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The selective formation of antimony‐carbon bonds via organic superbase catalysis under metal‐ and salt‐free conditions is reported. This novel approach utilizes electron‐deficient stibine, Sb(C6F5)3, to give upon base‐catalyzed reactions with weakly acidic aromatic and heteroaromatic hydrocarbons access to a range of new aromatic and heteroaromatic stibines, respectively, with loss of C6HF5. Also, the significantly less electron‐deficient stibines, Ph2SbC6F5 and PhSb(C6F5)2 smoothly underwent base‐catalyzed exchange reactions with a range of terminal alkynes to generate the stibines of formulae PhSb(C≡CPh)2, and Ph2SbC≡CR [R=C6H5, C6H4‐NO2, COOEt, CH2Cl, CH2NEt2, CH2OSiMe3, Sb(C6H5)2], respectively. These formal substitution reactions proceed with high selectivity as only the C6F5 groups serve as a leaving group to be liberated as C6HF5 upon formal proton transfer from the alkyne. Kinetic studies of the base‐catalyzed reaction of Ph2SbC6F5 with phenyl acetylene to form Ph2SbC≡CPh and C6HF5 suggested the empirical rate law to exhibit a first‐order dependence with respect to the base catalyst, alkyne and stibine. DFT calculations support a pathway proceeding via a concerted σ‐bond metathesis transition state, where the base catalyst activates the Sb‐C6F5 bond sequence through secondary bond interactions. [ABSTRACT FROM AUTHOR]

This work provides a detailed multi‐component analysis of aromaticity in monosubstituted (X = CH3, CH2−, CH2+, NH2, NH−, NH+, OH, O−, and O+) and para‐homodisubstituted (X = CH3, CH2, NH2, NH, OH, and O) benzene derivatives. We investigate the effects of substituents using single‐reference (B3LYP/DFT) and multireference (CASSCF/MRCI) methods, focusing on structural (HOMA), vibrational (AI(vib)), topological (ELFπ), electronic (MCI), magnetic (NICS), and stability (S0–T1 splitting) properties. The findings reveal that appropriate π‐electron‐donating and π‐electron‐accepting substituents with suitable size and symmetry can interact with the π‐system of the ring, significantly influencing π‐electron delocalization. While the charge factor has a minimal impact on π‐electron delocalization, the presence of a pz orbital capable of interacting with the π‐electron delocalization is the primary factor leading to a deviation from the typical aromaticity characteristics observed in benzene. [ABSTRACT FROM AUTHOR]

Cation‐π interactions are theoretically investigated for alkali metal cation (M+)‐circumcoronene (CC) complexes (M = Li, Na, K), in gas phase and in aqueous solution with consideration of micro‐ and global solvation models using the DFT/PBEh‐3c‐RI/TZVP method. The solvent effect on the M+–CC energy interaction regarding the cation size and the stability of inner‐ and outer‐sphere [M(H2O)n]+–CC complexes are calculated by means of geometry optimizations and potential energy (PE) curves. The PE curves, calculated as a function of perpendicular distance of M+ to the CC plane, predicted one energy minimum for each of the isolated M+–CC complexes. However, for microhydrated complexes, two minima assigned to two different surface complexations were obtained. Microhydrated Li+ and Na+ favored outer‐sphere complexation while inner‐sphere complexation was found more stable for microhydrated K+. These results illustrate nicely the key role, which the cation radius plays for the polarization of the water molecules and the aromatic system. [ABSTRACT FROM AUTHOR]

Time‐dependent density functional theory (TDDFT) was applied to gain insights into the electronic and vibrational spectroscopic properties of an important electron transport mediator, methyl viologen (MV2+). An organic dication, MV2+ has numerous applications in electrochemistry that include energy conversion and storage, environmental remediation, and chemical sensing and electrosynthesis. MV2+ is easily reduced by a single electron transfer to form a radical cation species (MV•+), which has an intense UV–visible absorption near 600 nm. The redox properties of the MV2+/MV•+ couple and light‐sensitivity of MV•+ have made the system appealing for photo‐electrochemical energy conversion (e.g., solar hydrogen generation from water) and the study of photo‐induced charge transfer processes through electronic absorption and resonance Raman spectroscopic measurements. The reported work applies leading TDDFT approaches to investigate the electronic and vibrational spectroscopic properties of MV2+ and MV•+. Using a conventional hybrid exchange functional (B3‐LYP) and a long‐range corrected hybrid exchange functional (ωB97X‐D3), including with a conductor‐like polarizable continuum model to account for solvation, the electronic absorption and resonance Raman spectra predicted are in good agreement with experiment. Also analyzed are the charge transfer character and natural transition orbitals derived from the TDDFT vertical excitations calculated. The findings and models developed further the understanding of the electronic properties of viologens and related organic redox mediators important in renewable energy applications and serve as a reference for guiding the interpretation of electronic absorption and Raman spectra of the ions. [ABSTRACT FROM AUTHOR]

