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Inspired by the brilliant photochemical and photophysical properties of organic molecule containing the halide substitution that could be potentially applied across various disciplines, in this work, effects of atomic electronegativity of halogen (F, Cl, and Br) on excited state hydrogen bond effects and excited state intramolecular proton transfer (ESIPT) reaction. We present the characteristic 4-chloro-2-[1-(4-methoxy-phenyl)-4,5-diphenyl-1H-imidazol-2-yl]-phenol (CMDIP) that is the main objective of this study to explore in detail the influence of the change of atomic electronegativity by photoexcitation. By comparing the structural changes and infrared (IR) vibrational spectra of the CMDIP derivatives (CMDIP-F, CMDIP-Cl, and CMDIP-Br) fluorophores in S0 and S1 states, combined with the preliminary detection of hydrogen bond interaction by core-valence bifurcation (CVB) index, we can conclude that the hydrogen bond could be strengthened in S1 state, which is favorable for the occurrence of ESIPT reactions. The charge recombination behavior of hydrogen bond induced by photoexcitation also further illustrates this point. Via constructing potential energy curves (PESs) based on restrictive optimization and searching transition state (TS) form, we confirm the change of atomic electronegativity has a regulatory effect on the ESIPT behavior for CMDIP derivatives, that is, the higher the atomic electronegativity is more conducive to the ESIPT reaction. [ABSTRACT FROM AUTHOR]

Gas-phase reactions involving simplest Criegee intermediate (CH2OO) have been the current hot topic due to its vital role in atmospheric chemistry. In this study, high-level ab initio calculations are used to investigate the energetics and kinetics for the reaction of CH2OO + ROH → ROCHO + H2O (R=CH3, CH3CH2 and (CH3)2CH). Energies of the stationary points are computed at the CCSD(T)/M06-2X/6-311++G(3d,3pd)//M06-2X/6-311++G(3d,3pd) level of theory. Reaction is going through a 1,2-addition and water elimination step leading to the formation of alkoxymethyl hydroperoxides and alkyl formates, respectively. The barrier heights for the 1,2-addition step with methanol, ethanol, and isopropanol were found to be − 3.1, − 3.7, and − 4.8 kcal mol−1, and water elimination steps were found to be 2.2, 1.5, and 1.6 kcal mol−1, respectively, relative to the energies of the starting reactants. The rate constants for addition and elimination channels were calculated using canonical variational transition state theory in conjugation with small-curvature tunneling and the interpolated single point energy method between the temperature range of 200 and 500 K. In addition, the thermochemistry analysis indicates that addition and elimination channels are thermodynamically feasible and the formation of alkyl formates is entropically more favored when compared to the formation of alkoxymethyl hydroperoxide along the reaction path in the potential energy surface. The pressure-dependent microcanonical rate constants for both addition and elimination channels were also estimated using the Rice–Ramsperger–Kassel–Marcus theory and discussed in this study. [ABSTRACT FROM AUTHOR]

The energy pathways of possible decomposition and isomerization reactions of iso-propyl (i-C3H7) and n-propyl (n-C3H7) radicals are studied by computational methods of quantum chemistry. The B3LYP, M062X, MP2, and CBS-QB3 methods are used to localize stationary points on the potential energy surface of a system containing propyl radicals. A number of intermediate compounds formed during the isomerization and decomposition of propyl radicals are identified and information is obtained on their structure and thermochemical parameters. Based on the results of the research, a diagram of the energy levels of the system under consideration is constructed. [ABSTRACT FROM AUTHOR]

In the last years, state-to-state molecular dynamics simulations of some basic elementary processes, occurring at the gas–surface interface in a wide range of temperatures and collision energies, have been performed by adopting new potential energy surfaces. In this contribution, our attention is mostly addressed to the role of long-range forces, determining the physisorption of gaseous molecules on the surface. Such forces, formulated in terms of the improved Lennard–Jones interaction potential model, control the formation of precursor or pre-reactive state that plays a crucial role in the dynamical evolution of molecules impinging on the surface in the range of low–intermediate collision kinetic energies. The study focuses on the collisions of H2, O2, N2 and CO, initially in their ground and excited vibro-rotational levels, on a graphite surface. The resulting dispersion coefficients, which control the capture of impinging molecules, are compared and found in good agreement with those available in the literature. New selectivity and peculiarities of scattered molecules, crucial to control the kinetics of elementary chemical processes occurring at the gas–surface interfaces under thermal and sub-thermal conditions, of interest in different applied fields, are highlighted. [ABSTRACT FROM AUTHOR]

The current state of the protein folding problem and other biopolymers folding is discussed. The concept of a multidimensional potential energy surface and free energy surface for linear polymers is detailed, taking into account the topology of the configuration space and the presence of symmetry elements with respect to the rearrangement of identical monomer units. The presence of kinematic connections for conformational movements in a viscous medium leads to a tendency for the formation of helical structures of linear polymers. The dynamic effects of viscosity also lead to an almost uniform distribution of energy dissipation rates across the nodes of the chain. The combination of free energy surface topography and viscosity effects provides a physical basis for advancing folding theory toward interpreting a variety of experimental observations and elucidating principles of amino acid code formation for 3D protein structures. The relationship between the denaturation temperature of the folded state of the biopolymer and the energy of nonvalent interactions between monomers in the chain is analyzed. [ABSTRACT FROM AUTHOR]

