Analytical Potential Energy Surface Research Articles

Modeling microsolvation clusters with electronic-structure calculations guided by analytical potentials and predictive machine learning techniques.

We propose a new methodology to study, at the density functional theory (DFT) level, the clusters resulting from the microsolvation of alkali-metal ions with rare-gas atoms. The workflow begins with a global optimization search to generate a pool of low-energy minimum structures for different cluster sizes. This is achieved by employing an analytical potential energy surface (PES) and an evolutionary algorithm (EA). The next main stage of the methodology is devoted to establish an adequate DFT approach to treat the microsolvation system, through a systematic benchmark study involving several combinations of functionals and basis sets, in order to characterize the global minimum structures of the smaller clusters. In the next stage, we apply machine learning (ML) classification algorithms to predict how the low-energy minima of the analytical PES map to the DFT ones. An early and accurate detection of likely DFT local minima is extremely important to guide the choice of the most promising low-energy minima of large clusters to be re-optimized at the DFT level of theory. In this work, the methodology was applied to the Li+Krn (n = 2-14 and 16) microsolvation clusters for which the most competitive DFT approach was found to be the B3LYP-D3/aug-pcseg-1. Additionally, the ML classifier was able to accurately predict most of the solutions to be re-optimized at the DFT level of theory, thereby greatly enhancing the efficiency of the process and allowing its applicability to larger clusters.

Kinetics theoretical study of the O(3P) + C2H6 reaction on an ab initio-based global potential energy surface

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.

Two conformational polymorphs of 4-methylhippuric acid.

4-Methylhippuric acid {systematic name: 2-[(4-methylbenzoyl)amino]ethanoic acid}, a p-xylene excreted metabolite with a backbone containing three rotatable bonds (R-bonds), is likely to produce more than one stable molecular structure in the solid state. In this work, we prepared polymorph I by slow solvent evaporation (plates with Z' = 1) and polymorph II by mechanical grinding (plates with Z' = 2). Potential energy surface (PES) analysis, rotating the molecule about the C-C-N-C torsion angle, shows four conformational energy basins. The second basin, with torsion angles near -73°, agree with the conformations adopted by polymorph I and molecules A of polymorph II, and the third basin at 57° matched molecules B of polymorph II. The energy barrier between these basins is 27.5 kJ mol-1. Superposition of the molecules of polymorphs I and II rendered a maximum r.m.s. deviation of 0.398 Å. Polymorphs I and II are therefore true conformational polymorphs. The crystal packing of polymorph I consists of C(5) chains linked by N-H...O interactions along the a axis and C(7) chains linked by O-H...O interactions along the b axis. In polymorph II, two molecules (A with A or B with B) are connected by two acid-amide O-H...O interactions rendering R22(14) centrosymmetric dimers. These dimers alternate to pile up along the b axis linked by N-H...O interactions. A Hirshfeld surface analysis localized weaker noncovalent interactions, C-H...O and C-H...π, with contact distances close to the sum of the van der Waals radii. Electron density at a local level using the Quantum Theory of Atoms in Molecules (QTAIM) and the Electron Localization Function (ELF), or a semi-local level using noncovalent interactions, was used to rank interactions. Strong closed shell interactions in classical O-H...O and N-H...O hydrogen bonds have electron density highly localized on bond critical points. Weaker delocalized electron density is seen around the p-methylphenyl rings associated with dispersive C-H...π and H...H interactions.

Spherical harmonics representation of the potential energy surface for the H2⋯H2 van der Waals complex.

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.

