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A full-dimensional analytical potential energy surface (PES) is developed for the Cl + CH3CN reaction following our previous work on the benchmark ab initio characterization of the stationary points. The spin–orbit-corrected PES is constructed using the Robosurfer program and a fifth-order permutationally invariant polynomial method for fitting the high-accuracy energy points determined by a ManyHF-based coupled-cluster/triple-zeta-quality composite method. Quasi-classical trajectory simulations are performed at six collision energies between 10 and 60 kcal mol−1. Multiple low-probability product channels are found, including isomerization to isonitrile (CH3NC), but out of the eight possible channels, only the H-abstraction has significant reaction probability; thus, detailed dynamics studies are carried out only for this reaction. The cross sections and opacity functions show that the probability of the H-abstraction reaction increases with increasing collision energy (Ecoll). Scattering angle, initial attack angle, and product relative translational energy distributions indicate that the mechanism changes with the collision energy from indirect/rebound to direct stripping. The distribution of initial attack angles shows a clear preference for methyl group attack but with different angles at different Ecoll values. Post-reaction energy distributions show that the energy transfer is biased toward the products' relative translational energy instead of their internal energy. Rotational and vibrational energy have about the same amount of contribution to the internal energy in the case of both products (HCl and CH2CN), i.e., both of them are formed with high rotational excitations. HCl is produced mostly in the ground vibrational state, while a notable fraction of CH2CN is formed with vibrational excitation. [ABSTRACT FROM AUTHOR]

Reactions between methanol and the hydroxyl radical are of significant interest for combustion-, atmospheric-, and astrochemistry. While the two primary product channels (the formation of H2O with either CH3O· or ·CH2OH) have been the subject of numerous studies, the possibility of other products has seen little attention. Here, we present a comprehensive thermochemical survey of the stationary points and plausible products of the reaction, featuring 29 geometries optimized at the UCCSD(T)-F12b/aug-cc-pVTZ level, followed by accurate composite ab initio computations for all stationary points (including ·CH2OH dissociation and isomerization) and five product channels, with a detailed evaluation of basis set convergence and efficiency. The computations reveal that the formation of methanediol and the hydroxymethoxy radical is thermodynamically favorable and the endothermicity of formaldehyde formation is low enough to be a plausible product channel. We also observe unexpectedly large energy deviations between the partially-spin-adapted ROHF-RCCSD(T) method and ROHF-UCCSD(T) as well as between UHF-UCCSDT(Q) and ROHF-UCCSDT(Q) results. [ABSTRACT FROM AUTHOR]

The article focuses on a discussion about scattering in extreme environments, specifically addressing the impact of molecular orientation on nuclear spin coupling measurements. Topics include the effect of relative orientation on measurement outcomes, the role of angular momentum in molecular collisions, and the absence of significant orientation effects in the studied system.

The article focuses on the discussion of scattering phenomena involving larger molecules, specifically addressing questions related to the phenylethynyl radical in the interstellar medium. Topics include the challenges in detecting the phenylethynyl radical due to its complex spectrum, potential formation mechanisms in space, and the role of experimental and computational methods in predicting its presence.

The article focuses on a general discussion about the scattering of larger molecules, particularly examining the differences in potential energy surfaces (PES) between HDO, H2O, and D2O. Topics include the reason for stronger back scattering in HDO compared to H2O and D2O, the role of symmetry and potential terms in scattering phenomena, and specific insights into the n10 term affecting back-scattering.

Developing global, high-dimensional potential energy surfaces (PESs) is a formidable task. Beside the challenges of PES fitting and fitting set generation, one also has to choose an electronic structure method capable of delivering accurate potential energy values for all geometries in the fitting set, even in regions far from equilibrium. Such regions are often plagued by Hartree–Fock (HF) convergence issues, and even if convergence is achieved, self-consistent field (SCF) procedures that are used to obtain HF solutions offer no guarantee that the solution found is the lowest-energy solution. We present a study of the reactant regions of CH3OH + OH·, C2H6 + F·, and CH3NH2 + Cl·, where the SCF procedure often converges to a higher-energy state or fails to converge, resulting in erratic post-HF energies and regions where no energy is obtained, both of which are major obstacles for PES development. We introduce a pragmatic method for automatically finding better HF solutions (dubbed ManyHF) and present evidence that it may extend the applicability of single-reference methods to some systems previously thought to require multireference methods. [ABSTRACT FROM AUTHOR]

