Formation of Magnesium and Aluminum Oxides from Water and Metal Hydrides: Creation of the Smallest Ruby

Watrous A.G., Westbrook B.R., Davis M.C., Fortenberry R.C.

ABSTRACT This work provides the first full set of vibrational and rotational spectral data needed to aid in the detection of AlH3OH2, SiH3OH (silanol), and SiH3NH2 (silylamine) in astrophysical or simulated laboratory environments through the use of quantum chemical computations at the CCSD(T)-F12b level of theory employing quartic force fields for the three molecules of interest. Previous work has shown that SiH3OH and SiH3NH2 contain some of the strongest bonds of the most abundant elements in space. AlH3OH2 also contains highly abundant atoms and represents an intermediate along the reaction pathway from H2O and AlH3 to AlH2OH. All three of these molecules are also polar with AlH3OH2 having the largest dipole of 4.58 D and the other two having dipole moments in the 1.10–1.30 D range, large enough to allow for the detection of these molecules in space through rotational spectroscopy. The molecules also have substantial infrared intensities with many of the frequencies being over 90 km mol−1 and falling within the currently uncertain 12–17 μm region of observed infrared spectra. The most intense frequency for AlH3OH2 is ν9 that has an intensity of 412 km mol−1 at 777.0 cm−1 (12.87 μm). SiH3OH has an intensity of 183 km mol−1 at 1007.8 cm−1 (9.92 μm) for ν5, and SiH3NH2 has an intensity of 215 km mol−1 at 1000.0 cm−1 (10.00 μm) for ν7.

Pardo J.R., Cabezas C., Fonfría J.P., Agúndez M., Tercero B., de Vicente P., Guélin M., Cernicharo J.

After the previous discovery of MgC3N and MgC4H in IRC +10216, a deeper Q-band (31.0–50.3 GHz) integration on this source had revealed two additional series of harmonically related doublets that we assigned on the basis of quantum mechanical calculations to the larger radicals MgC5N and MgC6H. The results presented here extend and confirm previous results on magnesium-bearing molecules in IRC +10216. We derived column densities of (4.7 ± 1.3) × 1012 for MgC5N and (2.0 ± 0.9) × 1013 for MgC6H, which imply that MgC5N/MgC3N = 0.5 and MgC6H/MgC4H = 0.9. Therefore, MgC5N and MgC6H are present with column densities not so different from those of the immediately shorter analogs. The synthesis of these large magnesium cyanides and acetylides in IRC +10216 can be explained for their shorter counterparts by a two-step process initiated by the radiative association of Mg+ with large cyanopolyynes and polyynes, which are still quite abundant in this source, followed by the dissociative recombination of the ionic complexes.

Turner A.M., Chandra S., Fortenberry R.C., Kaiser R.I.

Interstellar ices processed by galactic cosmic rays: Utilizing photoionization reflectron time-of-time mass spectrometry, irradiated interstellar ice analogues of acetylene and ammonia were found to form ethynamine and 2H-azirine.

Watrous A.G., Davis M.C., Fortenberry R.C.

Small, inorganic hydrides are likely hiding in plain sight, waiting to be detected toward various astronomical objects. AlH2OH can form in the gas phase via a downhill pathway, and the present, high-level quantum chemical study shows that this molecule exhibits bright infrared features for anharmonic fundamentals in regions above and below that associated with polycyclic aromatic hydrocarbons. AlH2OH along with HMgOH, HMgNH2, and AlH2NH2 are also polar with AlH2OH having a 1.22 D dipole moment. AlH2OH and likely HMgOH have nearly unhindered motion of the hydroxyl group but are still strongly bonded. This could assist in gas phase synthesis, where aluminum oxide and magnesium oxide minerals likely begin their formation stages with AlH2OH and HMgOH. This work provides the spectral data necessary to classify these molecules such that observations as to the buildup of nanoclusters from small molecules can possibly be confirmed.

Fortenberry R.C., DeYonker N.J.

