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In this work, the stability, aromaticity and radical character of pristine and eleven BN-doped armchair 5 and zigzag 5, 6, and 7 periacenes, were chosen for studying the effect of different doping schemes to stabilize the periacene, and to direct the open-shell density into specific regions of the PAH sheets. Ab initio multireference methods and different DFT functionals were used to analyze the singlet triplet (ST) energy. Moreover, a range of descriptors were used to characterize the open-shell character and aromaticity of the different doped structures. The overall results provide a good overview of the efficiency of the different doping topologies. In general, because of the closed-shell character of the borazine doping units, the role of the doping is to reduce the strong open-shell character of the pristine periacenes. Substitutions along the zigzag edges has a significant effect while doping along the armchair edges is not significant., (© 2025 Wiley Periodicals LLC.)

Polycyclic aromatic hydrocarbons (PAHs) exhibit intriguing characteristics that position them as promising candidates for advancements in organic semiconductor technologies. Notably, tetracene finds substantial utility in Electronics due to its application in organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). The strategic introduction of an isoelectronic boron-nitrogen (B,N) pair to replace a carbon-carbon pair in acenes introduces changes in the electronic structure, allowing for the controlled modulation of diradical characteristics. Consequently, this B,N substitution enables precise adjustments in chemical, optical, and electronic attributes. In this work, we undertook a systematic exploration of thermally activated delayed fluorescence (TADF) phenomena within a set of 77 B,N-substituted derivatives of tetracene. The primary objective was to identify and select prospective molecules for the fabrication of OLEDs. Employing multiconfigurational methods of computational quantum chemistry, we conducted an extensive investigation to unravel the potential candidates. As a result, we identified molecules that might exhibit the sought-after TADF behavior. Descriptors such as excitation energies, harmonic oscillator model of aromaticity (HOMA) and fractional occupation number weighted density (FOD) were assessed and indicated five candidates with stability comparable to that of pristine tetracene. This research not only contributes to a deeper understanding of the influence of B,N substitution on acene derivatives but also opens doors for the development of organic electronics by harnessing the properties of these selected molecules.

The thermal rate constants and H/D kinetic isotope effects (KIEs) of the reactions SiH 4 + H → SiH 3 + H 2 and SiH 4 + CH 3 → SiH 3 + CH 4 were calculated with the variational transition state theory including multidimensional tunneling corrections (VTST-MT). The ωB97X-D and CCSD(T) methods were employed to compute the barrier height and reaction energy. The ωB97X-D/aug-cc-pVTZ methodology was used to build reaction paths, and the CCSD(T)/CBS Q-5 approach was used to improve the energetics of the stationary points in the calculations of CVT/μOMT thermal rate constants. For the SiH 4 + H pathway, the CVT/μOMT rate constants and H/D KIEs are in excellent agreement with previous experimental and theoretical calculations; otherwise, for the SiH 4 + CH 3 pathway, which has few experimental and theoretical data available, it has provided, for the first time, rate constants and kinetic isotope effects using variational transition state theory with multidimensional tunneling corrections.

This work provides a detailed multi-component analysis of aromaticity in monosubstituted (X = CH 3 , C H 2 - , C H 2 + , NH 2 , NH - , NH + , OH, O - , and O + ) and para-homodisubstituted (X = CH 3 , CH 2 , NH 2 , 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 π ), electronic (MCI), magnetic (NICS), and stability (S 0 -T 1 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 p z 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., (© 2023 Wiley Periodicals LLC.)

Context: One of the more significant sub-mechanisms of H 2 /O 2 combustion involves the reaction of hydrogen peroxide with hydrogen atoms (H + H 2 O 2 ), resulting in the production of OH + H 2 O (R1) and H 2  + HO 2 (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. For (R1), the best results for the barrier height and reaction energies computed with the CASPT2(15,11)/aug-cc-pV6Z are 5.2 and - 70.3 kcal.mol -1 , respectively. CCSD(T)/aug-cc-pV5Z + CV (core-valence) calculations for (R2) give 9.7 and - 15.6 kcal.mol -1 to those parameters. The CVT/SCT rate constants of both paths agree well with the fitted rate constants from uncertainty-weighted statistical analysis of the 14-mechanism of H 2 /O 2 . The kinetic isotopic effect (k H /k D ) for the reaction D + H 2 O 2  → DH + HO 2 was found to be 0.47, which is in excellent agreement with the experimental value of 0.43., Methods: The structures of reactants, transition state, and products of (R1) and (R2) are calculated with the aug-cc-pVTZ basis set and M062X DFT, CCSD(T), and CASSCF methods. The barrier heights and reaction energies of (R1) and (R2) are computed using the M06-2X, CCSD(T), MRCI, and CASPT2 methods and various basis sets. The rate constants are calculated with the variational transition state theory including multidimensional tunneling corrections (VTST-MT), with potential energy surfaces built by the M06-2X/aug-cc-pVTZ approach., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