Regio- and stereochemistry in the intramolecular [4 + 2] and intermolecular [3 + 2] cycloaddition reactions in the synthesis of epoxypyrrolo[3,4-g]indazoles: a density functional theory study

Both intramolecular [4 + 2] and intermolecular [3 + 2] cycloaddition reactions for the synthesis of 1,3-dioxoepoxyisoindole and epoxypyrrolo[3,4-g]indazoles, respectively, experimentally explored by Alizadeh and co-workers, were studied theoretically at the M06-2X/6-311G(d,p) computational level in both gaseous and solution (acetonitrile, benzene and DMSO) phases. In the first stage, it was studied the intramolecular [4 + 2] cycloaddition reaction generating potentially two diastereomeric adducts and it was confirmed the formation of experimentally reported product, 1,3-dioxoepoxyisoindole DEI-1 by the local reactivity indices as well as potential energy surface analysis. In the second stage, the [3 + 2] cycloaddition reaction of DEI-1 with three nitrilimines including NI–H, NI-C and NI–N, which generates potentially four possible regio- and stereoisomeric adducts was studied. The local reactivity indices described satisfactorily the experimentally observed regioselectivities. The results of potential energy surface analysis indicated that among the four possible products, only two exo regioisomeric adducts (namely, epoxypyrrolo[3,4-g]indazoles, CA-x1 and CA-x2) can be formed. In fact, for the reaction of DEI-1 with NI–H, NI-C and NI–H, the ratio of CA-x1:CA-x2 was estimated to be 57:43, 86:14 and 39:61, respectively, which is in satisfactory agreement with the experimental outcomes. The structural variations during the reactions were also studied and the results indicated that the transition states are not completely synchronous.

Degradation of acetaminophen by activated peroxymonosulfate using Co(OH)2 hollow microsphere supported titanate nanotubes: Insights into sulfate radical production pathway through CoOH+ activation

Pharmaceuticals and personal care products (PPCPs) are of great concern due to their increasing health effects, so advanced treatment technologies for PPCPs removal are urgently needed. In this study, titanate nanotubes decorated Co(OH)2 hollow microsphere (CoM/TNTs) composites were synthesized by a two-step solvothermal method, and used to activate peroxymonosulfate (PMS) through heterogenous catalysis for acetaminophen (ACE) degradation in water. The optimum material (CoM/TNTs0.5) activated PMS system exhibited high ACE removal efficiency and quick kinetic, as 93.0% ACE was degraded even within 10 min. The two components in CoM/TNTs showed a synergetic effect on PMS activation for radicals production: Co(OH)+ from CoM was the primary active species to active PMS, while TNTs could offer abundant –OH groups for Co(OH)+ formation. Density functional theory (DFT) calculation further interpreted the mechanism of Co(OH)+ for PMS activation by means of reaction potential energy surface (PES) analysis. Both the scavenger quenching tests and electron paramagnetic resonance analysis revealed that the sulfate radical (SO4-·) played a dominant role in ACE degradation. Moreover, DFT calculation also suggested that the ACE atoms with high Fukui index (f-) represented the active sites for electrophilic attack by SO4-·. The toxicity analysis based on quantitative structure-activity relationship (QSAR) verified the reduced toxicity of transformation products. Furthermore, CoM/TNTs also had good reusability and stability over five cycles. This work provides deep insights into the reaction mechanisms of radical production and organics attack in cobalt-based PMS activation system.

A general code for fitting global potential energy surfaces via CHIPR method: Direct-Fit Diatomic and tetratomic molecules