ConspectusOur two groups have both independently and collaboratively been pushing quantum-chemical techniques to produce highly accurate predictions of anharmonic vibrational frequencies and spectroscopic constants for molecules containing atoms outside of the typical upper p block. Methodologies employ composite approaches, relying on various levels of coupled cluster theory-most often at the singles, doubles, and perturbative triples level-and quartic force field constructions of the potential portion of the intramolecular Watson Hamiltonian. Such methods are known to perform well for organic species, and we have extended this to molecules containing atoms outside of this realm.One notable atom that has received much attention in this application is magnesium. Mg is the second-most-abundant element in the Earth's mantle, and while molecules containing this element are among the confirmed astrochemicals, its further atomic abundance in the galaxy implies that many more molecules (both purely inorganic and organometallic) containing element 12 exist in astrophysical regions in chemical sizes between those of atoms and dust-sized nanocrystals. Our approach discussed herein is producing quality benchmarks and predicting novel data for magnesium-bearing molecules.The story is similar for Al and Si, which are also notably abundant in both rocky bodies and the universe at large. While Na, Sc, and Cu may not be as abundant as Mg, Al, and Si, molecules containing Na and transition metals have also previously been reported to be detected beyond the Earth. Consequently, the need to produce spectral reference data for molecules containing such atoms is growing. While several experimental groups (including, notably, the groups in Arizona, Boston, and France/Spain) have clearly led the way in detection of inorganic/organometallic molecules in space, computational support and even rational design can provide novel avenues for the detection of molecules containing atoms not typically studied in most laboratories. The application of quantum chemistry to other elements beyond carbon and its cronies at the top right of the periodic table promises a better understanding of the observable universe. It will also provide novel and fundamental chemical insights pushing the central science into new molecular territory.

Nanayakkara S., Freindorf M., Tao Y., Kraka E.

The unified reaction valley approach combined with the local vibrational mode and ring puckering analysis is applied to investigate the hydrogen evolution from water in the presence of small hydrides such as BH3, metal hydrides as AlH3, and their derivatives. We studied a series of reactions involving BH3, AlH3, B2H6, Al2H6, and AlH3BH3 with one- and two-water molecules, considering multiple reaction paths. In addition, the influence of the aqueous medium was examined. A general reaction mechanism was identified for most of the reactions. Those that deviate could be associated with unusually high reaction barriers with no hydrogen release. The charge transfer along the reaction path suggests that a viable hydrogen release is achieved when the catalyst adopts the role of a charge donor during the chemical processes. The puckering analysis showed that twistboat and boat forms are the predominant configurations in the case of an intermediate six-membered ring formation, which influences the activation barrier. The local mode analysis was used as a tool to detect the H-H bond formation as well as to probe catalyst regenerability. Based on the correlation between the activation energy and the change in the charge separation for cleaving O-H and B(Al)-H bonds, two promising subsets of reactions could be identified along with prescriptions for lowering the reaction barrier individually with electron-donating/withdrawing substituents.

Fortenberry R.C., Trabelsi T., Francisco J.S.

The astrophysical buildup of premineral nanocrystals from atoms to the smallest network-covalent solids will require observations of various small molecules containing the most common elements in minerals including aluminum and oxygen. The present work utilizes high-level quantum chemical quartic force field (QFF) approaches to produce anharmonic vibrational frequencies and spectroscopic constants for such species. The computed Beff for the astrochemically known AlOH molecule at 15780.5 MHz is a mere 40 MHz above the experimental value implying that the Beff for OAlOH at 5580.9 MHz is similarly accurate. The additional 7.31 D dipole moment in OAlOH implies that this molecule is a viable target for interstellar observation. Unlike the other anharmonic vibrational frequencies reported in this work, the Al-O-H bending frequencies in both AlOH and OAlOH are poorly described in the present QFF results. However, this failing actually highlights the fact that these bends are exceptionally floppy yet with counterintuitive exceedingly strong bonding. The Al-O bond energies are 128.2 and 107.2 kcal/mol, respective of AlOH and OAlOH, while the barriers to linearity are meager 16.6 and 380.7 cm-1 (0.1 and 1.1 kcal/mol).

Piani L., Marrocchi Y., Rigaudier T., Vacher L.G., Thomassin D., Marty B.

An unexpected source of Earth's water The abundances of Earth's chemical elements and their isotopic ratios can indicate which materials formed Earth. Enstatite chondrite (EC) meteorites provide a good isotopic match for many elements but are expected to contain no water because they formed in the hot inner Solar System. This would require Earth's water to be from a different source, such as comets. Piani et al. measured hydrogen contents and deuterium/hydrogen ratios (D/H) in 13 EC meteorites (see the Perspective by Peslier). They found far more hydrogen than is commonly assumed, with D/H close to that of Earth's mantle. Combining these data with cosmochemical models, they show that most of Earth's water could have formed from hydrogen delivered by EC meteorites. Science , this issue p. 1110 ; see also p. 1058

Turner A.M., Koutsogiannis A.S., Kleimeier N.F., Bergantini A., Zhu C., Fortenberry R.C., Kaiser R.I.