The ab initio intermolecular pair potentials of F 2 dimer were calculated for five leading stable configurations, using the symmetry-adapted perturbation theory. We employ an improved Lennard–Jones potential to best fit the potential energy surface of each leading configuration. The molecular anisotropy is characterized through the expansion of the degrees of freedom of the analytical potential energy surface (PES) using the spherical harmonics. The resulting analytical PES is used to calculate the second virial coefficients and compared with the experimental values and other theoretical works to test the quality of the presented intermolecular potential. Finally, we performed the theoretical computation of viscosity and self-diffusion transport properties for the F 2 ⋯ F 2 system. [ABSTRACT FROM AUTHOR]

Three-dimensional ab initio potential energy surfaces of the interacting triple states 3A2 and 3B1 of the O3 molecule are constructed within the diabatization approach implemented in the MOLPRO package. These two states are responsible for the strongest singlet–triplet transitions in the Wulf band of O3. The molecular orbitals are optimized by the CASSCF method with the active space CAS(18, 12) involving three electronic states (X1A1, 3A2, and 3B1). The correlation energy is computed by icMRCI(Q). The influence of the basis set size on the accuracy of both the adiabatic excitation energy and origins of the vibronic transitions is analyzed. [ABSTRACT FROM AUTHOR]

Artificial intelligence (AI) and machine learning (ML) methods have touched practically all aspects of our life. Their utility ranges from separating different quality agricultural produce to facial recognition to guiding us through most steps in our day-to-day life. In this perspective article, we demonstrate the utility of artificial neural network (ANN) method in fitting potential energy curves and surfaces and point out the potential applications to predicting and analyzing dynamical observables. Although the regression methods seem to be successful in fitting potential energy surfaces using limited ab initio data, the ANN method yields accurate fits of surfaces when enough number of ab initio points on the potential energy surface become available. The possibility of utilizing the ANN method for fitting excitation function data is pointed out and the implications are discussed. This perspective article illustrates how the artificial neural network can be used to interpolate accurately potential energy curves and surfaces for molecular systems and how the method can be extended to systems with avoided crossing of potential energy curves and to multidimensional excitation function data. [ABSTRACT FROM AUTHOR]

The article briefly reviews the history of the development of ideas about the dynamics of proteins and other biopolymers and notes the significant contribution of V.I. Goldansky in organizing and conducting these studies in Russia. The modern development of earlier ideas about the dynamics of biopolymers and protein folding is discussed. It is shown that folding is not an isolated problem and is related to the fundamental dynamic properties of linear polymers in the condensed phase. Analytical methods using approaches based on multidimensional geometry show that the viscosity of the medium is one of the most important factors that determines the rules for the movement of a representative point along the ultramultidimensional potential energy surface (PES). These rules lead to the concentration of trajectories in those regions of the configuration space of a macromolecule that correspond to relatively smooth PES regions, which is important for understanding the reasons for the stability of the results of calculations of large systems using the molecular dynamics (MD) method, despite the fundamental inaccuracy in determining the available force fields. This article also briefly describes a new approach to determine and study the properties of a multidimensional PES, which is based on the features of the topology of the configuration space of linear polymers (and biopolymers), symmetry with respect to permutations of identical chain links, and Morse theory for studying the topography of multidimensional surfaces. Under certain conditions, this approach gives observable analytical results for the topography of the PES and the free energy surface (FES) of a macromolecule and makes it possible to relate the rather heterogeneous results of experiments on protein folding from a unified point of view. At the same time, a new formulation appears for a number of fundamental and controversial issues related to the physical laws of the formation of living systems. In particular, a connection is traced between the temperature regime on the planet and the chemical realization of the energy of nonvalent interactions in a macromolecule, which are necessary for the formation of unique spatial structures of biopolymers. [ABSTRACT FROM AUTHOR]

In order to study the potential energy surfaces (PESs) of systems containing ethylene, hydrogen, and oxygen atoms, quantum chemical calculations are carried out using the hybrid B3LYP method of the density functional theory (DFT) and the composite CBS-QB3 method. An enthalpy diagram reflecting the PES of this reaction system is constructed. It is shown that the addition of a hydrogen atom to ethylene with the formation of an ethyl radical occurs through the formation of a van der Waals complex. The diagram of enthalpies of monomolecular reactions of decomposition and isomerization of the ethoxyl radical is presented in detail, and a conclusion is made about the probability of their occurrence. The global PES minimum of the system describing the sequential addition of hydrogen and oxygen atoms to ethylene is the hydroxyethyl radical. Intermediates CHCH2OH and CH2OCH2 are localized on the PES of the C2H4 + O system and possible pathways for their further transformation are analyzed. The data obtained make it possible to estimate the ranking of individual elementary reactions in the processes of combustion and oxidation of hydrocarbons and the probabilities of various directions for the transformation of chemical species in the studied systems. [ABSTRACT FROM AUTHOR]

The eleven monohydrates of 1, 2, 3, 4-tetrahydroisoquinoline (THIQ) are analyzed through natural bond orbital (NBO) analysis and QTAIM methods employing M06-2X functional in DFT and MP2 methods. Here, the role of OH bonds as an acceptor and donor is critically analyzed. The role of lone pairs of O is critically monitored in two of the complexes, where N–H···O hydrogen bonds are present. The relative contributions of rehybridisation and hyperconjugation are compared in detail. Popelier criteria are satisfied in all the complexes barring a few exceptions involving weak hydrogen bonds. At the bond critical points (BCP), four monohydrates show higher values of electron density (ρC) and negative values of total electron energy density (HC), while Laplacian (∇ 2 ρ C) remains positive. These complexes satisfy the criteria of partial covalency. All these are O–H⋅⋅⋅N-type bonds. Remaining h-bonds are weaker in nature. These are also confirmed by the smaller values of ρC at the respective BCP. The variation of potential energy density (VC) among the complexes seems to be the most important factor in determining the nature of non-covalent interactions. [ABSTRACT FROM AUTHOR]

