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Vibrational spectroscopy enables critical insight into the structural and dynamic properties of molecules. Presently, the majority of theoretical approaches to spectroscopy employ wavefunction‐based ab initio or density functional methods that rely on the harmonic approximation. This approximation breaks down for large molecules with strongly anharmonic bonds or for molecules with large internuclear separations. An alternative to these methods involves generating molecular anharmonic potential energy surfaces (potentials) and using them to extrapolate the vibrational frequencies. This study examines the efficacy of density functional theory (DFT) and the correlation consistent Composite Approach (ccCA) in generating anharmonic frequencies from potentials of small main group molecules. Vibrational self‐consistent field Theory (VSCF) and post‐VSCF methods were used to calculate the fundamental frequencies of these molecules from their potentials. Functional choice, basis set selection, and mode‐coupling are also examined as factors in influencing accuracy. The absolute deviations for the calculated frequencies using potentials at the ccCA level of theory were lower than the potentials at the DFT level. With DFT resulting in bending modes that are better described than those of ccCA, a multilevel DFT:ccCA approach where DFT potentials are used for single vibrational mode potentials and ccCA is used for vibrational mode‐mode couplings can be utilized for larger polyatomic systems. The frequencies obtained with this multilevel approach using VCIPSI‐PT2 were closer to experimental frequencies than the scaled harmonic frequencies, indicating the success of utilizing post‐VSCF methods to generate more accurate representations of computed infrared spectra. [ABSTRACT FROM AUTHOR]

AbstractDensity functional theory was employed to investigate methylidyne radical reaction on to the unsaturation of dipyrromethanol moiety through cycloaddition forming a six membered di(pyridin-2-yl)methanol followed by H-elimination via ring expansion. Time-dependent density functional theory (TD-DFT) simulations were performed to figure out the ultraviolet–visible spectrum of dipyrromethanol and di(pyridin-2-yl)methanol. Ultraviolet–visible spectrum of dipyrromethanol and di(pyridin-2-yl)methanol shows that two sharp absorption peaks were obtained at λmax = 161 nm and 206 nm for the reactant dipyrromethanol while a sharp peak was observed for the product di(pyridin-2-yl)methanol at shorter wavelength (λmax = 170 nm). The theoretical IR spectrum of dipyrromethanol and di(pyridin-2-yl)methanol shows that a significant red shift of 129 cm−1 was observed in the case of di(pyridin-2-yl)methanol due to presence of two strong intra-molecular hydrogen bonds, which indicates its higher stability than the dipyrromethanol. HOMO–LUMO energy gap calculations and natural bond orbital (NBO) analysis endorse the type of electronic transition for the desired product di(pyridin-2-yl)methanol is π–π*. The global reactivity analysis shows the electron donor and acceptor nature of dipyrromethanol and methylidyne radical, respectively. A potential energy surface is constructed via methylidyne radical addition, ring expansion, and hydrogen elimination transition states. Thermodynamics study and potential energy surface of the title reaction indicates its highly exothermicity and spontaneous in nature. [ABSTRACT FROM AUTHOR]

An accurate interatomic potential energy surface for ground state of SH2− is reported at the multireference configuration interaction level of theory using aug–cc–pVQZ basis set. The calculated ab initio energy points are utilized, after semi-empirically correcting the dynamical correlation by the many-body expansion-scaled external correlation method. To verify the accuracy of the potential energy surface, molecular reaction dynamics, including the relationship between reaction probability and collision energy, thermal rate constant, and integral scattering cross section are calculated by using the quantum time-dependent wave packet method. The final results show that our potential energy surface is accurate and the molecular reaction dynamics results are satisfactory, and the accurate potential energy surface and its dynamics results can also lay a foundation for the future experimental research of SH−-containing molecules. This study provides a reasonable ionic chemical dissociation channel for the dynamics, A global many-body expansion (MEB) PES is reported by fitting high-precision ab initio energies calculated at the multireference configuration interaction level with Davidson correction (MRCI(Q)) and the aug-cc-pVQZ basis set (AVQZ). By comparing with the integral cross section with experimental result, which have agreement with the previous experimental and theoretical values. Our theoretical results can provide effective support for future dynamic experiments. [ABSTRACT FROM AUTHOR]

Recently, surface‐hopping ab initio molecular dynamics (SH‐AIMD) simulations have come to be used to discuss the mechanisms and dynamics of excited‐state chemical reactions, including internal conversion and intersystem crossing. In dynamics simulations involving intersystem crossing, there are two potential energy surfaces (PESs) governing the motion of nuclei: PES in a spin‐pure state and PES in a spin‐mixed state. The former gives wrong results for molecular systems with large spin‐orbit coupling (SOC), while the latter requires a potential gradient that includes a change in SOC at each point, making the computational cost very high. In this study, we systematically investigate the extent to which the magnitude of SOC affects the results of the spin‐pure state‐based dynamics simulations for the hydride MH2 (M = Si, Ge, Sn, Pb) by performing SH‐AIMD simulations based on spin‐pure and spin‐mixed states. It is clearly shown that spin‐mixed state PESs are indispensable for the dynamics simulation of intersystem crossing in systems containing elements Sn and Pb from the fifth period onward. Furthermore, in addition to the widely used Tully's fewest switches (TFS) algorithm, the Zhu‐Nakamura (ZN) global switching algorithm, which is computationally less expensive, is applied to SH for comparison. The results from TFS‐ and ZN‐SH‐AIMD methods are in qualitative agreement, suggesting that the less expensive ZN‐SH‐AIMD can be successfully utilized to investigate the dynamics of photochemical reactions based on quantum chemical calculations. [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 potential energy surfaces and photoelectron spectra of the interstellar molecule H2S and its cation H2S+ were calculated using ab initio methods based on the MOLPRO software package. First, the potential energy surfaces of H2S expanded along the normal coordinates were obtained at the（U）CCSD/cc-pVQZ level of theory. The potential energy surfaces visually describe the effect of different vibrational modes coupling on the molecular energy change, and the combined effect of the asymmetric stretching vibration of the S-H bond and the in-plane bending vibration makes the potential energy change range of the system significantly larger. The vibrational multi-reference configuration interaction method was used to calculate the anharmonic vibrational frequencies and vibrational spectrum, and the calculations showed that strong Fermi resonances appear between the overtones and combination bands, resulting in a significant enhancement of the infrared intensity at the corresponding bands. Finally, the photoelectron spectrum of H2S X¹A1→ H2S+ X²B1 was calculated for the first time using the Raman wave function and the contracted invariant Krylov subspaces method. This study will help to further understand the internal structure of interstellar molecules and provide a reference for experimental studies and interstellar observations. [ABSTRACT FROM AUTHOR]