The formation of isomers of C2H2O—ketene (H2CCO), ethynol (HCCOH), and oxirene (c-CHCHO)—was investigated in interstellar ice analogs composed of carbon monoxide and water. Using tunable photoionization time-of-flight mass spectrometry to selectively ionize the isomer of interest, ketene and ethynol were detected as reaction products, but oxirene remains elusive. These findings demonstrate that organic compounds that are precursors to complex organic molecules can form without an organic source of carbon. Furthermore, we report the first plausible detection of ethynol in astrophysically relevant ices. These investigations were supported by theoretical calculations describing reaction energies, pathways, ionization energies, and harmonic frequencies.

Doerksen E.S., Fortenberry R.C.

After removal the geologically and astrophysically underabundant Be, B, and F atoms from consideration, the strongest X–Y bonds in HnX–YHm hydrides are for O bonding with Al, Si, and Mg. These are ...

Westbrook B.R., Fortenberry R.C.

The low-frequency vibrational fundamentals of D2h inorganic oxides are readily modeled by heuristic scaling factors at fractions of the computational cost compared to explicit anharmonic frequency computations. Oxygen and the other elements in the present study are abundant in geochemical environments and have the potential to aggregate into minerals in planet-forming regions or in the remnants of supernovae. Explicit quartic force field computations at the CCSD(T)-F12b/cc-pVTZ-F12 level of theory generate scaling factors that accurately predict the anharmonic frequencies with an average error of less than 1.0 cm-1 for both the metal-oxygen stretching frequencies and the torsion and antisymmetric stretching frequencies. Inclusion of hydrogen motions is less absolutely accurate but is similarly relatively predictive. The fundamental vibrational frequencies for the seven tetra-atomic inorganic oxides examined presently fall below 876 cm-1 and most of the hydrogenated species do as well. Additionally, ν6 for the SiO dimer is shown to have an intensity of 562 km mol-1, with each of the other molecules having one or more frequencies with intensities greater than 80 km mol-1, again with most in the low-frequency infrared range. These intensities and the frequencies computed in the present study should assist in laboratory characterization and potential interstellar or circumstellar observation.

Del Rio W.A., Fortenberry R.C.

• H 2 OO + may be an intermediate in the creation of molecular oxygen in comets. • The ground state is highly dipolar making it observable with radiotelescopes. • The O O bond is stronger here than even in hydrogen peroxide. The oxywater cation (H 2 OO + ) has been suggested as a possible intermediate in the abiotic formation of molecular oxygen in the gas phase potentially relating to the chemistry of comets or the upper atmosphere. Accurate spectroscopic data of the oxywater cation is required for astrochemical or atmospheric observations to have any ability to support or refute the role that this molecule could play in such processes. This work improves upon the existing literature by using a quartic force field approach at a high-level of theory and examines both the X ∼ 2 A ′ and 1 4 A ″ states of H 2 OO + . The weaker bonding in the quartet state makes it behave more like water than the doublet state, but both are notably different from gas phase water. Both states have four bright vibrational peaks corresponding to the two O H stretches, the out-of-plane bend, and the symmetric bend. These bands will probably still be in the same range as those from H 2 O in some cases making H 2 OO + non-trivial to differentiate from water in the infrared. Fortunately, both states of the molecule have large dipole moments (3.91 D & 1.96 D, respective of the X ∼ 2 A ′ and 1 4 A ″ states) allowing them to be detected, distinguished, and characterized in the laboratory or potentially using ground-based radio telescopes with the spectroscopic data generated here.

Cernicharo J., Cabezas C., Pardo J.R., Agúndez M., Bermúdez C., Velilla-Prieto L., Tercero F., López-Pérez J.A., Gallego J.D., Fonfría J.P., Quintana-Lacaci G., Guélin M., Endo Y.

We report on the detection of two series of harmonically related doublets in IRC +10216. From the observed frequencies, the rotational constant of the first series is B = 1380.888 MHz and that of the second series is B = 1381.512 MHz. The two series correspond to two species with a 2Σ electronic ground state. After considering all possible candidates, and based on quantum chemical calculations, the first series is assigned to MgC3N and the second to MgC4H. For the latter species, optical spectroscopy measurements support its identification. Unlike diatomic metal-containing molecules, the line profiles of the two new molecules indicate that they are formed in the outer layers of the envelope, as occurs for MgNC and other polyatomic metal-cyanides. We also confirm the detection of MgCCH that was previously reported from the observation of two doublets. The relative abundance of MgC3N with respect to MgNC is close to one while that of MgC4H relative to MgCCH is about ten. The synthesis of these magnesium cyanides and acetylides in IRC +10216 can be explained in terms of a two-step process initiated by the radiative association of Mg+ with large cyanopolyynes and polyynes followed by the dissociative recombination of the ionic complexes.