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 3 -carbon at the borders were removed. The harmonic oscillator model of aromaticity (HOMA) and the nucleus-independent chemical shift as geometric and magnetic indexes calculated with density functional theory were utilized to assess the aromaticity of the studied molecules. These indexes were compared with properties such as the excitation energy and natural orbitals occupation. The reactivity analyzed using the HOMA index combined with MR-AQCC revealed the radical character of certain structures as well as the weakening of their aromaticity. Moreover, the results suggest that the removal of sp 3 -carbon atoms and the addition of hydrogen atoms did not alter the π network and the excitation energies of the phenalene molecules.

In the present work, possible molecular models of the isolated manganese oxides and supported Mn 3 O x /Al 2 O 3 structures were built based on small clusters of passivated MnO x . The support was represented as a simplified model of the alumina tetramer cluster based on small fragments of AlO x H y . Combinations of MnO x H y and AlO x H y clusters were made to form both the isolated and supported manganese oxides clusters. The electronic structure of these systems was characterized by ab initio methods (DFT and CASPT2). It was observed that the vertical excitation energy of the isolated and supported Mn 3 O x H y clusters is significantly lower than that of the alumina cluster model, while both the isolated and supported Mn 3 O x H y wave function characters are qualitatively similar with respect to the ground state and electronic transition processes, suggesting that the alumina cluster behaves as an inert support, since there is little contribution of this component in the description of the low-lying electronic states. The present study also reports for the first time the spectroscopic parameters of several clusters containing the manganese transition metal atom.

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 3 B 1 state to be the ground state with three energetically close states within an energy range of 1 eV. These planar structures constitute saddle points. However, following the out-of-plane imaginary frequency yields more stable (by 0.22 to 0.53 eV) but nonplanar structures of C s symmetry. Of these, the 1 A' structure is the lowest in energy and is strongly deformed into an L shape. Following a further out-of-plane imaginary frequency in the nonplanar structures leads to the most stable but most deformed singlet structure of C 1 symmetry. In this structure, a bond is formed between the carbon atom with the dangling bond and a carbon of the cyclopentadienyl ring. This bond stabilizes the structure by more than 3 eV compared to the planar 3 B 1 structure. Higher excited states were calculated at the MR-CISD level, showing a grouping of four states low in energy and higher states starting around 3 eV.

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., (© 2022 Wiley Periodicals LLC.)

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, Al 2 O 3 , SiO 2 , and TiO 2 ) were studied by highly correlated molecular quantum chemistry methods. Through the electronic state's characterization, a semiconductor aspect was found for the titania oxide nanocluster (T e  < 0.8 eV) while the other agglomerates showed a characteristic of insulating material (T e  > 3.3 eV). From the stability index, the following stability order can be characterized: (SiO 2 ) 4  > (Al 2 O 3 ) 4  > (MgO) 4  > (TiO 2 ) 3 . Initial information of intermolecular and intramolecular forces caused by the studied clusters was calculated through the relative electrophilicity index, which classified the (MgO) 4 and (TiO 2 ) 3 clusters as the more reactive ones, in which the (MgO) 4 cluster was identified as a nucleophilic species, while the (TiO 2 ) 3 cluster as an electrophilic molecule., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