Transition state (TS) on the potential energy surface (PES) plays a key role in determining the kinetics and thermodynamics of chemical reactions. Inspired by the fact that the dynamics of complex systems are always driven by rare but significant transition events, we herein propose a TS search method in accordance with the Q‐learning algorithm. Appropriate reward functions are set for a given PES to optimize the reaction pathway through continuous trial and error, and then the TS can be obtained from the optimized reaction pathway. The validity of this Q‐learning method with reasonable settings of Q‐value table including actions, states, learning rate, greedy rate, discount rate, and so on, is exemplified in 2 two‐dimensional potential functions. In the applications of the Q‐learning method to two chemical reactions, it is demonstrated that the Q‐learning method can predict consistent TS and reaction pathway with those by ab initio calculations. Notably, the PES must be well prepared before using the Q‐learning method, and a coarse‐to‐fine PES scanning scheme is thus introduced to save the computational time while maintaining the accuracy of the Q‐learning prediction. This work offers a simple and reliable Q‐learning method to search for all possible TS and reaction pathway of a chemical reaction, which may be a new option for effectively exploring the PES in an extensive search manner. [ABSTRACT FROM AUTHOR]

The status of machine learning in chemistry and related disciplines is becoming increasingly prominent, but its application in undergraduate chemistry experiments has yet to be seen. Existing computational chemistry experiments mostly focus on the application of quantum chemistry methods to study molecular properties and chemical reactions. In order to popularize machine learning as a powerful tool, this article will introduce a comprehensive computational chemistry experiment suitable for senior undergraduate students or graduate students. Through this experiment, students can gain a preliminary understanding of the basic principles and operational procedures of machine learning, while also develop their ability to apply quantum chemistry and machine learning to solve chemical problems. [ABSTRACT FROM AUTHOR]

SO2 (Sulfur dioxide) is the major precursor to the production of sulfuric acid (H2SO4), contributing to acid rain and atmospheric aerosols. Sulfuric acid formed from SO2 generates light-reflecting sulfate aerosol particles in the atmosphere. This property has prompted recent geoengineering proposals to inject sulfuric acid or its precursors into the Earth’s atmosphere to increase the planetary albedo to counteract global warming. SO2 oxidation in the atmosphere by the hydroxyl radical HO to form HOSO2 is a key rate-limiting step in the mechanism for forming acid rain. However, the dynamics of the HO + SO2 → HOSO2 reaction and its slow rate in the atmosphere are poorly understood to date. Herein, we use photoelectron spectroscopy of cryogenically cooled HOSO2-anion to access the neutral HOSO2 radical near the transition state of the HO+SO2 reaction. Spectroscopic and dynamic calculations are conducted on the first ab initio-based full-dimensional potential energy surface to interpret the photoelectron spectra of HOSO2-and to probe the dynamics of the HO + SO2 reaction. In addition to the finding of a unique pre-reaction complex (HO⋯SO2) directly connected to the transition state, dynamic calculations reveal that the accessible phase space for the HO + SO2 → HOSO2 reaction is extremely narrow, forming a key reaction bottleneck and slowing the reaction rate in the atmosphere, despite the low reaction barrier. This study underlines the importance of understanding the full multidimensional potential energy surface to elucidate the dynamics of complex bimolecular reactions involving polyatomic reactants. [ABSTRACT FROM AUTHOR]

The impact of the chalcogen atomic electronegativity (O, S, and Se atoms) of new organic molecules on excited-state dynamical reactions is self-evident. Inspired by this kind of distinguished photochemical characteristic, in this work, we performed a computational investigation of chalcogen-substituted 3,6-bis(4,5-dihydroxyoxazo-2-yl)benzene-1,2-diol (BDYBD) derivatives (i.e., BDYBD-O, BDYBD-S, and BDYBD-Se). In this paper, we pay close attention to characteristic BDYBD derivatives that contain intramolecular double hydrogen bonds (O1–H2···N3 and O4–H5···N6). The main goal of this study was to explore how changes in atomic electronegativity affect the way hydrogen bonds interact and how excited molecules affect transfer protons. We go into further detail in the main text of the paper. By fixing our attention to geometrical variations and infrared (IR) vibrational spectra between the S0 and S1 states, exploring hydrogen bonding behaviors using the core-valence bifurcation (CVB) index, and simulating hydrogen bonding energy (EHB) via the atom in molecule (AIM) method, we clarified the photo-induced strengthened dual hydrogen bonding interactions that facilitate the excited-state dual-proton transfer (ESDPT) behavior of BDYBD derivatives. The reorganization of charge stemming from photoexcitation further verifies the tendencies of ESDPT reactions. We relied on constructing potential energy surfaces (PESs) by adopting a restrictive optimization approach, and herein, we finally clarify the gradual ESDPT mechanism of BDYBD derivatives. Particularly, we confirm that the variation in chalcogen atomic electronegativity has a regulatory effect on the ESDPT behavior of BDYBD derivatives; that is, the lower the atomic electronegativity, the more favorable it is for the gradual ESDPT reaction. [ABSTRACT FROM AUTHOR]

