Formation of thiocarbonic acid (HCS) - the sulfur counterpart of carbonic acid (HCO) - in interstellar analog ices.

The first experimental formation of thiocarbonic acid (HCS) is presented in this work from low-temperature interstellar ice analogs composed of hydrogen sulfide (HS) and carbon disulfide (CS) exposed to electron irradiation simulating the impact of galactic cosmic rays (GCRs) on interstellar ices. The recent attention brought to sulfur-bearing molecules, as well as the recent detection of carbonic acid (HCO) in the interstellar medium (ISM), invites the study of the interstellar detection of the sulfur counterpart, thiocarbonic acid. However, the interstellar formation pathways of thiocarbonic acid have remained elusive.

Reparameterized Semiempirical Anharmonic IR Spectra of Neutral PAHs: Benchmarking and Predictions for PAHs with More than Five Rings.

Recently developed, reparameterized PM6 methods can reproduce experimental polycyclic aromatic hydrocarbon (PAH) IR spectra with nearly the same accuracy as state-of-the-art quantum chemical methods but for notably less computational cost. The use of the () semiempirical method (as opposed to () for density functional theory or () for the most accurate coupled cluster theory) allows for full, explicit, quartic force field (QFF), anharmonic computations on PAHs. The anharmonicity also predicts the combination band and overtone frequencies in addition to the fundamentals.

Methanetetrol and the final frontier in ortho acids.

The rapid dissociation of methanetetrol (C(OH)) has been suggested as an impediment to its observation, despite the stability of its substituted derivative orthocarbonates (C(OR)). Here, the tunability of synchrotron-generated vacuum ultraviolet light and the sensitivity of photoionization are exploited to probe the exotic chemistry of the interstellar medium toward the discovery of this molecule. Laboratory-made model ices simulate the composition of astrophysical ices under conditions mimicking those found in dense interstellar molecular clouds with low temperature (5-10 K) and low pressure (<10Torr).

Missing interstellar sulfur in inventories of polysulfanes and molecular octasulfur crowns.

The disparity between predicted sulfur abundances and identified reservoirs of sulfur in cold molecular clouds, also known as the sulfur depletion problem, has remained an ongoing debate over decades. Here, we show in laboratory simulation experiments that hydrogen sulfide (HS) can be converted on ice-coated interstellar grains in cold molecular clouds through galactic cosmic rays processing at 5 K to sulfanes (HS; n = 2-11) and octasulfur (S). This locks the processed hydrogen sulfide as high-molecular weight sulfur-containing molecules thus providing a plausible rationale for the fate of the missing interstellar sulfur.

Investigating Formation Pathways and the Spectroscopic Constants of Aluminum Nitrides from AlH and NH.

Reactions of AlH and NH are shown to produce Al-N containing species that could potentially contribute to the composition of dust grains, like the Murchison CM2 chondritic meteorite. The present computational study couples explicitly correlated coupled cluster theory and density functional theory to produce a gas-phase chemical formation pathway from AlH and NH to AlNH, a stable, cubic structure with the proper Al/N ratio expected for larger aluminum nitride species. Rovibrational spectroscopic analyses of HAlNH, AlNH, AlNH, and AlNH reveal consistently intense vibrational transitions and large absorption cross sections of modes associated with spectral features typically associated with dust grain formation in circumstellar media.

Real-Time Detection of Hydrogen and Ammonia Isotopologues for Impurity Removal and Recovery of Tritium.

To accommodate gas measurements for impurity removal and recovery of tritium, a silver-coated optical or waveguide is employed for collecting Raman scattered signals to determine relative hydrogen and ammonia isotopologue populations in real time. The data and results presented here demonstrate an analytical methodology for the analysis of four ammonia and three hydrogen isotopologues in a hydrogen-deuterium exchange reaction by gas phase Raman spectroscopy. Standard chemometric modeling techniques effectively unravel the signatures of the isotopologues involved observed here; however, a sophisticated quantum chemical approach supports the spectral assignments.