The alternant polycyclic aromatic hydrocarbon pyrene has photophysical properties that can be tuned with different donor and acceptor substituents. Recently, a D (donor)‐Pyrene (bridge)‐A (acceptor) system, DPA, with the electron donor N,N‐dimethylaniline (DMA), and the electron acceptor trifluoromethylphenyl (TFM), was investigated by means of time‐resolved spectroscopic measurements (J. Phys. Chem. Lett. 2021, 12, 2226–2231). DPA shows great promise for potential applications in organic electronic devices. In this work, we used the ab initio second‐order algebraic diagrammatic construction method ADC(2) to investigate the excited‐state properties of a series of analogous DPA systems, including the originally synthesized DPAs. The additionally investigated substituents were amino, fluorine, and methoxy as donors and nitrile and nitro groups as acceptors. The focus of this work was on characterizing the lowest excited singlet states regarding charge transfer (CT) and local excitation (LE) characters. For the DMA‐pyrene‐TFM system, the ADC(2) calculations show two initial electronic states relevant for interpreting the photodynamics. The bright S1 state is locally excited within the pyrene moiety, and an S2 state is localized ~0.5 eV above S1 and characterized as a donor to pyrene CT state. HOMO and LUMO energies were employed to assess the efficiency of the DPA compounds for organic photovoltaics (OPVs). HOMO‐LUMO and optical gaps were used to estimate power conversion and light‐harvesting efficiencies for practical applications in organic solar cells. Considering the systems using smaller D/A substituents, compounds with the strong acceptor NO2 substituent group show enhanced CT and promising properties for use in OPVs. Some of the other compounds with small substituents are also found to be competitive in this regard. [ABSTRACT FROM AUTHOR]

The kinetics of a strategically synthesized burn rate enhancer (BRE) are investigated in its reaction with ammonium perchlorate (AP). The BRE is designed for multifunctionality by 1) producing reactive gases that couple with AP gas production; 2) trapping reactive gases with high surface area; and 3) exposing metal cations to incite exothermic reactions at a temperature coincident with AP decomposition. This study advances inorganic synthesis by introducing a metal inorganic framework (MIF) composed of an aluminum cation (Al+3) surrounded by inorganic "linker" molecules of oxidizing species. To increase surface area, a porous, amorphous MIF (a‐MIF) is synthesized by controlling solution properties of an acid–base precipitation reaction. Upon gas generation, high surface area and aluminum‐rich surface of a‐MIF accelerate AP decomposition and induce an exothermic reaction that is otherwise endothermic in thermal equilibrium analysis. AP decomposition rate is advanced by reducing peak onset temperature and increasing decomposition rate with addition of a‐MIF (i.e., from 17% min−1 at 401 °C to 18% min−1 at 365 °C). Enthalpy of AP decomposition increases from +240 J g−1 to −1040 J g−1. Results introduce an approach for increasing the decomposition rate of solid oxidizers by demonstrating a recipe for designing and synthesizing an MIF. [ABSTRACT FROM AUTHOR]

The biradicaloid character of different types of polycyclic aromatic hydrocarbons (PAHs) based on small band gaps is an important descriptor to assess their opto-electronic properties. In this work, the unpaired electron densities and numbers of unpaired electrons (NU values) calculated at the high-level multireference averaged quadratic coupled-cluster (MR-AQCC) method are used to develop a test set to assess the capabilities of different biradical descriptors based on density functional theory. A benchmark collection of 29 different compounds has been selected. The DFT descriptors contain primarily the fractional occupation number weighted electron density (FOD) based on simplified thermally-assisted-occupation density functional theory (TAO-DFT) calculations, but the singlet–triplet energy difference and other descriptors denoted as y0 and nLUNO have been considered as well. After adjustment of the literature-recommended finite temperatures, a very good, detailed agreement between unpaired density and FOD analysis is observed which is also manifested in excellent statistical correlations. The other two descriptors also show good correlations even though the absolute scaling is not satisfactory. A new linear fit of FOD data to the MR-AQCC reference values leads to an improved regression relation for determining the recommended finite temperature value in dependence of the Hartree–Fock exchange. This provides the basis for fast and reliable assessment of the biradical character of many classes of PAHs without the need for performing computationally extended MR calculations. [ABSTRACT FROM AUTHOR]

