The CH stretch overtone region (5750–6300 cm−1) of benzene and naphthalene is assigned herein using anharmonic quantum chemical computations, and the trend of how this extends to larger polycyclic aromatic hydrocarbons (PAHs) is established. The assignment of all experimental bands to specific vibrational states is performed for the first time. Resonance polyads and the inclusion of 3-quanta vibrational states are both needed to compute accurate vibrational frequencies with the proper density-of-states to match the experimental band shape. Hundreds of 3-quanta states produce the observed band structure in naphthalene, anthracene, and tetracene, and this number is expected to increase drastically for larger PAHs. The width and shape of the main peak are consistent from naphthalene to anthracene, necessitating further exploration of this trend to confirm whether it is representative of all PAHs in the CH stretch overtone region. Understanding observations of PAH sources in the 1–3 μm region from the NIRSpec instrument aboard JWST requires new computational data, and this study provides a benchmark and foundation for their computation.
Fluorine's hostile nucleosynthetic environment makes it one of the least common elements and, consequently, understudied both on the earth and in the interstellar medium (ISM). However, the presence of fluorine-containing species in both the ISM and in the earth's atmosphere necessitates the existence of a pathway out of this environment to form fluorine-containing molecules. To that end, the presence of fluorine and hydroperoxyl radical (HO2) in either of these environments may lead to the formation of fluorinated molecules like fluoro hydrogen peroxide (HOOF) on dust grains of protoplanetary disks in the planet-forming regions of ρ Oph and in the earth's atmosphere as a sink for other fluorine pollutants that have yet to be detected. This theoretical study utilizes explicitly correlated coupled cluster theory computed with core correlation and corrections from scalar relativity to provide the first anharmonic fundamental vibrational frequencies and rotational constants of HOOF for use as reference benchmarking of further computational or experimental study, as well as potential astrophysical observation. The ν6 bending frequency for HOOF at 454.4 cm–1 exhibits an anharmonic transition intensity of 78 km/mol, while the ν4 frequency at 738.2 cm–1 is 66 km/mol. Additionally, HOOF has a large net dipole moment of 2.12 D compared to the previously detected HF and HOOH molecules, 1.82 and 1.85 D, respectively, resulting from the electronegativity of the fluorine. Consequently, HOOF is a likely candidate for possible detection via vibrational and rotational spectroscopy to further the understanding of fluorine's small, but important, role in astrochemical and atmospheric environments.
The well-studied hydrogen sulfide molecule is shown here for the first time to form a S-S bond barrierlessly with sulfur atomic cation to produce stable H2SS+, a compound for which there is nearly no literature data. Previous work has shown that the reaction of hydrogen sulfide with neutral atomic sulfur will likely only take place at high pressures. Conversely, this work shows that hydrogen sulfide will readily bind with atomic sulfur cation first through the 1 4A″ state from association of H2S with S+(4S) and then will relax to the nearly degenerate 1 2A' or 1 2A″ states. S+(4S) + H2S lies 29.5 kcal/mol above the 1 4A″ H2SS+ minimum. The 1 4A″ H2SS+ minimum in the S-S bond is also directly intersected by the doublet potential energy surface. As the S-S bond shortens in the association, the 1 2A' and 1 2A″ states split, falling 33.5 and 26.4 kcal/mol, respectively, below the 1 4A″ state. Hence, this work is opening the door for novel synthesis of S-S bonds or potential removal of the common H2S toxin/pollutant through concatenation and subsequent precipitation.
The interstellar presence of protonated nitrous oxide has been suspected for some time. Using established high-accuracy quantum chemical techniques, spectroscopic constants and fundamental vibrational frequencies are provided for the lower energy O-protonated isomer of this cation and its deuterated isotopologue. The vibrationally-averaged B0 and C0 rotational constants are within 6 MHz of their experimental values and the D(J) quartic distortion constants agree with experiment to within 3%. The known gas phase O-H stretch of NNOH(+) is 3330.91 cm(-1), and the vibrational configuration interaction computed result is 3330.9 cm(-1). Other spectroscopic constants are also provided, as are the rest of the fundamental vibrational frequencies for NNOH(+) and its deuterated isotopologue. This high-accuracy data should serve to better inform future observational or experimental studies of the rovibrational bands of protonated nitrous oxide in the interstellar medium and the laboratory.