This investigation provides accurate rate constant values for a set of elementary reactions relevant to mixtures between molecular hydrogen (H 2 ) and carbon monoxide (CO) such as syngas. We considered intermediates and products including formaldehyde (H 2 CO), hydroxymethylene (c-HCOH and t-HCOH) and methanol (CH 3 OH). The calculations were performed employing the improved canonical variational transition state theory with small-curvature tunneling corrections based on high-level electronic structure results. This study demonstrates for the first time that H 2 can act as an effective catalyst to the reaction from t-HCOH to H 2 CO. In this case, the adiabatic barrier height for the reaction decreases from 30.6 kcal⋅mol - 1 to 18.1 kcal⋅mol - 1 in the presence of H 2 . The results obtained here can improve the comprehension regarding processes such as the combustion of hydrogen-rich syngas., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

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 main results were acquired from calculations employing the M06-2X density functional for the electronic structure calculations and the canonical variational theory with small curvature tunneling for the chemical kinetic calculations. The rise of ε affects both the thermochemistry and the kinetics of the reaction, increasing the barrier height and decreasing the rate constant for the reaction occurring at room temperature. For example, the rate constant at 300 K is 5-10× 10 - 53 cm 3 ⋅molecule - 1 ⋅s - 1 for low dielectric constant (ε < 3) and around 2-4× 10 - 53 cm 3 ⋅molecule - 1 ⋅s - 1 for ε between 7 and 40. Our results indicate that the ε variation allows a fine tuning to the rate of the reaction., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

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 31.6 to 79.9 kcal/mol depending on the polarizing power of the ligand trans to CF 3 . Based on the analysis of the Laplacian of the density, we propose that the charge-shift bond character plays an important role in the stability of the molecules studied, especially for those containing I-O bonds. New insights on the trans influence and on possible ways to fine-tune the stability of these reagents are provided., (© 2021 Wiley-VCH GmbH.)

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 4 + Au + ↔ AuCH 2 + + H 2 . The global reaction is modeled as a two-step process, CH 4 + Au + ↔ HAuCH 3 + ↔ AuCH 2 + + H 2 . Moreover, the barrierless dissociation of the initial adduct between reactants, AuCH 4 + , is discussed as well. Higher-order relativistic treatments are used to provide corrections beyond the commonly considered scalar effects of relativistic effective core potentials (RECPs). Although the scalar relativistic contributions predominate, lowering the forward barrier heights by 48.4 and 36.1 kcal mol -1 , the spin-orbit coupling effect can still provide additional reductions of these forward barrier heights by as much as 9% (1.0 and 2.2 kcal mol -1 ). The global reaction proceeds rapidly at low temperatures to the intermediate attained after the first hydrogen transfer, HAuCH 3 + . The relativistic corrections beyond the ones from RECPs are still able to double the rate constant of the CH 4 + Au + → HAuCH 3 + process at 300 K, while the reverse reaction becomes five times slower. The formation of global products from this intermediate only becomes significant at much higher temperatures (∼1500 K upward). The scalar relativistic contributions decrease the dissociation energy of the initial adduct, AuCH 4 + , into the global products by 105.8 kcal mol -1 , while the spin-orbit effect provides an extra lowering of 1.8 kcal mol -1 .

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 2 → HCO + CO reaction in a supersonic flow reactor, measuring rate constants that increase rapidly below 100 K. Mechanistically, tunneling is shown to occur by CH insertion into the C-O bond, with rate calculations accurately reproducing the experimental values. To exclude the possibility of H-atom tunneling, CD was used in additional experiments and calculations. Surprisingly, the equivalent CD + CO 2 reaction accelerates at low temperature as zero-point energy effects remove the barrier to product formation. In conclusion, heavy-particle tunneling effects might be responsible for the observed reactivity increase at lower temperatures for the CH + CO 2 reaction, while the equivalent effect for the CD + CO 2 reaction results instead from a submerged barrier with respect to reactants.

