Significance Methanediol [CH 2 (OH) 2 ] represents a pivotal atmospheric volatile organic compound and plays a fundamental role in aerosol growth. Although sought for decades, methanediol has never been identified due to the inherent dehydration tendency of two adjacent hydroxyl groups (OH) at the same carbon atom. Here, we prepare and identify methanediol via processing of low-temperature ices followed by sublimation into the gas phase. These findings open up a concept to synthesize and characterize unstable geminal diols—critical organic transients in Earth’s atmosphere. The excited state dynamics of oxygen may also lead to methanediol in methanol-rich interstellar ices in cold molecular clouds, followed by sublimation in star-forming regions and prospective detection of these reactive intermediates in the gas phase by radiotelescopes.
ABSTRACT Isolated MgSiO3 and Mg2SiO4 molecules are shown here to exhibit bright infrared (IR) features that fall close to unattributed astronomical lines observed toward objects known to possess crystalline enstatite and forsterite, minerals of the same respective empirical formulae. These molecules are therefore tantalizing candidates for explaining the origin of such features. Furthermore, the C2v monomer minima of each formula set have dipole moments on the order of 10.0 D or larger making them desirable candidates for radioastronomical observation as enabled through rotational spectroscopic data further provided in this high-level CCSD(T)-F12/cc-pVTZ-F12 quantum chemical study. Astrophysical detection of these molecules could inform the build-up pathways for creating nanocrystals from small molecules in protoplanetary discs or could show the opposite in explaining the destruction of enstatite and forsterite minerals in supernovae events or other high-energy stellar processes. This work also shows that the lowest energy isomers for molecules containing the geologically necessary elements Mg and Si have oxygen bonded between any of the other heavier elements making oxygen the glue for pre-mineralogic chemistry.
A new aluminum-bearing species, OAlNO, which has the potential to impact the chemistry of the Earth's upper atmosphere, is characterized via high-level, ab initio, spectroscopic methods. Meteor-ablated aluminum atoms are quickly oxidized to aluminum oxide (AlO) in the mesosphere and lower thermosphere (MLT), where a steady-state layer of AlO then builds up. Concurrent formation of nitric oxide (NO) in the same region of the atmosphere will lead to the bimolecular formation of the OAlNO molecule. Molecular orbital analysis provides fundamental insights into the chemical bonding and energetic arrangement of the triplet (1 3A″) ground state and singlet (1 1A') excited-state species of OAlNO. Additionally, unpaired electrons on the terminal oxygen atom of triplet (1 3A″) OAlNO cause it to be reactive to atmospheric species, potentially impacting climate science and high-altitude chemistry. The triplet (1 3A″) ground-state species exhibits a large permanent dipole moment useful for rotational spectroscopic detection; however, similar rotational constants to the singlet (1 1A') excited-state species will hamper differentiation in a spectrum. Strong infrared intensities will assist in detection and discrimination of the different spin states and isomers. Repulsive electronic excited states of OAlNO will lead to photolysis of the Al-N bond and formation of various electronic states of AlO + NO through nonadiabatic pathways. Reaction through the OAlNO intermediate represents a means for the production of electronically excited AlO, leading to new chemistry in the atmosphere. Excitation to higher-lying electronic states will lead to fluorescence with a minor Stokes shift, useful for laboratory investigation. Such physical properties of this molecule will allow for new, unexplored chemical pathways in the MLT to be considered.
The oxywater cation (H2OO+), previously shown to form barrierlessly in the gas phase from water cations and atomic oxygen, is proposed here potentially to possess a 2A″ ←4A″ excitation leading to the H2⋯O2+ complex. This complex could then easily decompose into molecular hydrogen and the molecular oxygen cation. The present quantum chemical study shows that the necessary electronic transition takes place in the range of 1.92 eV (645 nm), in the orange-red range of the visible and solar spectrum, and dissociation of the complex only requires 5.8 kcal/mol (0.25 eV). Such a process for the abiotic, gas phase formation of O2 would only need to be photocatalyzed by visible wavelength photons. Hence, such a process could produce O2 at the mesosphere/stratosphere boundary as climate change is driving more water into the upper atmosphere, in the comet 67P/Churyumov-Gerasimenko where surprisingly high levels of O2 have been observed, or at gas-surface (ice) interfaces.