Dynamics and mechanisms of the F− + CH3Br(v = 0) → Br− + CH3F (SN2 via Walden inversion, front-side attack, and double inversion), F− + inverted-CH3Br (induced inversion), HF + CH2Br− (proton abstraction), and FH⋯Br− + 1CH2 reactions are investigated using a high-level global ab initio potential energy surface, the quasiclassical trajectory method, as well as non-standard configuration- and mode-specific analysis techniques. A vector-projection method is used to identify inversion and retention trajectories; then, a transition-state-attack-angle-based approach unambiguously separates the front-side attack and the double-inversion retention pathways. The Walden-inversion SN2 channel becomes direct rebound dominated with increasing collision energy as indicated by backward scattering, initial back-side attack preference, and the redshifting of product internal energy peaks in accord with CF stretching populations. In the minor retention and induced-inversion pathways, almost the entire available energy transfers into product rotation–vibration, and retention mainly proceeds with indirect, slow double inversion following induced inversion with about 50% probability. Proton abstraction is dominated by direct stripping (evidenced by forward scattering) with CH3-side initial attack preference, providing mainly vibrationally ground state products with significant zero-point energy violation. [ABSTRACT FROM AUTHOR]

We describe a composite ab initio approach to determine the best technically feasible relative energies of stationary points considering additive contributions of the CCSD(T)/complete-basis-set limit, core and post-CCSD(T) correlation, scalar relativistic and spin–orbit effects, and zero-point energy corrections. The importance and magnitude of the different energy terms are discussed using examples of atom/ion + molecule reactions, such as X + CH4/C2H6 and X− + CH3Y/CH3CH2Cl [X, Y = F, Cl, Br, I, OH, etc.]. We test the performance of various ab initio levels and recommend the modern explicitly-correlated CCSD(T)-F12 methods for potential energy surface (PES) developments. We show that the choice of the level of electronic structure theory may significantly affect the reaction dynamics and the CCSD(T)-F12/double-zeta PESs provide nearly converged cross sections. Trajectory orthogonal projection and an Eckart-transformation-based stationary-point assignment technique are proposed to provide dynamical characterization of the stationary points, thereby revealing front-side complex formation in SN2 reactions and transition probabilities between different stationary-point regions. [ABSTRACT FROM AUTHOR]

We report a story where CCSD(T) breaks down at certain geometries of the potential energy surface (PES) of the OH− + CH3I reaction. To solve this problem, we combine CCSD-F12b and Brueckner-type BCCD(T) methods to develop a full-dimensional analytical PES providing method- and basis-converged statistically-accurate SN2 and proton-transfer cross sections. [ABSTRACT FROM AUTHOR]

Accuracy of the different levels of electronic structure theory is frequently studied for stationary-point properties; however, little is known about the effects of the electronic structure methods and basis sets on the dynamics of chemical reactions. Here we report such an investigation for the F- + CH3I SN2 and proton-transfer reactions by developing 20 different analytical potential energy surfaces (PESs) obtained at the HF/DZ, HF/TZ, HF-D3(BJ)/DZ, HF-D3(BJ)/TZ, MP2/DZ, MP2/TZ, MP2-F12/DZ, MP2-F12/TZ, CCSD/DZ, CCSD-F12b/DZ, CCSD(T)/DZ, CCSD(T)-F12b/DZ, OQVCCD(T)/DZ, B97-1/TZ, PBE0/TZ, PBE0-D3(BJ)/TZ, M06-2X/TZ, M06-2X-D3(0)/TZ, B2PLYP/TZ, and B2PLYP-D3(BJ)/TZ levels of theory, where DZ and TZ denote the aug-cc-pVDZ and aug-cc-pVTZ basis sets with a relativistic effective core potential and the corresponding bases for iodine. Millions of quasiclassical trajectories on these PESs reveal that (a) in the case of standard methods, increasing the basis from DZ to TZ decreases the SN2 cross sections by 20-30%; (b) the explicitly correlated F12 reactivity is converged with a DZ basis; (c) the quasi-variational OQVCCD(T) and the CCSD(T) methods provide virtually the same cross sections; (d) the above DFT functionals give significantly larger SN2 cross sections than the ab initio methods; (e) retention SN2 cross sections show striking method and basis dependence and double inversion is substantially enhanced with a TZ basis or F12 methods; (f) the TZ basis doubles the DZ proton-transfer reactivity; (g) at a high collision energy ab initio methods show dominance of backward scattering, in agreement with experiment, whereas most DFT functionals provide slight forward preference; and (h) at high energy the ab initio correlation (DFT) methods slightly underestimate (overestimate) the CH3F internal energy excitations, whereas the broad experimental distribution is qualitatively reproduced. [ABSTRACT FROM AUTHOR]