Polycyclic (anti)aromatic hydrocarbons: interstellar formation and spectroscopic characterization of biphenylene and benzopentalene.

Formation of biphenylene (CH) and its isomer benzopentalene, CH, may act as a consumption route for -benzyne (-CH) in interstellar clouds such as TMC-1. MRCI-F12 and CCSD(T)-F12 potential energy surfaces show that -CH dimerization is possible through a -symmetry single-bond association to a (CH) precursor before isomerization to (CH) and subsequently CH. Formation of a bimolecular product set from either species is energetically hindered, allowing (CH) and CH to stabilize radiatively.

The Formation of MgS & MgO Monomers and Dimers from Magnesium, Oxygen, and Sulfur Hydrides.

The reaction of SH + MgH is shown in this quantum chemical study to be an energetically downhill pathway leading to the astronomically known MgS molecule as well as H. Hence, the formation of MgS in the gas phase is now shown to be a possible contributor to this diatomic molecule's observed astronomical abundance. Similarly, MgO can form through a related process with OH + MgH, but the energy costs are higher, implying that MgH could be consumed in reactions with SH faster than those with OH.

Pinpointing the CN Stretch Frequency of Neutral Cyano-Polycyclic Aromatic Hydrocarbons: A Laboratory and Quantum Chemical Spectroscopic Study of 9-Cyanoanthracene.

The CN stretch frequency of neutral, gas-phase 9-cyanoanthracene is 2207 cm (4.531 μm) based on high-resolution infrared absorption experiments coupled with a new hybrid anharmonic quantum chemical methodology. A broad band (full-width at half-maximum of 47 cm) is observed and assigned to multiple transitions, including the CN stretch fundamental and various combination bands that gather intensity from strong anharmonic coupling with the bright CN stretch.

Quantum Chemical Determination of Molecular Dye Candidates for Non-Invasive Bioimaging.

Molecular dyes containing carbazole-based π bridges and/or julolidine-based donors should be promising molecules for intense SWIR emission with potential application to molecular bioimaging. This study stochastically analyzes the combinations of more than 250 organic dyes constructed within the D-π-D (or equivalently D-B-D) motif. These dyes are built from 22 donors (D) and 14 π bridges (B) and are computationally examined using density functional theory (DFT).

Infrared Spectroscopy of Isocyano-Polycyclic Aromatic Hydrocarbons: The NC Stretch.

Anharmonic computations reveal an intense, narrow (20 cm, 0.043 μm) absorption feature at approximately 2160 cm (4.63 μm) in the vibrational spectra of 14 prototypical singly isocyano-substituted polycyclic aromatic hydrocarbons (NC-PAHs) attributed to the NC stretching mode.

Computing Accurate & Reliable Rovibrational Spectral Data for Aluminum-Bearing Molecules.

The difficulty of quantum chemically computing vibrational, rotational, and rovibrational reference data via quartic force fields (QFFs) for molecules containing aluminum appears to be alleviated herein using a hybrid approach based upon CCSD(T)-F12b/cc-pCVTZ further corrected for conventional CCSD(T) scalar relativity within the harmonic terms and simple CCSD(T)-F12b/cc-pVTZ for the cubic and quartic terms: the F12-TcCR+TZ QFF. Aluminum containing molecules are theorized to participate in significant chemical processes in both the Earth's upper atmosphere as well as within circumstellar and interstellar media. However, experimental data for the identification of these molecules are limited, showcasing the potential for quantum chemistry to contribute significant amounts of spectral reference data.

Battle of the CH motions: aliphatic versus aromatic contributions to astronomical PAH emission and exploration of the aliphatic, aromatic, and ethynyl CH stretches.

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.

Computational Mechanistic Analysis of the Formation of the Magnesium Silicate Monomers MgSiO and MgSiO.

