Mass-independent oxygen isotopic partitioning during gas-phase SiO2 formation

Mechanisms and Thermochemistry of Reactions of SiO and Si2O2 with OH and H2O

This paper reports on computational studies of gas-phase reactions of SiO and Si₂O₂. The oxidation of SiO can initiate efficient formation of silica or silicate dust particles in a wide range of environments. Both OH radicals and H₂O molecules are often present in these environments, and their reactions with SiO and the smallest SiO cluster, Si₂O₂, affect the efficiency of eventual dust formation. Density functional theory calculations on these reactions, benchmarked against accurate coupled cluster calculations, indicate that the Si₂O₂ + OH reaction should be faster than SiO + OH. The reaction SiO + H₂O → SiO₂ + H₂ is both endothermic and has high activation energies to reaction. Instead, the formation of molecular complexes is efficient. The reaction of Si₂O₂ with H₂O, which has been suggested as efficient for producing Si₂O₃, might not be as efficient as previously thought. If the H₂O molecules dissociate to form OH radicals, oxidation of SiO and Si₂O₂ could be accelerated instead.

Counterintuitive Chemistry: Carbene Stabilization of Zero-Oxidation State Main Group Species

Carbenes have evolved from transient laboratory curiosities to a robust, diverse, and surprisingly impactful ligand class. A variety of different carbenes have significantly contributed to the development of low-oxidation state main group chemistry. This Perspective focuses upon advances in the chemistry of carbene complexes containing main group element cores in the formal oxidation state of zero, including their diverse synthetic strategies, unusual bonding and structural motifs, and utility in transition metal coordination chemistry and activation of small molecules.

Oxygen and magnesium mass-independent isotopic fractionation induced by chemical reactions in plasma

Both the physical effect and the chemical conditions at the origin of the oxygen isotope variations in the solar system have been puzzling questions for 50 y. The data reported here bring the MIF effect (the mass-independent fractionation originally identified on ozone) back to the center of the debate. Similar to Ti isotopes, we observe that the MIF effect for O and Mg is triggered by redox reactions in plasmas. These observations reinforce the idea of a universal mechanism observable in photochemical reactions when molecular collisions involving indistinguishable isotopes yield a symmetrical complex stabilized as a chemical product.

Enrichment or depletion ranging from −40 to +100% in the major isotopes ¹⁶O and ²⁴Mg were observed experimentally in solids condensed from carbonaceous plasma composed of CO₂/MgCl₂/Pentanol or N₂O/Pentanol for O and MgCl₂/Pentanol for Mg. In NanoSims imaging, isotope effects appear as micrometer-size hotspots embedded in a carbonaceous matrix showing no isotope fractionation. For Mg, these hotspots are localized in carbonaceous grains, which show positive and negative isotopic effects so that the whole grain has a standard isotope composition. For O, no specific structure was observed at hotspot locations. These results suggest that MIF (mass-independent fractionation) effects can be induced by chemical reactions taking place in plasma. The close agreement between the slopes of the linear correlations observed between δ²⁵Mg versus δ²⁶Mg and between δ¹⁷O versus δ¹⁸O and the slopes calculated using the empirical MIF factor η discovered in ozone [M. H. Thiemens, J. E. Heidenreich, III. Science 219, 1073–1075; C. Janssen, J. Guenther, K. Mauersberger, D. Krankowsky. Phys. Chem. Chem. Phys. 3, 4718–4721] attests to the ubiquity of this process. Although the chemical reactants used in the present experiments cannot be directly transposed to the protosolar nebula, a similar MIF mechanism is proposed for oxygen isotopes: at high temperature, at the surface of grains, a mass-independent isotope exchange could have taken place between condensing oxides and oxygen atoms originated form the dissociation of CO or H₂O gas.