Density functional theory calculations were used to reveal the mechanism for the fluorination reaction of active Lewis acid sites on alumina structures, which is important in understanding the pyrophoric processes involving Al particles. In this reaction, hydroxyl groups of active sites are replaced by fluorine anions. Alumina structures were represented by three aluminum aqua hydroxo clusters (labeled AlOOH), in which the Al atom had different coordination spheres, particularly four, five, or six. The F-bearing molecules HF, CH3F, and CF4 were taken as reactants for the fluorination reactions. The overall reaction was represented by four reaction steps as follows: (i) formation of the reaction complex, (ii) activation of the transition state (TS), (iii) deactivation of the TS with a formation of the product complex, and (iv) its decomplexation to individual products. The active reaction center of the TS structure is four-membered, in which two bonds break heterolytically and two form. The lowest reaction barriers were observed for the HF molecule, while the two other molecules had significantly higher reaction barriers. Similarly, the largest overall reaction energies (in absolute value) were found for HF, especially for the five- and six-coordinated Al centers. While the positive charge on the Al center remained almost constant throughout the reaction steps, large charge changes were observed for carbon bearing molecules with a formation of the carbenium cations in the TS step. Realizing the important role of HF in promoting exothermic reactions will enable new molecular design strategies for transforming energy release properties of aluminum powder fuels. [ABSTRACT FROM AUTHOR]

Extended quantum chemical calculations were performed for the tetracene dimer to provide benchmark results, analyze the excimer survival process, and explore the possibility of using long-range-corrected (LC) time-dependent second-order density functional tight-biding (DFTB2) for this system. Ground- and first-excited-state optimized geometries, vertical excitations at relevant minima, and intermonomer displacement potential energy curves (PECs) were calculated for these purposes. Ground-state geometries were optimized with the scaled-opposite-spin (SOS) second-order Møller–Plesset perturbation (MP2) theory and LC-DFT (density functional theory) and LC-DFTB2 levels. Excited-state geometries were optimized with SOS-ADC(2) (algebraic diagrammatic construction to second-order) and the time-dependent approaches for the latter two methods. Vertical excitations and PECs were compared to multireference configuration interaction DFT (DFT/MRCI). All methods predict the lowest-energy S0 conformer to have monomers parallel and rotated relative to each other and the lowest S1 conformer to be of a displaced-stacked type. LC-DFTB2, however, presents some relevant differences regarding other conformers for S0. Despite some state-order inversions, overall good agreement between methods was observed in the spectral shape, state character, and PECs. Nevertheless, DFT/MRCI predicts that the S1 state should acquire a doubly excited-state character relevant to the excimer survival process and, therefore, cannot be completely described by the single reference methods used in this work. PECs also revealed an interesting relation between dissociation energies and the intermonomer charge-transfer interactions for some states. [ABSTRACT FROM AUTHOR]

Aluminum powder was thermally stressed by annealing and quenching, then the powder was non-uniformly dispersed in air and examined for dust combustion behavior as a function of stress-altering conditions. An explosion chamber with a powder injector, spark gap igniter, pressure sensor, spectrometer, and high-speed camera was used for experimentation. Aluminum powder was annealed to 573 K, held for 15 min, and quenched at a rate of 200 K/min (pre-stressed, PS) or 900 K/min (super-quenched, SQ). The untreated (UN), PS, and SQ Al powders were injected into the chamber, and pressure, temperature, and flame spreading behavior were analysed. SQ Al powder exhibited lower pressurization rates than that of PS Al, which was also lower than that of UN Al. Surface modifications to the stress-altered powders may affect their dispersion and suspension in the air environment, which affects flame spreading and pressurization rate. Specifically, annealing powders caused the removal of surface hydration that had two effects: increased the surface energy of the particles (confirmed with density functional theory calculations) and decreased surface roughness (suggested from previous work revealing loss of a nanostructure at the surface with annealing). These two surface modifications may inhibit powder dispersion such that pressurization rate is reduced compared with UN Al powder. [ABSTRACT FROM AUTHOR]

