Search for Chirality in Hydrogenated Magnesium Nanosilicates: A DFT and TD-DFT Investigation

The formation of silicate grains in the interstellar medium (ISM), especially those containing chiral surfaces such as clinopyroxenes, is poorly understood. Moreover, silicate interactions with various forms of hydrogen-proton (H+), neutral H (HI), and molecular hydrogen (H2) are of high importance as hydrogen comprises >90% of the ISM gas budget, and these species play important roles in the formation of new molecules in space. Furthermore, silicate surfaces catalyze the formation of H2 in the interstellar medium formed on dust grain surfaces by H-H association. The technical difficulty of in situ laboratory investigations of nanosilicate nucleation using astrophysically relevant environmental conditions makes computational chemistry a useful tool for studying potential nanosilicate structures. Furthermore, chiral surfaces interacting with chiral organic molecules could serve as templates that lead to the enantiomeric excess of l-amino acids and d-polyols detected in carbonaceous meteorites. However, in order for this effect to take place, an excess of one chiral form of a mineral is required to break the symmetry. This symmetry-breaking event could have been due to the asymmetric absorption of circularly polarized light by the nanosilicate as it traverses star-forming regions. We investigate this possibility using a metastable chiral form of an enstatite dimer as an input for density functional theory (DFT) and time-dependent (TD)-DFT calculations to obtain various properties and circular dichroism spectra. All in all, twenty-six magnesium nanosilicate structures were studied using varying degrees of hydrogenation: none, with HI, with H+, and with H2. The HSE06/aug-cc-pVQZ level of theory was used for the DFT calculations. TD-DFT calculations utilized the CAM-B3LYP/cc-pVQZ and ωB97X-D3/cc-pVQZ functional and basic set pairings. Results show that (1) all twenty-six structures have absorption bands that fall within the 0.6-28.3 μm range available with the newly launched James Webb Space Telescope and (2) there is a small enantioselective effect by UV-CPL on the eight chiral enstatite dimers (predicted g-values of up to 0.007). While the observed effect is small, it opens up the possibility that it is the inorganic material that becomes enantiomerically biased by UV-CPL, driving chiral enhancements in meteoric organic molecules.

Toward size-dependent thermodynamics of nanoparticles from quantum chemical calculations of small atomic clusters: a case study of (B2O3)n

We present a method for obtaining canonical partition functions and, accordingly, temperature-dependent thermodynamics of arbitrary-sized (nano) particles from electronic structure calculations of the corresponding small size atomic clusters. The guiding idea here is to extrapolate the basic properties underlying the thermochemistry of clusters (electronic energies, rotational constants, and vibrational frequencies) rather than the thermodynamic functions themselves. The thus obtained scaling dependences for these basic properties expressed in a simple analytical form provide an efficient tool for fast evaluation of the size-selected thermochemical data for particles of any nuclearity. To exemplify the performance of the methodology, neutral stoichiometric boron oxide clusters are considered. To this end, the geometry and various physical properties of the energetically lowest-lying (B₂O₃)_n (n = 1,…,8) structures are found using density functional theory and the authors' multistage hierarchical procedure customized for global optimization of quite large cluster structures. With these data and based on the physically consistent scaling regularities for the principal cluster properties, the size-selected thermodynamic functions of boron oxide particles in the gas phase, such as enthalpy, entropy, and specific heat capacity, are derived. The variation of these characteristics with increasing cluster size is discussed in detail as well. To facilitate handling of the temperature and size dependences we have found here in further chemical kinetic and equilibrium modeling, the tabulated thermodynamic functions of interest are fitted for n = 1,…,1000 to the standard seven-parameter Chemkin polynomials.

Complex Organic Matter Synthesis on Siloxyl Radicals in the Presence of CO

Heterogeneous phase astrochemistry plays an important role in the synthesis of complex organic matter (COM) as found on comets and rocky body surfaces like asteroids, planetoids, moons and planets. The proposed catalytic model is based on two assumptions: (a) siliceous rocks in both crystalline or amorphous states show surface-exposed defective centers such as siloxyl (Si-O•) radicals; (b) the second phase is represented by gas phase CO molecules, an abundant C₁ building block found in space. By means of quantum chemistry; (DFT, PW6B95/def2-TZVPP); the surface of a siliceous rock in presence of CO is modeled by a simple POSS (polyhedral silsesquioxane) where a siloxyl (Si-O•) radical is present. Four CO molecules have been consecutively added to the Si-O• radical and to the nascent polymeric CO (pCO) chain. The first CO insertion shows no activation free energy with ΔG_200K = −21.7 kcal/mol forming the SiO-CO• radical. The second and third CO insertions show ΔG200K‡ ≤ 10.5 kcal/mol. Ring closure of the SiO-CO-CO• (oxalic anhydride) moiety as well as of the SiO-CO-CO-CO• system (di-cheto form of oxetane) are thermodynamically disfavored. The last CO insertion shows no free energy of activation resulting in the stable five member pCO ring, precursor to 1,4-epoxy-1,2,3-butanone. Hydrogenation reactions of the pCO have been considered on the SiO oxygen or on the carbons and oxygens of the pCO chains. The formation of the reactive aldehyde SiO-CHO on the siliceous surface is possible. In principle, the complete hydrogenation of the (CO)₁₋₄ series results in the formation of methanol and polyols. Furthermore, all the SiO-pCO intermediates and the lactone 1,4-epoxy-1,2,3-butanone product in its radical form can be important building blocks in further polymerization reactions and/or open ring reactions with H (aldehydes, polyols) or CN (chetonitriles), resulting in highly reactive multi-functional compounds contributing to COM synthesis.