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. In relative terms, the variational effects on rate constants are shown to be almost as important at high temperatures as quantum tunneling corrections at the lowest temperatures investigated. The activation energies fitted by using modified and traditional Arrhenius' equations for the forward rate constants from 298 to 2000 K are, respectively, equal to 2.64 and 3.89 kcal mol -1 , while similar fittings provided, respectively, 1.96 and 3.22 kcal mol -1 , considering now forward rate constants from a temperature range of 298-373 K. This last activation energy determination (3.22 kcal mol -1 ) is in better agreement with the commonly referenced experimental value of 2.0 ± 0.4 kcal mol -1 , also obtained from traditional fittings in the range 298-373 K, than the value attained from a broader temperature range fitting (3.89 kcal mol -1 ). However, some additional care must be considered along these comparisons once the experimental reaction rate measurements have been done for the trimolecular H + CO + M → HCO + M reaction instead. Anyway, the usage of appropriate temperature ranges is fundamental for reliable activation energy comparisons, although the remaining deviation between theory and experiment is still large and is possibly caused by the different pressure regimes assessed in each case. Finally, we roughly estimated that the high-pressure limit for the HCO decomposition into H and CO can be achieved along temperatures ranging from ∼246 and ∼255 K downward, respectively, at pressures of 1.1 and 9.6 atm, although further experiments should be carried out in order to improve these estimates. On the other hand, pressures larger than 9.8 × 10 4 atm are required for the aforementioned dissociation reaction to attain the high-pressure limit at 700 K. Therefore, the rate constants determined here are probably applicable in combustion studies at lower temperatures., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

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. In this sense, the present work focuses on the characterization of the low-lying doublet electronic states correlated to the first dissociation channel of the alkaline earth monofluorides diatomic molecules MF (M = Be, Mg and Ca). The developed state-of-the-art methodology was based on a qualitative analysis of the diatomic electronic structure, employing a hypothetical potential energy curve or by a simple molecular orbital diagram combined with bond order analysis. The potential energy curves, excitation and dissociation energies, and various sets of spectroscopic parameters were calculated by the MRCI/cc-pV5Z methodology. Transition probabilities for emission and radiative lifetimes among the characterized electronic states were also calculated for the (A) 2 Π ⟶ (X) 2 Σ + electronic transition. Comparing the spectroscopy properties, we were able to indicate the CaF molecule as the best candidate molecule for laser cooling devices among the studied molecules., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

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.

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 3 ) in nanoclusters formed by simple metals (Na, Mg), main group elements (Al), or transition metals (Ti, Cu). The ε 3 function parameters are molecular properties derived from the wave function. These parameters can be divided into kinetic and thermodynamic descriptors, in which the kinetic descriptors are the ionization potential and electronic excitation energy, while the atomization free Gibbs energy is the thermodynamic one. This simple ε 3 function was able to identify the possible magic numbers of the studied clusters across the periodic table in a good agreement with previous experimental and theoretical works.

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 2 )(NC 5 H 4 X-3) 2 with M = Pd(II), Pt(II); Y = F, Cl, Br; X = Cl, Br, I; and NC 5 H 4 X-3 = 3-halopyridine. As a new tool for the quantitative assessment of metal-halogen bonding, we introduced our local vibrational mode analysis, complemented by energy and electron density analyses and electrostatic potential studies at the density functional theory (DFT) and coupled-cluster single, double, and perturbative triple excitations (CCSD(T)) levels of theory. We could for the first time quantify the various attractive contacts and their contribution to the dimer stability and clarify the special role of halogen bonding in these systems. The largest contribution to the stability of the dimers is either due to halogen bonding or nonspecific interactions. Hydrogen bonding plays only a secondary role. The metal can only act as halogen acceptor when the monomer adopts a (quasi-)planar geometry. The best strategy to accomplish this is to substitute the halo-pyridine ring with a halo-diazole ring, which considerably strengthens halogen bonding. Our findings based on the local mode analysis provide a solid platform for fine-tuning of existing and for design of new metal-halogen-bonded materials.

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 3 - and OF 3 - as the only exceptions. For several systems, we found that a weak noncovalent bonded complex is more stable than a hypervalent one, due to the high energetic cost to accommodate an extra ligand, which can surpass the stability gained by 3c-4e bonding.

The potential energy curves (PECs) for the interaction of 3 CH 2 with 3 O 2 in singlet and triplet potential energy surfaces (PESs) leading to singlet and triplet Criegee intermediates (CH 2 OO) 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 3 CH 2 with 3 O 2 on the singlet surface leads to a flat long-range attractive PEC lacking any maxima or minima along the curve. The triplet surface stems into a maximum along the curve resulting in a transition state with an energy barrier of 5.3 kcal/mol at CASSCF(4,4)/cc-pVTZ level. The resulting 3 CH 2 OO is less stable than the 1 CH 2 OO. In this study, the biradical character (β) is used as a measure to understand the difference in the topology of the singlet and triplet PECs and the relation of the biradical nature of the species with their structures. The 3 CH 2 OO has a larger biradical character than 1 CH 2 OO, and because of the larger bond order of 1 CH 2 OO, the C-O covalent bond becomes harder to break, thereby stabilizing 1 CH 2 OO. Thus, this study provides insights into the shape of the PEC obtained from the reaction between 3 CH 2 and 3 O 2 in terms of their bonding nature and from the shape of the curves, the temperature dependence or independence of the rate of the reaction is discussed.

