Heats of Formation of Medium-Sized Organic Compounds from Contemporary Electronic Structure Methods

Direct Measurements of Covalently Bonded Sulfuric Anhydrides from Gas-Phase Reactions of SO3 with Acids under Ambient Conditions

Sulfur trioxide (SO3) is an important oxide of sulfur and a key intermediate in the formation of sulfuric acid (H2SO4, SA) in the Earth's atmosphere. This conversion to SA occurs rapidly due to the reaction of SO3 with a water dimer. However, gas-phase SO3 has been measured directly at concentrations that are comparable to that of SA under polluted mega-city conditions, indicating gaps in our current understanding of the sources and fates of SO3. Its reaction with atmospheric acids could be one such fate that can have significant implications for atmospheric chemistry. In the present investigation, laboratory experiments were conducted in a flow reactor to generate a range of previously uncharacterized condensable sulfur-containing reaction products by reacting SO3 with a set of atmospherically relevant inorganic and organic acids at room temperature and atmospheric pressure. Specifically, key inorganic acids known to be responsible for most ambient new particle formation events, iodic acid (HIO3, IA) and SA, are observed to react promptly with SO3 to form iodic sulfuric anhydride (IO3SO3H, ISA) and disulfuric acid (H2S2O7, DSA). Carboxylic sulfuric anhydrides (CSAs) were observed to form by the reaction of SO3 with C2 and C3 monocarboxylic (acetic and propanoic acid) and dicarboxylic (oxalic and malonic acid)-carboxylic acids. The formed products were detected by a nitrate-ion-based chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (NO3--CI-APi-TOF; NO3--CIMS). Quantum chemical methods were used to compute the relevant SO3 reaction rate coefficients, probe the reaction mechanisms, and model the ionization chemistry inherent in the detection of the products by NO3--CIMS. Additionally, we use NO3--CIMS ambient data to report that significant concentrations of SO3 and its acid anhydride reaction products are present under polluted, marine and polar, and volcanic plume conditions. Considering that these regions are rich in the acid precursors studied here, the reported reactions need to be accounted for in the modeling of atmospheric new particle formation.

Enthalpy of formation of 6-phenyl-1,5-diazabicyclo[3.1.0]hexane by combustion calorimetry and theoretical approach for efficient prediction of thermochemistry of diaziridines

The combustion energy and standard molar enthalpy of formation of crystalline 6-phenyl-1,5-diazabicyclo[3.1.0]hexane (PDABH) were determined using an isoperibolic calorimeter with a static bomb. PDABH is the first diaziridine for which the experimental value of the enthalpy of formation was obtained. This value was validated by the theoretical values of gas phase enthalpy of formation and enthalpy of sublimation. The gas phase enthalpy of formation was calculated using the DLPNO-CCSD(T1)/CBS method in conjunction with isodesmic-type reactions. This method was chosen in comparison to another high quality evaluative method (G4), which has been shown to provide unreliable results for cyclic nitrogen containing compounds. The descriptors of the molecular electrostatic potential (MEP) were used to estimate the enthalpy of sublimation of PDABH. The proposed MEP model is based on experimental enthalpies of sublimation for 75 compounds structurally similar to PDABH. The high-level ab initio calculations of gas phase enthalpies of formation combined with enthalpies of sublimations estimated using descriptors of MEP allow predicting the enthalpies of formation of diaziridines in the solid phase.

Benchmarking First-Principles Reaction Equilibrium Composition Prediction

The availability of thermochemical properties allows for the prediction of the equilibrium compositions of chemical reactions. The accurate prediction of these can be crucial for the design of new chemical synthesis routes. However, for new processes, these data are generally not completely available. A solution is the use of thermochemistry calculated from first-principles methods such as Density Functional Theory (DFT). Before this can be used reliably, it needs to be systematically benchmarked. Although various studies have examined the accuracy of DFT from an energetic point of view, few studies have considered its accuracy in predicting the temperature-dependent equilibrium composition. In this work, we collected 117 molecules for which experimental thermochemical data were available. From these, we constructed 2648 reactions. These experimentally constructed reactions were then benchmarked against DFT for 6 exchange-correlation functionals and 3 quality of basis sets. We show that, in reactions that do not show temperature dependence in the equilibrium composition below 1000 K, over 90% are predicted correctly. Temperature-dependent equilibrium compositions typically demonstrate correct qualitative behavior. Lastly, we show that the errors are equally caused by errors in the vibrational spectrum and the DFT electronic ground state energy.