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview. [ABSTRACT FROM AUTHOR]

Benchmark ab initio calculations have been performed for poly(p-phenylenevinylene) (PPV) dimers, a paradigmatic material for studying excitation energy transfer mechanisms. Second-order Møller–Plesset perturbation theory was utilized with the scaled opposite spin approach (SOS-MP2) and correlation consistent basis sets to determine the geometric properties and interaction energies in the ground state. Vertical excitations and optimized structures for the S1 state were computed using the SOS second-order algebraic diagrammatic construction method. For the ground state properties, extrapolation to the complete basis set (CBS) limit and correction for the basis set superposition error (BSSE) were performed. While all results computed with different basis sets and considering BSSE correction or not agreed at the CBS limit, a strong bias was observed either using augmented basis sets or BSSE corrections, proving that these approaches are not advisable for calculating intermolecular distances and interaction energies with smaller basis sets. The lower states for vertical excitations were largely local excitons where the hole/electron pair was confined to single chains. For higher excited states, interchain charge transfer (CT) states were also observed. Geometry optimization of the S1 state led to significant reductions in the intermolecular distances and energetic stabilization, with Stokes shifts between 1.4 eV and 0.9 eV (with increasing chain length), and significant CT values between 0.5e and 0.4e. [ABSTRACT FROM AUTHOR]

In this work we discuss the calculation of the spin-density matrix from fundamental spin principles as implemented in the COLUMBUS Program System employing the graphical unitary group approach (GUGA). First, a general equation for the spin-density matrix is derived in terms of the one-and two-particle reduced density matrices, quantities that are spin-independent and readily available within the GUGA formalism. Next, the evaluation of this equation using the Shavitt loop values is discussed. Finally, the spatially resolved counterpart of the spin-density matrix, the spin distribution, is calculated for the phenalenyl radical and structures produced by heteroatoms with mono- and di-substitutions. The physical meaning of the spin-density along with its computational description using various methods is discussed putting special emphasis on negative contributions to the spin-density and their quantification via a spin-promotion index. [ABSTRACT FROM AUTHOR]

The chemical stability and the low‐lying singlet and triplet excited states of BN‐n‐acenes (n = 1–7) were studied using single reference and multireference methodologies. From the calculations, descriptors such as the singlet‐triplet splitting, the natural orbital (NO) occupations and aromaticity indexes are used to provide structural and energetic analysis. The boron and nitrogen atoms form an isoelectronic pair of two carbon atoms, which was used for the complete substitution of these units in the acene series. The structural analysis confirms the effects originated from the insertion of a uniform pattern of electronegativity difference within the molecular systems. The covalent bonds tend to be strongly polarized which does not happen in the case of a carbon‐only framework. This effect leads to a charge transfer between neighbor atoms resulting in a more strengthened structure, keeping the aromaticity roughly constant along the chain. The singlet‐triplet splitting also agrees with this stability trend, maintaining a consistent gap value for all molecules. The BN‐n‐acenes molecules possess a ground state with monoconfigurational character indicating their electronic stability. The low‐lying singlet excited states have charge transfer character, which proceeds from nitrogen to boron. [ABSTRACT FROM AUTHOR]

Progress in the hydrogen fuel field requires a clear understanding and characterization of how materials of interest interact with hydrogen. Due to the inherently quantum mechanical nature of hydrogen nuclei, any theoretical studies of these systems must be treated quantum dynamically. One class of material that has been examined in this context are dihydrogen complexes. Since their discovery by Kubas in 1984, many such complexes have been studied both experimentally and theoretically. This particular study examines the rotational dynamics of the dihydrogen ligand in the Fe(H)2(H2)(PEtPh2)3 complex, allowing for full motion in both the rotational degrees of freedom and treating the quantum dynamics (QD) explicitly. A "gas-phase" global potential energy surface is first constructed using density functional theory with the Becke, 3-parameter, Lee-Yang-Parr functional; this is followed by an exact QD calculation of the corresponding rotation/libration states. The results provide insight into the dynamical correlation of the two rotation angles as well as a comprehensive analysis of both ground- and excited-state librational tunneling splittings. The latter was computed to be 6.914 cm-1--in excellent agreement with the experimental value of 6.4 cm-1. This work represents the first full-dimensional ab initio exact QD calculation ever performed for dihydrogen ligand rotation in a coordination complex. [ABSTRACT FROM AUTHOR]