In the teaching of university-level physical chemistry, the Arrhenius formula elucidates the relationship between the rate constant of a reaction, temperature, and activation energy. However, it is difficult for students to understand and master the formula simply from its expression. Taking the reaction of isopropyl nitrate with Cl atoms, OH and NO3 radicals as examples, the rate constants of the reactions at different temperatures were calculated based on the potential energy surface by the Transition State Theory. The change trends of rate constants with temperature for the reactions with different energy barriers have been visually analyzed. We hope to help students understand Arrhenius formula in essence by analyzing specific cases, so as to broaden their knowledge and cultivate their scientific inquiry ability. [ABSTRACT FROM AUTHOR]

Hydrogen spillover, involving the surface migration of dissociated hydrogen atoms from active metal sites to the relatively inert catalyst support, plays a crucial role in hydrogen‐involved catalytic processes. However, a comprehensive understanding of how H atoms are driven to spill over from active sites onto the catalyst support is still lacking. Here, we examine the atomic‐scale perspective of the H spillover process on a Pt/Cu(111) single atom alloy surface using machine‐learning accelerated molecular dynamics calculations based on density functional theory. Our results show that when an impinging H2 dissociates at an active Pt site, the Pt atom undergoes deactivation due to the dissociated hydrogen atoms that attach to it. Interestingly, collisions between H2 and sticking H atoms facilitate H spillover onto the host Cu, leading to the reactivation of the Pt atom and the realization of a continuous H spillover process. This work underscores the importance of the interaction between gas molecules and adsorbates as a driving force in elucidating chemical processes under a gaseous atmosphere, which has so far been underappreciated in thermodynamic studies. [ABSTRACT FROM AUTHOR]

Hydrogen spillover, involving the surface migration of dissociated hydrogen atoms from active metal sites to the relatively inert catalyst support, plays a crucial role in hydrogen‐involved catalytic processes. However, a comprehensive understanding of how H atoms are driven to spill over from active sites onto the catalyst support is still lacking. Here, we examine the atomic‐scale perspective of the H spillover process on a Pt/Cu(111) single atom alloy surface using machine‐learning accelerated molecular dynamics calculations based on density functional theory. Our results show that when an impinging H2 dissociates at an active Pt site, the Pt atom undergoes deactivation due to the dissociated hydrogen atoms that attach to it. Interestingly, collisions between H2 and sticking H atoms facilitate H spillover onto the host Cu, leading to the reactivation of the Pt atom and the realization of a continuous H spillover process. This work underscores the importance of the interaction between gas molecules and adsorbates as a driving force in elucidating chemical processes under a gaseous atmosphere, which has so far been underappreciated in thermodynamic studies. [ABSTRACT FROM AUTHOR]

Central‐atom effects on bimolecular nucleophilic substitution (SN2) reactions are well‐known in chemistry, however, the atomic‐level SN2 dynamics at phosphorous (P) centers has never been studied. We investigate the dynamics of the F−+PH2Cl reaction with the quasi‐classical trajectory method on a novel full‐dimensional analytical potential energy surface fitted on high‐level ab initio data. Our computations reveal intermediate dynamics compared to the F−+CH3Cl and the F−+NH2Cl SN2 reactions: phosphorus as central atom leads to a more indirect SN2 reaction with extensive complex‐formation with respect to the carbon‐centered one, however, the title reaction is more direct than its N‐centered pair. Stereospecificity, characteristic at C‐center, does not appear here either, due to the submerged front‐side‐attack retention path and the repeated entrance‐channel inversional motion, whereas the multi‐inversion mechanism discovered at nitrogen center is also undermined by the deep Walden‐well. At low collision energies, 6 % of the PH2F products form with retained configuration, mostly through complex‐mediated mechanisms, while this ratio reaches 24 % at the highest energy due to the increasing dominance of the direct front‐side mechanism and the smaller chance for hitting the deep Walden‐inversion minimum. Our results suggest pronounced central‐atom effects in SN2 reactions, which can fundamentally change their (stereo)dynamics. [ABSTRACT FROM AUTHOR]

A theoretical approach based on Periodic Boundary Conditions (PBC) and a Linear Combination of Atomic Orbitals (LCAO) in the framework of the density functional theory (DFT) is used to investigate the molecular mechanism that rules the piezoelectric behavior of poly(vinylidene fluoride) (PVDF) polymer in the crystalline β-phase. We present several computational tests highlighting the peculiar electrostatic potential energy landscape the polymer chains feel when they change their orientation by a rigid rotation in the lattice cell. We demonstrate that a rotation of the permanent dipole through chain rotation has a rather low energy cost and leads to a lattice relaxation. This justifies the macroscopic strain observed when the material is subjected to an electric field. Moreover, we investigate the effect on the molecular geometry of the expansion of the lattice parameters in the (a, b) plane, proving that the rotation of the dipole can take place spontaneously under mechanical deformation. By band deconvolution of the IR and Raman spectra of a PVDF film with a high content of β-phase, we provide the experimental phonon wavenumbers and relative band intensities, which we compare against the predictions from DFT calculations. This analysis shows the reliability of the LCAO approach, as implemented in the CRYSTAL software, for calculating the vibrational spectra. Finally, we investigate how the IR/Raman spectra evolve as a function of inter-chain distance, moving towards the isolated chain limit and to the limit of a single crystal slab. The results show the relevance of the inter-molecular interactions on the vibrational dynamics and on the electro-optical features ruling the intensity pattern of the vibrational spectra. [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]

Photophysical properties of a series of bis(arylydene)cycloalkanone dyes with various donor substituents are studied using quantum chemistry. Their capacity for luminescence and nonradiative relaxation through trans–cis isomerization is related to their structure, in particular, to the donor capacity of the substituents and the degree of conjugation due to the central cycloalkanone moiety. It is shown that cyclohexanone central moiety introduces distortions and disrupts the conjugation, thus leading to a nonmonotonic change in their properties. The increasing donor capacity of the substituents causes increase in the HOMO energy (rise in the oxidation potential) and decrease in the HOMO–LUMO gap, which results in the red shift of the absorption spectra. The ability of the excited dye to relax through fluorescence or through trans–cis isomerization is governed by the height of the barrier between the Franck–Condon and S1–S0 conical intersection regions on the potential energy surface of the lowest π-π* excited state. This barrier also correlates with the donor capacity of the substituents and the degree of conjugation between the central and donor moieties. The calculated fluorescence and trans–cis isomerization rates are in good agreement with the observed fluorescence quantum yields. [ABSTRACT FROM AUTHOR]