Strong anharmonic coupling between vibrational states in polycyclic aromatic hydrocarbons (PAH) produces highly mixed vibrational transitions that challenge the current understanding of the nature of the astronomical mid-infrared PAH emission bands. Traditionally, PAH emission bands have been characterized as either aromatic or aliphatic, and this assignment is used to determine the fraction of aliphatic carbon in astronomical sources. In reality, each of the transitions previously utilized for such an attribution is highly mixed with contributions from both aliphatic and aromatic CH motions as well as non-CH motions such as CC stretches. High-resolution gas-phase IR absorption measurements of the spectra of the aromatic molecules indene and 2-ethynyltoluene at the Canadian Light Source combined with high-level anharmonic quantum chemical computations reveal the complex nature of these transitions, implying that the use of these features as a marker for the aliphatic fraction in astronomical sources is not uniquely true or actually predictive. Further, the presence of aliphatic, aromatic, and ethynyl CH groups in 2-ethynyltoluene provides an internally consistent opportunity to simultaneously study the spectroscopy of all three astronomically important groups. Finally, this study makes an explicit connection between fundamental quantum mechanical principles and macroscopic astronomical chemical physics, an important link necessary to untangle the lifecycle of stellar and planetary systems.
The reaction of water molecules with alane (AlH3) or magnesium hydride (MgH2) ultimately produces metal oxide clusters with surrounding hydrogen atoms. This quantum chemical study shows that such reactions proceed initially barrierlessly by creating dative-bonded intermediates, then go through a transition state, where two of the hydrogen atoms come within close proximity of one another, and then eject a hydrogen molecule, producing a strongly favored product. The hydrogen molecule can dissipate the kinetic energy, promoting the formation of the metal hydroxide products as opposed to simply breaking the intermediate apart. This reaction then proceeds consecutively, ultimately producing, in this study, a hydrogenated corundum monomer and hydrogenated periclase dimer, two notable terrestrial minerals. These clusters can likely then react with each other, self-catalyzing a larger reaction network. Such processes may complement or even compete with other means of producing inorganic oxide covalent network molecular clusters or even larger mineral grains.
Molecular cations are present in various astronomical environments, most notably in cometary atmospheres and tails where sunlight produces exceptionally bright near-UV to visible transitions. Such cations typically have longer-wavelength and brighter electronic emission than their corresponding neutrals. A robust understanding of their near-UV to visible properties would allow these cations to be used as tools for probing the local plasma environments or as tracers of neutral gas in cometary environments. However, full spectral models are not possible for characterization of small, oxygen containing molecular cations given the body of molecular data currently available. The five simplest such species (H2O+, CO+2 , CO+, OH+, and O+2 ) are well characterized in some spectral regions but are lacking robust reference data in others. Such knowledge gaps hinder fully quantitative models of cometary spectra, specifically, hindering accurate estimates of physical-chemical processes originating with the most common molecules in comets. Herein the existing spectral data are collected for these molecules and highlight the places where future work is needed, specifically where the lack of such data would greatly enhance the understanding of cometary evolution.
Abstract HSS has yet to be observed in the gas phase in the interstellar medium (ISM). HSS has been observed in cometary material and in high abundance. However, its agglomeration to such bodies or dispersal from them has not been observed. Similarly, HSO and HOS have not been observed in the ISM, either, even though models support their formation from reactions of known sulfur monoxide and hydrogen molecules, among other pathways. Consequently, this work provides high-level, quantum chemical rovibrational spectroscopic constants and vibrational frequencies in order to assist in interstellar searches for these radical molecules. Furthermore, the HSO−HOS isomerization energy is determined to be 3.63 kcal mol −1 , in line with previous work, and the dipole moment of HOS is 36% larger at 3.87 D than HSO, making the less stable isomer more rotationally intense. Finally, the S−S bond strength in HSS is shown to be relatively weak at 30% of the typical disulfide bond energy. Consequently, HSS may degrade into SH and sulfur atoms, making any ISM abundance of HSS likely fairly low, as recent interstellar surveys have observed.