McGuire B.A.

To date, 204 individual molecular species, comprised of 16 different elements, have been detected in the interstellar and circumstellar medium by astronomical observations. These molecules range in size from two atoms to seventy, and have been detected across the electromagnetic spectrum from cm-wavelengths to the ultraviolet. This census presents a summary of the first detection of each molecular species, including the observational facility, wavelength range, transitions, and enabling laboratory spectroscopic work, as well as listing tentative and disputed detections. Tables of molecules detected in interstellar ices, external galaxies, protoplanetary disks, and exoplanetary atmospheres are provided. A number of visual representations of this aggregate data are presented and briefly discussed in context.

Kodolányi J., Vollmer C., Hoppe P., Müller M.

Dotson J.R., Palmer C.Z., Fortenberry R.C.

Firth R.A., Fortenberry R.C.

One of the most abundant Al-containing molecules detected in the interstellar medium (ISM) is AlOH. Over the past several years, there have been various pathways proposed for the formation of AlOH in the ISM, including reactions between AlO and H2 or H2O. However, these pathways include an energetic barrier from a transition state that likely prevents the reaction from progressing efficiently in the low temperature/low pressure environment of the ISM. Recently, a barrierless pathway for formation of AlOH from AlO and AlH has been proposed for the formation of AlOH. Even so, only one of these species really needs to contain an aluminum atom. To account for this, alternative but related pathways reacting the known interstellar molecule AlO with XH and AlH with XO (X = Mg, Si, P, or S) to form AlOH are explored with high accuracy quantum chemical calculations via CCSD(T)-F12b/cc-pVTZ-F12. Each third row element has at least one pair of reactants that lead to exothermic formation of AlOH. These reactions can go on to form other aluminum oxides and aluminum oxide clusters that may, in part, lead to the formation of interstellar dust grains.

Fortenberry R.C., McGuire B.A.

Abstract The formation of silicon monosulfide (SiS) in space appears to be a difficult process, but the present work shows that a previously excluded pathway may contribute to its astronomical abundance. Reaction of the radicals SH + SiH produces SiS with a submerged transition state and generates a stabilizing H2 molecule as a product to dissipate the kinetic energy. Such is a textbook chemical reaction for favorable gas-phase chemistry. While previously proposed mechanisms reacting atomic sulfur and silicon with SiH, SH, and H2S will still be major contributors to the production of SiS, an abundance of SiS in certain regions could be a marker for the presence of SiH where it has previously been unobserved. These quantum chemically computed reaction profiles imply that the silicon-chalcogen chemistry of molecular clouds, shocked regions, or protoplanetary disks may be richer than previously thought. Quantum chemical spectral data for the intermediate cis- and trans-HSiSH are also provided to aid in their potential spectroscopic characterization.

Fortenberry R.C.

Firth R.A., Bell K.M., Fortenberry R.C.

Fortenberry R.C.

Wang L., Jiang X., Trabelsi T., Wang G., Francisco J.S., Zeng X., Zhou M.

Palmer C.Z., Fortenberry R.C.

Abstract A dust nucleating agent may be present in interstellar or circumstellar media that has gone seemingly undetected and unstudied for decades. Some analyses of the Murchison CM2 meteorite suggest that at least some of the aluminum present within condensed as aluminum nitrides instead of the long-studied, but heretofore undetected suite of aluminum oxides. The present theoretical study utilizes explicitly correlated coupled cluster theory and density functional theory to provide a formation pathway from alane (AlH3) and ammonia to the cyclic structure Al2N2H4, which has the proper Al/N ratio expected of bulk aluminum nitrides. Novel rovibrational spectroscopic constants are computed for alane and the first two formed structures, AlNH6 and AlNH4, along the reaction pathway for use as reference in possible laboratory or observational studies. The ν 8 bending frequency for AlNH6 at 755.7 cm−1 (13.23 μm) presents a vibrational transition intensity of 515 km mol−1, more intense than the antisymmetric C−O stretch of carbon dioxide, and contains a dipole moment of 5.40 D, which is ∼3× larger than that of water. Thus, the present reaction pathway and rovibrational spectroscopic analysis may potentially assist in the astrophysical detection of novel, inorganic species which may be indicative of larger dust grain nucleation.

Trabelsi T., Esposito V.J., Francisco J.S.

Flint A.R., Watrous A.G., Westbrook B.R., Patel D.J., Fortenberry R.C.