Revisiting nuclear tunnelling in the aqueous ferrous–ferric electron transfer

We find that golden-rule quantum transition-state theory predicts nearly an order of magnitude less tunnelling than some of the previous estimates. This may indicate that the spin-boson model of electron transfer is not valid in the quantum regime.

Interferometric observations of SiO thermal emission in the inner wind of M-type AGB stars IK Tauri and IRC+10011

CONTEXT

Asymptotic giant branch (AGB) stars go through a process of strong mass loss that involves pulsations of the atmosphere, which extends to a region in which the conditions are adequate for dust grains to form. Radiation pressure acts on these grains which, coupled to the gas, drive a massive outflow. The details of this process are not clear, including which molecules are involved in the condensation of dust grains.

AIMS

We seek to study the role of the SiO molecule in the process of dust formation and mass loss in M-type AGB stars.

METHODS

Using the IRAM NOEMA interferometer we observed the ²⁸SiO and ²⁹SiO J = 3 - 2, v = 0 emission from the inner circumstellar envelope of the evolved stars IK Tau and IRC+10011. We computed azimuthally averaged emission profiles to compare the observations to models using a molecular excitation and ray-tracing code for SiO thermal emission.

RESULTS

We observe circular symmetry in the emission distribution. We also find that the source diameter varies only marginally with radial velocity, which is not the expected behaviour for envelopes expanding at an almost constant velocity. The adopted density, velocity, and abundance laws, together with the mass-loss rate, which best fit the observations, give us information concerning the chemical behaviour of the SiO molecule and its role in the dust formation process.

CONCLUSIONS

The results indicate that there is a strong coupling between the depletion of gas-phase SiO and gas acceleration in the inner envelope. This could be explained by the condensation of SiO into dust grains.

Propylene Oxide Formation on a Silica Surface with Peroxo Defects: Implications in Astrochemistry

The formation of the chiral molecule propylene oxide (CH₃CHCH₂O) recently detected in the interstellar medium (ISM) is proposed to take place on an amorphous silicate grain surface where peroxo defects are present. A computational analysis conducted at the DFT and MP2-F12 levels of theory on a neat amorphous silica model supports such a hypothesis resulting in (a) strong thermodynamic driving forces and low activation energies allowing the synthesis of CH₃CHCH₂O at low temperatures, (b) chemical defects on silica surfaces promoting heterogeneous catalysis of the increasing molecular complexity found in interstellar and circumstellar medium, and (c) chemical defects that have implications on understanding how processing phases modify the nature of the reactive groups on a silica surface affecting the surface catalytic activity.

Theoretical Study of Silicon Monoxide Reactions with Ammonia and Methane

High-accuracy calculations were performed to study the mechanisms of the reactions between the diatomic silicon monoxide (SiO) with NH₃ and CH₄. These reactions are relevant to the SiO-related astrochemistry and atmospheric chemistry as well as the activation of the N-H and C-H bonds by the SiO triple bond. Energetic data used in the construction of potential energy surfaces describing the SiO + NH₃/CH₄ reactions were obtained at the coupled-cluster theory with extrapolation to the complete basis set limit (CCSD(T)/CBS) using DFT/B3LYP/aug-cc-pVTZ optimized geometries. Standard heats of formation of a series of small Si-molecules were predicted. Insertion of SiO into the N-H bond is exothermic with a small energy barrier of ∼8 kcal/mol with respect to the SiO + NH₃ reactants, whereas the C-H bond activation by SiO involves a higher energy barrier of 45 kcal/mol. Eight product channels are opened in the SiO + NH₃ reaction including dehydrations giving HNSi/HSiN and dehydrogenations. These reactions are endothermic by 16-119 kcal/mol (calculated at 298.15 K) with the CCSD(T)/CBS energy barriers of 21-128 kcal/mol. The most stable set of products, HNSi + H₂O, was also the product of the reaction pathway having lowest energy barrier of 21 kcal/mol. Ten product channels of the SiO + CH₄ reaction including decarbonylation, dehydration, dehydrogenation, and formation of Si + CH₃OH are endothermic by 19-118 kcal/mol with the energy barriers in the range of 71-126 kcal/mol. The formation of H₂CSiO + H₂O has the lowest energy barrier of 71 kcal/mol, whereas the most stable set of products, SiH₄ + CO, is formed via a higher energy barrier of 90 kcal/mol. Accordingly, while SiO can break the N-H bond of ammonia without the assistance of other molecules, it is not able to break the C-H bond of methane.

