Abstract We present the Open-Shell Organic Systems (OSOS) database, a diverse benchmark set comprising 9934 atomization, isomerization, and H-abstraction energies computed at the CCSD(T)/CBS limit via W1-F12 theory. Historically, large-scale data sets (e.g., QM9 and ANI-1x) have been heavily biased toward closed-shell species. The OSOS database addresses this gap by providing a diverse and accurate set of reaction energies dedicated exclusively to organic doublet radical systems for benchmarking and training purposes. The data set covers the chemical space spanned by localized and delocalized organic radicals containing up to seven non-hydrogen atoms (C, N, O, and F). The database consists of 3360 radical total atomization energies, 3360 H-abstractions, 2292 H-shifts, and 922 constitutional isomerizations. We use this database to assess the performance of density functional theory (DFT). We find that most density functionals systematically overestimate radical atomization energies; therefore, the inclusion of dispersion corrections generally leads to a deterioration in performance. Furthermore, delocalized radicals present a significantly greater challenge for most DFT methods than localized species, leading to larger overestimations of their atomization energies. For localized radicals, errors increase in the order: O-centered < C-centered < N-centered. Remarkably, the deep-learning meta-GGA Skala functional demonstrates exceptional accuracy, significantly outperforming conventional functionals across all rungs of Jacob’s Ladder and consistently achieving mean absolute deviations below 1 kcal mol–1 for the abstraction, shift, and isomerization reactions. The OSOS database establishes a rigorous standard to guide the development and validation of next-generation density functionals and machine-learning potentials for doublet radicals.
Abstract Enantiorecognition is conventionally rationalized through stereochemical models, such as lock-and-key, induced-fit, and three-point interaction schemes, in which a chiral selector discriminates between enantiomers through geometry and noncovalent interactions. Nevertheless, such models are blind to electron spin. Chiral-induced spin selectivity (CISS) brings electron spin explicitly into this picture. Here, we argue that spin-selective charge transfer can provide an additional, predominantly kinetic contribution to enantiorecognition, most clearly expressed out of equilibrium, where it affects adsorption, reaction, or charge-transfer rates. We organize this view as a generator-and-reader scheme: a chiral element under a nonequilibrium drive generates a handedness-locked spin polarization, which is then read either by a ferromagnet, through exchange, or by a second chiral element, through spin matching, with no external magnet required. Recent experiments on magnetic adsorption, electrochemical junctions, organic photoelectrochemical transistors, and magnetization-controlled growth of helical aggregates from achiral units converge on this generator-and-reader structure. Across these platforms (spanning molecular, axial, and emergent mesoscopic chirality), a common signature recurs: a handedness-dependent, spin-selective current at a chiral charge-transfer interface. Rather than replacing stereochemical models, this framework adds a spin-resolved electronic coordinate to them: one that becomes experimentally visible when chiral recognition is coupled to nonequilibrium charge transfer.
Superthermal (“hot”) oxygen atoms deposit their energy in planetary upper atmospheres and in hypersonic shock layers through collisions with N2, but the rate at which they do so is set not by the magnitude of the collision cross section, which is dominated by forward diffraction, but by transport-weighted moments of the differential cross section (DCS). We derive momentum-transfer and viscosity cross sections, mean collisional deflections, and energy-relaxation rates for O(3P) + N2 from our previously reported quantum coupled-channel DCSs, for 16O, 17O, and 18O over collision energies 0.31−4.0 eV. The momentum-transfer cross section is well represented by σm = 9.90 × 10−16 Ek−0.385 cm2 (Ek in eV), a factor of 28−32 below the collision cross section, with mean deflection ⟨1 − cos θ⟩ = 0.031−0.036. Rotationally inelastic channels supply 72−88% of it, so transport treatments built on elastic angular distributions underestimate the stopping power of N2 several-fold. The heavy-isotope enhancement of the integral elastic cross section (1−5%) does not survive transport weighting. σm is isotope-independent to within 0.4% above 0.6 eV, because the enhancement resides in the transport-inert forward cone, so isotopic effects in thermalization are kinematic (+2.9% recoil per collision for 18O relative to 16O). The mean energy loss per collision rises from 10 to 68 meV, and Monte Carlo slowing down gives ≈120 collisions and a stopping column of 3.7 × 1015 cm−2 to thermalize a nascent 3.48 eV atom in pure N2, some twenty times the isotropic hard-sphere estimate. A loss law that satisfies detailed balance in a 460 K bath raises the number of collisions by 6.5%. Comparing the isotopes at equal velocity instead of equal energy shifts σm by 1.5−2.8%, and this shift is purely kinematic.