Silicate grains comprise a large fraction of cosmic dust, motivating a need to understand how they form. The current body of work on silicates generally reflects the abundance of silicate grains, yet models for their formation often do not consider silicate chemistry on the smallest scale, which can form species available for dust grain nucleation processes. In order to expand upon previous attempts to bridge this gap in silicate chemistry, novel gas-phase reaction pathways for the magnesium silicate monomers enstatite (MgSiO) and forsterite (MgSiO) from MgH, HO, and SiO are presently computed using highly accurate quantum chemical calculations.

Formation of Paraldehyde (CHO) in Interstellar Analog Ices of Acetaldehyde Exposed to Ionizing Radiation.

Acetaldehyde (CHCHO) plays a crucial role in the synthesis of prebiotic molecules such as amino acids, sugars, and sugar-related compounds, and in the progress of chain reaction polymerization in deep space. Here, we report the first formation of the cyclic acetaldehyde trimer - paraldehyde (CHO) - in low-temperature interstellar analog ices exposed to energetic irradiation as proxies of galactic cosmic rays (GCRs). Utilizing vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry and isotopic substitution experiments, paraldehyde was identified in the gas phase during the temperature-programmed desorption of the irradiated acetaldehyde ices based on the calculated adiabatic ionization energies and isomer-specific dissociative fragmentation patterns upon photoionization.

Vibrational and Rovibrational Spectroscopic Data for the Ground and First-Excited States of Phosgene (COCl), Formic Acid (HCOOH), and Chloroformic Acid (ClCOOH).

While "black box" quantum chemical computations for the determination of rovibronic spectral data are not quite at hand, the present work utilizes the titular molecules to showcase how excited-state quantum chemical methods can be conjoined to quartic force field (QFF) anharmonic rovibrational treatments to provide novel and useful predictions for such data. This work employs hybrid QFFs with explicitly correlated coupled cluster theory along with the equation-of-motion formalism to generate harmonic force constants and time-dependent density functional theory (TD-DFT) to produce anharmonic force constants for the generation of electronically excited-state rovibrational spectral data, in effect, rovibronic spectral data. Specific spectroscopic results from this work show that the fundamental C═O stretch in phosgene as well as in - and -formic acid drop from the region of around 1800 cm to close to 1100 cm or less in the first excited states of each molecule.

The fundamental vibrational frequencies and spectroscopic constants of the COH isomers: molecules known in simulated interstellar ice analogues.

While -glyoxal may not be easily observable in astronomical sources through either IR or radioastronomy due to its symmetry, its conformer along with the -HCOCO epoxide isomer should be ready targets for astrochemical detection. The present quantum chemical study shows that not only are both molecular isomers strongly polar, they also have notable IR features and low isomerisation energies of 4.1 kcal mol and 10.

Picking up Good Vibrations through Quartic Force Fields and Vibrational Perturbation Theory.

Quartic force fields (QFFs) define sparse potential energy surfaces (compared to semiglobal surfaces) that are the cheapest and easiest means of computing anharmonic vibrational frequencies, especially when utilized with second-order vibrational perturbation theory (VPT2). However, flat and shallow potential surfaces are exceedingly difficult for QFFs to treat through a combination of numerical noise in the often numerically computed derivatives and in competing energy factors in the composite energies often utilized to provide high-level spectroscopic predictions. While some of these issues can be alleviated with analytic derivatives, hybrid QFFs, and intelligent choices in coordinate systems, the best practice is for predicting good molecular vibrations via QFFs is to understand what they cannot do, and this manuscript documents such cases where QFFs may fail.

Rovibrational analysis of AlCO3, OAlO2, and HOAlO2 for possible atmospheric detection.

