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Hydroperoxymethyl thioformate (or HPMTF) is a compound relevant to the chemistry of sulfur in the marine atmosphere. The chemical cycling of this molecule in the atmosphere is still uncertain due in part to the lack of accurate knowledge of its photolytic behavior. Only approximations based on the properties of its chromophores are used in previous studies. In this work, we calculated the absorption spectra of the molecule in gas and aqueous phases using the Nuclear Ensemble Approach (NEA) and the CASPT2 method. Furthermore, we used such information to obtain relative photolysis rates. We found that the chromophore approximation overestimates the photolysis rates in the gas phase by twice the value obtained with the NEA-CASPT2 protocol. Furthermore, for the aqueous phase, we predict a lower role of photolysis as compared to the gas phase.

Molecular switches which can be triggered by light to interconvert between two or more well-defined conformation differing in their chemical or physical properties are fundamental for the development of materials with on-demand functionalities. Recently, a novel molecular switch based on a the azodicarboxamide core has been reported. It exhibits a volume-conserving conformational change upon excitation, making it a promising candidate for embedding in confined environments. In order to rationally implement and efficiently utilize the azodicarboxamide molecular switch, detailed insight into the coordinates governing the excited-state dynamics is needed. Here, we report a detailed comparative picture of the molecular motion at the atomic level in the presence and absence of explicit solvent. Our hybrid quantum mechanics/molecular mechanics (QM/MM) excited state simulations reveal that, although the energy landscape is slightly modulated by the solvation, the light-induced motion is dominated by a bending-assisted pedalo-type motion independent of the solvation. To support the predicted mechanism, we simulate time-resolved IR spectroscopy from first principles, thereby resolving fingerprints of the light-induced switching process. Our calculated time-resolved data are in good agreement with previously reported measured spectra.

Curved (non-planar) aromatic compounds have attracted significant research attention in the fields of basic chemistry and materials science. The contribution of the quinoidal structure in the curved π-conjugated structures has been proposed to be the key for materials functions. In this study, the curve effect on the quinoidal contribution was investigated in Kekulé-type singlet diradicals (S-DR1-4) as a sensitive probe for quinoidal structures in curved π-conjugated molecules. The quinoidal contribution in S-DR1-4 was found to increase with increasing the curvature of the curved structure, which was quantitatively analyzed using NBO analysis and the natural orbital occupation numbers computed by the CASSCF method. The curve effect on the singlet-triplet energy gap was examined by the CASPT2 method. The singlet-triplet energy gaps for the highly π-conjugated diradicals were determined for the first time using the CASPT2 method. Substantial quinoidal contribution was found in the curved structures of the delocalized singlet diradicals S-DR1-4, in contrast to its absence in the corresponding triplet states T-DR1-4.

The purpose of the work is to calculate accurate values of molecular properties of tetracyanoquinodimethane (TCNQ) and anions using the complete active space self-consistent field and complete active space second-order perturbation theory methods. The accuracy has been evaluated using several basis sets and active spaces. The calculated properties have, in many cases, been confirmed by experimental data (within parentheses), e.g., 9.54 eV (9.61 eV) and 3.36 eV (3.38 eV) for the ionization potential and electron affinity, respectively, of TCNQ; 3.12 eV (3.01 eV) and 3.54 eV (3.42 or 3.60 eV) for transition energies to the two lowest-lying excited singlet states of TCNQ; − 0.03, 0.46 and 1.44 eV (0, 0.5 and 1.4 eV) for electronic energies in electron attachment of TCNQ forming TCNQ - ; and 3.88 eV (3.71 eV) for the transition energy to the second lowest-lying excited singlet state of TCNQ 2 - . Further, the calculations have brought insight into some experimental observations, e.g., the shape of the fluorescence spectrum of TCNQ at 3–4 eV.

The nine-valence-electron HMgN- and HNMg- anions have been investigated for the first time theoretically using CASSCF (complete active space self-consistent field) and CASPT2 (multiconfiguration second-order perturbation theory) methods in conjunction with the contracted atomic natural orbital (ANO) basis sets. The structures of the low-lying electronic states of HMgN- and HNMg- were predicted. The possible unimolecular conversions between HMgN- and HNMg-were discussed. The calculated results indicated that the ground-state of HMgN-is linear, while the ground-state HNMg- is bent, which is in contradiction to Walsh?s rules predicting linear structures for the HXY systems containing 10 or less valence electrons. nema

In this work, we studied the main decomposition reactions on the ground state of nitromethane (CH3NO2) with the CASPT2 approach. The energetics of the main elementary reactions of the title molecule have been analyzed on the basis of Gibbs free energies obtained from standard expressions of statistical thermodynamics. In addition, we describe a mapping method (orthogonalized 3D representation) for the potential energy surfaces (PESs) by defining an orthonormal basis consisting of two Rn orthonormal vectors (n, internal degrees of freedom) that allows us to obtain a set of ordered points in the plane (vector subspace) spanned by such a basis. Geometries and harmonic frequencies of all species and orthogonalized 3D representations of the PESs have been computed with the CASPT2 approach. It is found that all of the analyzed kinetically controlled reactions of nitromethane are endergonic. For such a class of reactions, the dissociation of nitromethane into CH3 and NO2 is the process with the lower activation energy barrier (ΔG); that is, the C-N bond cleavage is the most favorable process. In contrast, there exists a dynamically controlled process that evolves through a roaming reaction mechanism and is an exergonic reaction at high temperatures: CH3NO2 → [CH3…NO2]* → [CH3ONO]* → CH3O + NO. The above assertions are supported by CASPT2 mappings of the potential energy surfaces (PESs) and classical trajectories obtained by “on-the fly” CASSCF molecular dynamics calculations.