Until about a decade or so, till the fast-paced development and usage of computers, organic chemistry students often chose experimental projects during an internship. Presently, there are software packages that solve quantum chemical equations and present the possibility of a 'black-box' type of approach to getting exposed to computational organic chemistry. Encouraging as this may seem, Indian university students can find the entry to and an understanding of computational chemistry daunting due to a lack of exposure to research. This article attempts to bridge the gap for such students by providing a gentle introduction to the field of quantum chemistry. [ABSTRACT FROM AUTHOR]

Collisions of metastable pionic helium atoms with helium atoms of the medium lead to the destruction of these states, as well as to shifts and broadening of E1 spectral lines of transitions between the pionic helium states. In the paper, in order to obtain the interaction potential matrix ( , the unrestricted Hartree–Fock calculations of the potential energy surface (PES) are performed taking into account electron–electron correlations within the second-order perturbation theory (MP2). Numerically solved with this potential is the system of coupled-channel equations. Due to strong channel-coupling in the vicinity of small distances between the colliding subsystems, there arises degeneracy of the matrix solution there, which results in the numerical solutions "forgetting" the left-hand boundary conditions. In order to eliminate this solution degeneracy, various techniques are used. Cross sections and collisional transition rates are calculated (). It is found that the collisional transition rate ( , —the principal quantum number and the angular momentum respectively) for the density of the medium is lower than s , which indicates that it is possible to ignore the effect of collisional quenching of the pionic helium long-lived states in precision-laser-spectroscopic experiments. [ABSTRACT FROM AUTHOR]

Gaussian process regression (GPR) has been finding an increased use in computational chemistry, including for applications where high accuracy of machine learning is required, such as potential energy surfaces or functionals for density functional theory. The quality of a GPR model critically depends on the choice of hyperparameters of the kernel. When the data are sparse, optimization of the hyperparameters by the commonly used methods such as maximum likelihood estimation (MLE) criterion can lead to overfitting. We show that choosing hyperparameters (in this case, the kernel length parameter and the regularization parameter) based on a criterion of the completeness of the basis in the corresponding linear regression problem is superior to MLE. We show that this is facilitated by the use of high-dimensional model representation (HDMR) whereby a low-order HDMR representation can provide reliable reference functions and allow generating large synthetic test data sets needed for basis parameter optimization even when the original data are few. This is expected to be particularly useful when fitting potential energy surfaces where a sufficiently low-order HDMR can provide a good approximation. An example of a 15-dimensional PES of UF6 is presented. [ABSTRACT FROM AUTHOR]

The title compound, 4-(4-methoxyphenethyl)-5-(p-tolyl)-2,4-dihydro-3H-1,2,4-triazol-3-one (C18H19N3O2), was characterized by single crystal X-ray diffraction. In order to compare the experimental and theoretical compatibility, the DFT and HF modeling technique was also used. When the theoretical and experimental results are compared, it is seen that the geometric parameters from both investigation techniques are quite compatible. X-ray diffraction (XRD) analysis shows that the structure has crystallized in the orthorhombic space group Pna21. The planes of the triazole and benzyl rings in the molecule make dihedral angles of 46.14(1)ο (C1-C6) and 43.89(1)ο (C12-C17). The molecules in the asymmetric unit are linked by intermolecular N–H···O hydrogen bonds, forming a three-dimensional network. Frontier molecular orbital (FMO) analysis was performed to describe intramolecular interactions. For the molecule the energy difference between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) was calculated as − 4.95 eV. Potential energy surface (PES) analysis was performed by a semi-empirical method to determine the stable states of molecular structure and to compare them with XRD geometry. In order to determine the thermodynamic properties of the molecular structure enthalpy, heat capacity and entropy values were calculated for selected temperature values. Triazole compound were synthesized and characterized by elemental analyses, Molecular Hirshfeld Surface, Frontier Molecular Orbitals, Potential Energy Surface, Thermodynamic Properties analysis, and single crystal X-ray diffraction. [ABSTRACT FROM AUTHOR]

Derivatives of 1,2,4-triazole exhibit antimicrobial, anticonvulsant, anti-inflammatory, immunomodulatory, and other types of activity, which makes it possible to create effective drugs on their basis. Understanding the reaction mechanism for the formation of triazoles helps to control the chemical process and conduct targeted synthesis. Quantum-chemical modeling of the mechanism of interaction of diformylhydrazine with o- and p-aminophenols was carried out using the combined approach CCSD (T)/6–31+G*//B3LYP/6–311++G**. The elementary stages of the reaction, possible intermediate compounds, and transition states have been established. The obtained results have been compared with the data from NMR spectroscopy. [ABSTRACT FROM AUTHOR]

The mechanistic, thermochemical, and kinetic study of the 4-hydroxy-2-pentanone (4H2P) + OH radical reaction is performed for the first time by employing quantum theoretical calculations. The potential energy diagram was evaluated for five possible reaction pathways at the CCSD(T)/cc-pVTZ//BH&HLYP/cc-pVTZ level of theory. Theoretical rate coefficients of five abstraction pathways are computed as a function of temperature (210–350 K) utilizing the canonical variational transition state theory (CVT) with small-curvature tunneling (SCT). A three-parameter modified Arrhenius equation is used to fit rate coefficients. The thermodynamic quantities like reaction enthalpy and Gibbs free energy are calculated at the BH&HLYP/cc-pVTZ level of theory. According to thermodynamic analysis, the hydrogen abstraction from the –CH group adjacent to the hydroxyl group occurs more favorably and is the dominant pathway with minimum barrier height. The structure–activity relationship is explored by comparing rate coefficients of the titled reaction with the literature values of similar species. The subsequent fate of the alkyl radical (CH3C(O)CH2C·(OH)CH3) is further studied in a NO-rich environment resulting in the formation of acetone, NO2, and oxygen as the major final products. [ABSTRACT FROM AUTHOR]