The fundamental unit for comprehending the physicochemical properties of water, the Zundel cation configuration H5O2+, has yet to be exhaustively evaluated in terms of its interaction with terahertz (THz) electromagnetic waves, characterized by sub-picosecond oscillation periods or pulse widths. In this study, we embark on an investigation of the broad resonance and high-field nonresonant effects of intense THz radiation (ITR) on Zundel cations, utilizing a multifaceted methodological approach that includes density functional theory (DFT) calculations, finite difference time domain (FDTD) algorithm of the Schrödinger equation, and ab initio molecular dynamics (AIMD) simulations. Our analysis reveals that the proton potential energy surface (PES) varies in response to the external electric (E) field, suggesting that the interaction frequency of the central proton with the electromagnetic wave encompasses the THz band. This resonance effect is associated with proton behavior that may oscillate or demonstrate periodic tunneling. Moreover, our work uncovers the high-field nonresonant effects of ITR on Zundel cations, manifesting in proton transfer and vibrational excitation of the system. Our findings contribute to the understanding of the interaction between Zundel species and electromagnetic waves by presenting a microscopic view of proton transfer as informed by wavefunction evolution. [ABSTRACT FROM AUTHOR]

Based on an analytical full‐dimensional potential energy surface (PES), named PES‐2022, fitted to high‐level ab initio calculations previously developed by our group and specifically developed to describe this polyatomic reactive process, an exhaustive kinetics analysis was performed in the temperature range 50–2000 K, that is, interstellar, atmospheric and combustion conditions. Using the competitive canonical unified theory with multidimensional tunneling corrections of small curvature, CCUS/SCT, and low‐ and high‐pressure limit (LPL and HPL) models, in this wide temperature range we found that the overall rate constants increase with temperature at T > 300 K and T < 200 K, showing a V‐shaped temperature dependence, reproducing the experimental evidence when the HPL model was used. The increase of the rate constant with temperature at low temperatures was due to the strong contribution of the tunneling factor. The title reaction evolves by two paths, H2O + CH2OH (R1) and H2O + CH3O (R2), and the branching ratio analysis showed that the R2 path was dominant at T < 200 K while the R1 path dominated at T > 300 K, with a turnover temperature of ∼260 K, in agreement with previous theoretical estimations. Three kinetics isotope effects (KIEs), 13CH3OH, CH318OH, and CD3OH, were theoretically studied, reproducing the experimental evidence. The kinetics analysis in the present paper together with the dynamics study previously reported showed the capacity of the PES‐2022 to understand this important chemical process. [ABSTRACT FROM AUTHOR]

Inspired by the distinguished regulated photochemical and photophysical properties of 2-(2′-hydroxyphenyl)benzazole derivatives, in this work, the novel bis(2′-benzothiazolyl)hydroquinone (BBTHQ) fluorophore is explored, looking at its photo-induced behaviors associated with different substituted atomic electronegativities, i.e., BBTHQ-SO, BBTHQ-SS and BBTHQ-Se compounds. From the structural changes, infrared (IR) vibrational variations and simulated core-valence bifurcation (CVB) indexes for the dual hydrogen bonds for the three BBTHQ derivatives, we see that low atomic electronegativity could be conducive to enhancing hydrogen bonding effects in the S1 state. Particularly, the O4-H5⋯N6 of BBTHQ-SO and the O1-H2⋯N3 of BBTHQ-SSe could be strengthened to be more intensive in the S1 state, respectively. Looking into the charge recombination induced by photoexcitation, we confirm a favorable ESDPT trend deriving from the charge reorganization of the dual hydrogen bonding regions. By constructing the potential energy surfaces (PESs) along with the ESDPT paths for the BBTHQ-SO, BBTHQ-SS and BBTHQ-Se compounds, we not only unveil stepwise ESDPT behaviors, but also present an atomic electronegativity-regulated ESDPT mechanism. [ABSTRACT FROM AUTHOR]

In this work the rovibrational spectrum of the acetylide anion HCC − is investigated using high-level electronic structure methods and variational rovibrational calculations. Using a composite approach the potential energy surface and dipole surface is constructed from explicitly correlated coupled-cluster accounting for corrections due to core-valence correlation, scalar relativistic effects and higher-order excitation effects. Previous approaches for approximating the latter are critically evaluated. Employing the composite potential, accurate spectroscopic parameters determined from variational calculations are presented. In comparison to the few available reference data the present results show excellent agreement with ground state rotational constants within 0.005% of the experimental value. Intensities determined from the variational calculations suggest the bending fundamental transition ν 2 around 510 cm − 1 to be the best target for detection. The rather weak CD stretching fundamental ν 1 in deuterated isotopologues show a second-order resonance with the (0 , 2 0 , 1) state and the consequences are discussed in some detail. The spectroscopic parameters and band intensities provided for a number of vibrational bands in isotopologues of the acetylide anion should facilitate future spectroscopic investigations. [ABSTRACT FROM AUTHOR]