A thermally stable FLP-CO2 adduct of pronounced nucleophilic properties that forms a range of Lewis acid–base adducts with strong Lewis acids is reported. Upon addition of Tf2O, it generates a cationic triflate, which undergoes C–O bond cleavage to give the formal FLP adduct of the elusive dication C2O32+. [ABSTRACT FROM AUTHOR]

Born-Oppenheimer direct dynamics simulations were performed to study atomistic details of the F + CH3CN→HF + CH2CN H-atom abstraction reaction. The simulation trajectories were calculated with a combined M06-2X/MP2 algorithm utilizing the 6-311++G** basis set. The experiments were performed at 300 K, and assuming the accuracy of transition state theory (TST), the trajectories were initiated at the F… HCH2CN abstraction TS with a 300 K Boltzmann distribution of energy and directed towards products. Recrossing of the TS was negligible, confirming the accuracy of TST. HF formation was rapid, occurring within 0.014 ps of the trajectory initiation. The intrinsic reaction coordinate (IRC) for reaction involves rotation of HF about CH2CN and then trapping in the CH2CN… HF post-reaction potential energy well of ~10 kcal/mol with respect to the HF + CH2CN products. In contrast to this IRC, five different trajectory types were observed: the majority proceeded by direct H-atom transfer and only 11% approximately following the IRC. The HF vibrational and rotational quantum numbers, n and J, were calculated when HF was initially formed and they increase as potential energy is released in forming the HF +CH2CNproducts. The population of the HF product vibrational states is only in qualitative agreement with experiment, with the simulations showing depressed and enhanced populations of the n = 1 and 2 states as compared to experiment. Simulations with an anharmonic zero-point energy constraint gave product distributions for relative translation, HF rotation, HF vibration, CH2CN rotation, and CH2CN vibration as 5%, 11%, 60%, 7%, and 16%, respectively. In contrast, the experimental energy partitioning percentages to HF rotation and vibration are6%and 41%. Comparisons are made between the current simulation and those for other F + H-atom abstraction reactions. The simulation product energy partitioning and HF vibrational population for F +CH3CN→HF+CH2CNresemble those for other reactions. A detailed discussion is given of possible origins of the difference between the simulation and experimental energy partitioning dynamics for F +CH3CN→HF +CH2CN. The F +CH3CNreaction also forms the CH3C(F)N intermediate, in which the F-atom adds to the C#8801;N bond. However, this intermediate and F… CH3CN and CH3CN… F van der Waals complexes are not expected to affect the F + CH3CN→HF + CH2CN product energy partitioning. [ABSTRACT FROM AUTHOR]

Fluorescence spectra and lifetimes were determined for 16 synthetic flavylium cation analogues of anthocyanin plant pigments in dry acetonitrile acidified with trifluoroacetic acid (TFA). Phosphorescence was also observed from the lowest excited triplet state for all of the flavylium cations at 77 K in a rigid TFA-acidified isopropanol glass. The fluorescence quantum yields and lifetimes depend in a systematic manner on the nature and position of the substituents on the flavylium chromophore and three specific substitution patterns associated with significant decreases in the fluorescence quantum yield were identified. A 4′-bromo or 4′-iodo substituent in the B-ring of the flavylium cation produced a small but normal heavy-atom effect, reducing the fluorescence quantum yield and the phosphorescence lifetime relative to analogues without the halogen atom. In contrast, three flavylium cations with a 3′-bromo substituent exhibited an "inverse" heavy atom effect, i.e., an increase in the fluorescence quantum yield rather than a decrease, which was rationalized on the basis of the nodal properties of the natural transition orbitals (NTOs) involved in the S0→S1 radiative transition. [ABSTRACT FROM AUTHOR]