Thermochemistry of per- and polyfluoroalkyl substances

The determination of gas phase thermochemical properties of per- and polyfluoroalkyl substances (PFAS) is central to understanding the long-range transport behavior of PFAS in the atmosphere. Prior gas-phase studies have reported the properties of perfluorinated sulfonic acid (PFOS) and perfluorinated octanoic acid (PFOA). Here, this study reports the gas phase enthalpies of formation of short- and long-chain PFAS and their precursor molecules determined using density functional theory (DFT) and ab initio approaches. Two density functionals, two ab initio methods and an empirical method were used to compute enthalpies of formation with the total atomization approach and an isogyric reaction. The performance of the computational methods employed in this work were validated against the experimental enthalpies of linear alkanoic acids and perfluoroalkanes. The gas-phase determinations will be useful for future studies of PFAS in the atmosphere, and the methodological choices will be helpful in the study of other PFAS.

"In Vitro" and "In Vivo" Diagnostic Check for the Thermochemistry of Metal-Organic Compounds

Volatile metal β-diketonates are of interest from both practical and theoretical perspectives (manufacturing of film materials, catalysis, and the nature of metal-ligand bonding). Knowledge of their reliable thermochemical properties is essential for effective applications. However, there is an unacceptable scattering of the available data on the enthalpies of formation. In this work, we proposed "in vitro" and "in vivo" diagnostic tools to verify the available enthalpies of formation in both the crystalline and gaseous states for metal tris-β-diketonates. The "in vitro" procedure involved high-level quantum-chemical calculations and was applied to define a consistent data set on the enthalpies of formation for iron(III) β-diketonates. This data set has provided the basis for "in vivo" structure-property-based diagnostics to evaluate the robustness of the thermochemical data for β-diketonate tris-complexes with metals other than iron.

Strong Bases Design: Predicted Limits of Basicity

Brønsted superbases have wide applications in organic chemistry due to their ability to activate C-H bonds. The strongest neutral bases to date are substituted aminophosphazenes developed in the late 1980s by Reinhard Schwesinger. Since then, much effort has been expended to create even stronger neutral bases. In this article, the reasons for the instability of very basic compounds are investigated by means of high-level quantum-chemical calculations. Theoretical basicity limits are suggested for solutions as well as for the gas phase. A record-breaking superbase most likely to be synthesizable and stable at ambient conditions is proposed. Hexamethylphosphoramide is considered a reliable ionizing solvent for superbases.

Gas-phase thermochemistry of polycyclic aromatic hydrocarbons: an approach integrating the quantum chemistry composite scheme and reaction generator

We introduce a protocol aimed at predicting the accurate gas-phase enthalpies of formation of polycyclic aromatic hydrocarbons (PAHs). Automatic generation of a dataset of equilibrated chemical reactions preserving the number of carbon atoms in each hybridization state on each side of equations is at the core of our scheme. The performed tests suggest the recommended enthalpy of formation to be derived via a two-step scheme. First, we consider the reactions with a minimal sum of the total number of particles involved, N, and the absolute difference between the total number of products and reactants, |ΔN|. Second, among these reactions, we identify the one with the smallest absolute reaction enthalpy change, . This approach has been applied to predict the gas-phase enthalpies of formation of 113 PAHs via the Feller-Peterson-Dixon approach. Our calculated values provide the mean absolute deviations of 1.7, 1.9, 4.2, 8.1, and 18.5 kJ mol^-1 with respect to the literature group-based error corrected (GBEC) G3MP2B3, ATOMIC (HC), group equivalent M06-2X, GBEC B3LYP, and G4MP2 values. Our predicted values give the mean signed and mean absolute errors of -7.5 and 12.9 kJ mol^-1 with respect to the experimental enthalpies of formation. The combination of our predicted and the experimental values provide the solid-state enthalpies of formation, , which are not available for a few species. Approaching these values as well as , producing large discrepancies from the experimental side, would be indispensable for testing and further tuning of computational chemistry approaches.