The title compound, 4-(4-methoxyphenethyl)-5-(p-tolyl)-2,4-dihydro-3H-1,2,4-triazol-3-one (C18H19N3O2), was characterized by single crystal X-ray diffraction. In order to compare the experimental and theoretical compatibility, the DFT and HF modeling technique was also used. When the theoretical and experimental results are compared, it is seen that the geometric parameters from both investigation techniques are quite compatible. X-ray diffraction (XRD) analysis shows that the structure has crystallized in the orthorhombic space group Pna21. The planes of the triazole and benzyl rings in the molecule make dihedral angles of 46.14(1)ο (C1-C6) and 43.89(1)ο (C12-C17). The molecules in the asymmetric unit are linked by intermolecular N–H···O hydrogen bonds, forming a three-dimensional network. Frontier molecular orbital (FMO) analysis was performed to describe intramolecular interactions. For the molecule the energy difference between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) was calculated as − 4.95 eV. Potential energy surface (PES) analysis was performed by a semi-empirical method to determine the stable states of molecular structure and to compare them with XRD geometry. In order to determine the thermodynamic properties of the molecular structure enthalpy, heat capacity and entropy values were calculated for selected temperature values. Triazole compound were synthesized and characterized by elemental analyses, Molecular Hirshfeld Surface, Frontier Molecular Orbitals, Potential Energy Surface, Thermodynamic Properties analysis, and single crystal X-ray diffraction. [ABSTRACT FROM AUTHOR]

The results of the quantum chemical study of the reaction of propionitrile with methine radical at parameters characteristic of deep space are presented. The surface profile of the potential energy, including the most energetically favorable paths, is constructed, and reaction kinetic constants are calculated. The proposed mechanism predicts the formation of many cyclic and acyclic products with dominating linear molecules. [ABSTRACT FROM AUTHOR]

We present a full-dimensional potential energy surface for Ar–SO2 which involves three intramolecular Q 1 , Q 2 and Q 3 normal modes for the ν 1 symmetric stretching, ν 2 bending and ν 3 asymmetric stretching vibrations of SO2. The intermolecular potential was computed at the [CCSD(T)]-F12a level with aug-cc-pVTZ basis set plus the midpoint bond functions (3s3p2d1f1g). Three vibrationally averaged potentials of Ar–SO2 with SO2 in the ground state as well as the ν 1 and ν 3 excited states were generated by integrating three intramolecular coordinates. Each potential has a global minimum with the non-planar geometry and two saddle points. The radial discrete variable representation (DVR)/angular finite basis representation (FBR) method and Lanczos algorithm were utilized to calculate the rovibrational bound states and energy levels of Ar–SO2. The vibrational band origin shifts for this complex in the ν 1 and ν 3 regions of SO2 were determined to be − 0.0970 and − 0.7537 cm−1, respectively. The calculated origin shifts as well as the microwave and infrared transition frequencies agree well with available experimental results. [ABSTRACT FROM AUTHOR]

The advance of computing and the development of modern quantum chemistry models such as Density Functional Theory (DFT) have allowed scientists to perform fast in silico studies with accurate results. It also allowed for the achievement of empirically unattainable quantities such as Potential Energy Surfaces (PES), a fundamental construct in various applications, such as the study of weakly bound systems. One of DFT's current weaknesses is a reliable description of PESs, due to a lack of suitable exchange-correlation functionals. In general, other post-Hartree-Fock methods are employed, such as n th-order Møller-Plesset's Perturbation Theory (MPn) or Coupled Cluster Theory (CCSD(T)) with large basis sets. Despite producing good results, these methods demand much computational power when applied to large systems. This work presents a novel approach of PES description of the H2O2–Kr system using DFT by optimizing a long-range parameter present in some DFT functionals, obtaining results similar to those of the MPn methods with somewhat less computational time necessary. [ABSTRACT FROM AUTHOR]

Present work studies the dynamical properties of the O + O2 collision by the quasi-classical trajectory method on the double many-body expansion potential energy surface. Our study includes both the exchange and dissociation reactions. For different rovibrational levels of O2, the integral cross-sections (ICSs) distributions of both channels as a function of translational energy were obtained in a range of collision energy from 0.1 to 30 eV, and the dissociation rate was calculated in the temperature range of 1000–20 000 K. Appreciable differences are found for the excitation functions between the two channels as expected for the dissociation with no barrier and exchange having a small barrier. The initial collision energy and vibrational excitation both play an enormous role in the ICSs for both channels, while the rotational excitation of the reagent has a weak effect on it. The results show that vibrationally excited states have some contribution to the dissociation rate. [ABSTRACT FROM AUTHOR]

Two spectra of 18O-enriched ozone in the 3400–5600 cm−1 range were recorded with the Fourier spectrometer of the University of Reims. The experimental centers of 12 rotational–vibrational bands of the 16O16O18O isotopologue were determined from the spectra analysis. They are compared with theoretical calculations based on the potential energy surface of the molecule. [ABSTRACT FROM AUTHOR]