Few experimental studies on the CH + CO 2 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. The advanced theoretical study performed here shows that a CH insertion mechanism is capable of explaining all the experimental data available. Hence, the strong deviations from a traditional Arrhenius behavior ascribed to the rate-determining elementary reaction (the CH insertion step) account for the kinetic particularities observed experimentally. A change in the preferred product channel as temperatures increase (from HCO + CO to H + 2CO) is also predicted to occur due to the HCO decomposition, although the CH depletion rates in typical conditions are not affected by this additional step., Competing Interests: The authors declare no competing financial interest., (Copyright © 2019 American Chemical Society.)

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.

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. The DFT/MRCI method has been used as a benchmark method to evaluate the performance of the other ones. All methods produce useful lists of excited states. However, an overestimation of excitation energies and an inverted ordering of states, especially concerning the bright HOMO-LUMO excitation, are observed. In the pyrene-based systems, the first bright state appears among the first four states, whereas the number of dark states is significantly larger for the coronene-based systems. Fluorescence emission properties are addressed by means of geometry optimization in the S1 state. The inter sheet distances for the S1 state decrease in comparison to the corresponding ground-state values. These reductions are largest for the pyrene dimer and decrease significantly for the larger dimers. Several minima have been determined on the S1 energy surface for most of the dimers. The largest variability in emission energies is found for the pyrene dimer, whereas in the other cases a more regular behavior of the emission spectra is observed.

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. Zinc phthalocyanine shows a complex water dependence behavior in the mixture. At least three distinct steps were observed: (i) until 30% water zinc phthalocyanine is essentially in the monomeric form, changing to (ii) small slipped cofacial-aggregates around 30% to 40% water and finally to (iii) a staircase arrangement of large aggregates at higher water percent. The staircase arrangement is driven by the intermolecular coordination between the pyrrolic nitrogen lone-pairs and the central metal zinc. The water-Zn coordination governs the fluorescence quenching by a static mechanism. These results have direct relevance in the better understanding on the behavior of zinc phthalocyanine in vivo and when incorporated in drug delivery systems for clinical applications in photodynamic therapy., (Copyright © 2019 Elsevier B.V. All rights reserved.)

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. Both TD-B3LYP and NEVPT2 agree well with the benchmark data. SORCI performs better for coronene than for pyrene. SOS-ADC(2) does reasonably well in terms of excitation energies for smaller systems, but the error increases somewhat as the size of the system gets bigger. The natural transition orbital analysis for SOS-ADC(2) results indicated that at least two configurations were essential to characterize most of the lower-case electronic states. TD-CAM-B3LYP gives the largest errors for excitation energies and also gives an incorrect order of the lowest two states in circum-1-pyrene. A strong density increase of dark states was observed in the UV spectra with increasing size except for the lowest few states which remained well separated. An extrapolation of the UV spectra to infinite PAH size for S 1 , S 2 , and the first bright state based on the coronene series was made. The extrapolated excitation energies closest to experimental measurements on graphene quantum dots were obtained by TD-CAM-B3LYP.

The progressive scaling down of the silicon-based electronics has allowed to develop devices at nanometer scale, requiring new engineering techniques guided by fundamental chemical and physical concepts. Particularly, the nanostructured cluster systems are promising materials since their physical-chemical properties are sensitive to its shape, size, and chemical components, such that completely different materials can be produced by the simple addition or removal of a single atom. These size-tunable properties can open a new area in materials science and engineering. In the present work, quantum chemical methods were used to study the chemical substitution effects caused by subvalent (aluminum) and supervalent (phosphorus) atoms in the physical-chemical properties of some small silicon clusters, which demonstrate high stability, called magic numbers. The changes in the electronic structure and chemical acceptance to the dopants were evaluated with respect to ionization potential, electronic excitation energy, stability, and reactivity parameters. Taken together, these results enable to identify the most stable silicon-doped clusters. Regarding electrophilic reactions, Si 10 P is the most favorable system, while for nucleophilic reactions, none of the doped clusters resulted in higher stability.