Comparison of classical reaction paths and tunneling paths studied with the semiclassical instanton theory

Comparison of classical reaction paths and semiclassical instanton paths for a proton transfer reaction mechanism.

H2 Formation on Cosmic Grain Siliceous Surfaces Grafted with Fe+ : A Silsesquioxanes-Based Computational Model

Cosmic siliceous dust grains are involved in the synthesis of H₂ in the inter-stellar medium. In this work, the dust grain siliceous surface is represented by a hydrogen Fe-metalla-silsesquioxane model of general formula: [Fe(H₇ Si₇ O_12-n )(OH)_n ]⁺ (n=0,1,2) where Fe⁺ behaves like a single-site heterogeneous catalyst grafted on a siliceous surface synthesizing H₂ from H. A computational analysis is performed using two levels of theory (B3LYP-D3BJ and MP2-F12) to quantify the thermodynamic driving force of the reaction: [Fe-T7H₇ ]⁺ +4H→[Fe-T7H₇ (OH)₂ ]⁺ +H₂ . The general outcomes are: 1) H₂ synthesis is thermodynamically strongly favored; 2) Fe-H / Fe-H₂ barrier-less formation potential; 3) chemisorbed H-Fe leads to facile H₂ synthesis at 20≤T≤100 K; 4) relative spin energetics and thermodynamic quantities between the B3LYP-D3BJ and MP2-F12 levels of theory are in qualitative agreement. The metalla-silsesquioxane model shows how Fe⁺ fixed on a siliceous surface can potentially catalyze H₂ formation in space.

Quantum dynamics of hydrogen atoms on graphene. II. Sticking

Following our recent system-bath modeling of the interaction between a hydrogen atom and a graphene surface [Bonfanti et al., J. Chem. Phys. 143, 124703 (2015)], we present the results of converged quantum scattering calculations on the activated sticking dynamics. The focus of this study is the collinear scattering on a surface at zero temperature, which is treated with high-dimensional wavepacket propagations with the multi-configuration time-dependent Hartree method. At low collision energies, barrier-crossing dominates the sticking and any projectile that overcomes the barrier gets trapped in the chemisorption well. However, at high collision energies, energy transfer to the surface is a limiting factor, and fast H atoms hardly dissipate their excess energy and stick on the surface. As a consequence, the sticking coefficient is maximum (∼0.65) at an energy which is about one and half larger than the barrier height. Comparison of the results with classical and quasi-classical calculations shows that quantum fluctuations of the lattice play a primary role in the dynamics. A simple impulsive model describing the collision of a classical projectile with a quantum surface is developed which reproduces the quantum results remarkably well for all but the lowest energies, thereby capturing the essential physics of the activated sticking dynamics investigated.

Quantum tunneling observed without its characteristic large kinetic isotope effects

Classical transition-state theory is fundamental to describing chemical kinetics; however, quantum tunneling is also important in explaining the unexpectedly large reaction efficiencies observed in many chemical systems. Tunneling is often indicated by anomalously large kinetic isotope effects (KIEs), because a particle's ability to tunnel decreases significantly with its increasing mass. Here we experimentally demonstrate that cold hydrogen (H) and deuterium (D) atoms can add to solid benzene by tunneling; however, the observed H/D KIE was very small (1-1.5) despite the large intrinsic H/D KIE of tunneling (≳ 100). This strong reduction is due to the chemical kinetics being controlled not by tunneling but by the surface diffusion of the H/D atoms, a process not greatly affected by the isotope type. Because tunneling need not be accompanied by a large KIE in surface and interfacial chemical systems, it might be overlooked in other systems such as aerosols or enzymes. Our results suggest that surface tunneling reactions on interstellar dust may contribute to the deuteration of interstellar aromatic and aliphatic hydrocarbons, which could represent a major source of the deuterium enrichment observed in carbonaceous meteorites and interplanetary dust particles. These findings could improve our understanding of interstellar physicochemical processes, including those during the formation of the solar system.

Trends in the adsorption and reactivity of hydrogen on magnesium silicate nanoclusters

We investigate the potential role of ultrasmall silicate grains in interstellar hydrogen-based chemistry by modelling H adsorption and H₂ formation/dissociation on nanosilicates.