The cross second virial coefficient B 12 for the interaction between water (H2O) and carbon monoxide (CO) was obtained with low uncertainty at temperatures from 200 K to 2000 K employing a new intermolecular potential energy surface (PES) for the H2O–CO system. This PES was fitted to interaction energies determined for about 58 000 H2O–CO configurations using high-level quantum-chemical ab initio methods up to coupled cluster with single, double, and perturbative triple excitations [CCSD(T)]. The cross second virial coefficient B 12 was extracted from the PES using a semiclassical approach. An accurate correlation of the calculated B 12 values was used to determine the dilute gas cross isothermal Joule–Thomson coefficient, ϕ 12 = B 12 - T (d B 12 / d T) . The predicted values for both B 12 and ϕ 12 agree reasonably well with the few existing experimental data and older calculated values and should be the most accurate estimates of these quantities to date. [ABSTRACT FROM AUTHOR]

The scattering cross sections of rotational excitations of O3 by collisions with noble gas atoms (Ar and He) have been calculated using the two-body scheme implemented in the MOLSCAT2020 package. All O3 energy levels up to Jmax = Kmax = 5 and Jmax = Kmax = 7 that correspond to 18 and 32 coupled channel equations have been included. Dependences of the scattering cross sections on the collision energy are analyzed, and the most probable excited O3 rotational states are determined. The rate coefficients are calculated at temperatures of 5–300 K. The construction of the radial part of the angular coefficients of the interaction potential is also discussed. [ABSTRACT FROM AUTHOR]

The rate constants of ethynyl (C2H) radical reaction with isobutane (kiso) and neopentane (kneo) were calculated for the first time using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) in the temperature range of 150–5000 K. The geometries and frequencies of all the stationary points were calculated at the M06-2X/Aug-cc-pVTZ level of theory. Thermodynamics (ΔH°298 and ΔG°298) results govern that the title reactions are highly exothermic and spontaneous in nature. The rate constants obtained over the temperature range of 150–300 K were used to derive the modified Arrhenius expressions: kiso = 6.84 × 10–19 T2.5 exp[1496/T] and kneo = 1.72 × 10–17 T2.0 exp[938/T] cm3 molecule−1 s−1. Branching ratio calculation for the reaction of C2H radical with isobutane shows that the tertiary-H abstraction channel is dominant (> 95%) over the complete temperature range. [ABSTRACT FROM AUTHOR]

A mechanism has been proposed for the reaction of 1,3-dichloropropene with dipotassium propane-1,3-bis(dithiolate) in the system hydrazine hydrate–potassium hydroxide on the basis of DFT quantum chemical calculations at the B3LYP/6-311++G(d,p) level of theory. The first stage of this reaction has been found to be nucleophilic substitution of one chlorine atom of 1,3-dichloropropene linked to the sp3-carbon atom by the sulfur atom of propane-1,3-bis(thiolate) to give the corresponding monosubstitution product. Further transformations of the latter could lead to the formation of six-, seven-, and eight-membered sulfur heterocycles, as well as of a linear structure as a result of reaction of the monosubstitution product with the second 1,3-dichloropropene molecule. [ABSTRACT FROM AUTHOR]

Absorption in the 1–0 band of CO broadened by Ar is examined using the asymptotic line-wing theory (ALWT). A semiclassical expression is derived for the line shape, where the motion of the centers of mass is considered classical and other variables remain quantum. The line shape parameters are fitted to reach agreement between calculated and experimental data on the absorption in the CO band wing. The classical potential parameters are found from the temperature dependence of the second virial coefficient. A qualitative agreement is found between the quantum potential parameters and intermolecular potential surfaces from quantum-chemical calculations. [ABSTRACT FROM AUTHOR]

The practical and theoretical aspects of the conversion of sulfur-containing components during the hydroconversion of heavy oil feedstock in the presence of nanosized MoS2-based catalytic systems are considered. Thermodynamic calculation of the temperature dependences of the equilibrium composition of the products of hydrogenation reactions of sulfur-containing compounds demonstrated that thiophene is the most stable product in a wide temperature range. Quantum-chemical techniques have shown that chemisorption of a hydrogen molecule on valence-unsaturated Mo atoms brings about the breaking of the H–H bond, migration of H atoms to other valence-unsaturated Mo atoms, as well as to S atoms. The study of the interaction of the Н2S molecule with the Mo2S4 and Mo3S6 clusters showed that the chemisorption of Н2S occurs on the valence unsaturated Mo atoms, followed by the abstraction and migration of the Н atoms over the cluster surface, and the unsaturated S atom plays the role of a donor, hydrogen carrier. It was shown that sulfur-containing compounds (mercaptans, thiophene, and dibenzothiophene) are also chemisorbed through S atoms on valence-unsaturated Mo atoms located on the faces of MoS2 clusters. [ABSTRACT FROM AUTHOR]

We investigate the minimum-energy path for the rotation of formal C=N double bonds in molecules with guanidine-like substructures as present in the chemical class of neonicotinoids. The transitions between the E- and Z-isomers of several neonicotinoids using scans of the torsional potential energy hypersurfaces are quantified at the DFT-level of theory. The validity of using this ansatz is checked by single-point CCSD(T) calculations for model systems like nitroguanidine. A combined approach of theory and experiment permits to unambiguously identify the relevant isomers present at ambient conditions. As an example, MP2-GIAO predictions of the NMR spectra of E- and Z-Clothianidin are experimentally confirmed by low-temperature NMR-experiments identifying for the first time the hitherto unknown Z-Isomer of Clothianidin. [ABSTRACT FROM AUTHOR]