Aromaticity is a multivariable concept in organic chemistry that plays a central role for understanding the structure, stability, and reactivity of polycyclic aromatic hydrocarbons (PAHs). Several types of PAHs are characterized as singlet biradicaloid species and their chemical stability is intimately linked to the degree of aromatic character. In this study, theoretically designed routes to tune the biradical character (and thereby its chemical stability) of nitrogen-substituted octacenes have been investigated on the basis of the high-level multireference averaged quadratic coupled-cluster MR-AQCC method necessary for the appropriate description of polyradicaloid systems. The influence of nitrogen centers on the aromaticity of octacene is probed through structural (HOMA) and electron localization (ELF) indices by comparing the N- against NH-doping cases. These analyses reveal that the aromaticity and biradical character of octacene is only slightly affected by replacing one pair of CH groups with N atoms, i.e., by N-doping. However, a significant aromatic stabilization can be obtained when NH-doping is applied at the inner octacene rings; this is also accompanied by an overall decrease of the open-shell character, as evidenced by the gradual quenching of the unpaired electrons and increase in the singlet-triplet splittings when the NH doping groups are moved toward the center of the octacene molecule. Our findings aid in the rational design of new PAH compounds with balanced biradicaloid character and chemical stability which is important, e.g., for practical applications in organic solar cells based on the singlet-fission mechanism.

Zethrenes are interesting polycyclic aromatic hydrocarbons (PAHs), which possess unique optoelectronic and magnetic properties because of their singlet open-shell biradicaloid character, making them promising candidates for application in organic electronics. Tuning their properties is a key task in order to develop efficient compounds for practical use by balancing the desired biradicaloid character against its chemical instability. In this work, high-level theoretical multireference methods appropriate for the correct description of polyradicaloid systems are used to develop rules for doping of zethrenes by means of nitrogen taking heptazethrene (HZ) as a benchmark example. The results of the quantum chemical calculations have been concentrated on a series of quantitative descriptors such as unpaired densities and singlet-triplet (S-T) splittings. They clearly indicate different regions in the HZ where N-doping can either lead to strong enhancement of the biradicaloid character or to strong quenching towards a closed shell state. A wide scale of varying open-shell character is accessible from the different doping positions. It is shown that the S-T splittings correlate well with the total number of unpaired electrons in the medium range of biradicaloid character. For pronounced biradical character the S-T splitting decays to about zero with a margin of ±0.15 eV. In the opposite closed-shell limit, much larger S-T splittings of up to 3 eV are computed., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications.

Membrane-based gas separation technology is of crucial importance in the current economy and nanoporous graphene, given its single-atomic layer, is an essential building-block material to achieve efficiency towards permeability and selectivity for such processes. Classically, pore size is the main feature that governs the diffusion energy barrier. Its nature, nevertheless, is also affected by other non-negligible physical mechanisms not yet discussed. Here we propose a theoretical study on the role of non-covalent interactions towards H2 diffusion through two graphene-based membranes. Symmetry-Adapted Perturbation Theory (SAPT) was used to investigate the total interaction energy and its physically meaningful components (electrostatics, exchange, induction and dispersion). The study reveals the importance of quantum effects such as polarization and electron delocalization in order to counterbalance the abiding idea of pore size being the dominant factor accounting for the energy barrier. These results have important implications for the rational design of efficient nanoporous devices for separation applications.

The reaction of methanol (CH 3 OH) with atomic nitrogen was studied considering three elementary reactions, the hydrogen abstractions from the hydroxyl or methyl groups (R1 and R3, respectively) and the C-O bond break (R2). Thermochemical properties were obtained using ab initio methods and density functional theory approximations with aug-cc-pVXZ (X = T and Q) basis sets. The minimum energy path was built with a dual-level methodology using the BB1K functional as the low-level and the CCSD(T) as the high-level. This surface was used to calculate the thermal rate constants in the frame of variational transitional state theory considering the tunneling effects. Our results indicate the dehydrogenation of the methyl group (R3) as the dominant path with k R3 = 7.5 × 10 -27 cm 3 ·molecule -1 ·s -1 at 300 K. The thermal rate constants were fitted to a modified Arrhenius equation for use in mechanism studies of the methanol decomposition.

