Thermal rate constants and kinetic isotope effects of the H + HO reactions: barrier height and reaction energy from single- and multireference methods.

Context: One of the more significant sub-mechanisms of H/O combustion involves the reaction of hydrogen peroxide with hydrogen atoms (H + HO), resulting in the production of OH + HO (R1) and H + HO (R2) paths. Previous experimental and ab initio calculations reveal some variations in the barrier height for (R1). To improve the energetics of both (R1) and (R2), single reference and multireference ab initio methods are employed, and the rate constants and H/D kinetic isotope effects (KIEs) are calculated as a function of temperature.

A multi-descriptor analysis of substituent effects on the structure and aromaticity of benzene derivatives: π-Conjugation versus charge effects.

This work provides a detailed multi-component analysis of aromaticity in monosubstituted (X = CH, C , C , NH, NH, NH, OH, O, and O) and para-homodisubstituted (X = CH, CH, NH, 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 (ELF), electronic (MCI), magnetic (NICS), and stability (S-T 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.

Stability and Reactivity of the Phenalene and Olympicene Isomers.

The phenalene (triangulene) and olympicene molecules belong to the polycyclic aromatic hydrocarbon class, which have attracted substantial technological interest due to their unique electronic properties. Electronic structure calculations serve as a valuable tool in investigating the stability and reactivity of these molecular systems. In the present work, the multireference calculations, namely, the complete active space second-order perturbation theory and multireference averaged quadratic coupled cluster (MR-AQCC), were employed to study the reactivity and stability of phenalene and olympicene isomers, as well as their modified structures where the sp-carbon at the borders were removed.

Electronic Structure of Small Isolated and Supported Manganese Oxide Clusters.

In the present work, possible molecular models of the isolated manganese oxides and supported MnO/AlO structures were built based on small clusters of passivated MnO. The support was represented as a simplified model of the alumina tetramer cluster based on small fragments of AlOH. Combinations of MnOH and AlOH clusters were made to form both the isolated and supported manganese oxides clusters.

High-Level Multireference Investigations on the Electronic States in Single-Vacancy (SV) Graphene Defects Using a Pyrene-SV Model.

The nonplanar character of graphene with a single carbon vacancy (SV) defect is investigated utilizing a pyrene-SV model system by way of complete-active-space self-consistent field theory (CASSCF) and multireference configuration interaction singles and doubles (MR-CISD) calculations. Planar structures were optimized with both methods, showing the B state to be the ground state with three energetically close states within an energy range of 1 eV. These planar structures constitute saddle points.

Structural stability and the low-lying singlet and triplet states of BN-n-acenes, n = 1-7.

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.

Electronic structure and physicochemical properties of the metal and semimetal oxide nanoclusters.

Clusters are physical entities composed of a few to thousands of atoms with capabilities to develop novel materials, like cluster-assembled materials. In this sense, knowing the electronic structure and physicochemical properties of the isolated clusters can be useful to understand how they interact with other chemical species by intermolecular forces, as free, embedded, and saturated clusters, and by intramolecular forces, acting as support clusters. In this way, in the present work, the electronic structure and physicochemical properties of metal oxide nanoclusters (MgO, AlO, SiO, and TiO) were studied by highly correlated molecular quantum chemistry methods.

Accurate rate constants for elementary reactions of molecular hydrogen and carbon monoxide mixtures and the role of the H rich environment.

This investigation provides accurate rate constant values for a set of elementary reactions relevant to mixtures between molecular hydrogen (H) and carbon monoxide (CO) such as syngas. We considered intermediates and products including formaldehyde (HCO), hydroxymethylene (c-HCOH and t-HCOH) and methanol (CHOH). The calculations were performed employing the improved canonical variational transition state theory with small-curvature tunneling corrections based on high-level electronic structure results.

Relating Bond Strength and Nature to the Thermodynamic Stability of Hypervalent Togni-Type Iodine Compounds.