On the origin of reactivity variation upon sequential ligation: the [Re(Cl)x]+/CH4 (x = 1-3) couples

The potential of [ReCl_x]⁺ (x = 1-3) in activating methane has been explored by using a combination of gas-phase experiments and high-level quantum calculations. When the number of Cl ligands increases, the reactivity towards methane activation varies accordingly. While [ReCl_x]⁺ (x = 1-2) are able to dehydrogenate methane by a three-state reactivity scenario, [ReCl₃]⁺ shows inertness towards methane at ambient conditions. Furthermore, the product ion [ClRe(H)CH]⁺ of the [ReCl]⁺/CH₄ couple could continue to activate methane and liberate molecular dihydrogen but another product ion [Cl₂ReCH₂]⁺ is unreactive with methane. Obviously, the nature and the number of ligands make a difference to the reactivity towards methane activation. The associated reaction mechanism and the electron origins for the rather different reactivities are discussed in detail. Finally and more importantly, instructive information concerning the rational design of Re-catalysts for methane conversion is obtained.

Enthalpy formation of fluorene: a challenging problem for theory or experiment?

The large discrepancy between the experimental enthalpy of formation of fluorene and theoretical value calculated by the G3(MP2) method was revealed more than ten years ago. Three years later, a new experimental study of this compound was undertaken to ascertain whether there is any significant error in the thermochemical data. However, after this research, the agreement between theory and experiment was improved only slightly. In this work we decided to calculate the enthalpy of formation of fluorene using the high-level DLPNO-CCSD(T₁)/CBS method which shows better results compared to Gn theories. To examine the accuracy of the available experimental data, the calculations were performed not only for fluorene but also for eleven fluorene derivatives. The discrepancy of about 9 kJ mol^-1 between the experimental and theoretical enthalpies of formation of fluorene was confirmed by the present calculations, whereas good agreement was observed for the fluorene derivatives. It is highly unlikely that this discrepancy may disappear when using a higher-level theory. The possible reason for such inconsistency might be the experimental difficulty associated with the glass transition discovered in the stable crystalline state of fluorene. In this case, new experiments using the latest methods, such as differential scanning calorimetry combined with X-ray powder diffraction, are needed to gain deeper insight into the solid phase transformations of fluorene.

On the origins of the mechanistic variants in the thermal reactions of Sx+ (x = 1-3) with benzene

The S-π interaction between sulfur atom(s) and aromatic ring prevails in chemical and biochemical processes. The thermal gas-phase reactions of the S_n⁺ (n = 1-3) ions with benzene have been explored by using Quadrupole-Ion Trap (Q-IT) mass spectrometry complemented by quantum chemical calculations. Charge transfer was found to be the only reaction channel for S₂⁺/C₆H₆, while both charge transfer and bond activation are available for the S⁺/C₆H₆ and S₃⁺/C₆H₆ couples. Upon interrogating the associated electronic origins, multiple factors were found to matter for these processes. In contrast to the σ-type two-center three-electron (2c-3e) S-π hemibond as reported previously, unusual S-π hemibonds were addressed for the S_n⁺/C₆H₆ couples, i.e. the 2c-3e π(S061Eπ) and the three-center three-electron (3c-3e) σ(S₂061Eπ) hemibonds. Such S-π interaction was found to be responsible for the charge transfer processes in S⁺/C₆H₆ and S₂⁺/C₆H₆, but uninvolved in any transformation for S₃⁺/C₆H₆.