Harnessing aluminum oxidation energy requires navigating the particle's passivation shell composed of alumina. The shell is a barrier to aluminum oxidation but can also exothermically react with halogenated species and therefore contribute to the overall energy generated during aluminum particle combustion. Fluorination reactions with alumina have been studied because fluorine is abundant in binder formulations that commonly surround aluminum particles in an energetic mixture. However, iodine has emerged as an alternative halogenated-based binder or oxidizer because iodine gas provides ancillary benefits such as chemical neutralization of biological agents or sterilization of contaminated environments. This study used density functional theory (DFT) calculations to evaluate potential reaction pathways for aluminum–iodine combustion. Relative to fluorinated fragments such as HF and F−, the adsorption energies associated with HI and I− are nearly triple the exchange reaction energy available from fluorination reactions with alumina (−189 and −278 kJ mol−1 for HI and I−, respectively). However, exchange reactions between iodinated species and the alumina surface are energetically unfavorable. These results explain that through adsorption, alumina surface exothermic reactions with iodine are more energetic than with fluorine fragments. Experiments performed with differential scanning calorimetry (DSC) confirm the higher magnitude of energy generated for iodination compared with fluorination reactions with alumina. Additionally, strong adsorption energies can promote synthesis of new shell chemistries. Adsorption in solution will promote alumina dissolution and iodine precipitation reactions to produce hydroxyl complexes and iodinated species synthesized on the surface of the particle, thereby replacing alumina with alternative passivation shell chemistry. [ABSTRACT FROM AUTHOR]

Tuning of the electronic spectra of carbon dots by means of inserting heteroatoms into the π-conjugated polycyclic aromatic hydrocarbon (PAH) system is a popular tool to achieve a broad range of absorption and emission frequencies. Especially nitrogen atoms have been used successfully for that purpose. Despite the significant progress achieved with these procedures, the prediction of specific shifts in the UV-vis spectra and the understanding of the electronic transitions is still a challenging task. In this work, high-level quantum chemical methods based on multireference (MR) and single-reference (SR) methods have been used to predict the effect of different nitrogen doping patterns inserted into the prototypical PAH pyrene on its absorption spectrum. Furthermore, a simple classification scheme based on valence bond (VB) theory and the Clar sextet rule in combination with the harmonic oscillator measure of aromaticity (HOMA) index was applied to arrange the different doping structures into groups and rationalize their electronic properties. The results show a wide variety of mostly redshifts in the spectra as compared to the pristine pyrene case. The most interesting doping structures with the largest red shifts leading to absorption energies below one eV could be readily explained by the occurrence of diradical VB structures in combination with Clar sextets. Moreover, analysis of the electronic transitions computed with MR methods showed that several of the low-lying excited states possess double-excitation character, which cannot be realized by the popular SR methods and, thus, are simply absent in the calculated spectra. [ABSTRACT FROM AUTHOR]

Humic substances (HSs) as a major part of soil organic matter (SOM) are complex combinations of natural organic matter, which are ubiquitous in the environment and occur predominantly in soils, residues and natural water. They play a crucial role in the fate and behaviour of contaminants in the environment. In this work, an HS nanopore model was based on a structural motif containing polar and nonpolar domains. The polar domain was represented by the carboxyl groups that predominate in the composition of HSs, whereas the nonpolar domain was constructed by aliphatic chains. The effect of hydrophobic/hydrophilic interactions on sorption in HSs was analysed by the inclusion of the nonpolar polycyclic aromatic hydrocarbon naphthalene and the polar herbicide 2‐methyl‐4‐chlorophenoxyacetic acid (MCPA) in the nanopore. Water cluster was also included to allow the formation of the water molecular bridges (WaMB), which stabilized the whole system. Thermodynamic potential‐of‐mean‐force (PMF) calculations by means of classical molecular dynamics simulations were performed to investigate the effect of polar and nonpolar domains in the adsorption mechanism. The polar MCPA molecule was stabilized at the hydrophilic water–HS interface, whereas nonpolar naphthalene was stabilized inside of the hydrophobic nanovoid of the HS nanopore model. The obtained results demonstrated that the adsorption strength of HSs regarding polar and nonpolar species is similar, but the adsorption mechanisms differ. Highlights: The work elucidates the adsorption mechanism of polar and nonpolar species in SOM at the molecular scale.The derived HS model was suitable to describe and explain the adsorption properties of SOM by means of constrained molecular dynamics simulations.Adsorption mechanisms for polar and nonpolar species in SOM differ. Polar molecules prefer the water–SOM interface, whereas nonpolar molecules are trapped in hydrophobic nanovoids of SOM.Molecular dynamics simulation is an effective method providing a detailed description of the adsorption processes in SOM. [ABSTRACT FROM AUTHOR]