The role of second-order saddle in the isomerization dynamics was investigated by considering the isomerization of guanidine. The potential energy profile for the reaction was mapped using the ab initio wavefunction method. The isomerization path involved a torsional motion about the imine (C-N) bond in a clockwise or an anticlockwise fashion resulting in two degenerate transition states corresponding to a barrier of 23.67 kcal/mol. An alternative energetically favorable path ( 1 kcal/mol higher than the transition states) by an in-plane motion of the imine (N-H) bond via a second-order saddle point on the potential energy surface was also obtained. The dynamics of the isomerization was investigated by ab initio classical trajectory simulations. The trajectories reveal that isomerization happens via the transition states as well as the second-order saddle. The dynamics was found to be nonstatistical with trajectories exhibiting recrossing and the higher energy second-order saddle pathway preferred over the traditional transition state pathway. Wavelet based time-frequency analysis of internal coordinates indicate regular dynamics and existence of long-lived quasi-periodic trajectories in the phase space. [ABSTRACT FROM AUTHOR]

Although several studies have attempted to deduce by theoretical calculations the reaction mechanism of carmustine with DNA, no study has presented data on the reaction sites and the behavior of the electrons of the participating molecules. Thus, the objective of this paper is to apply the NBO (Natural Bond Orbital) model from the donor-acceptor point of view and the QTAIM (Quantum Theory: Atoms in Molecules) theory from the Laplacian of electronic density and the energy partition formalism IQA (Interacting Quantum Atoms) to quantify the interactions present in the alkylation between carmustine and DNA. The results showed that the decomposition of carmustine forms the chloroethyl ion and the 2-chloroethyldiazene hydroxide molecule, which subsequently breaks down to form 2-chloroethanol and N2. The NBO, QTAIM, and IQA data demonstrated that the ionic carbon of the chloroethyl ion suffers a nucleophilic attack from the O6-guanine. In the alkylation between intermediate 1,O6-ethanoguanine and cytosine, the bond between the CO of the intermediate ring breaks with a subsequent nucleophilic attack of the N of the cytosine with the C of the ring of the molecule 1,O6-ethanoguanine, forming a bridge of ethane between the guanine and cytosine bases. [ABSTRACT FROM AUTHOR]

Based on a recently developed analytical full-dimensional potential energy surface describing the gas-phase O(3P) + C2H6 reaction (Espinosa-Garcia et al. in Phys Chem Chem Phys 22:22,591, 2020), thermal rate constants and kinetic isotope effects (KIEs) were studied in the temperature range 200–3000 K using three different kinetics tools: variational transition-state theory with multidimensional tunnelling corrections (VTST/MT), ring polymer molecular dynamics (RPMD), and quasi-classical trajectory (QCT) calculations. Except the last method, which failed in the description at low temperatures, as was expected due to its classical nature, the other two methods present rate constants with differences between them of 3% at low temperatures and 25% at high temperatures, simulate the experimental measurements in the intermediate temperature range, 500–1000 K, and are intermediate between two reviews of experimental measurements at low and high temperatures, where the experimental evidence shows differences of a factor of about 5. This result shows that both methods capture quantum effects, such as zero-point energy, tunnelling, and recrossing effects. The kinetics of the title reaction presents a non-Arrhenius behavior, where the activation energy increases with temperature. Two KIEs were analyzed. For the H/D isotopes, the KIE decreases with temperature, from 46.55 to 1.18 in the temperature range 200–3000 K, while the 12C/13C KIE presents values close to unity. Unfortunately, no experimental information is available for comparison. [ABSTRACT FROM AUTHOR]

We perform a study of the molecular anisotropy for the H2⋯H2 van der Waals system using a spherical harmonics expansion. We use six leading stable configurations to construct our analytical potential energy surface (PES) from ab initio calculations guided qualitatively by the symmetry-adapted perturbation theory (SAPT) analyses. We extrapolate the energies of the PES performed at the CCSD(T)/aug-cc-pVnZ (n = 2 and 3) levels to the complete basis set (CBS) limit. To best fit the shallow potential energy surface of each leading configuration with the intermolecular distance, it was employed an extended version of the Rydberg potential. To assess the quality of our extrapolated analytical PES, we calculate the second virial coefficients, which are in relatively good agreement with the experimental data. As a result, the spherical harmonics coefficients obtained might be of considerable relevance in spectroscopy and dynamics applications. [ABSTRACT FROM AUTHOR]

Folding of model polypeptide sequences consisting of 50 and 150 amino-acid residues with the alanine and glycine repeat was explored using Langevin dynamics techniques at different values of the viscosity of a virtual medium. The starting conformations corresponded to a totally extended chain structure. The change in the dissipative properties of the medium was simulated under varying parameters of the Langevin thermostat. It is shown that there is a sufficiently high and threshold sensitivity of the result of chain folding to the viscosity of the medium. The folding of the considered polypeptide sequences when the viscosity of the medium is already on the order of viscosity of liquefied gases occurs mainly in alpha-helical conformations. With a decrease in the effective viscosity of the medium below the critical value, disordered structures are formed. It is noteworthy that in a virtual environment energy effects are not an unambiguous criterion for determining conformation as a result of folding of the polypeptide chain. The observed effects of viscosity, which lead to the correlation of conformational motions due to the action of dissipative forces and the selection of possible conformational relaxation paths of the chain, directly affect the folding result in accordance with the predictions of the analytic theory. [ABSTRACT FROM AUTHOR]

The interaction energy of the ozone molecule with atoms of noble gases (Ar and He) is analyzed. The coupled cluster methods CCSD(T) and CCSD(T)-F12 with orbital basis sets including electrons from diffuse orbitals (aug-cc-pVXZ for X = D, T, Q, 5, ... CBS) have been used. The geometrical configurations corresponding to the structures with global minima are established. The full three-dimensional grids of the interaction potentials with more than 4800(5300) points have been fitted for the O3–Ar(He) complexes with root-mean-square values of 0.421(0.567) cm–1. The interaction energy was additionally calculated for the O3–He complex with one internal coordinate r1 of ozone stretched to Δr1 = 0.2 and 0.4a0. [ABSTRACT FROM AUTHOR]