The reaction HO + SO → H + SO2 (Rt) and its reverse (R‐t) play an important role in environment and the combustion of sulfur‐containing fuels. However, their kinetics is of high uncertainty as its reaction profile is complicated with multiple deep complexes and channels. In this work, the kinetics and mechanisms of Rt and R‐t are studied comprehensively based on a newly developed full‐dimensional accurate potential energy surface (PES) with the aid of machine learning. This highly accurate PES is interfaced with the software Gaussian. Then reliable information, including the energy, structures, and vibrational frequencies of the stationary points, as well as the minimum energy path and variational analysis can be efficiently determined. The variational transition state theory (VTST) and Rice−Ramsperger−Kassel−Marcus (RRKM) theory are employed to obtain the rate coefficients of each elementary reaction. The temperature‐ and pressure‐dependent rate coefficients of Rt are derived by the RRKM‐based master equation with hindered rotor and free rotor model considered. In addition, the effect of isotope substitution for the hydrogen is investigated on the reaction kinetics. Meanwhile, the quasi‐classical trajectory (QCT) calculation is performed on the PES‐2020 to obtain the temperature‐dependent reaction kinetics. [ABSTRACT FROM AUTHOR]

We demonstrate that a single polycyclic π‐scaffold can undergo sequential multistep excited‐state structural evolution along the bent, planar, and twisted conformers, which coexist to produce intrinsic multiple fluorescence emissions in room‐temperature solution. By installing a methyl or trifluoromethyl group on the ortho‐site of N,N′‐diphenyl‐dihydrodibenzo[a,c]phenazine (DPAC), the enhanced steric effects change the fluorescence emission of DPAC from a dominant red band to well‐resolved triple bands. The ultra‐broadband triple emissions of ortho‐substituted DPACs range from ≈350 to ≈850 nm, which is unprecedented for small fluorophores with molecular weight of <500. Ultrafast spectroscopy and theoretical calculations clearly reveal that the above dramatic changes originate from the influence of steric hindrance on the shape of excited state potential energy surface (S1 PES). Compared to the steep S1 PES of parental DPAC, the introduction of ortho‐substituent is shown to make the path of structural evolution in S1 wider and flatter, so the ortho‐substituted derivatives exhibit slower structural transformations from bent to planar and then to twisted forms, yielding intrinsic triple emission. The results provide the proof of concept that the bent, planar, and twisted emissive states can coexist in the same S1 PES, which greatly expand the fundamental understanding of the excited‐state structural relaxation. [ABSTRACT FROM AUTHOR]

We demonstrate that a single polycyclic π‐scaffold can undergo sequential multistep excited‐state structural evolution along the bent, planar, and twisted conformers, which coexist to produce intrinsic multiple fluorescence emissions in room‐temperature solution. By installing a methyl or trifluoromethyl group on the ortho‐site of N,N′‐diphenyl‐dihydrodibenzo[a,c]phenazine (DPAC), the enhanced steric effects change the fluorescence emission of DPAC from a dominant red band to well‐resolved triple bands. The ultra‐broadband triple emissions of ortho‐substituted DPACs range from ≈350 to ≈850 nm, which is unprecedented for small fluorophores with molecular weight of <500. Ultrafast spectroscopy and theoretical calculations clearly reveal that the above dramatic changes originate from the influence of steric hindrance on the shape of excited state potential energy surface (S1 PES). Compared to the steep S1 PES of parental DPAC, the introduction of ortho‐substituent is shown to make the path of structural evolution in S1 wider and flatter, so the ortho‐substituted derivatives exhibit slower structural transformations from bent to planar and then to twisted forms, yielding intrinsic triple emission. The results provide the proof of concept that the bent, planar, and twisted emissive states can coexist in the same S1 PES, which greatly expand the fundamental understanding of the excited‐state structural relaxation. [ABSTRACT FROM AUTHOR]

Electronic structure calculations on small systems such as H2, H2O, LiH, and BeH2 with chemical accuracy are still a challenge for the current generation of noisy intermediate‐scale quantum (NISQ) devices. One of the reasons is that due to the device limitations, only minimal basis sets are commonly applied in quantum chemical calculations, which allows one to keep the number of qubits employed in the calculations at a minimum. However, the use of minimal basis sets leads to very large errors in the computed molecular energies as well as potential energy surface shapes. One way to increase the accuracy of electronic structure calculations is through the development of small basis sets better suited for quantum computing. In this work, we show that the use of adaptive basis sets, in which exponents and contraction coefficients depend on molecular structure, provides an easy way to dramatically improve the accuracy of quantum chemical calculations without the need to increase the basis set size and thus the number of qubits utilized in quantum circuits. As a proof of principle, we optimize an adaptive minimal basis set for quantum computing calculations on an H2 molecule, in which exponents and contraction coefficients depend on the HH distance, and apply it to the generation of H2 potential energy surface on IBM‐Q quantum devices. The adaptive minimal basis set reaches the accuracy of the double‐zeta basis sets, thus allowing one to perform double‐zeta quality calculations on quantum devices without the need to utilize twice as many qubits in simulations. This approach can be extended to other molecular systems and larger basis sets in a straightforward manner. [ABSTRACT FROM AUTHOR]

A new two-dimensional potential energy surface of the interaction between HC3S+ molecule and helium atom is mapped using an explicitly correlated coupled cluster approach with a single, double and perturbative triple excitation-F12a approach with an augmented-correlation-consistent polarized valence triple-zeta Gaussian basis set. The deepest potential well is identified at θ = 95∘ and R = 6.0 bohr with a depth of 100.67 cm−1. Cross-sections are determined for total energy up to 300 cm−1 using a coupled states method. Thermal rate coefficients are derived from the determined cross-sections for temperatures ranging between 3 K and 50 K. A clear predominance of | Δ j | = 2 processes is observed. [ABSTRACT FROM AUTHOR]

A new global many-body expansion potential energy surface (PES) for the ground electronic state of HO2 is constructed. All ab initio energy points are employed using aug-cc-pV(Q + d)Z and aug-cc-pV(5 + d)Z basis sets at the multi-reference conformational interaction level and then extrapolated to the complete basis set limit. The topographic characteristics of the global PES for HO2 (X 2A″) are discussed in detail and compared with experimental results, showing good agreement. We subsequently consider the O(3P) + OH(2Π) → O2(3 Σ g − ) + H(2S) reaction using the time-dependent wave packet method and calculate dynamics behaviors, including the vibration–rotation distribution and reaction probabilities. The results indicate that the new PES is suitable for dynamics investigations of O(3P) + OH(2Π) → O2(3 Σ g − ) + H(2S) and can be used as a component for PESs to construct larger oxygen/hydrogen containing systems. [ABSTRACT FROM AUTHOR]