Anthocyanins, which are responsible for most of the red, blue and purple colors of fruits and flowers, are very efficient at absorbing and dissipating light energy via excited state proton transfer or charge-transfer mediated internal conversion without appreciable excited triplet state formation. During the maturation of red wines, grape anthocyanins are slowly transformed into pyranoanthocyanins, which have a much more chemically stable pyranoflavylium cation chromophore. Development of straightforward synthetic routes to mono- and disubstituted derivatives of the pyranoflavylium cation chromophore has stimulated theoretical and experimental studies that highlight the interesting absorption and emission properties and redox properties of pyranoflavylium cations. Thus, p-methoxyphenyl substitution enhances the fluorescence quantum yield, while a p-dimethylaminophenyl substituent results in fast decay via a twisted intramolecular charge-transfer (TICT) state. Unlike anthocyanins and their synthetic analogs (flavylium cations), a variety of pyranoflavylium cations form readily detectable excited triplet states that sensitize singlet oxygen formation in solution and exhibit appreciable two-photon absorption cross sections for near-infrared light, suggesting a potential for applications in photodynamic therapy. These excited triplet states have microsecond lifetimes in solution and excited state reduction potentials of at least 1.3 V vs. SCE, features that are clearly desirable in a triplet photoredox catalyst. [ABSTRACT FROM AUTHOR]

Theoretical descriptions of excited state proton transfer (ESPT) have had various degrees of success. This work presents a theoretical description of the photodissociation of the 7‐hydroxyflavylium cation (7‐HF), the fundamental chromophoric moiety of anthocyanin natural plant pigments. ESPT of 7‐HF is promoted by a significant shift of charge away from the OH group in the first singlet excited state, leading smoothly to the excited conjugate base and a protonated water cluster. Several factors contribute to the consistency of the results of the present study: (1) the theoretical approach (TD‐DFT with the B3‐LYP functional and def2‐TZVP basis set utilizing Grimme's D3 dispersion correction); (2) the modeling of the solvent effect combining hydrogen bonding of the photoacid to a cluster of discrete water molecules in a water‐like continuum solvent (COSMO); (3) the large S1‐S2 energy gap of flavylium cations; and (4) the electrostatics of the ESPT in which a proton is transferred from a cationic photoacid to water without Coulombic interaction between the proton and the conjugate base. [ABSTRACT FROM AUTHOR]

Environmental context: Immobilisation of organic chemicals in soil organic matter can strongly influence their availability in the environment. We show that the presence of water clusters, called water molecule bridges, hampers the release of organic molecules from soil organic matter. Moreover, water molecule bridges are sensitive to changes in environmental conditions (e.g., temperature or moisture) which affect the release of organic molecules into the environment. Water molecule bridges (WaMB) can stabilise the supramolecular structure of soil organic matter (SOM) by connecting individual SOM molecular units. WaMB are hypothesised to act as a desorption barrier and thus to physically immobilise molecules in SOM. To test this hypothesis, we prepared two sets of soil samples – aged samples with WaMB developed, and vacuumed samples, in which WaMB were disrupted. The samples were spiked with phenol and then stored under controlled humidity. The degree of phenol immobilisation in SOM was assessed by desorption kinetics of phenol into a gas phase. This was compared with the thermal stability (T*) of WaMB obtained by modulated differential scanning calorimetry (MDSC) and the results were related to computer modelling, which provided the stability and solvation energies of phenol-WaMB-SOM models. The desorption kinetics of phenol was best described by a first-order model with two time constants ranging between 1 and 10 h. In aged samples, the time constants correlated with T*, which showed that the desorption time increased with increasing WaMB stability. Molecular modelling proposed that phenol molecules are preferentially locked in nanovoids with polar OH groups pointed to WaMB in the most stable configurations. Both findings support the hypothesis that WaMB can act as a desorption barrier for phenol. Environmental context. Immobilisation of organic chemicals in soil organic matter can strongly influence their availability in the environment. We show that the presence of water clusters, called water molecule bridges, hampers the release of organic molecules from soil organic matter. Moreover, water molecule bridges are sensitive to changes in environmental conditions (e.g., temperature or moisture) which affect the release of organic molecules into the environment. [ABSTRACT FROM AUTHOR]

The time-dependent density functional theory (TDDFT) and the resolution of the identity-coupled cluster singles and doubles method (RI-CC2) calculations are performed on the excited-state intramolecular proton transfer in malonaldehyde, o-hydroxybenzaldehyde, salicylic acid, 7-hydroxy-1-indanone, and 2-(2'-hydroxyphenyl)benzothiazole. The results obtained indicate that both RI-CC2 and TDDFT methods are good candidates for the description of excited-state intramolecular proton transfer (ESPIT) potential energy surfaces.