A general program to fit global adiabatic potential energy surfaces of up to tetratomic molecules to ab initio points and available spectroscopic data for simple diatomics is reported. It is based on the Combined-Hyperbolic-Inverse-Power-Representation (CHIPR) method. The final form describes all dissociating fragments and long-range/valence interactions, while obeying the system permutational symmetry. The code yields as output a Fortran 90 subroutine that readily evaluates the potential and gradient at any arbitrary geometry. Program summaryProgram title: CHIPR-4.0CPC Library link to program files:http://dx.doi.org/10.17632/8wdv87gt5x.2Licensing provisions: GPLv3Programming language: Fortran 90Journal reference of previous version: C. M. R. Rocha, A. J. C. Varandas, Comput. Phys. Commun. 247 (2020) 106913Does the new version supersede the previous version?: YesReasons for the new version: CHIPR-4.0 is a major extension of the previous CHIPR-3.0 code [1] which, besides diatomics and triatomics, also includes the possibility of fitting global adiabatic potential energy surfaces (PESs) of A4-, AB3-, A2B2-, ABC2- and ABCD-type tetratomic molecules [3]. Additionally, a new feature is that it allows the user to fine-tune ab initio diatomic curves using available spectroscopic data.Summary of revisions:• Implementation of CHIPR’s polynomial form and permutation operators for tetratomic molecules• Implementation of a direct-fit module to fine-tune ab initio potential energy curves using spectroscopic data• Automatic global minimum search on the diatomic, triatomic or tetratomic final PESs• Automatic harmonic vibrational analysis at the optimum molecular geometriesNature of problem: This version of the CHIPR code, CHIPR-4.0, provides a set of subroutines to fit global adiabatic potential energy surfaces of up to tetratomic molecules using ab initio and (for diatomics) optional fine-tuning experimental data.Solution method: CHIPR-4.0 uses the Combined-Hyperbolic-Inverse-Power-Representation (CHIPR) [2] method to interpolate and extrapolate ab initio data points.Additional comments including restrictions and unusual features: For triatomic and tetratomic fits, the user must supply subroutines containing the analytic forms defining the sum of two- and two-plus-three-bodies, respectively. These are utilized internally to calculate (from ab initio data) the corresponding three- and four-body energies to be fitted. The current version of this code is strictly based on single-sheeted analytical PESs. High-accuracy in diatomic refinements is currently limited to the 1Σ single-curve case.

Photocatalytic transformation fate and toxicity of ciprofloxacin related to dissociation species: Experimental and theoretical evidences

Chemical speciation of ionizable antibiotics greatly affects its photochemical kinetics and mechanisms; however, the mechanistic impact of chemical speciation is not well understood. For the first time, the impact of different dissociation species (cationic, zwitterionic and anionic forms) of ciprofloxacin (CIP) on its photocatalytic transformation fate was systematically studied in a UVA/LED/TiO2 system. The dissociation forms of CIP at different pH affected the photocatalytic degradation kinetics, transformation products (TPs) formation as well as degradation pathways. Zwitterionic form of CIP exhibited the highest degradation rate constant (0.2217 ± 0.0179 min−1), removal efficiency of total organic carbon (TOC) and release of fluoride ion (F−). Time-dependent evolution profiles on TPs revealed that the cationic and anionic forms of CIP mainly underwent piperazine ring dealkylation, while zwitterionic CIP primarily proceeded through defluorination and piperazine ring oxidation. Moreover, density functional theory (DFT) calculation based on Fukui index well interpreted the active sites of different CIP species. Potential energy surface (PES) analysis further elucidated the reaction transition state (TS) evolution and energy barrier (ΔEb) for CIP with different dissociation species after radical attack. This study provides deep insights into degradation mechanisms of emerging organic contaminants in advanced oxidation processes associated to their chemical speciation.

A global potential energy surface for the ground electronic state of SSiH

An analytical global potential energy surface for the ground electronic state of SSiH is here reported. The function is based upon ab initio energies calculated at Davidson-corrected multi-reference configuration interaction level with the aug-cc-pV(T + d)Z and aug-cc-pV (Q + d)Z basis sets and subsequently extrapolated to the complete basis set (CBS) limit. The analytical representation follows the Aguado and Paniagua many-body expansion methodology. A total of 2487 ab initio points were used in the fitting. The topological features of the so constructed potential are here discussed. A quasi-classical trajectory study for the SH + Si → SSi + H is carried out using the new function. Excitation function, rate constants and details of the reactive collisions are also discussed.