Inspired by the excellent photochemical properties of 2‐(2′‐hydroxyphenyl)benzazole and its derivatives, in this work, the novel 2‐bis(benzothia‐zolyl)naphthalene‐diol (2‐BBTND) fluorophore is explored about its photo‐induced behaviors. The intramolecular double hydrogen bonding interaction and the excited state intramolecular double proton transfer (ESDPT) behavior of 2‐BBTND fluorophore are investigated in combination with different polar solvent environments. From the structural changes and charge recombination induced by photoexcitation, we can affirm that strong polar solvent environment promotes the excited state dynamical reaction for 2‐BBTND compound. By constructing potential energy surfaces (PESs) in S0 and S1 states, we clarify that 2‐BBTND fluorophore should undergo the stepwise ESDPT reaction after photoexcitation. Combined with the size of potential energy barriers along with reaction paths in different solvents, we finally propose a new solvent‐polarity‐dependent stepwise ESDPT for 2‐BBTND fluorophore. [ABSTRACT FROM AUTHOR]

The UV absorption spectrum of all-trans-octatetraene is reinvestigated theoretically, focussing on the strongly dipole-allowed 1 A g – 1 B u transition. The dynamical calculations rely on ab initio multistate CASPT2 (MS-CASPT2) computations of the underlying potential energy surfaces and coupling elements, and the multiconfiguration time-dependent Hartree (MCTDH) method for solving the time-dependent Schrödinger equation for the nuclear motion. The vibronic structure of the absorption band near 2700 Å is well reproduced, better than ever before by a purely ab initio approach. The C–C single bond and C = C double bond stretching modes dominate the vibrational structure of the transition. While the latter resembles nearly harmonic motion on weakly coupled potential energy surfaces, the time-dependent calculations reveal a subpicosecond electronic population transfer from the 1 B u state to the lower-lying 2 A g state proceeding on a time scale of ∼ 50 fs . The details of this internal conversion process and its relation to the conical intersection between the two excited states are discussed, paying particular attention to the impact of the 1 B u – 2 A g energy gap on the dynamics. [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 open‐source statistical mechanics software described here, Arkane–Automated Reaction Kinetics and Network Exploration–facilitates computations of thermodynamic properties of chemical species, high‐pressure limit reaction rate coefficients, and pressure‐dependent rate coefficient over multi‐well molecular potential energy surfaces (PES) including the effects of collisional energy transfer on phenomenological kinetics. Arkane can use estimates to fill in information for molecules or reactions where quantum chemistry information is missing. The software solves the internal energy master equation for complex unimolecular reaction systems. Inputs to the software include converged electronic structure computations performed by the user using a variety of supported software packages (Gaussian, Molpro, Orca, TeraChem, Q‐Chem, Psi4). The software outputs high‐pressure limit rate coefficients and pressure‐dependent phenomenological rate coefficients, as well as computed thermodynamic properties (enthalpy, entropy, and constant pressure heat capacity) with added energy corrections. Some of the key features of Arkane include treatment of 1D, 2D or ND hindered internal rotation modes, treatment of free internal rotation modes, quantum tunneling effect consideration, transition state theory (TST) and Rice‐Ramsperger‐Kassel‐Marcus (RRKM) rate coefficient computations, master equation solution with four implemented methods, inverse‐Laplace transform of high‐pressure limit rate coefficients into the energy domain, energy corrections based on bond‐additivity or isodesmic reactions, automated and efficient PES exploration, and PES sensitivity analysis. The present work describes the design of Arkane, how it should be used, and refers to the theory that it employs. Arkane is distributed via the RMG‐Py software suite (https://github.com/ReactionMechanismGenerator/RMG‐Py). [ABSTRACT FROM AUTHOR]

TDDFT scan calculations were performed for s-carborane-anthracene derivatives (o-CB-X-Ant where X=-H, -CH3, -C2H5 and tert-butyl or -tBu) in order to understand the interplay between the steric effects, S1 potential energy surface (PES) and photophysical properties. The results show that all systems exhibit three local minima on the S1 PES, which correspond to the emissive LE and TICT state, along with the nonemissive CT state respectively. In the case of the unsubstituted system (o-CB-H-Ant), and -CH3 and -C2H5 substituted cases, S1 PES is predicted to be quite flat for certain conformations indicating that it is possible for these systems to reach the nonemissive CT state without a large energy penalty. In comparison, conformational pathways for the nonemissive CT state are predicted to be energetically unfavorable for o-CB-tBu-Ant as a result of both steric and electronic effects. These results provide a mechanism for the enhanced emission of s-CB-fluorophore molecules with bulky ligands. [ABSTRACT FROM AUTHOR]

The LiNa2 reactive system has recently received great attention in the experimental study of ultracold chemical reactions, but the corresponding theoretical calculations have not been carried out. Here, we report the first globally accurate ground-state LiNa2 potential energy surface (PES) using a Gaussian process model based on only 1776 actively selected high-level ab initio training points. The constructed PES had high precision and strong generalization capability. On the new PES, the quantum dynamics calculations on the Li(2S) + Na2(v = 0, j = 0) → LiNa + Na reaction were carried out in the 0.001–0.01 eV collision energy range using an improved time-dependent wave packet method. The calculated results indicate that this reaction is dominated by a complex-forming mechanism at low collision energies. The presented dynamics data provide guidance for experimental research, and the newly constructed PES could be further used for ultracold reaction dynamics calculations on this reactive system. [ABSTRACT FROM AUTHOR]