Several two-dimensional (2D) materials such as graphene, molybdenum disulfide, or boron nitride are emerging as alternatives for lubrication additives to control friction and wear at the interface. On the other hand, the initiative to accelerate materials discovery through data-driven computational methods has identified numerous novel topologies and families of 2D materials that can potentially be designed as low-friction additives. Hence, generating a structure–property (friction) correlations for 2D material-based additives that present a large variation in atomic composition is the next big challenge. Herein, we present a machine learning (ML) method using the Bayesian modeling and transfer learning approach to predict the maximum energy barrier (MEB) of the potential surface energy (correlated to intrinsic friction) of ten different 2D materials that were previously unexplored for their tribological properties. The descriptors (or properties) required to train the ML model with high accuracy are identified by taking into account the established physical models for dissipation in 2D materials. As a result, a difference of less than 8% in MEB values as predicted via the ML model presented here and the PES profiles generated using molecular dynamics simulations, for a select few 2D materials, was obtained. The model also enabled the identification of material properties that present the highest sensitivity to the corrugated potential, hence enabling the development of design routes for the synthesis of 2D materials with optimal tribological properties. [ABSTRACT FROM AUTHOR]

Collisions of the metastable antiprotonic helium atoms with the atoms of the medium cause, among other processes, transitions between hyperfine structure (HFS) states, as well as shifts and broadening of microwave M1 spectral lines. To obtain the interaction potential matrix ( , the potential energy surface (PES) is calculated by the unrestricted Hartree–Fock method with consideration of the electron correlations within the second-order of perturbation theory (MP2). With this potential, the system of equations of close coupling of the HFS channels is numerically solved; the cross sections and transition rates, shifts, and broadenings of M1 spectral lines are calculated; they are then used to solve the kinetic equation that determines the time evolution of the HFS state density matrix. The results of these calculations are compared with the experimental data and with the results of model calculations. [ABSTRACT FROM AUTHOR]

In this work, chemical computations were carried out to study of tautomeric conversion of 5-fluorouracil (A → products) at CBS-QB3, M06-2x, and B3LYP levels of theory. These 11 tautomers connected to each other via 10 transition state molecules and four pathways. In addition, the stationary points (including stable structures and transition states), in which playing a significant role on ground potential energy surfaces of isomerization processes, were found. Multichannel RRKM steady-state approximation calculations have been performed to calculate the rate constants at 1 atm pressure and over a range of temperature from 268.15 to 310.15 K to cover the in vivo and atmospheric conditions. The stability ranking of tautomers obtained and demonstrated that di-keto form of 5-fluorouracil is the most stable tautomer. The energy barriers for tautomeric conversions were calculated. The information reported in this study may help with the identification of drug in vivo and in the laboratory. [ABSTRACT FROM AUTHOR]

Herein the performance of a modification within the hybrid algorithm implemented in the AUTOMATON program is introduced and evaluated. For the creation of the initial population, AUTOMATON combines a probabilistic automata procedure with a genetic algorithm used to evolve this population. The proposed modification is addressed to efficiently identify the minimum energy structures of systems composed of more than one type of atom and with a low computational cost. The effectiveness of this approach is evaluated in the determination of the minimum energy structures of Be6B11−. The modification, aimed to explore the potential energy surface, consisted of filling the cells first with Be atoms in the process of creating the initial population. This order obeys the structural pattern established in the Be–B clusters reported to date. The results show that this variation not only identifies a more significant number of viable isomers but also to find a better putative global minimum than those previously reported in the literature. Therefore, it is recommended to be used as a complement to the standard searching process. [ABSTRACT FROM AUTHOR]

In the present research, the nuclear deformation of even–even rare-earth Er–Os isotopes with N = 102 has been calculated using a new empirical equation and an interacting boson model (IBM-1). The potential energy surface, reduced transition probabilities B(E2) and energy level have been calculated for these nuclei. The properties of gamma, beta and yrast band have been also calculated and compared with the available experimental data. The results of both models have been indicated the similarity or dissimilarity with the published experimental data. We have been studied the ratio E γ (I + 2) / E γ (I) versus the spin (I) to determine the character of the ground-state band. From the outcome of our investigation, it is possible to conclude that the 170Er/172Yb/174Hf/176W nuclei show a rotational SU(3) character and 178Os shows X(5) character. Results of this calculation are in good agreement with the corresponding available experimental data. [ABSTRACT FROM AUTHOR]

The kinetics and mechanisms of atmospheric reaction of methanimine (H2CNH) with nitroxyl on the singlet surface were studied by employing DFT, MP2, and CCSD(T) methods along with the 6-311++G(3df, 3pd) and aug-cc-pVTZ basis sets. Also, thermodynamic parameters and rate constants have been computed in the temperature range of 300–3000 K using conventional transition state theory and RRKM theories at MP2/6-311++G(3df,3pd) level. The results show that seven adducts can be produced. Two of them are highly thermodynamically stable (CH4+N2O and HNCNH+H2O). In kinetic point of view, HCN + HNOH and H2CN + H2NO adducts (due to passing one corresponding low-level transition state) are favorable pathways of title reaction. [ABSTRACT FROM AUTHOR]

Machine learning has been applied to various fields and is envisaged as the technology of the future. We discuss here, the applications of machine learning methods to represent potential energy surfaces - an important aspect of chemical dynamics. We illustrate the process of machine learning using simple examples, and demonstrate how it can be extended to complicated problems. [ABSTRACT FROM AUTHOR]

