly correlated coupled cluster theory with the accurate rotational constants provided by canonical high-level coupled cluster theory to produce rovibrational spectra for use in the potential observation of HOSH and HSSH. The ν6 mode for HSSH at 886.1 cm−1 is within 0.2 cm−1 of the gas-phase experiment, and the B0 rotational constant for HSSH of 6979.5 MHz is within 9.0 MHz of the experimental benchmarks, implying that the unknown spectral features (such as the first overtones and combination bands) provided herein are similarly accurate. Notably, a previous experimentally-attributed 2ν1 mode, at 7041.8 cm−1, has been reassigned to the ν1+ν5 combination band based on the present work’s ν1+ν5 value at 7034.3 cm−1. The most intense vibrational transitions for each molecule are the torsions, with HOSH having a more intense transition of 72 km/mol compared to HSSH’s intensity of 14 km/mol. Furthermore, HOSH has a larger net dipole moment of 1.60 D compared to HSSH’s 1.15 D. While HOSH may be the more likely candidate of the two for possible astronomical observation via vibrational spectroscopy due to the notable difference in their intensities, both HSSH and HOSH have large enough net dipole moments to be detectable by rotational spectroscopy to discover the role these molecules may have as possible molecular sulfur sinks in the dense ISM.

Fluoro Hydrogen Peroxide: A Plausible Molecular Form of Naturally-Occurring Fluorine

“…In order to assess the viability and persistence of the HOOF molecule for potential astronomical or atmospheric observation, the bond strengths of the HOOF are computed in this work, in kcal/mol, at the F12-TZ level and given in the following scheme: Comparing the O–O bond strength in HOOF to the previously computed O–O bond strength of HOOH, first observed in the ISM in 2011, shows good agreement with a difference of only 3.0 kcal/mol, suggesting this bond will persist after formation. The O–F bond strength, however, is much weaker at −31.53 kcal/mol, which is a 30% difference from the O–O bond strength of HOOH, calling into question its persistence.…”

“…In order to assess the viability and persistence of the HOOF molecule for potential astronomical or atmospheric observation, the bond strengths of the HOOF are computed in this work, in kcal/mol, at the F12-TZ level and given in the following scheme: 3. The data for both HOOH and HOSH come from a previous theoretical study 71 that also utilizes the F12-TZ QFF method to compute accurate vibrational frequencies. HOOH exhibits more intense vibrational transitions than either HOSH and HOOF with HOOH's largest being the torsional motion, ν 4 at 164 km/mol and its second largest being the antisymmetric H − O − O bend, ν 6 , at 119 km/mol.…”

“…Additionally, a comparison of the vibrational frequencies and dipole moments between HOOF and other peroxides such as HOOH and the theorized hydrogen thioperoxide (HOSH) molecule are given in Table . The data for both HOOH and HOSH come from a previous theoretical study that also utilizes the F12-TZ QFF method to compute accurate vibrational frequencies. HOOH exhibits more intense vibrational transitions than either HOSH and HOOF with HOOH’s largest being the torsional motion, ν 4 at 164 km/mol and its second largest being the antisymmetric H – O – O bend, ν 6 , at 119 km/mol.…”

Spectroscopy and Photochemistry of OAlNO and Implications for New Metal Chemistry in the Atmosphere

“…The present rovibrational study utilizes a QFF method based on CCSD­(T) again within the explicitly correlated F12b formalism , conjoined to the corresponding cc-pVTZ-F12 basis set. This QFF is typically called “F12-TZ.” While the F12-TZ QFF has been used in the past to provide accurate anharmonic fundamental frequencies, − one of its major downfalls is its inability to calculate high-accuracy rotational constants. ,, In order to overcome the accuracy shortcomings in the rotational constants, a composite method is employed that combines the accuracy in the anharmonic frequencies from the F12-TZ method and the higher accuracies for the rotational constants achieved by a previous and well documented composite method . This QFF method utilized herein uses the CCSD­(T)-F12b level with the inclusion of explicit core–electron correlation in the cc-pCVTZ-F12 basis set and scalar relativistic corrections also present in the previous composite methodology.…”

“…One drawback of the methodology implemented in the present vibrational study is the inability to effectively model the very floppy bends of the 1 3 A″ state of OAlNO, much like previously studied Alcontaining species, 68 and even HOOH. 58,87 For that reason, for the computed vibrational spectra, the untrustworthy cubic and quartic terms of coordinates 4 and 5 were removed for the 1 3 A″ state of OAlNO and the ∠(Al−N−O 2 ) and in-plane linear bend, respectively, from the VPT2 calculations. Even then, the vibrational spectrum contains some positive anharmonicities for ν 4 −ν 6 for OAlNO, but this is expected for near-prolate molecules.…”