The lack of observational data for the AlO molecule in the mesosphere/lower thermosphere may be due to ablated aluminum reacting quickly to form other species. Previously proposed reaction pathways show that aluminum could be ablated in the atmosphere from meteoritic activity, but there currently exist very limited spectroscopic data on the intermediates in these reactions, limiting the possible detection of said molecules. As such, rovibrational spectroscopic data are computed herein using quartic force field methodology at four different levels of theory for the neutral intermediates AlCO3, OAlO2, and HOAlO2.

Assigning the CH stretch overtone spectrum of benzene and naphthalene with extension to anthracene and tetracene using 2- and 3-quanta anharmonic quantum chemical computations.

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.

Theoretical Rotational and Vibrational Spectral Data for the Hypermagnesium Oxide Species MgO and MgO.

While magnesium is astronomically observed in small molecules, it largely serves as a contributor to silicate grains, though how these grains form is not well-understood. The smallest hypermagnesium oxide compounds (Mg O/Mg O ) may play a role in silicate formation, but little vibrational reference data exist. As such, anharmonic spectroscopic data are computed for Mg O, Mg O, and Mg O using quartic force fields (QFFs).

Bridging the gap: viable reaction pathways from tetrahedrane to benzyne.

The addition of sp-carbon-containing molecules to polycyclic sp tetrahedrane (-CH) results in the formation of both -benzyne (-CH) and benzene (-CH). Since both -CH and -CH have been detected in the interstellar medium (ISM), providing additional pathways for their possible astrochemical formation mechanisms can lead to the discovery of other molecules, such as -CH, benzvalyne, and vinylidene (:CCH). Addition of diatomic carbon (C), the ethynyl radical (CH), vinylidene, and acetylene (HCCH) to -CH is undertaken in individual pathways through high-level quantum chemical computations at the CCSD(T)-F12b/cc-pVTZ-F12 level of theory.

The infrared absorption spectrum of phenylacetylene and its deuterated isotopologue in the mid- to far-IR.

Anharmonicity strongly influences the absorption and emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules. Here, IR-UV ion-dip spectroscopy experiments together with detailed anharmonic computations reveal the presence of fundamental, overtone, as well as 2- and 3-quanta combination band transitions in the far- and mid-infrared absorption spectra of phenylacetylene and its singly deuterated isotopologue. Strong absorption features in the 400-900 cm-1 range originate from CH(D) in-plane and out-of-plane wags and bends, as well as bending motions including the C≡C and CH bonds of the acetylene substituent and the aromatic ring.

Interstellar formation of glyceric acid [HOCHCH(OH)COOH]-The simplest sugar acid.

Glyceric acid [HOCHCH(OH)COOH]-the simplest sugar acid-represents a key molecule in biochemical processes vital for metabolism in living organisms such as glycolysis. Although critically linked to the origins of life and identified in carbonaceous meteorites with abundances comparable to amino acids, the underlying mechanisms of its formation have remained elusive. Here, we report the very first abiotic synthesis of racemic glyceric acid via the barrierless radical-radical reaction of the hydroxycarbonyl radical (HOĊO) with 1,2-dihydroxyethyl (HOĊHCHOH) radical in low-temperature carbon dioxide (CO) and ethylene glycol (HOCHCHOH) ices.

Performance of EOM-CCSD(T)(a)*-Based Quartic Force Fields in Computing Fundamental, Anharmonic Vibrational Frequencies of Molecular Electronically Excited States with Application to the ″ State of :CCH (Vinylidene).

Highly accurate anharmonic vibrational frequencies of electronically excited states are not as easily computed as their ground electronic state counterparts, but recently developed approximate triple excited state methods may be changing that. One emerging excited state method is equation of motion coupled cluster theory at the singles and doubles level with perturbative triples computed via the (a)* formalism, EOMEE-CCSD(T)(a)*. One of the most employed means for the ready computation of vibrational anharmonic frequencies for ground electronic states is second-order vibrational perturbation theory (VPT2), a theory based on quartic force fields (QFFs),fourth-order Taylor series expansions of the potential portion of the internuclear Watson Hamiltonian.