Abstract Nitrous oxide (N2O) and ethylene (C2H4) blending fuel is one of the most representative nitrous oxide fuel blend propellants. To address deficiencies in the study of the C2H4/N2O model at intermediate temperatures, this work investigates the C2H4 + N2O potential energy surface using quantum chemistry at the CCSD(T)/aug-cc-pVQZ//B3LYP/6-311++G(d,p) level. Rate constants over 333–2000 K and 0.1–100 atm are calculated based on RRKM/ME theory. According to our calculation results, the addition reaction C2H4 + N2O → CH2CH2N2O is identified as the dominant chain-initiation step under the present investigated conditions, which decomposes via two primary pathways. The CH2CH2N2O → CH2O + CH2NN pathway competes strongly with the CH2CH2N2O → CH3CHO + N2 channel due to their close energy barriers. Well-skipping reactions of C2H4 + N2O → CH3CHO + N2 and C2H4 + N2O → CH2O + CH2NN, as well as a newly computed pathway CH2NN → CH2 + N2, are also important under combustion-relevant conditions. These obtained reaction pathways and their related rate constants are incorporated into four different models, i.e., GRI 3.0 model, Konnov 2009 model, Glarborg 2018 model, and Zhang 2023 model. The updated reactions significantly improve all their predictions of ignition delay times at intermediate temperatures. The impact of the theoretical calculation results on the mechanism of N2O/C2H4 oxidation is analyzed. At intermediate temperatures, chain-initiation reactions between N2O and C2H4 successfully bypass the unimolecular decomposition of N2O that requires high activation energy. Therefore, C2H4 tends to react directly with N2O, and aldehyde-related reactions are responsible for chain propagation and OH formation. In contrast, at high temperatures, the decomposition of N2O is activated, and the reactions between C2H4 and reactive radicals become another important pathway for fuel consumption. Consequently, the contribution of the direct reaction between C2H4 and N2O is weakened.
Abstract In hydrocarbon fuel oxidation, propanal is a key intermediate and oxygenated pollutant. Its combustion rate constants are mostly estimated by analogy, bringing large uncertainties to kinetic models. This work comprehensively investigates H-abstraction from propanal by H/CH3/C2H5/OH radicals. High-precision rate constants and branching ratios were determined by multistructural variational transition state theory with small-curvature tunneling across 240–2000 K. Our findings emphasize that conformational flexibility and torsional anharmonicity greatly affect both the rate constants and branching ratios. Across the full temperature range, α-site abstraction (R1α) is the primary process in the propanal + H system. For propanal + CH3, the rate constant of β-site channel (R2β) is dominant. In the propanal + C2H5 system, the α-site reaction (R3α) remains prominent, whereas for propanal + OH, the branching ratios of the β-site and γ-site channels (R4β, R4γ) increase with temperature, while that of the α-site (R4α) decreases. Excellent agreement with available experimental data is achieved by the fitted expression kR4 = 0.0017 × T4.835 exp(2621.61/T) (in cm3 mol–1 s–1) for the total rate constant of propanal + OH. The new rate constants were adopted to update Veloo et al.’s kinetic model, followed by sensitivity analysis. The revised model enables more reliable simulation of propanal combustion and emission behaviors.