Rational design of BODIPY-carbazole analogues in the context of D-π-A approach for facile charge transport: A DFT/TD-DFT study

In this paper, we have examined the influence of torsional rigidity on the optoelectronic properties by imposing furan, pyrrole and thiophene unit on the BODIPY-carbazole based donor-acceptor systems employing density functional theory (DFT) formalism. We have designed 12 small conjugated molecules based on the donor (carbazole)-acceptor (BODIPY) approach using furan, pyrrole and thiophene unit as the bridging units. To study the torsional rigidity imparted by the bridging units we have performed potential energy surface (PES) analysis. Our study explores that among the bridging units furan and thiophene impart maximum and minimum rigidity on the systems respectively. Different parameters viz. distortion energy (ΔEdis), HOMO-LUMO gap (ΔH−L values), ionization potential (IP), electron affinity (EA), bond length alteration (BLA) parameters, dipole moment values, reorganization energies for holes (λh) and electrons (λe), electronic coupling matrix element (V), charge transfer rate (kct), hopping mobility (μhop), radiative decay rate (kr) etc. have been calculated. The absorption and emission spectra of the BODIPY based compounds have been studied using TD-DFT. NTO analysis have also been performed for the dominant electronic transitions. Our calculations predict that compounds possessing pyrrole unit as the bridging unit and compounds in which BODIPY unit is meso substituted with pyrrole unit possesses greater amount of conjugation and as a result exhibit facile charge transport.

Accurate Global Potential Energy Surfaces for the H + CH3OH Reaction by Neural Network Fitting with Permutation Invariance.

The H + CH3OH reaction, which plays an important role in combustion and the interstellar medium, presents a prototypical system with multi channels and tight transition states. However, no globally reliable potential energy surface (PES) has been available to date. Here we develop global analytical PESs for this system using the permutation-invariant polynomial neural network (PIP-NN) and the high-dimensional neural network (HD-NN) methods based on a large number of data points calculated at the level of the explicitly correlated unrestricted coupled cluster single, double, and perturbative triple level with the augmented correlation corrected valence triple-ζ basis set (UCCSD(T)-F12a/AVTZ). We demonstrate that both machine learning PESs are able to accurately describe all dynamically relevant reaction channels. At a collision energy of 20 kcal/mol, quasi-classical trajectory calculations reveal that the dominant channel is the hydrogen abstraction from the methyl site, yielding H2 + CH2OH. The reaction of this major channel takes place mainly via the direct rebound mechanism. Both the vibrational and rotational states of the H2 product are relatively cold, and large portions of the available energy are converted into the product translational motion.

Effect of substituents in hydroxyl radical-mediated degradation of azo pyridone dyes: Theoretical approaches on the reaction mechanism

Understanding the degradation behavior of azo dyes in photocatalytic wastewater treatment is of fundamental and practical importance for their application in textile-processing and other coloration industries. In this study, quantum chemistry, as density functional theory, was used to elucidate different degradation pathways of azo pyridone dyes in a hydroxyl radical (HO•)-initiated photocatalytic system. A series of substituted azo pyridone dyes were synthesized by changing the substituent group in the para position of the benzene moiety, ranging from strong electron-donating to strong electron-withdrawing groups. The effect of dye molecular structure on the photocatalytic degradation reaction mechanism was analyzed and quantification of substituent effects on the thermodynamic and kinetics parameters was performed. Potential energy surface analysis revealed the most susceptible reaction site for the HO• attack. The calculated reaction barriers are found to be strongly affected by the nature of substituent group with a good correlation using Hammett σp constants and experimentally determined reaction rates. The stability of pre-reaction complexes and transition state complexes were analyzed applying the distortion-interaction model. The increased stability of the transition state complexes with the distancing from the substituent group has been established.

Analytic intermolecular potential energy surface and first-principles prediction of the rotational profiles for a symmetric top ion-atom complex: A case study of H3O+-Ar.

We presented the first three-dimensional (3D) ab initio intermolecular potential energy surface (PES) for the H3O+-Ar complex. The electronic structure computations were carried out at the explicitly correlated coupled cluster theory-F12 with an augmented correlation-consistent triple zeta basis set. Analytic 3D PES was obtained by least-squares fitting the multi-dimensional Morse/Long-Range (mdMLR) potential model to interaction energies, where the mdMLR function form was applied to the nonlinear ion-atom case for the first time. The 3D PES fitting to 1708 points has root-mean-square deviations of 0.19 cm-1 with only 108 parameters for interaction energies less than 500 cm-1. With the 3D PES of the H3O+-Ar complex, we employed the combined radial discrete variable representation/angular finite basis representation method and Lanczos algorithm to calculate rovibrational energy levels. The rotational profiles of the O-H anti-stretching vibrational bands of v3 +(S)←0+ and v3 -(A)←0- for the H3O+-Ar complex were predicted and were in good agreement with the experimental results.