The carriers of the diffuse interstellar bands (DIBs) is one of the longest standing problems in astronomical spectroscopy. This completely unattributed absorption spectrum stretches from the UV to near-IR and can be seen in multiple interstellar sightlines with the strongest features at 443 nm and 578 nm. Additionally, the number of known features grows with every observation. No known molecular spectra have been matched to the DIBs as observed in the interstellar medium. It has been hypothesized that the molecular carriers of the DIBs may play a role in many aspects of astrochemistry including the potential for better understanding of the origins of the chemical building blocks of life. The current approach to answering the problem of the carriers of the DIBs, a form of experimental guess and test, has yet to prove fruitful. The use of chemical theory and computation in answering this problem seems promising, but has been held up by development of the necessary computing capabilities. With state-of-the-art computational techniques we have examined several proposed carriers of the DIBs. In this work, we report on our findings including a molecule that our methods predict to have a strong transition at 442 nm.
New high-level ab initio quartic force field (QFF) methods are explored which provide spectroscopic data for the electronically excited states of the carbon monoxide, water, and formaldehyde cations, sentinel species for expanded, recent cometary spectral analysis. QFFs based on equation-of-motion ionization potential (EOM-IP) with a complete basis set extrapolation and core correlation corrections provide assignment for the fundamental vibrational frequencies of the A˜2B1 and B˜2A1 states of the formaldehyde cation; only three of these frequencies have experimental assignment available. Rotational constants corresponding to these vibrational excitations are also provided for the first time for all electronically excited states of both of these molecules. EOM-IP-CCSDT/CcC computations support tentative re-assignment of the ν1 and ν3 frequencies of the B˜2B2 state of the water cation to approximately 2409.3 cm-1 and 1785.7 cm-1, respectively, due to significant disagreement between experimental assignment and all levels of theory computed herein, as well as work by previous authors. The EOM-IP-CCSDT/CcC QFF achieves agreement to within 12 cm-1 for the fundamental vibrational frequencies of the electronic ground state of the water cation compared to experimental values and to the high-level theoretical benchmarks for variationally-accessible states. Less costly EOM-IP based approaches are also explored using approximate triples coupled cluster methods, as well as electronically excited state QFFs based on EOM-CC3 and the previous (T)+EOM approach. The novel data, including vibrationally corrected rotational constants for all states studied herein, provided by these computations should be useful in clarifying comet evolution or other remote sensing applications in addition to fundamental spectroscopy.
ABSTRACT The recent radioastronomical detection of magnesium dicarbide (MgC2) towards the carbon-rich star IRC+10216 leads to questions about whether this molecule can be observed in other wavelengths, especially with the wealth of IR data being produced by JWST. This present, theoretical spectral characterization, unfortunately, implies that mid-IR observations of MgC2 are unlikely due to small IR transition intensities, overlap with polycyclic aromatic hydrocarbon IR features, low frequencies/long wavelengths, or the relatively small column densities. In spite of this, the full set of fundamental anharmonic vibrational frequencies are provided for each of the 24Mg, 25Mg, and 26Mg isotopologues as are the complete rotational constants for the same set for additional laboratory characterization. Most notably and with regards to 24MgC2, the B0 and C0 (11452.7 and 9362.7 MHz) rotational constants are uniquely provided for the first time. The experimentally derived A0, (B + C)/2, and (B − C)/4 values are within 0.7 % of the presently computed anharmonic results implying similar accuracy for the remaining spectroscopic constants.