Abstract Planar boron clusters often display π-bonding patterns analogous to those found in aromatic hydrocarbons, making them intriguing inorganic counterparts of classical hydrocarbon systems. Herein, we report zinc-group element-stabilized boron analogues of the cyclopropenyl cation (C3H3+), formulated as M3B3+ (M = Zn, Cd, and Hg). Global minimum searches reveal that these clusters adopt highly symmetric D3h structures with a 1A1′ electronic ground state and exhibit pronounced dynamic stability. Chemical bonding analyses demonstrate that M3B3+ reproduces the fundamental bonding motif of the cyclopropenyl cation, consisting of covalent M-B interactions and a three-center two-electron (3c–2e) π bond delocalized over the B3 core. The resulting π-electron system satisfies Hückel’s (4n + 2) rule, giving rise to π aromaticity, as supported by magnetic aromaticity indices. Notably, in M3B3+ (M = Zn and Cd), the vertical T1 excited states exhibit σ-antiaromaticity, highlighting an unusual aromaticity reversal upon electronic excitation.
Abstract We report E2(η6-Be6H6) (E═Si, Ge, Sn, Pb), a new family of doubly aromatic inverse sandwich clusters in which a tetrel diatomic is sandwiched between a beryllium hydride ring. All four members adopt a singlet D6h global minimum, separated from the nearest competing isomer by 31.3, 29.1, 19.8, and 8.4 kcal·mol–1 for E═Si, Ge, Sn, and Pb, respectively, with large HOMO–LUMO gaps of 4.3–4.8 eV and dynamic stability confirmed up to at least 500 K. The electronic structure is strongly polarized: the tetrel atoms carry large negative charges (−2.1 to −1.8 |e|) and interact with the Be6H6 ring through predominantly ionic but non-negligible covalent E–Be contacts, while the ring is sustained by six Be–H–Be three-center two-electron bonds and the E2 fragment retains significant E–E bonding character. A delocalized 6σ/6π bonding pattern spans the full inverse sandwich framework, and diatropic σ and π ring currents─with total ring-current strengths of 15.6–19.5 nA·T–1─confirm σ/π double Hückel aromaticity throughout the series. These results extend the Be6H6-based inverse sandwich family to group 14 and show that the aromatic electron count in these systems is governed by the electron-accepting capacity of the axial fragment.
Abstract Spiro and fused skeletons are particularly noteworthy for their intriguing biological properties and their prevalence in natural products and drug candidates. Although meaningful progress has been made in the synthesis of these compounds, a comprehensive mechanistic understanding remains essential. In this study, we present a theoretical investigation of the competing mechanisms in Cs2CO3-catalyzed (3 + 2) versus (3 + 4) annulations involving aminoalkyl α,β-unsaturated esters and azadienes. Our calculations reveal that for benzofuran-derived azadienes, the (3 + 2) annulation is energetically favored over the (3 + 4) pathway, leading to spirocycles. In contrast, benzothiophene-derived azadienes preferentially follow the (3 + 4) route to afford fused diazepines. The overall mechanism begins with Cs2CO3 deprotonating the relatively acidic N–H group of the aminoalkyl α,β-unsaturated ester to generate a nitrogen anion. This anion then nucleophilically attacks the azadiene, forming carbanion and nitrogen anion intermediates, which subsequently diverge into two pathways. In Pathway A, the carbanion intermediate undergoes α-regioselective (3 + 2) annulation, followed by protonation with CsHCO3 to yield spirobenzofurans. In Pathway B, the nitrogen anion intermediate undergoes γ-regioselective (3 + 4) annulation, and subsequent protonation by CsHCO3 gives the seven-membered ring benzothiophene-diazepine product. To elucidate the origin of the heteroatom-controlled regioselectivity, we employed three complementary approaches: Parr functions (Pk–) and local nucleophilicity indices (Nk) to assess the nucleophilicity of the competing reactive sites, and distortion/interaction analysis to dissect the energetic contributions. The results reveal that replacement of O by S modulates the nucleophilicity of the N1 and C2 sites in opposite directions. Moreover, the distortion/interaction analysis demonstrates that the O → S substitution selectively increases the distortion energy for the (3 + 2) pathway but decreases it for the (3 + 4) pathway, providing a quantitative physical origin for the switchable regioselectivity. Collectively, these findings establish a complete mechanistic picture of the heteroatom-controlled regiodivergence.