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part II. Kinetic Analysis.

Monolignols are precursor units and primary products of lignin pyrolysis. The currently available global (lumped) and semidetailed kinetic models, however, are lacking the comprehensive decomposition kinetics of these key intermediates in order to advance toward the fundamentally based detailed chemical-kinetic models of biomass pyrolysis. para-Coumaryl alcohol (HOPh-CH═CH-CH2OH, p-CMA) is the simplest of the three basic monolignols containing a typical side-chain double bond and both alkyl and phenolic type OH groups. The two other monomers additionally contain one and two methoxy groups, respectively, attached to the benzene ring. Previously, we developed a detailed fundamentally based mechanism for unimolecular decomposition of p-CMA (as well as its truncated allyl and cinnamyl alcohol models) and explored its reactivity toward H radicals generated during pyrolysis. The reactions of p-CMA with pyrolytic OH radicals is another set of key reactions particularly important for understanding the formation mechanisms of a wide variety of oxygenates in oxygen-deficit (anaerobic) conditions and the role of the lignin side groups in pyrolysis pathways. In Part I of the current study (J. Phys. Chem. A, 2019, 123, 2570-2585), we reported a detailed potential energy (enthalpy) surface analysis of the reaction OH + p-CMA with suggestions for a variety of chemically activated, unimolecular, and bimolecular reaction pathways. In Part II of our work, we provide a detailed kinetic analysis of the major reaction channels to evaluate their significance and possible impacts on product distributions. Temperature- and pressure-dependent rate constants are calculated using the quantum Rice-Ramsperger-Kassel method and the master equation analysis for falloff and stabilization. Enthalpies of formation, entropies, and heat capacities are calculated using density functional theory and higher-level composite methods for stable molecules, radicals, and transition-state species. A significant difference between well depths for the chemically activated adduct radicals, [p-CMA-OH]*, is found for the α- and β-carbon addition reactions to generate the 1,3- and 1,2-diol radicals, respectively. This is due to the synergistic effect from conjugation of the proximal radical center with the aromatic ring and the strong H-bonding interaction between vicinal OH groups in the β-adduct (1,2-diol radical). Both adducts undergo isomerization and low-energy transformations, however, with different kinetic efficiencies because of the difference in stabilization energies. Reaction pathways include dissociation, intramolecular abstraction, atom and group transfers, and elimination. Of particular interest is a roaming-like low-energy dehydration reaction to form O-centered intermediate radicals. The kinetic analysis demonstrated the feasible formation of various products detected in pyrolysis experiments, suggesting that the gas-phase reactions of OH radicals can be a key process to form major products and complex oxygenates during lignin pyrolysis. Our preliminary experiments involving pyrolysis of the vaporized monomers support this basic statement. A novel mechanism for the formation of benzofuran, identified in experimentation, is also provided based on the potential conversions of hydroxyphenylacetaldehyde and corresponding isomers, which are kinetically favored products.

An ab Initio/Transition State Theory Study of the Reactions of Ċ5H9 Species of Relevance to 1,3-Pentadiene, Part II: Pressure Dependent Rate Constants and Implications for Combustion Modeling.