Learning Molecular Representations for Thermochemistry Prediction of Cyclic Hydrocarbons and Oxygenates

Accurate thermochemistry estimation of polycyclic molecules is crucial for kinetic modeling of chemical processes that use renewable and alternative feedstocks. In kinetic model generators, molecular properties are estimated rapidly with group additivity, but this method is known to have limitations for polycyclic structures. This issue has been resolved in our work by combining a geometry-based molecular representation with a deep neural network trained on ab initio data. Each molecule is transformed into a probabilistic vector from its interatomic distances, bond angles, and dihedral angles. The model is tested on a small experimental dataset (200 molecules) from the literature, a new medium-sized set (4000 molecules) with both open-shell and closed-shell species, calculated at the CBS-QB3 level with empirical corrections, and a large G4MP2-level QM9-based dataset (40 000 molecules). Heat capacities between 298.15 and 2500 K are calculated in the medium set with an average deviation of about 1.5 J mol^-1 K^-1 and the standard entropy at 298.15 K is predicted with an average error below 4 J mol^-1 K^-1. The standard enthalpy of formation at 298.15 K has an average out-of-sample error below 4 kJ mol^-1 on a QM9 training set size of around 15 000 molecules. By fitting NASA polynomials, the enthalpy of formation at higher temperatures can be calculated with the same accuracy as the standard enthalpy of formation. Uncertainty quantification by means of the ensemble standard deviation is included to indicate when molecules that are on the edge or outside of the application range of the model are evaluated.

Accuracy of DLPNO-CCSD(T): Effect of Basis Set and System Size

The DLPNO-CCSD(T) method is designed to study large molecular systems at significantly reduced cost relative to its canonical counterpart. However, the error in this approach is also size-extensive and relies on cancellation of errors for the calculation of relative energies. This work provides a direct comparison of canonical CCSD(T) and TightPNO DLPNO-CCSD(T) calculations of reaction energies and barriers of a broad range of chemical reactions. The dataset includes acidities, anion binding affinities, enolization, Diels-Alder, nucleophilic substitution, and atom transfer reactions and complements existing theoretical datasets in terms of system size as well as new reaction types (e.g., anion binding affinities and chlorine atom transfer reactions). The performance of DLPNO-CCSD(T) was further examined with respect to systematic variation of basis set and system size and amounts of nonbonded interaction present in the system. The errors in the DLPNO-CCSD(T) were found to be relatively insensitive to the choice of basis set for small systems but increase monotonically with system size. Additionally, calculations of barriers appear to be more challenging than reaction energies with errors exceeding 5 kJ mol^-1 for many Diels-Alder reactions. Further tests on three realistic organic reactions reveal the impact of the DLPNO approximation in calculating absolute and relative barriers that are important for predictions such as stereoselectivity.

Gas-Phase Thermochemistry of MX3 and M2X6 (M = Sc, Y; X = F, Cl, Br, I) from a Composite Reaction-Based Approach: Homolytic versus Heterolytic Cleavage

A domain-based local-pair natural-orbital coupled-cluster approach with single, double, and improved linear-scaling perturbative triple correction via an iterative algorithm, DLPNO-CCSD(T₁), was applied within the framework of the Feller-Peterson-Dixon approach to derive gas-phase heats of formation of scandium and yttrium trihalides and their dimers via a set of homolytic and heterolytic dissociation reactions. All predicted heats of formation moderately depend on the reaction type with the most and least negative values obtained for homolytic and heterolytic dissociation, respectively. The basis set size dependence, as well as the influence of static correlation effects not covered by the standard (DLPNO-)CCSD(T) approach, suggests that exploitation of the heterolytic dissociation reactions with the formation of M³⁺ and X^- ions leads to the most robust heats of formation. The gas-phase formation enthalpies ΔH_f°(0 K)/ΔH_f°(298.15 K) and absolute entropies S°(298.15 K) were obtained for the first time for the Sc₂F₆, Sc₂Br₆, and Sc₂I₆ species. For ScBr₃, ScI₃, Sc₂Cl₆, and Y₂Cl₆, we suggest a reexamination of the experimental heats of formation available in the literature. For other compounds, the predicted values were found to be in good agreement with the experimental estimates. Extracted MX₃ (M = Sc, Y; X = F, Cl, Br, and I) 0 K atomization enthalpies indicate weaker bonding when moving from fluorine to iodine and from yttrium to scandium. Likewise, the stability of yttrium trihalide dimers degrades when going from fluorine to iodine. Respective scandium trihalide dimers are less stable, with 0 K dimer dissociation energy decreasing in the row fluorine - chlorine - bromine ≈ iodine. Correlation of the (n - 1)s²p⁶ electrons on bromine and iodine, inclusion of zero-point energy, relativistic effects, and the effective-core-potential correction as well as amelioration of the DLPNO localization inaccuracy are shown to be of similar magnitude, which is critical if accurate heats of formation are a goal.