CHONDRITES AND THEIR COMPONENTS: RECORDS OF EARLY SOLAR SYSTEM PROCESSES

Chondrites consist of three major components: refractory inclusions (Ca,Al‐rich inclusions [CAIs] and amoeboid olivine aggregates), chondrules, and matrix. Here, I summarize recent results on the mineralogy, petrology, oxygen, and aluminum‐magnesium isotope systematics of the chondritic components (mainly CAIs in carbonaceous chondrites) and their significance for understanding processes in the protoplanetary disk (PPD) and on chondrite parent asteroids. CAIs are the oldest solids originated in the solar system: their U‐corrected Pb‐Pb absolute age of 4567.3 ± 0.16 Ma is considered to represent time 0 of its evolution. CAIs formed by evaporation, condensation, and aggregation in a gas of approximately solar composition in a hot (ambient temperature >1300 K) disk region exposed to irradiation by solar energetic particles, probably near the protoSun; subsequently, some CAIs were melted in and outside their formation region during transient heating events of still unknown nature. In unmetamorphosed, type 2–3.0 chondrites, CAIs show large variations in the initial 26Al/27Al ratios, from <5 × 10–6 to ~5.25 × 10–5. These variations and the inferred low initial abundance of 60Fe in the PPD suggest late injection of 26Al by a wind from a nearby Wolf–Rayet star into the protosolar molecular cloud core prior to or during its collapse. Although there are multiple generations of CAIs characterized by distinct mineralogies, textures, and isotopic (O, Mg, Ca, Ti, Mo, etc.) compositions, the 26Al heterogeneity in the CAI‐forming region(s) precludes determining the duration of CAIs formation using 26Al‐26Mg systematics. The existence of multiple generations of CAIs and the observed differences in CAI abundances in carbonaceous and noncarbonaceous chondrites may indicate that CAIs were episodically formed and ejected by a disk wind from near the Sun to the outer solar system and then spiraled inward due to gas drag. In type 2–3.0 chondrites, most CAIs surrounded by Wark–Lovering rims have uniform Δ17O (= δ17O−0.52 × δ18O) of ~ −24‰; however, there is a large range of Δ17O (from ~−40 to ~ −5‰) among them, suggesting the coexistence of 16O‐rich (low Δ17O) and 16O‐poor (high Δ17O) gaseous reservoirs at the earliest stages of the PPD evolution. The observed variations in Δ17O of CAIs may be explained if three major O‐bearing species in the solar system (CO, H2O, and silicate dust) had different O‐isotope compositions, with H2O and possibly silicate dust being 16O‐depleted relative to both the Genesis solar wind Δ17O of −28.4 ± 3.6‰ and even more 16O‐enriched CO. Oxygen isotopic compositions of CO and H2O could have resulted from CO self‐shielding in the protosolar molecular cloud (PMC) and the outer PPD. The nature of 16O‐depleted dust at the earliest stages of PPD evolution remains unclear: it could have either been inherited from the PMC or the initially 16O‐rich (solar‐like) MC dust experienced O‐isotope exchange during thermal processing in the PPD. To understand the chemical and isotopic composition of the protosolar MC material and the degree of its thermal processing in PPD, samples of the primordial silicates and ices, which may have survived in the outer solar system, are required. In metamorphosed CO3 and CV3 chondrites, most CAIs exhibit O‐isotope heterogeneity that often appears to be mineralogically controlled: anorthite, melilite, grossite, krotite, perovskite, and Zr‐ and Sc‐rich oxides and silicates are 16O‐depleted relative to corundum, hibonite, spinel, Al,Ti‐diopside, forsterite, and enstatite. In texturally fine‐grained CAIs with grain sizes of ~10–20 μm, this O‐isotope heterogeneity is most likely due to O‐isotope exchange with 16O‐poor (Δ17O ~0‰) aqueous fluids on the CO and CV chondrite parent asteroids. In CO3.1 and CV3.1 chondrites, this process did not affect Al‐Mg isotope systematics of CAIs. In some coarse‐grained igneous CV CAIs, O‐isotope heterogeneity of anorthite, melilite, and igneously zoned Al,Ti‐diopside appears to be consistent with their crystallization from melts of isotopically evolving O‐isotope compositions. These CAIs could have recorded O‐isotope exchange during incomplete melting in nebular gaseous reservoir(s) with different O‐isotope compositions and during aqueous fluid–rock interaction on the CV asteroid.

Use of Isotope Effects To Understand the Present and Past of the Atmosphere and Climate and Track the Origin of Life

Stable isotope ratio measurements have been used as a measure of a wide variety of processes, including solar system evolution, geological formational temperatures, tracking of atmospheric gas and aerosol chemical transformation, and is the only means by which past global temperatures may be determined over long time scales. Conventionally, isotope effects derive from differences of isotopically substituted molecules in isotope vibrational energy, bond strength, velocity, gravity, and evaporation/condensation. The variations in isotope ratio, such as ¹⁸ O/¹⁶ O (δ¹⁸ O) and ¹⁷ O/¹⁶ O (δ¹⁷ O) are dependent upon mass differences with δ¹⁷ O/δ¹⁸ O=0.5, due to the relative mass differences (1 amu vs. 2 amu). Relations that do not follow this are termed mass independent and are the focus of this Minireview. In chemical reactions such as ozone formation, a δ¹⁷ O/δ¹⁸ O=1 is observed. Physical chemical models capture most parameters but differ in basic approach and are reviewed. The mass independent effect is observed in atmospheric species and used to track their chemistry at the modern and ancient Earth, Mars, and the early solar system (meteorites).

Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition

Dust grains of organic matter were the main reservoir of C and N in the forming Solar System and are thus considered to be an essential ingredient for the emergence of life. However, the physical environment and the chemical mechanisms at the origin of these organic grains are still highly debated. In this study, we report high-precision triple oxygen isotope composition for insoluble organic matter isolated from three emblematic carbonaceous chondrites, Orgueil, Murchison, and Cold Bokkeveld. These results suggest that the O isotope composition of carbonaceous chondrite insoluble organic matter falls on a slope 1 correlation line in the triple oxygen isotope diagram. The lack of detectable mass-dependent O isotopic fractionation, indicated by the slope 1 line, suggests that the bulk of carbonaceous chondrite organics did not form on asteroidal parent bodies during low-temperature hydrothermal events. On the other hand, these O isotope data, together with the H and N isotope characteristics of insoluble organic matter, may indicate that parent bodies of different carbonaceous chondrite types largely accreted organics formed locally in the protosolar nebula, possibly by photochemical dissociation of C-rich precursors.

A light carbon isotope composition for the Sun

Measurements by the Genesis mission have shown that solar wind oxygen is depleted in the rare isotopes, ¹⁷O and ¹⁸O, by approximately 80 and 100‰, respectively, relative to Earth's oceans, with inferred photospheric values of about -60‰ for both isotopes. Direct astronomical measurements of CO absorption lines in the solar photosphere have previously yielded a wide range of O isotope ratios. Here, we reanalyze the line strengths for high-temperature rovibrational transitions in photospheric CO from ATMOS FTS data, and obtain an ¹⁸O depletion of δ¹⁸O = -50 ± 11‰ (1σ). From the same analysis we find a carbon isotope ratio of δ¹³C = -48 ± 7‰ (1σ) for the photosphere. This implies that the primary reservoirs of carbon on the terrestrial planets are enriched in ¹³C relative to the bulk material from which the solar system formed, possibly as a result of CO self-shielding or inheritance from the parent cloud.

Heavier congeners of CO and CO2 as ligands: from zero-valent germanium ('germylone') to isolable monomeric GeX and GeX2 complexes (X = S, Se, Te)

The first isolable germanium chalcogenide complexes 2–5 representing heavier congeners of CO and CO₂ were synthesised from the germylone adduct 1.

In contrast to molecular CO and CO₂, their heavier mono- and dichalcogenide homologues, EX and EX₂ (E = Si, Ge, Sn, Pb; X = O, S, Se, Te), are important support materials (e.g., SiO₂) and/or semiconductors (e.g., SiS₂) and exist typically as insoluble crystalline or amorphous polymers under normal conditions. Herein, we report the first successful synthesis and characterisation of an extraordinary series of isolable monomeric GeX and GeX₂ complexes (X = S, Se, Te), representing novel classes of compounds and heavier congeners of CO and CO₂. This could be achieved by solvent-dependent oxidation reactions of the new zero-valent germanium (‘germylone’)–GaCl₃ precursor adduct (bis-NHC)Ge⁰→GaCl₃1 (bis-NHC = H₂C[{NC(H) Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C(H)N(Dipp)}C:]₂, Dipp = 2,6-iPr₂C₆H₃) with elemental chalcogens, affording the donor–acceptor stabilised monomeric germanium(iv) dichalcogenide (bis-NHC)Ge^IV( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> X) Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> X→GaCl₃ (X = S, 2; X = Se, 3) and germanium(ii) monochalcogenide complexes (bis-NHC)Ge^II Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> X→GaCl₃ (X = Se, 4; X = Te, 5), respectively. Moreover, the reactivity of 4 and 5 towards elemental sulphur, selenium, and tellurium has been investigated. In THF, the germanium(ii) monoselenide complex 4 reacts with activated elemental selenium to afford the desired germanium(iv) diselenide complex 3. Unexpectedly, both reactions of 4 and 5 with elemental sulphur, however, lead to the formation of germanium(iv) disulfide complex 2 under liberation of elemental Se and Te as a result of further oxidation of the germanium centre and replacement of the Se and Te atoms by sulphur atoms. All novel compounds 1–5 have been fully characterised, including single-crystal X-ray diffraction analyses, and studied by DFT calculations.