The SN2 and proton transfer (PT) pathways for OH − + NH 2 Cl reaction are represented by employing various electronic structure computations. Both back-side SN2 and PT channels are exothermic and stationary points of PESs are below the reactant asymptote. Overall, the PES is similar to the C-centered SN2 reactions. Conversely, ion-dipole complex was not found for OH − + NH 2 Cl system. The N–HOH/NH–Cl hydrogen bond characterizes on either side of the reaction barrier of nitrogen complexes. Moreover, a halogen-bonded complex (HO−–ClNH2) and two types of H-bond complexes (HONH2–Cl− and Cl−–HONH2) were described, predicting an important role in dynamics. The PT pathway may be the major channel in the title system, which is contradictory to OH − + CH 3 Cl and F − + NH 2 Cl reactions. Here, MP2, B3LYP and CAM-B3LYP methods show overall excellent consistency with CCSD(T)/CBS energies and are recommended to carry out dynamics simulations. The PES of OH– + NH2Cl reaction shows two main pathways, SN2 and proton transfer, of which the later may be more competitive than SN2 pathway. A halogen-bond complex HO–ClNH2 is obtained, that could play an important role in dynamics. MP2, B3LYP and CAM-B3LYP methods are recommended to perform chemical dynamics simulations. [ABSTRACT FROM AUTHOR]

The results of a CCSD(T)‐F12/cc‐pVTZ‐f12//ωB97XD/cc‐pVTZ quantum‐chemical study of the potential energy surface (PES) for the reaction of propionitrile with methylidyne are combined with Rice‐Ramsperger‐Kassel‐Marcus (RRKM) calculations of the reaction rate constants and product branching ratios in the deep space conditions corresponding to the zero‐pressure limit at various collision energies. The most energetically favorable reaction pathways have been identified. The reaction outcome has been shown to strongly depend on the branching in the entrance reaction channel, between CH additions and insertions into various C‐H and C‐C bonds. For instance, CH addition to the N atom predominantly leads to 3H‐pyrrole + H (p9), with CH2NC + C2H4 (p2) also being a significant product. CH addition to the triple C≡N bond mostly results in 2‐methylene‐2H‐azirine + CH3 (p13), whereas CH insertions into C‐H bonds in the CH3 and CH2 groups of propionitrile form CH2CN + C2H4 (p1) and CH2CHCN + CH3 (p7) respectively. Less likely CH insertions into single C‐C bonds yield CH3CHCHCN + H (p5) and CH2CHCH2CN + H (p8). The results indicate that the methylidyne + propionitrile reaction may represent a critical step toward the formation of heterocyclic N‐containing molecules in the interstellar medium and in planetary atmospheres. [ABSTRACT FROM AUTHOR]

The nitrate radical is one of the most significant oxidants in troposphere at the nighttime. Furthermore, acrylic acid is reported to be one of 189 hazardous air pollutants. In order to investigate the atmospheric chemistry and environmental impact of acrylic acid with nitrate radical, in this work, the atmospheric reaction mechanisms of acrylic acid by NO3 radical have been discussed by the CCSD(T)/cc‐pVTZ//M06‐2X/6‐311++G(d, p) dual‐level of theory at the first time. The energy barriers of NO3 and CH2CHCOOH reaction have also been calculated accurately. The additions between NO3 with CH2CHCOOH would produce the dominant intermediates CH2NO3CHCOOH and CH2CHNO3COOH via low barriers. All hydrogen abstraction channels have higher barriers than that of additions. Therefore, addition pathways play an important role in this reaction. The total and branching rate constants have been computed by conventional transition state theory via the Variflex code. These rate constants of NO3 and acrylic acid reaction would be useful to complete structure–activity relationships of reaction rate. The calculated atmospheric lifetime of acrylic acid by NO3 is about 3.66 days at night. All findings will help us gain a better understanding of atmospheric chemistry of acrylic acid. [ABSTRACT FROM AUTHOR]

In this study, a computational chemistry exploration experiment for senior undergraduate or beginning graduate students is designed. The rotational isomerization reaction of an allyl cation is investigated using density functional theory (DFT) calculations. The theoretical experiment involves molecular geometry optimization, transition state location, and establishment of intrinsic reaction coordinates (IRCs). This design can help students understand the basic concepts and operations of computational chemistry. Furthermore, the concepts of molecular microstructures and the notion of "old bonds are about to break and new bonds are about to form" in the transition state theory are discussed. This experiment will also aid the understanding of the differences between reaction thermodynamics and kinetics through the construction of potential energy surfaces. Further investigations of charge population analysis and frontier orbital analysis will aid the understanding of electronic structures of molecules as well as the concept of reaction reactive sites. [ABSTRACT FROM AUTHOR]

A new global potential energy surface (PES) of the ground state of the HN2 system is built using the neural network method. The aug-cc-pVQZ basis set for the H atom, cc-pwCVQZ basis set for the N atom and multi-reference configuration interaction method are employed in the ab initio calculations. The topographic features of the PES are discussed and compared with available theoretical studies. In addition, the initial state (v0 = 0, j0 = 0) dynamic studies of the N + NH(³Σ -) → N2(X¹ Σg +)+H reaction are performed using a time-dependent wave packet method, based on the newly constructed PES. The dynamics properties, such as reaction probabilities, integral crosssections, differential cross-sections, the rovibrational state distribution of product and rate constants, are calculated at the state-to-state level of theory and compared with available theoretical and experimental results. The differential cross-section indicates that the forward abstraction mechanism dominates the reaction with the increase of collision energy. [ABSTRACT FROM AUTHOR]

The intermolecular potential energy surfaces (PESs) of Ne-CO complex have been constructed by using the CCSD(T)-F12a method with the aug-cc-pVQZ basis set. These PESs were subsequently applied to investigate the microwave and infrared spectra via bound state calculations. Firstly, compared to the available microwave data, the pure rotational transitions with K = 0 and J = 4 for this complex can be well reproduced. Furthermore, a set of accurate spectroscopic parameters were deduced for the fundamental (vCO = 1) and overtone (vCO = 2) bands. The vibrational shift for the normal isotopologue (Ne-12C16O) was determined to be -0.0683 cm-1, which is in excellent agreement with the experimental observation of -0.0711cm-1. Isotope effects were also investigated, and the determined rotational constants and vibrational shifts display a fairly good linear relationship, which is classified into two types according to the 12C and 13C in the CO monomer. [ABSTRACT FROM AUTHOR]