Aims. The detection of c-C3HC2H and possible future detection of c-C3HCN provide new molecules for reaction chemistry in the dense interstellar medium (ISM) where R-C2H and R-CN species are prevalent. Determination of chemically viable c-C3HC2H and c-C3HCN derivatives and their prominent spectral features can accelerate potential astrophysical detection of this chemical family. This work characterizes three such derivatives: c-C3(C2H)2, c-C3(CN)2, and c-C3(C2H)(CN). Methods. Interstellar reaction pathways of small carbonaceous species are well replicated through quantum chemical means. Highly accurate cc-pVXZ-F12/CCSD(T)-F12 (X = D,T) calculations generate the energetics of chemical formation pathways as well as the basis for quartic force field and second-order vibrational perturbation theory rovibrational analysis of the vibrational frequencies and rotational constants of the molecules under study. Results. The formation of c-C3(C2H)2 is as thermodynamically and, likely, as stepwise favorable as the formation of c-C3HC2H, rendering its detectability to be mostly dependent on the concentrations of the reactants. Both c-C3(C2H)2 and c-C3(C2H)(CN) will be detectable through radioastronomical observation with large dipole moments of 2.84 D and 4.26 D, respectively, while c-C3(CN)2 has an exceedingly small and likely unobservable dipole moment of 0.08 D. The most intense frequency for c-C3(C2H)2 is v2 at 3316.9 cm–1 (3.01 μm), with an intensity of 140 km mol–1. The mixed-substituent molecule c-C3(C2H)(CN) has one frequency with a large intensity, v1, at 3321.0 cm–1 (3.01 μm), with an intensity of 82 km mol–1. The molecule c-C3(CN)2 lacks intense vibrational frequencies within the range that current instrumentation can readily observe. Conclusions. Both c-C3(C2H)2 and c-C3(C2H)(CN) are viable candidates for astrophysical observation, with favorable reaction profiles and spectral data produced herein, but c-C3(CN)2 will not be directly observable through any currently available remote sensing means, even if it forms in large abundances.

Harwick O.A., Fortenberry R.C.

Aluminum forms strong bonds to oxygen, nitrogen, and hydrogen, but these motifs are relatively rare in the interstellar medium (ISM). However, no chemical rationale accounts for this observation. The cyclic molecules AlO2, HAlO2, and (HN)OAlH are quintessential examples of the aforementioned phenomenon, and the latter two are isoelectronic with each other. Matrix-isolation spectroscopic data and prior theoretical computations exist for cyclic AlO2, although it has yet to be observed astronomically. However, high-level theoretical data provided herein generate useful predictions for and insights into spectral data for these three molecules. Anharmonic vibrational frequencies and rotational constants are determined in this work for these molecules via quartic force fields at the CCSD(T)-F12b/cc-pVTZ-F12 level of theory and with canonical CCSD(T) for consideration of basis set convergence, core electron correlation, and relativity (CcCR). The present work finds that these aluminum oxides are weak-to-moderate infrared emitters but are much stronger microwave emitters with dipole moments of 4.95 D, 4.55 D, and 3.76 D respective of cyclic AlO2 (c-AlO2), cyclic HAlO2 (c-HAlO2), and cyclic (HN)OAlH (c-(HN)OAlH). Correlation to argon-matrix experiments for c-AlO2 is within the expected matrix shift for the ν3 bend computed here to be 527.8 cm−1. Astrophysical detection of these molecules could imply their role in the creation or degradation of aluminum-containing nanocrystals and interstellar dust of importance for the formation of rocky bodies, and the spectral data computed in this work should be able to assist in such classification.

Flint A.R., Fortenberry R.C.

Abstract Five substituted cyclopropenylidene derivatives (c-C3HX, X=CN, OH, F, NH2), all currently undetected in the interstellar medium (ISM), are found herein to have mechanistically viable, gas-phase formation pathways through neutral–neutral additions of ·X onto c-C3H2. The detection and predicted formation mechanism of c-C3HC2H introduces a need for the chemistry of c-C3H2 and any possible derivatives to be more fully explored. Chemically accurate CCSD(T)-F12/cc-pVTZ-F12 calculations provide exothermicities of additions of various radical species to c-C3H2, alongside energies of submerged intermediates that are crossed to result in product formation. Of the novel reaction mechanisms proposed, the addition of the cyano radical is the most exothermic at -16.10 kcal mol−1. All five products are found to or are expected to have at least one means of associating barrierlessly to form a submerged intermediate, a requirement for the cold chemistry of the ISM. The energetically allowed additions arise as a result of the strong electrophilicity of the radical species as well as the product stability gained through substituent-ring conjugation.