In recent years, ozone and its isotopologues have been a topic of interest in many fields of research, due to its importance in atmospheric chemistry and its anomalous isotopic enrichment-or the so-called "mass-independent fractionation." In the field of potential energy surface (PES) creation, debate over the existence of a potential barrier just under the dissociation threshold (referred to as a "potential reef") has plagued research for some years. Recently, Dawes and co-workers [Dawes, Lolur, Li, Jiang, and Guo (DLLJG) J. Chem. Phys. 139, 201103 (2013)] created a highly accurate global PES, for which the reef is found to be replaced with a (monotonic) "plateau." Subsequent dynamical calculations on this "DLLJG" PES have shown improved agreement with experiment, particularly the vibrational spectrum. However, it is well known that reaction dynamics is also highly influenced by the rovibrational states, especially in cases like ozone that assume a Lindemann-type mechanism. Accordingly, we present the first significant step toward a complete characterization of the rovibrational spectrum for various isotopologues of ozone, computed using the DLLJG PES together with the ScalIT suite of parallel codes. Additionally, artificial neural networks are used in an innovative fashion-not to construct the PES function per se but rather to greatly speed up its evaluation.

The present work focuses on establishing a function to rank the stability of small silicon clusters to characterize their magic numbers. This function is composed by a thermodynamic descriptor, the atomization Gibbs free energy, and indirect kinetic descriptors, the highest occupied molecular orbital energy and the lowest excitation energy of each system. The silicon clusters geometries were optimized using density functional theory within a hybrid meta-GGA approximation (M06), while the electronic energy was corrected by single-point calculation using CASPT2 level of theory to obtain the molecular properties. Both methodologies were combined with polarized diffused triple zeta, 6-311++G(3df,3pd), basis set for all atoms. Some molecular properties and their combinations were considered to create the aforementioned function to represent the clusters chemical stability and their magic numbers. The chosen stability ranking function, called ε 3 , presents results in agreement with the previous mass spectrometry experimental data identifying 4, 6, 7 and 10 as magic numbers for small silicon clusters. We believe this stability ranking function can be useful to study other intramolecular atomic and molecular clusters. Graphical abstract Stability ranking function, ε 3 1 , applied on Si n (n = 2 - 11) clusters showing Fukui's functions for the Si n (n = 2 - 11) obtained by the electronic density difference through CASPT2//M06/6-311++G(3df,3pd) with an isosurface value equal to 0.003.

This study addresses the rational design of a magnetic molecularly imprinted polymer (magnetic-MIP) for the selective recognition of the hormone levothyroxine. The theoretical study was carried out by the density functional theory (DFT) computations considering dispersion interaction energies, and using the D2 Grimme's correction. The B97-D/def2-SV(P)/PCM method is used not only for studying the structure of the template the and monomer-monomer interactions, but also to assess the stoichiometry, noncovalent binding energies, solvation effects and thermodynamics properties such as binding energy. Among the 13 monomers studied in silico, itaconic acid is the most suitable according to the thermodynamic values. In order to assess the efficiency of the computational study, three different magnetic-MIPs based on itaconic acid, acrylic acid and acrylamide were synthesized and experimentally compared. The theoretical results are in agreement with experimental binding studies based on laser confocal microscopy, magneto-actuated immunoassay and electrochemical sensing. Furthermore, and for the first time, the direct electrochemical sensing of L-thyroxine preconcentrated on magnetic-MIP was successfully performed on magneto-actuated electrodes within 30 min with a limit of detection of as low as 0.0356 ng mL -1 which cover the clinical range of total L-thyroxine. Finally, the main analytical features were compared with the gold standard method based on commercial competitive immunoassays. This work provides a thoughtful strategy for magnetic molecularly imprinted polymer design, synthesis and application, opening new perspectives in the integration of these materials in magneto-actuated approaches for replacing specific antibodies in biosensors and microfluidic devices., (Copyright © 2018 Elsevier B.V. All rights reserved.)