DFT Study on the Electronic Properties, Spectroscopic Profile, and Biological Activity of 2-Amino-5-trifluoromethyl-1,3,4-thiadiazole with Anticancer Properties

Extensive investigation on the molecular and electronic structure of 2-amino-5-trifluoromethyl-1,3,4-thiadiazole in the ground state and in the first excited state has been performed. The energy barrier corresponding to the conversion between imino and amino tautomers has been calculated, which indicates the existence of amino tautomer in solid state for the title compound. The FT-Raman and FT-IR spectra were recorded and compared with theoretical vibrational wavenumbers, and a good coherence has been observed. The MESP map, dipole moment, polarizability, and hyperpolarizability have been calculated to comprehend the properties of the title molecule. High polarizability value estimation of the title compound may enhance its bioactivity. Natural bonding orbital analysis has been done on monomer and dimer to investigate the charge delocalization and strength of hydrogen bonding, respectively. Strong hydrogen bonding interaction energies of 17.09/17.49 kcal mol^-1 have been calculated at the B3LYP/M06-2X functional. The UV-vis spectrum was recorded and related to the theoretical spectrum. The title compound was biologically examined for anticancer activity by studying the cytotoxic performance against two human cancer cell lines (A549 and HeLa) along with the molecular docking simulation. Both molecular docking and cytotoxic performance against cancer cell lines show positive outcomes, and the title compound appears to be a promising anticancer agent.

Accurate prediction of norbornadiene cycle enthalpies by DLPNO-CCSD(T1 )/CBS method

The DLPNO-CCSD(T₁ )/CBS method combined with simple reactions containing small reference species leads to an improvement in the accuracy of theoretically evaluated enthalpies of formation of medium-sized polyalicyclic hydrocarbons when compared with the widely used composite approach. The efficiency of the DLPNO-CCSD(T₁ )/CBS method is most vividly demonstrated by comparing with the results of G4 calculations for adamantane. The most important factor in choosing appropriate working reaction is the same number of species on both sides of the equation. Among these reactions, the reactions with small enthalpy change usually provide a better cancellation of errors. The DLPNO-CCSD(T₁ )/CBS method was used to calculate the enthalpies of formation of compounds belonging to the norbornadiene cycle (norbornadiene, quadricyclane, norbornene, nortricyclane, and norbornane). The most reliable experimental enthalpies of formation are recommended for these compounds by comparing calculated values with conflicting experimental data.

Evaluating the Viability of Successive Ring-Expansions Based on Amino Acid and Hydroxyacid Side-Chain Insertion

The outcome of ring-expansion reactions based on amino/hydroxyacid side-chain insertion is strongly dependent on ring size. This manuscript, which builds upon our previous work on Successive Ring Expansion (SuRE) methods, details efforts to better define the scope and limitations of these reactions on lactam and β-ketoester ring systems with respect to ring size and additional functionality. The synthetic results provide clear guidelines as to which substrate classes are more likely to be successful and are supported by computational results, using a density functional theory (DFT) approach. Calculating the relative Gibbs free energies of the three isomeric species that are formed reversibly during ring expansion enables the viability of new synthetic reactions to be correctly predicted in most cases. The new synthetic and computational results are expected to support the design of new lactam- and β-ketoester-based ring-expansion reactions.

Data Science Approach to Estimate Enthalpy of Formation of Cyclic Hydrocarbons

In spite of increasing importance of cyclic hydrocarbons in various chemical systems, studies on the fundamental properties of these compounds, such as enthalpy of formation, are still scarce. One of the reasons for this is the fact that the estimation of the thermodynamic properties of cyclic hydrocarbon species via cost-effective computational approaches, such as group additivity (GA), has several limitations and challenges. In this study, a machine learning (ML) approach is proposed using a support vector regression (SVR) algorithm to predict the standard enthalpy of formation of cyclic hydrocarbon species. The model is developed based on a thoroughly selected dataset of accurate experimental values of 192 species collected from the literature. The molecular descriptors used as input to the SVR are calculated via alvaDesc software, which computes in total 5255 features classified into 30 categories. The developed SVR model has an average error of approximately 10 kJ/mol. In comparison, the SVR model outperforms the GA approach for complex molecules and can be therefore proposed as a novel data-driven approach to estimate enthalpy values for complex cyclic species. A sensitivity analysis is also conducted to examine the relevant features that play a role in affecting the standard enthalpy of formation of cyclic species. Our species dataset is expected to be updated and expanded as new data are available to develop a more accurate SVR model with broader applicability.