The bond strength and nature of a set of 32 Togni-like reagents have been investigated at the M062X/def2-TZVP(D) level of theory in acetonitrile described with the SMD continuum solvent model, to rationalize the main factors responsible for their thermodynamic stability in different conformations, and trifluoromethylation capabilities. For the assessment of bond strength, we utilized local stretching force constants and associated bond strength orders, complemented with local features of the electron density to access the nature of the bonds. Bond dissociation energies varied from 31.

The influence of the environment in chemical reactivity: the HCOOH formation from the HO + CO reaction.

The reaction between carbon monoxide and water was studied occurring in an aerosol medium rich in methanol. This environment is plausible for the primitive and prebiotic Earth atmosphere. The chemical environment is expressed in terms of dielectric constant (ε) and the chemical system was modeled employing the polarizable continuum model (PCM).

The relativistic effects on the methane activation by gold(I) cations.

The reactivity of gold has been investigated for a long time. Here, we performed an in-depth analysis of relativistic effects over the chemical kinetic properties of elementary reactions associated with methane activation by gold(I) cations, CH + Au ↔ AuCH + H. The global reaction is modeled as a two-step process, CH + Au ↔ HAuCH ↔ AuCH + H.

Tunneling Enhancement of the Gas-Phase CH + CO Reaction at Low Temperature.

The rates of numerous activated reactions between neutral species increase at low temperatures through quantum mechanical tunneling of light hydrogen atoms. Although tunneling processes involving molecules or heavy atoms are well known in the condensed phase, analogous gas-phase processes have never been demonstrated experimentally. Here, we studied the activated CH + CO → HCO + CO reaction in a supersonic flow reactor, measuring rate constants that increase rapidly below 100 K.

Accurate Rate Constants for the Forward and Reverse H + CO ↔ HCO Reactions at the High-Pressure Limit.

The forward and reverse H + CO ↔ HCO reactions are important for combustion chemistry and have been studied from a wide variety of theoretical and experimental techniques. However, most of the chemical kinetic investigations concerning these processes are focused on low pressures or fall-off regions. Hence, a high-level electronic structure treatment was employed here in order to provide accurate rate constant values for these reactions at the high-pressure limit along temperatures from 50 to 4000 K.

The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry.

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.

Multireference study of ionic/covalent electronic states of MF (M = Be, Mg and Ca).

Ultracold environments composed by atoms or molecules offer an opportunity to study chemical reactions at the quantum-state level, for simulation of solid-state systems, as qubits in quantum computing, and for test fundamental symmetries. Those ultracold conditions formed by molecules can be obtained from cryogenic buffer gas, via supersonic expansion, followed by deceleration or from the laser cooling process. Diatomic alkaline earth monofluoride molecules have been shown as great candidates for the laser cooling process.

Metal-Halogen Bonding Seen through the Eyes of Vibrational Spectroscopy.

Incorporation of a metal center into halogen-bonded materials can efficiently fine-tune the strength of the halogen bonds and introduce new electronic functionalities. The metal atom can adopt two possible roles: serving as halogen acceptor or polarizing the halogen donor and acceptor groups. We investigated both scenarios for 23 metal-halogen dimers trans-M(Y)(NCHX-3) with M = Pd(II), Pt(II); Y = F, Cl, Br; X = Cl, Br, I; and NCHX-3 = 3-halopyridine.

Identification of Magic Numbers in Homonuclear Clusters: The ε Stability Ranking Function.

With the rise of cluster-assembled materials, an index that is able to rank and identify stable clusters or molecules is of great interest in materials sciences and engineering. In the present work, we applied a stability ranking function (ε) in nanoclusters formed by simple metals (Na, Mg), main group elements (Al), or transition metals (Ti, Cu). The ε function parameters are molecular properties derived from the wave function.

A Proposal for the Mechanism of the CH + CO Reaction.

Few experimental studies on the CH + CO global reaction propose H, CO, and HCO as major products. However, the reaction mechanisms behind this process have not yet been elucidated. Moreover, some intriguing kinetic particularities were noticed in these previous investigations.

Pushing 3c-4e Bonds to the Limit: A Coupled Cluster Study of Stepwise Fluorination of First-Row Atoms.