Critical temperatures for the ignition of cylindrical hot spots up to 0.5 μm in diameter in the secondary explosive cyclotetramethylene tetranitramine (HMX) have been computed using the kinetic Monte Carlo (kMC) method. By combining the thermal bit transfer model with effective single-step Arrhenius kinetics for the onset of thermal explosion, we are able to observe the homogeneous nucleation and growth of sustained deflagration reactions in HMX at the mesoscale. The kMC simulations capture the stochastic onset of thermal explosion seen in quantum molecular dynamics simulations at the nanometer length scales that are required to adequately resolve the temperature profile across a deflagration front. Deflagration velocities and hot spot critical temperatures have been evaluated for four sets of single-step Arrhenius kinetics for HMX that account for the effects of self-heating on the acceleration of the reaction rate in different ways. It is shown that the single-step Arrhenius kinetics proposed by Manner et al., Burnham and Weese, and Henson et al. give roughly similar hot spot critical temperatures, but a reparametrization of the 2001 Henson-Smilowitz kinetics to include the effects of self-heating on the reaction rate gives rise to significantly different behavior, with hot spot critical temperatures that are about 700 K higher than those from the other models. The origins of this behavior are discussed.
Abstract Vibrational Raman optical activity (VROA) is an information-rich technique with which to explore the molecular stereochemistry of chiral molecules. The molecular properties needed to model a VROA spectrum can be computed using coupled-cluster (CC) theory. We report VROA spectra computed with CC theory in an origin-independent manner for the first time, applying the approach to several derivatives of β-propiolactone (oxetan-2-one), a strained, four-membered heterocycle with naturally occurring derivatives exhibiting a variety of stereochemistries. The spectra were theoretically benchmarked in several ways. First, a comparison of VROA spectra computed using CC2 theory was made to evaluate the performance of a large number of one-electron basis sets. Second, spectra computed with CC2 and CCSD were compared to gauge the effectiveness of the less costly CC2 method. Third, VROA spectra determined from CC2 chiroptical tensors computed using different geometries and vibrational Hessians were compared with one another. Finally, VROA spectra for β-propiolactones substituted with methyl, ethyl, and propyl groups were produced with CC2 using geometries and Hessians determined by B3LYP/aug-cc-pVTZ computations. For structures monosubstituted at the 3-position (adjacent to the carbonyl), an intense peak occurs at ≈1130 cm–1 corresponding largely to the C–C stretch between the ring and the alkyl substituent. For structures monosubstituted at the 4-position, a similar peak corresponding largely to the C–C stretch between the ring and the substituent appears at ≈1115 cm–1. For disubstituted species of (3S,4R) stereochemistry, a mode involving ring-breathing motion and asymmetrical wagging of the H atoms on C3 and C4 (on the same side of the β-lactone ring) produces a large negative signal at ≈1130 cm–1. The overall spectrum is found to be dominated by the configuration at the 3-position and less sensitive to that at the 4-position. The results suggest that VROA can be productively employed to interrogate the stereochemistry at the 3-position. Unfortunately, no clear VROA signal was identified that characterizes the stereochemistry at the 4-position in disubstituted species.