Domain-based local pair natural orbital methods within the correlation consistent composite approach

Ab initio composite approaches have been utilized to model and predict main group thermochemistry within 1 kcal mol^-1 , on average, from well-established reliable experiments, primarily for molecules with less than 30 atoms. For molecules of increasing size and complexity, such as biomolecular complexes, composite methodologies have been limited in their application. Therefore, the domain-based local pair natural orbital (DLPNO) methods have been implemented within the correlation consistent composite approach (ccCA) framework, namely DLPNO-ccCA, to reduce the computational cost (disk space, CPU (central processing unit) time, memory) and predict energetic properties such as enthalpies of formation, noncovalent interactions, and conformation energies for organic biomolecular complexes including one of the largest molecules examined via composite strategies, within 1 kcal mol^-1 , after calibration with 119 molecules and a set of linear alkanes. © 2019 Wiley Periodicals, Inc.

Superhalogen and Superacid

A superhalogen F@C₂₀ (CN)₂₀ and a corresponding Brønsted superacid were designed and investigated on DFT and DLPNO-CCSD(T) levels of theory. Calculated compounds have outstanding electron affinity and deprotonation energy, respectively. We consider superacid H[F@C₂₀ (CN)₂₀ ] to be able to protonate molecular nitrogen. The stability of these structures is discussed, while some of the previous predictions concerning neutral Brønsted superacids of record strength are doubted. © 2019 Wiley Periodicals, Inc.

Gas Phase Silver Thermochemistry from First Principles

Domain-based local pair natural orbital coupled cluster approach with single, double, and perturbative triple excitations, DLPNO-CCSD(T), has been applied within a framework of a reduced version of the reaction-based Feller-Peterson-Dixon (FPD) scheme to predict gas phase heats of formation and absolute entropies of silver inorganic and organometallic compounds. First, we evaluated all existing experimental data currently limited by thermodynamic functions of 10 silver substances (AgH, AgF, AgBr, AgI, Ag₂, Ag₂S, Ag₂Se, Ag₂Te, AgCN, AgPO₂). The mean average deviation between computed and experimental heats of formation was found to be 1.9 kcal/mol. Notably, all predicted heats of formation turned out to be within the error bounds of their experimental counterparts. Second, we predicted heats of formation and entropies for additional 90 silver species with no experimental data available, substantially enriching silver thermochemistry. Combination of gas phase heats of formation Δ H_f and entropies S° of AgNO₂, AgSCN, Ag₂SO₄, and Ag₂SeO₄ obtained in this work, with respective solid-state information, resulted in accurate sublimation thermochemistry of these compounds. Complementation of predicted Δ H_f with heats of formation of some neutrals and positive ions produced 33 silver bond strengths of high reliability. Obtained thermochemical data are promising for developing the concepts of silver chemistry. In addition, derived heats of formation and bond dissociation enthalpies, due to their high diversity, are found to be relevant for testing and training of computational chemistry methods.

A Benchmark of Density Functional Approximations For Thermochemistry and Kinetics of Hydride Reductions of Cyclohexanones

The performance of density functionals and wavefunction methods for describing the thermodynamics and kinetics of hydride reductions of 2-substituted cyclohexanones has been evaluated for the first time. A variety of exchange correlation functionals ranging from generalized gradient approximations to double hybrids have been tested and their performance to describe the facial selectivity of hydride reductions of cyclohexanones has been carefully assessed relative to the CCSD(T) method. Among the tested methods, an approach in which single-point energy calculations using the double hybrid B2PLYP-D3 functional on ωB97X-D optimized geometries provides the most accurate transition state energies for these kinetically-controlled reactions. Moreover, the role of torsional strain, temperature, solvation, noncovalent interactions on the stereoselectivity of these reductions was elucidated. Our results indicate a prominent role of the substituent on the cis/trans ratios driven by the delicate interplay between torsional strain and dispersion interactions.