The hydrogen abstraction reaction OH + H2S→H2O + SH plays an important role in acid rain formation, air pollution and climate change. In this work, the product energy disposals of the reaction and its isotopic variants OD + H2S and OD + D2S are calculated on a new ab-initio-based ground electronic state potential energy surface (PES) using the quasi-classical trajectory method. The PES is developed by fitting a total of 72,113 points calculated at the level of UCCSD(T)-F12a/aug-cc-pVTZ and using the fundamental invariant-neural network method, resulting in a total RMSE of 4.14 meV. The product H2O formed in the OH + H2S reaction at 298 K is found to be largely populated in the first overtone states of its symmetric and asymmetric stretching modes, while the vibrational distributions of the products HOD and D2O in the isotopically substituted reactions are visibly different. The computed product vibrational state distributions agree reasonably well with experimental results and are rationalized by the sudden vector projection model. [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]

Machine learning (ML) models, neural networks, and Gaussian processes have been used to predict the potential energy surface taking C2‐He (both static and dynamic scenario) and NCCN‐He collision systems. The surface is restricted to ∽125 points where traditional spline becomes inefficacious. Quantum dynamics is performed by solving close‐coupling equation to compute cross sections benchmarking the performance of the ML models. The current study forms a basis for any future investigation of larger molecules where conventional fitting fails due to sparser ab initio points and cuts down the computational time without compromising on the quality of the surface. [ABSTRACT FROM AUTHOR]

Based on 17,326 ab initio points, a global potential energy surface (PES) of the ground state of the CsH2 system was constructed using the neural network method. In ab initio calculations, the AVQZ basis set was used for the H atom, the def2-QZVP basis set was adopted for the Cs atom, and a multi-reference configuration interaction method was employed. The topographical characteristics of the newly developed PES were discussed in detail and no wells or barriers were detected. In addition, the dynamical calculations of the H + CsH reaction were performed using the time-dependent wave packet method, based on the new PES. Dynamics properties, such as reaction probability, integral cross section and differential cross-section were examined. The differential cross-section results indicated that the forward abstraction mechanism dominated the abstraction channel, and the 'rebound' reaction mechanism played a primary role in the exchange channel. Highlights Using the PIP-NN method, A global potential energy surface for the ground state of the CsH2 system was built based on 17,326 ab initio points. Dynamic calculations for the abstraction and exchange channels of the H + CsH reaction were performed using the time-dependent wave packet method and the dynamic properties of these two channels were discussed in detail. The direct abstraction mechanism plays a dominant role in the abstraction channel and the 'rebound' mechanism dominates the exchange channel. [ABSTRACT FROM AUTHOR]

Predicting the whole process of a chemical reaction while solving kinetic equations presents an opportunity to realize an on-the-fly kinetic simulation that directly discovers chemical reactions with their product yields. Such a simulation avoids the combinatorial explosion of reaction patterns to be examined by narrowing the search space based on the kinetic analysis of the reaction path network, and would open a new paradigm beyond the conventional two-step approach, which requires a reaction path network prior to performing a kinetic simulation. The authors addressed this issue and developed a practical method by combining the artificial force induced reaction method with the rate constant matrix contraction method. Two algorithms are available for this purpose: a forward mode with reactants as the input and a backward mode with products as the input. This article first numerically verifies these modes for known reactions and then demonstrates their application to the actual reaction discovery. Finally, the challenges of this method and the prospects for ab initio reaction discovery are discussed. [ABSTRACT FROM AUTHOR]

Here, it is shown that the M3B12 (M = Cu-Au) clusters' global minima consist of an elongated planar B12 fragment connected by an in-plane linear M3 fragment. This result is striking since this B12 planar structure is not favored in the bare cluster, nor when one or two metals are added. The minimum energy structures were revealed by screening the potential energy surface using genetic algorithms and density functional theory calculations. Chemical bonding analysis shows that the strong electrostatic interactions with the metal compensate for the high energy spent in the M3 and B12 fragment distortion. Furthermore, metals participate in the delocalized π-bonds, which infers an aromatic character to these species. [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]

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]

Methyl isocyanate (CH3NCO) is an important agricultural and industrial precursor and emission from wildfires. In this paper, we study thermodynamic, kinetic and mechanistic approach of CH3NCO with OH radical and Cl atom using M06-2X level in combination with 6-311++G(d,p) basis set, and corrected by relatively accurate the single point CCSD(T) method. The rate constants of the title reactions are calculated in the temperature range of 200 and 400 K. The reactions exhibit a positive temperature depending on rate constants and fitted to the three-parameter Arrhenius expressions: kOH = 2.76 × 10−19 T2.65 exp[−486/T] and kCl = 3.51 × 10−18 T2.28 exp[95/T] with Wigner's and kOH = 4.95 × 10−19 T2.57 exp[−504/T] and kCl = 1.20 × 10−17 T2.11 exp[63/T] with Eckart's tunnelling corrections between 200 and 400 K. The calculated rate constants for the CH3NCO + OH/Cl reaction are in very good agreement with the previously reported experimental values. Thermochemical data (ΔH°298 K and ΔG°298 K) illustrate the feasibility and spontaneity of the title reactions. The atmospheric degradation mechanism of CH3NCO is discussed. The OH/Cl-driven tropospheric lifetimes and global warming potential (GWP) for CH3NCO were calculated. It was concluded that CH3NCO existed in the Earth's atmosphere for a few days and its GWP is not significantly large. The ozone formation potential of CH3NCO was also reported in this work. [ABSTRACT FROM AUTHOR]