Abstract Femtosecond pulse excitation gave the time evolution of the transient absorption spectra (TAS) of trans- and cis-stilbene (t- and c-St) in methanol. Global analysis of t-St TAS gives the spectrum of the singlet excited state of t-St, 1t*, with a lifetime, 46 ps, consistent with previous measurements. The spectrum of the singlet excited state of c-St, 1c*, has bands at 360 and 630 nm in agreement with early literature spectra in other solvents. However, the 1c* spectrum obtained on multiexponential global analysis of c-St TAS is much more intense in the UV relative to the visible in disagreement with the observed TAS. Although the lifetime of the 630 nm band, 0.47 ps, agrees with the reported lifetimes, monoexponential fits in the UV region yield lifetimes that are significantly longer. We attribute this discrepancy to interference by the twisted phantom singlet, 1p*, whose growth and decay occur in the same UV region. The 360 nm band rises together with a weak 405 nm band that decays faster and may be the Franck–Condon 1c* transient or a 1c* transient on the way to dihydrophenanthrene formation. Also observed on exciting c-St is the TAS of 1t*. A mechanism involving two-photon sequential excitation of c-St accounts for this result.
Abstract In this work, the collision-induced association of a benzene molecule with a pre-existing benzene–hexafluorobenzene (Bz–HFB) dimer and the ensuing dissociation dynamics of the resulting trimer complexes are investigated using computational chemical dynamics simulations. Trajectories were computed at 300, 500, 700, 1000, 1200, 1500, 1800, and 2000 K to examine the competition between trimer formation, dimer exchange, and dissociation. Stable trimer formation is found to be intrinsically rare and becomes increasingly unfavorable with increasing temperature, occurring only at 300 K and between 500 and 1500 K, while no stable trimers are observed at 1800 and 2000 K. Subsequently, the trimers undergo either partial dissociation to yield dimer products or complete fragmentation into three monomers. The formation of new dimers is considerably more probable than trimer formation. Among the product dimers, Bz(2)-HFB forms significantly more frequently than Bz(1)-Bz(2), reflecting the stronger Bz···HFB interaction. The ensuing dissociation of the newly formed Bz-HFB dimers is faster than that of the benzene dimers, consistent with trends reported for unimolecular dissociation. The dissociation of the original Bz-HFB complex closely resembles that of the isolated dimer. These results provide a detailed molecular-level understanding of association-induced competing reaction pathways in weakly bound aromatic clusters, providing new insights into the dynamics of collision-driven cluster growth and fragmentation in the combustion environment.
Abstract Heterogeneous reactions occurring in atmospheric aerosols are crucial components of multiple chemical reaction cycles. Sulfate species exert substantial influence on many heterogeneous reaction mechanisms due to their abundance and speciation. Despite the importance of sulfate species on heterogeneous reactions, quantitative descriptions of interfacial affinity and sulfate species partitioning at interfaces remain incomplete. Here, we quantify the interfacial properties of HSO4–, using cetyltrimethylammonium hydrogen sulfate (CTAHS) reverse micelles (RMs), as model marine aerosol particles. Infrared spectra (IR) of CTAHS RMs were collected and analyzed using multivariate curve resolution-alternating least-squares (MCR-ALS). MCR-ALS analysis reveals that four distinct species contribute to the observed IR spectra, which we assign as CTA+-HSO4– contact pairs (CPs), interfacial HSO4–, core HSO4–, and core SO42–. Computational studies reveal that the CTA+-HSO4– CPs exist in two distinct orientations at the interface. Spectral contributions from each chemical species are used to determine the quantitative interfacial affinity (χInt.) of HSO4–. Our observations indicate that the cationic charge of the interface alters the ionic distribution of sulfate species compared to air–water interfaces, placing H3O+ below HSO4–.