Titanocene Selenide Sulfides Revisited: Formation, Stabilities, and NMR Spectroscopic Properties

[TiCp₂S₅] (phase A), [TiCp₂Se₅] (phase F), and five solid solutions of mixed titanocene selenide sulfides [TiCp₂Se_xS_5−x] (Cp = C₅H₅⁻) with the initial Se:S ranging from 1:4 to 4:1 (phases B–E) were prepared by reduction of elemental sulfur or selenium or their mixtures by lithium triethylhydridoborate in thf followed by the treatment with titanocene dichloride [TiCp₂Cl₂]. Their ⁷⁷Se and ¹³C NMR spectra were recorded from the CS₂ solution. The definite assignment of the ⁷⁷Se NMR spectra was based on the PBE0/def2-TZVPP calculations of the ⁷⁷Se chemical shifts and is supported by ¹³C NMR spectra of the samples. The following complexes in varying ratios were identified in the CS₂ solutions of the phases B–E: [TiCp₂Se₅] (5₁), [TiCp₂Se₄S] (4₁), [TiCp₂Se₃S₂] (3₁), [TiCp₂SSe₃S] (3₆), [TiCp₂SSe₂S₂] (2₅), [TiCp₂SSeS₃] (1₂), and [TiCp₂S₅] (0₁). The disorder scheme in the chalcogen atom positions of the phases B–E observed upon crystal structure determinations is consistent with the spectral assignment. The enthalpies of formation calculated for all twenty [TiCp₂Se_xS_5−x] (x = 0–5) at DLPNO-CCSD(T)/CBS level including corrections for core-valence correlation and scalar relativistic, as well as spin-orbit coupling contributions indicated that within a given chemical composition, the isomers of most favourable enthalpy of formation were those, which were observed by ⁷⁷Se and ¹³C NMR spectroscopy.

Computational Investigation of RO2 + HO2 and RO2 + RO2 Reactions of Monoterpene Derived First-Generation Peroxy Radicals Leading to Radical Recycling

The oxidation of biogenically emitted volatile organic compounds (BVOC) plays an important role in the formation of secondary organic aerosols (SOA) in the atmosphere. Peroxy radicals (RO₂) are central intermediates in the BVOC oxidation process. Under clean (low-NO _x) conditions, the main bimolecular sink reactions for RO₂ are with the hydroperoxy radical (HO₂) and with other RO₂ radicals. Especially for small RO₂, the RO₂ + HO₂ reaction mainly leads to closed-shell hydroperoxide products. However, there exist other known RO₂ + HO₂ and RO₂ + RO₂ reaction channels that can recycle radicals and oxidants in the atmosphere, potentially leading to lower-volatility products and enhancing SOA formation. In this work, we present a thermodynamic overview of two such reactions: (a) RO₂ + HO₂ → RO + OH + O₂ and (b) R'O₂ + RO₂ → R'O + RO + O₂ for selected monoterpene + oxidant derived peroxy radicals. The monoterpenes considered are α-pinene, β-pinene, limonene, trans-β-ocimene, and Δ³-carene. The oxidants considered are the hydroxyl radical (OH), the nitrate radical (NO₃), and ozone (O₃). The reaction Gibbs energies were calculated at the DLPNO-CCSD(T)/def2-QZVPP//ωB97X-D/aug-cc-pVTZ level of theory. All reactions studied here were found to be exergonic in terms of Gibbs energy. On the basis of a comparison with previous mechanistic studies, we predict that reaction a and reaction b are likely to be most important for first-generation peroxy radicals from O₃ oxidation (especially for β-pinene), while being less so for most first-generation peroxy radicals from OH and NO₃ oxidation. This is because both reactions are comparatively more exergonic for the O₃ oxidized systems than their OH and NO₃ oxidized counterparts. Our results indicate that bimolecular reactions of certain complex RO₂ may contribute to an increase in radical and oxidant recycling under high HO₂ conditions in the atmosphere, which can potentially enhance SOA formation.