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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 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]