To better understand why hypervalent F, O, N, C, and B compounds are rarely stable, we carried out a systematic study of 28 systems, including anionic, cationic, and neutral molecules, held together by covalent, hypervalent, and noncovalent bonds. Molecular geometries, frequencies, atomic charges, electrostatic potentials, energy and electron densities, Mayer bond orders, local stretching force constants, and bond strength orders (BSOs) were derived from high accuracy CCSD(T) calculations and utilized to compare the strength and nature of hypervalent bonds with other types of bonds. All hypervalent molecules studied in this work were found to be either first-order transition states or unstable to dissociation, with F and OF as the only exceptions.

Potential Energy Curves for Formation of the CHO Criegee Intermediate on the CH + O Singlet and Triplet Potential Energy Surfaces.

The potential energy curves (PECs) for the interaction of CH with O in singlet and triplet potential energy surfaces (PESs) leading to singlet and triplet Criegee intermediates (CHOO) are studied using electronic structure calculations. The bonding mechanism is interpreted by analyzing the ground state multireference configuration interaction (MRCI) wave function of the reacting species and at all points along the PES. The interaction of CH with O on the singlet surface leads to a flat long-range attractive PEC lacking any maxima or minima along the curve.

Emission Energies and Stokes Shifts for Single Polycyclic Aromatic Hydrocarbon Sheets in Comparison to the Effect of Excimer Formation.

Emission spectra of paradigmatic single-sheet polycyclic aromatic hydrocarbons (PAHs), pyrene, circum-1-pyrene, coronene, circum-1-coronene, and circum-2-coronene and Stokes shifts were computed and compared with previously calculated comparable data for relaxed excimer structures using the SOS-ADC(2), TD-B3LYP, and TD-CAM-B3LYP methods with multireference DFT/MRCI data as the benchmark. Vertical emission transitions and Stokes shifts were extrapolated to infinite PAH size. Comparison of Stokes shifts computed from theoretical monomer and dimer data confirms assumptions that relaxed excimers are responsible for the unusually large Stokes shifts in carbon dots observed in experimental investigations.

High-level theoretical benchmark investigations of the UV-vis absorption spectra of paradigmatic polycyclic aromatic hydrocarbons as models for graphene quantum dots.

Five paradigmatic polycyclic aromatic hydrocarbons (PAHs) (pyrene, circum-1-pyrene, coronene, circum-1-coronene, and circum-2-coronene) are used for studying the performance of three single-reference methods {scaled opposite-spin-algebraic diagrammatic construction to second-order [SOS-ADC(2)], time-dependent (TD)-B3LYP, and TD-Coulomb-attenuating method (CAM)-B3LYP} and three multireference (MR) methods [density functional theory/multireference configuration interaction (DFT/MRCI), strongly contracted-n-electron valence state perturbation theory to second order (NEVPT2), and spectroscopy oriented configuration interaction (SORCI)]. The performance of these methods was evaluated by comparison of the calculated vertical excitation energies with experiments, where available. DFT/MRCI performs best and thus was used as a benchmark for other approaches where experimental values were not available.

Excited states and excitonic interactions in prototypic polycyclic aromatic hydrocarbon dimers as models for graphitic interactions in carbon dots.

The study of electronically excited states of stacked polycyclic aromatic hydrocarbons (PAHs) is of great interest due to promising applications of these compounds as luminescent carbon nanomaterials such as graphene quantum dots (GQDs) and carbon dots (CDs). In this study, the excited states and excitonic interactions are described in detail based on four CD model dimer systems of pyrene, coronene, circum-1-pyrene and circum-1-coronene. Two multi-reference methods, DFT/MRCI and SC-NEVPT2, and two single-reference methods, ADC(2) and CAM-B3LYP, have been used for excited state calculations.

Theoretical and experimental studies concerning monomer/aggregates equilibrium of zinc phthalocyanine for future photodynamic action.

Monomeric zinc phthalocyanine has been studied as a promising active photosensitizer in photodynamic therapy against cancer, in which its aggregate form is non-active. This paper aimed to describe the monomer/aggregates equilibrium of zinc phthalocyanine in binary water/DMSO mixtures. To reach this aim theoretical calculation, electronic absorption, static and time-resolved fluorescence, and resonance light scattering was used.