The reaction of 3 CH 2 with 3 O 2 is of fundamental importance in combustion, and the reaction is complex as a result of multiple extremely exothermic product channels. In the present study, direct dynamics simulations were performed to study the reaction on both the singlet and triplet potential energy surfaces (PESs). The simulations were performed at the UM06/6-311++G(d,p) level of theory. Trajectories were calculated at a temperature of 300 K, and all reactive trajectories proceeded through the carbonyl oxide Criegee intermediate, CH 2 OO, on both the singlet and triplet PESs. The triplet surface leads to only one product channel, H 2 CO + O( 3 P), while the singlet surface leads to eight product channels with their relative importance as CO + H 2 O > CO + OH + H ∼ H 2 CO + O( 1 D) > HCO + OH ∼ CO 2 + H 2 ∼ CO + H 2 + O( 1 D) > CO 2 + H + H > HCO + O( 1 D) + H. The reaction on the singlet PES is barrierless, consistent with experiment, and the total rate constant on the singlet surface is (0.93 ± 0.22) × 10 -12 cm 3 molecule -1 s -1 in comparison to the recommended experimental rate constant of 3.3 × 10 -12 cm 3 molecule -1 s -1 . The simulation product yields for the singlet PES are compared with experiment, and the most significant differences are for H, CO 2 , and H 2 O. The reaction on the triplet surface is also barrierless, inconsistent with experiment. A discussion is given of the need for future calculations to address (1) the barrier on the triplet PES for 3 CH 2 + 3 O 2 → 3 CH 2 OO, (2) the temperature dependence of the 3 CH 2 + 3 O 2 reaction rate constant and product branching ratios, and (3) the possible non-RRKM dynamics of the 1 CH 2 OO Criegee intermediate.

In this work CASPT2 calculations of polyacenes (from naphthalene to heptacene) were performed to find a methodology suitable for calculations of the absorption spectra, in particular of the L a (B 2u state) and L b (B 3u state) bands, of more extended systems. The effect of the extension of the active space and of freezing σ orbitals was investigated. The MCSCF excitation energy of the B 2u state is not sensitive to the size of the active space used. However, the CASPT2 results depend strongly on the amount of σ orbitals frozen reflecting the ionic character of the B 2u state. On the other hand, the excitation energies of the B 3u state are much more sensitive to the size of the active space used in the calculations reflecting its multiconfigurational character. We found a good agreement with experimental data for both bands by including 14 electrons in 14 π orbitals in the active space followed by the CASPT2(14,14) perturbation scheme in which both σ and π orbitals are included.

The association/dissociation reaction path for ozone (O 2 + O ↔ O 3 ) is notoriously difficult to describe accurately using ab initio electronic structure theory, due to the importance of both strong and dynamic electron correlations. Experimentally, spectroscopic studies of the highest lying recorded vibrational states combined with the observed negative temperature dependence of the kinetics of oxygen isotope exchange reactions confirm that the reaction is barrierless, consistent with the latest potential energy surfaces. Previously reported potentials based on Davidson-corrected internally contracted multireference configuration interaction (MRCI) suffer from a spurious reef feature in the entrance channel even when extrapolated towards the complete basis set limit. Here, we report an analysis of comparisons between a variety of electronic structure methods including internally contracted and uncontracted MRCI (with and without Davidson corrections), as well as full configuration interaction quantum Monte Carlo, fixed-node diffusion Monte Carlo, and density matrix renormalization group.

Acenes are fascinating polyaromatic compounds that combine impressive semiconductor properties with an open-shell character by varying their molecular sizes. However, the increasing chemical instabilities related to their biradicaloid structures pose a great challenge for synthetic chemistry. Modifying the π-bond topology through chemical doping allows modulation of the electronic properties of graphene-related materials. In spite of the practical importance of these techniques, remarkably little is known about the basic question - the extent of the radical character created or quenched thereby. In this work, we report a high-level computational study on two acene oligomers doubly-doped with boron and nitrogen, respectively. These calculations demonstrate precisely which the chemical route is in order to either quench or enhance the radical character. Moving the dopants from the terminal rings to the central ones leads to a remarkable variation in the biradicaloid character (and thereby also in the chemical stability). This effect is related to a π-charge transfer involving the dopants and the radical carbon centers at the zigzag edges. This study also provides specific guidelines for a rational design of large polyaromatic compounds with enhanced chemical stability.