Abstract Metal-free triplet–triplet annihilation upconversion (TTA-UC) under long-wavelength excitation rules out metal toxicity and improves biocompatibility, showing great potential for biomedical use of photon upconversion materials. Intermolecular hydrogen bonding commonly occurs in biological environments, yet its effects on organic upconversion performance have rarely been clarified. Herein, we present a cationic Nile blue derivative (SeNB) as an effective sensitizer that produces far-red to visible upconversion luminescence, and adopt this molecule as a model to explore hydrogen-bonding impacts on organic upconversion. Transient electronic spectroscopy measurements show solvent hydrogen bonding weakens upconversion by disrupting excited states of both the sensitizer and annihilator, and by slowing intermolecular triplet energy transfer via the solvent cage effect. Such interactions directly quench the triplet state of SeNB, greatly reducing intersystem crossing rates and triplet quantum yields. The diketopyrrolopyrrole (DPP) annihilator carries hydrogen-bond accepting sites, and undergoes accelerated triplet quenching and shortened triplet lifetimes under hydrogen-bonding interactions. At low annihilator content, solvent cage effects driven by hydrogen bonding hinder Dexter-type triplet energy transfer from the sensitizer to the annihilator, which further lowers upconversion efficiency. This study demonstrates that Nile blue derivatives are effective sensitizers for TTA-UC, but also reveals that intermolecular hydrogen bonding severely suppresses their upconversion performance in protic solvents. These findings provide fundamental insights into solvent-regulated photophysical behaviors of ionic sensitizers and highlight the need for molecular engineering strategies to mitigate hydrogen-bonding effects for future applications in biological environments.
Abstract Accurate modeling of nonbonded interactions remains challenging because conventional force fields often provide inadequate descriptions of polarization and charge transfer. Here, we introduce a polarizable split-charge equilibration (PSQ) framework that combines localized charge flow with dipolar polarization, thereby mitigating the unphysical long-range charge delocalization associated with polarizable charge-equilibration models. To enable practical large-scale simulations, we develop an efficient PSQ implementation through algorithmic optimization and parallelization, achieving performance comparable to conventional approaches despite the model’s greater formal complexity. We combine PSQ with the universal nonbond potential for van der Waals interactions and an explicit four-body hydrogen-bond term to form the PUxH framework. PUxH achieves near-chemical accuracy in predicting small-molecular pair interactions and accurately reproduces the bulk properties of water, ammonia, and benzene. Overall, PUxH provides a transferable, physically grounded, and computationally efficient framework for modeling nonbonded interactions in molecular and condensed-phase systems.
Abstract We performed infrared photodissociation spectroscopy and theoretical computations on gas-phase protonated water–acetonitrile clusters, H+[(H2O)m(CH3CN)n] (m = 1, n = 1–3; m = 2, n = 1–6; and m = 3, n = 5), to determine their cluster structures and compared their infrared spectra with previously reported spectra of protonated water clusters in acetonitrile solutions. For the (m = 2, n = 4) cluster, which serves as a model for the protonated water dimer in an acetonitrile solution, the band center of the hydrogen-bonded OH stretch absorption was observed at a lower frequency than that in the solution spectra. This indicates that the formation of the second and subsequent solvation shells significantly weakens the hydrogen bonding between the protonated water dimer (ion core) and the first solvation shell in the solution phase. On the other hand, the spectrum of the (m = 3, n = 5) cluster, a model of the protonated water trimer with a completed first solvation shell, showed a much better agreement with that of the solution phase, suggesting a tendency where an increase in the protonated cluster size reduces the influence of the second solvation shell.
Abstract Mass accommodation is a critical process, controlling uptake coefficients of gaseous species in aerosols and clouds. The effective mass accommodation coefficient (αeff) was developed to account for kinetic limitations of bulk diffusion and reactions with a simple analytical equation. In this work, αeff is extended by including surface reactions and bulk accommodation. Systematic simulations over a wide range of kinetic parameters demonstrate excellent agreement between the αeff method and a benchmark kinetic multilayer model once quasi-equilibrium is achieved in the effective penetration depth. The αeff method can quantify the contributions of surface and bulk processes to overall uptake. Comparisons with two other models (Wilson et al., 2022; Prophet and Wilson, 2025) show that the αeff method more accurately predicts uptake coefficients when uptake is limited by bulk diffusion of volatile species or when surface reactions dominate under high surface coverage conditions. The developed method provides a useful tool for analyzing experimental measurements and developing uptake parametrizations for large-scale models.