The temperature- and pressure-dependence of rate constants for several radicals and unsaturated hydrocarbons reactions (1,3-C5H8/1,4-C5H8/cyC5H8 + Ḣ, C2H4 + Ċ3H5-a, C3H6 + Ċ2H3) are analyzed in this paper. The abstraction reactions of these systems are also calculated and compared with available literature data. Ċ5H9 radicals can be produced via Ḣ atom addition reactions to the pentadiene isomers and cyclopentene, and also by H-atom abstraction reactions from 1- and 2-pentene and cyclopentane. Comprehensive Ċ5H9 potential energy surface (PES) analyses and high-pressure limiting rate constants for related reactions have been explored in part I of this work ( J. Phys. Chem. A 2019, 123 (22), 9019-9052). In this work, a chemical kinetic model is constructed based on the computed thermochemistry and high-pressure limiting rate constants from part I, to further understand the chemistry of different C5H8 molecules. The most important channels for these addition reactions are discussed in the present work based on reaction pathway analyses. The dominant reaction pathways for these five systems are combined together to generate a simplified Ċ5H9 PES including nine reactants, 25 transition states (TSs), and nine products. Spin-restricted single point energies are calculated for radicals and TSs on the simplified PES at the ROCCSD(T)/aug-cc-pVTZ level of theory with basis set corrections from MP2/aug-cc-pVXZ (where X = T and Q). Temperature- and pressure-dependent rate constants are calculated using RRKM theory with a Master Equation analysis, with restricted energies used for minima on the simplified Ċ5H9 PES and unrestricted energies for other species, over a temperature range of 300-2000 K and in the pressure range 0.01-100 atm. The rate constants calculated are in good agreement with those in the literature. The chemical kinetic model is updated with pressure-dependent rate constants and is used to simulate the species concentration profiles for Ḣ atom addition to cyclopentane and cyclopentene. Through detailed analyses and comparisons, this model can reproduce the experimental measurements of species qualitatively and quantitatively with reasonably good agreement.

Automatically growing global reactive neural network potential energy surfaces: A trajectory-free active learning strategy.

An efficient and trajectory-free active learning method is proposed to automatically sample data points for constructing globally accurate reactive potential energy surfaces (PESs) using neural networks (NNs). Although NNs do not provide the predictive variance as the Gaussian process regression does, we can alternatively minimize the negative of the squared difference surface (NSDS) given by two different NN models to actively locate the point where the PES is least confident. A batch of points in the minima of this NSDS can be iteratively added into the training set to improve the PES. The configuration space is gradually and globally covered without the need to run classical trajectory (or equivalently molecular dynamics) simulations. Through refitting the available analytical PESs of H3 and OH3 reactive systems, we demonstrate the efficiency and robustness of this new strategy, which enables fast convergence of the reactive PESs with respect to the number of points in terms of quantum scattering probabilities.

Crystal structure, vibrational, electrical, optical and DFT study of C2H10N2(IO3)2.HIO3

The reinvestigation of the EDA–HIO3–H2O system using a different stoichiometric ratio gives rise to a new iodate salt C2H10N2(IO3)2.HIO3 denoted as EBIMIA. In this study, we reported the structural properties of ethylenediammonium bis iodate mono iodic acid using X-ray powder and single crystal diffraction at room temperature. The Hirshfeld and the potential energy surface analysis reveal that I⋯O and N–H⋯O are the most noticeable interactions that took place inside the crystal and contribute to the cohesion and stability of the synthesized compound. The DSC measurement shows that this iodate salt undergoes two structural phase transitions, the first occurs at T = 290 K while the second occurs at T = 363 K. However, the dielectric analysis confirms only the second transition because it lies in the studied temperature domain 338–413K. Besides, the impedance data obey a circuit model consisting of a parallel combination of a bulk resistance and CPE. The frequency dispersion of the conductivity follows Jonscher’s law and the charge carrier transport may be interpreted using the correlation barrier hopping mechanism (CBH). Finally, the electronic properties and the vibrational analysis of this novel iodate salt are studied using DFT and compared to the experimental data given by the FT-IR, Raman and UV–visible spectroscopies.