For the first time, a 15-dimensional analytical form was obtained and the potential energy of the SF6 molecule in the ground electronic state was found. An optimal grid of geometries was constructed, which, taking into account the full symmetry of the molecule, unambiguously determines the potential energy surface of the sixth order. Using the MP2 method with the cc-pVTZ base set, the potential energy surface of the fourth order was calculated. [ABSTRACT FROM AUTHOR]

for the first time the question is raised concerning the effect of ion-molecular associations on the structural flexibility of radical ions of aromatic compounds with respect to pseudorotation. It is shown within the DFT method that the complex structure of the potential energy surface and structural flexibility of the Jahn-Teller benzene cation are preserved during the formation of complexes with a hydrogen cyanide or acetonitrile molecule. The pseudorotation barrier of the radical cation in considered complexes depends on the relative orientation of particles and varies from 0.1 kcal/mol to ∼2 kcal/mol. [ABSTRACT FROM AUTHOR]

Isoliquiritigenin, liquiritigen, uralenol, and neouralenol are four natural licorice flavonoid compounds extracted from licorice, which have the ability of scavenging free radicals. In this paper, the density functional theory (DFT) was used to study the microscopic reaction mechanism of the four kinds of licorice flavonoids respectively scavenging ·OOCl3C in vivo. At the level of M06-2X/6-311+G(d,p), the geometries of all stationary points for hydrogen atom transfer (HAT) and radical adduct formation (RAF) pathways were optimized. The thermodynamic parameters and kinetic parameters of each reaction pathway were obtained, and the potential energy surface information of each reaction pathway was obtained too. Using the continuum solvation model based on solute electron density (SMD), the influence of the solvation effect on the reaction was calculated. The main mechanisms and reactive sites for the capture of ·OOCl3C by four licorice flavonoids, liquiritigen, isoliquiritigenin, uralenol, and neouralenol, were determined. Research indicates liquiritigen captures ·OOCl3C by HAT mechanism. The HAT pathway on the B′ ring of liquiritigen is the main reactive site. Isoliquiritigenin captures ·OOCl3C in the body through HAT and RAF mechanism. The RAF pathway is the dominant reaction pathway. The C1 site on the carbon–carbon double bond linking the two benzene rings is the main reactive site. Uralenol and neouralenol capture ·OOCl3C by HAT mechanism. The main reactive site of uralenol is B6 site, and the main reactive site of neouralenol is B′ 2 site. By this study, the potential energy surface information difficult to capture by experimental research is obtained, which provides a reliable theoretical basis for the further screening of highly active natural free radical scavengers of flavonoids in licorice and provides the support for the improvement of the development and application technology of licorice. [ABSTRACT FROM AUTHOR]

Structural parameters, energies, and spectroscopic characteristics of two series of layerwise hydrogenated aluminum clusters Al44Hn (n = 27–44) and Al89Hm (m = 15, 24, 39, and 63) have been calculated by the density functional theory method. It has been shown that increasing number of H atoms in both series entails rapid enhancement of structural distortions up to cooperative rearrangements accompanied by a change in the shape and composition of the surface layer and internal core of the cluster. At the end of the first series Al44Hn, several surface atoms migrate to the outer sphere of the cage to form valence-unsaturated "outer-surface" AlHn and Al2Hn moieties, which can be active sites at the stages of deeper hydrogenation. Simultaneously, the inner core [Al]5 disintegrates, and its atoms are introduced into the surface layer. A family of "inverted" Al42H42 isomers with the hollow [Al42] cage has been localized; the isomers contain the endohedral AlH4 group and "inner" Al-H bonds with their hydrogen end directed to the center of the inner cavity. At the end of the second series, five alanate groups AlH4 and two Al3H2 fragments bonded to the surface through hydrogen bridges are formed in the outer sphere of the A l 89 H 63 − cluster. The results are of interest for DFT modeling of hydrogenation of nanosized aluminum clusters at the molecular level. [ABSTRACT FROM AUTHOR]

The dynamics and kinetics of the reaction of CH3+ cation with atomic oxygen in its lowest triplet and singlet states in the presence of N2 molecules as the bath gas is theoretically investigated. The potential energies over both surfaces at the CCSD(T)/aug-cc-pVTZ level of theory are explored. Both singlet and triplet surfaces characterize by barrierless initiation step. The entrance channel to form the energized adducts are governed by the capture probability once the centrifugal barrier is surmounted. Multiwell-multichannel mechanisms are found for both singlet and triplet potential energy surfaces. One-dimensional chemical master equation is solved to explore the dynamics and kinetics of the title reaction. The fractional populations of the stationary points as function of time are analyzed to determine the role of the energized intermediates on the dynamics of the title reaction. No significant temperature or pressure dependence for the title reaction was observed over a wide range of temperature (300-3000 K) and pressure (0.1-4.5 atm). The results indicate cis and trans-HCOH+, CH2O+, and H atom are the major products for both surfaces. CVT method was used to explore the importance of tunneling in hydrogen transfer isomerization reactions of CH2O+ to trans-HCOH+ and HOC+ to HCO+ using small-curvature approximation. [ABSTRACT FROM AUTHOR]

The method of time resolved magnetic field effects in the recombination fluorescence of spin-correlated radical ion pairs is used to detect for the first time the perfluorobenzocyclobutene radical anion. The quantum chemical analysis of the potential energy surface testifies that the particle is structurally flexible with respect to the pseudo-rotation coordinate. The calculated values of hyperfine interaction constants averaged over global minima are close to those estimated from the experimental data of time resolved magnetic field effects. [ABSTRACT FROM AUTHOR]