Hydrofluoroolefins (HFOs) are refrigerants designed to have low global warming potential (GWP), but, depending on their structure, HFOs can react with O3 to produce CHF3, a greenhouse gas with a GWP of 15,500 with a 100-year time horizon. In particular, the most recent mass spectrometric chamber study of the ozonolysis of the common refrigerant trans-CHF═CHCF3 (HFO-1234ze(E)) reports a CHF3 yield of 0.079 at 298 K and 1 atm. A recent quantum chemical (ωB97X-D/cc-pVTZ) and RRKM/master equation (ME) study quantified CHF3 formation due to the isomerization of the chemically activated CF3CHOO Criegee intermediate (CI) to trifluoroacetic acid (TFA), which then decomposes to CHF3 and CO2. The predicted CHF3 yield of 0.636 at 298 K and 1 atm was far higher than the experimental result. We have revised the theoretical model of HFO ozonolysis by using the DLPNO-CCSD(T1)/cc-pVTZ quantum chemical method and incorporating other CI reaction pathways. We find that, relative to the 0-K energy of the more stable CI conformer, the transition structure (TS) for the decomposition of TFA to HF and an α-lactone has an energy of -66.5 kcal mol-1, while the TS for the decomposition of TFA to CHF3 and CO2 is -41.8 kcal mol-1. RRKM/ME simulations of trans-CHF═CHCF3 ozonolysis based on our new chemical mechanism predict a CHF3 yield of 0.076 at 298 K and 1 atm. Revised predictions of CHF3 yield from the ozonolysis of CH2═CHCF3 (HFO-1243zf) and cis-CHCF3═CHCF3 (HFO-1336mzz(Z)) differ substantially from experimental measurements, underscoring the need for both more accurate modeling and additional experimental work.
Secondary organic aerosols (SOA) derived from biogenic volatile organic compounds (BVOCs) affect climate and human health; however, there is uncertainty associated with the extent of their impact, in part due to the complexity of their physicochemical composition and varied heterogeneous chemistry. α-Pinene oxide is an oxidation product of α-pinene, the second most abundant BVOC, and it has been found to undergo rapid hydrolysis, forming campholenic aldehyde, trans-carveol, and trans-soberol, which can then react further. Although a mechanism for α-pinene oxide hydrolysis has been proposed and the kinetics have been studied previously, the rate constants for the initial step in this reaction have not been determined because the reaction occurred too quickly to detect any α-pinene oxide. Herein, Raman spectroscopy was used to probe α-pinene oxide hydrolysis in bulk aqueous solutions, expanding on previous kinetic analyses by making measurements on a shorter time scale (<5 min reaction time) and testing a wider range of acidity conditions. Campholenic aldehyde and pinol were the primary products, with pinol being produced only under highly acidic conditions. The pseudo-first-order rate constants for the initial consumption of α-pinene oxide were calculated to be on the order of 10-2-10-3 s-1. Additionally, full characterization of the experimental and density functional theory (DFT)-predicted Raman spectra and vibrational modes for α-pinene oxide and subsequent products is provided, highlighting signature vibrational modes that can be used for the identification of these chemical species. This work further develops the understanding of α-pinene oxide chemistry relevant to α-pinene-derived SOA.
Cyclo[n]silicons (Sin), all-silicon rings composed solely of silicon atoms, represent a new class of molecular silicon allotropes analogous to cyclo[n]carbons. Here we present a systematic theoretical study on representative Sin (n = 8, 10, 12, 14), investigating their geometric structure, bonding configurations, and aromaticity. All examined silicon rings can exist as stable crown-like structures in the gas phase. Electronic structure and bonding analyses reveal that Si-Si bonding is dominated by σ interactions but also contains π contributions. Real-space analyses indicate that electron delocalization becomes more pronounced as the ring size decreases. Magnetic criteria further reveal that Si8 and Si10 exhibit antiaromatic magnetic responses mainly associated with σ-conjugation, whereas Si12 and Si14 show much weaker magnetic responses and are close to nonaromatic. These results establish the fundamental bonding and aromaticity patterns of all-silicon cyclic molecules and provide theoretical insight into the design of silicon-based molecular rings.