Understanding the mechanism of [3 + 2] cycloaddition reaction of benzoisothiazole-2,2-dioxide-3-ylidene with nitrones

A theoretical investigation was carried out on the mechanism and regioselectivity of the [3 + 2] cycloaddition (32CA) reaction of benzoisothiazole-2,2-dioxide-3-ylidene with nitrone using Molecular Electron Density Theory through DFT calculations at B3LYP and wB97xD functionals with 6-31G(d,p) basis set in the gas phase and toluene. Hard and soft acids and bases principle, Parr functions, as well as potential energy surface analysis were utilized for studying the regio- and stereoselectivity of this reaction, and the obtained results are consistent with the experimental observations. The electron localization function analysis of the nitrone shows that there is a double bond between N and C in the absence of any carbenoid or pseudoradical center. Therefore, a zwitterionic structure for nitrone 2 can be suggested, expected that its 32CA reaction is zw-type. The noncovalent interactions topological analysis shows that the first sigma bond has been formed before reaching the TS, which is in line with a two-stage one-step mechanism.

Exploring the Nature of the Energy Barriers on the Mechanism of the Zirconocene-Catalyzed Ethylene Polymerization: A Quantitative Study from Reaction Force Analysis

Ethylene polymerization mediated by methyl-bis(cyclopentadienyl)-zirconium or zirconocene catalyst, [ZrCp₂CH₃]⁺, is one of the most popular catalytic reaction for polyethylene production. Rationalizing the major effects that control the polymer growth result in a challenge for computational studies. Through quantum chemical calculations, we characterized the zirconocene ethylene polymerization reaction mechanism: chain initiation (I; first ethylene insertion) [ZrCp₂CH₂CH₂CH₃]⁺, chain propagation (P; from second (P₁) to ninth (P₉) ethylene insertion) [ZrCp₂ (CH₂)₂₀CH₃]⁺, and chain termination processes (T; β-hydrogen elimination from P₅ or P₉) [ZrHCp₂ (H₂C═CH(CH₂)₁₈CH₃]⁺ are analyzed through the potential energy surface (PES) and reaction force analysis (RFA). The RFA approach involves pulling out the portion of an activation barrier that corresponds to distorting reactants into the geometries they adopt in a transition state structure until it reaches the structural relaxation toward the equilibrium geometry of the product. Because the interactions between the zirconocene and the ethylene molecule are influenced by a combination of several kinds of steric and electronic effects, it is indispensable to understanding these interactions in order to rationalize and predict in a quantitative manner the reaction barrier heights and the concomitant polymer growth. In the present work, we employ a simple procedure within the framework of the RFA and the density functional steric energy decomposition analysis (EDA) approach to quantitatively separate the different types of interactions; steric (ΔE_s), electrostatic (ΔE_e), and quantum (ΔE_q) effects in order to predict the impact of each factor on the course of the polymerization process as well as for the polymer control and design.

Ab initio relativistic potential energy surfaces of benzene-Xe complex with application to intermolecular vibrations.

The benzene-Xe (BXe) complex in its electronic ground state is studied using ab initio methods. Since this complex contains the heavy Xe atom, the relativistic effects cannot be neglected. We test two different approaches that describe the scalar relativistic effects in the framework of the coupled-cluster level of theory with single, double, and perturbative triple excitations, used for the interaction energy calculations. The first one is based on the small core pseudopotential (PP), and the second one is based on the explicit treatment of scalar relativistic effects using the Douglas-Kroll-Hess (DKH) Hamiltonian. A few basis sets are tested with the PP and DKH, and for each one, the analytical potential energy surface (PES) is constructed. It is shown that the difference between PESs determined with PP and DKH methods is small, if the orbitals of the 4d subshell in Xe are correlated. We select the most appropriate approach for the calculation of the potential energy surface of BXe, with respect to accuracy and computational cost. The optimal level of theory includes a small Dunning's basis set for the benzene monomer and a larger PP basis set for Xe supplemented by midbond functions. The PES obtained using such an approach provides a reasonable accuracy when compared to the empirical one derived from the microwave spectra of BXe. The empirical and the theoretical values of intermolecular vibrational energies agree within 0.5 cm-1 up to second overtones. The vibrational energy level pattern of BXe is characterized by a distinct polyad structure.