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.

Metal dithiolene complexes—M(dmit)2—are key building blocks for magnetic, conducting, and optical molecular materials, with singular electronic structures resulting from the mixing of the metal and dmit ligand orbitals. Their use in the design of magnetic and conducting materials is linked to the control of the unpaired electrons and their localized/delocalized nature. It has been recently found that UV–Vis light can control the spin distribution of some [Cu(dmit)2]−2 salts in a direct and reversible way. In this work, we study the optical response of these salts and the origin of the differences observed in the EPR spectra under UV–Vis irradiation by means of wave function-based quantum chemistry methods. The low-lying states of the complex have been characterized and the electronic transitions with a non-negligible oscillator strength have been identified. The population of the corresponding excited states promoted by the UV–Vis absorption produces significant changes in the spin distribution, and could explain the changes observed in the system upon illumination. The interaction between neighbor [Cu(dmit)2]−2 complexes is weakly ferromagnetic, consistent with the relative orientation of the magnetic orbitals and the crystal packing, but in disagreement with previous assignments. Our results put in evidence the complex electronic structure of the [Cu(dmit)2]−2 radical and the relevance of a multideterminantal approach for an adequate analysis of their properties.

The possibility of the formation of glycine (Gly) from fundamental gas molecules in cold interstellar media was studied using quantum chemical methods, transition state theory and microcanonical molecular dynamics simulations with surface hopping dynamics (NVE-MDSH). This theoretical study emphasized five photochemical pathways in the lowest singlet-excited ( S 1 ) state, thermochemical processes after non-radiative S 1 → S 0 relaxations, and photo-to-thermal energy conversion in the NVE ensemble. The optimized reaction pathways suggested that to generate a reactive singlet dihydroxy carbene (HOCOH) intermediate, photochemical pathways involving the H 2 O…CO van der Waals and H 2 O−OC hydrogen bond precursors (Ch (1)_Step (1)) possess considerably lower energy barriers than the S 0 state pathways. The Gibbs free energy barriers (∆ G ǂ ) calculated after the non-radiative S 1 →S 0 relaxations indicated higher spontaneous temperatures ( T s ) for the formation of the HOCOH intermediate (Ch (1)_Step (1)) than for Gly formation (Ch (1)_Step (2) and Ch (4)). Although the termolecular reaction in Ch (4) possesses a low energy barrier, and is thermodynamically favourable, the high exothermic S 1 →S 0 relaxation energy leads to the separation of the weakly associated H 2 O…CH 2 NH…CO complex into single molecules. The NVE-MDSH results also confirmed that the molecular processes after the S 1 →S 0 relaxations are thermally selective, and because the non-radiative S 1 →S 0 relaxation temperatures are exceedingly higher than T s , the formation of Gly on consecutive reaction pathways is non-synergistic with low yields and several side products. Based on the theoretical results, photo-to-thermal control strategies to promote desirable photochemical products are proposed. They could be used as guidelines for future theoretical and experimental research on photochemical reactions.

The present study provides new insights into the topography of the potential energy hypersurfaces (PEHs) of the thymine nucleobase in order to rationalize its main ultrafast photochemical decay paths by employing two methodologies based on the complete active space self-consistent field (CASSCF) and the complete active space second-order perturbation theory (CASPT2) methods: (i) CASSCF optimized structures and energies corrected with the CASPT2 method at the CASSCF geometries and (ii) CASPT2 optimized geometries and energies. A direct comparison between these strategies is drawn, yielding qualitatively similar results within a static framework. A number of analyses are performed to assess the accuracy of these different computational strategies under study based on a variety of numerical thresholds and optimization methods. Several basis sets and active spaces have also been calibrated to understand to what extent they can influence the resulting geometries and subsequent interpretation of the photochemical decay channels. The study shows small discrepancies between CASSCF and CASPT2 PEHs, displaying a shallow planar or twisted 1(ππ*) minimum, respectively, and thus featuring a qualitatively similar scenario for supporting the ultrafast bi-exponential deactivation registered in thymine upon UV-light exposure. A deeper knowledge of the PEHs at different levels of theory provides useful insight into its correct characterization and subsequent interpretation of the experimental observations. The discrepancies displayed by the different methods studied here are then discussed and framed within their potential consequences in on-the-fly non-adiabatic molecular dynamics simulations, where qualitatively diverse outcomes are expected.