A full-dimensional analytical potential energy surface (PES) is developed for the Cl + CH3CN reaction following our previous work on the benchmark ab initio characterization of the stationary points. The spin-orbit-corrected PES is constructed using the Robosurfer program and a fifth-order permutationally invariant polynomial method for fitting the high-accuracy energy points determined by a ManyHF-based coupled-cluster/triple-zeta-quality composite method. Quasi-classical trajectory simulations are performed at six collision energies between 10 and 60 kcal mol-1. Multiple low-probability product channels are found, including isomerization to isonitrile (CH3NC), but out of the eight possible channels, only the H-abstraction has significant reaction probability; thus, detailed dynamics studies are carried out only for this reaction. The cross sections and opacity functions show that the probability of the H-abstraction reaction increases with increasing collision energy (Ecoll). Scattering angle, initial attack angle, and product relative translational energy distributions indicate that the mechanism changes with the collision energy from indirect/rebound to direct stripping. The distribution of initial attack angles shows a clear preference for methyl group attack but with different angles at different Ecoll values. Post-reaction energy distributions show that the energy transfer is biased toward the products' relative translational energy instead of their internal energy. Rotational and vibrational energy have about the same amount of contribution to the internal energy in the case of both products (HCl and CH2CN), i.e., both of them are formed with high rotational excitations. HCl is produced mostly in the ground vibrational state, while a notable fraction of CH2CN is formed with vibrational excitation., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Reactions between methanol and the hydroxyl radical are of significant interest for combustion-, atmospheric-, and astrochemistry. While the two primary product channels (the formation of H 2 O with either CH 3 O· or ·CH 2 OH) have been the subject of numerous studies, the possibility of other products has seen little attention. Here, we present a comprehensive thermochemical survey of the stationary points and plausible products of the reaction, featuring 29 geometries optimized at the UCCSD(T)-F12b/aug-cc-pVTZ level, followed by accurate composite ab initio computations for all stationary points (including ·CH 2 OH dissociation and isomerization) and five product channels, with a detailed evaluation of basis set convergence and efficiency. The computations reveal that the formation of methanediol and the hydroxymethoxy radical is thermodynamically favorable and the endothermicity of formaldehyde formation is low enough to be a plausible product channel. We also observe unexpectedly large energy deviations between the partially-spin-adapted ROHF-RCCSD(T) method and ROHF-UCCSD(T) as well as between UHF-UCCSDT(Q) and ROHF-UCCSDT(Q) results.

Dynamics and mechanisms of the F - + CH 3 Br(v = 0) → Br - + CH 3 F (S N 2 via Walden inversion, front-side attack, and double inversion), F - + inverted-CH 3 Br (induced inversion), HF + CH 2 Br - (proton abstraction), and FH⋯Br - + 1 CH 2 reactions are investigated using a high-level global ab initio potential energy surface, the quasiclassical trajectory method, as well as non-standard configuration- and mode-specific analysis techniques. A vector-projection method is used to identify inversion and retention trajectories; then, a transition-state-attack-angle-based approach unambiguously separates the front-side attack and the double-inversion retention pathways. The Walden-inversion S N 2 channel becomes direct rebound dominated with increasing collision energy as indicated by backward scattering, initial back-side attack preference, and the redshifting of product internal energy peaks in accord with CF stretching populations. In the minor retention and induced-inversion pathways, almost the entire available energy transfers into product rotation-vibration, and retention mainly proceeds with indirect, slow double inversion following induced inversion with about 50% probability. Proton abstraction is dominated by direct stripping (evidenced by forward scattering) with CH 3 -side initial attack preference, providing mainly vibrationally ground state products with significant zero-point energy violation.

Since the pioneering reaction dynamics studies of H + H 2 in the 1970s, theory increased the system size by one atom in every decade arriving to six-atom reactions in the early 2010s. Here, we take a significant step forward by reporting accurate dynamics simulations for the nine-atom Cl + ethane (C 2 H 6 ) reaction using a new high-quality spin-orbit-ground-state ab initio potential energy surface. Quasi-classical trajectory simulations on this surface cool the rotational distribution of the HCl product molecules, thereby providing unprecedented agreement with experiment after several previous failed attempts of theory. Unlike Cl + CH 4 , the Cl + C 2 H 6 reaction is exothermic with an adiabatically submerged transition state, allowing testing of the validity of the Polanyi rules for a negative-barrier reaction.

We describe a composite ab initio approach to determine the best technically feasible relative energies of stationary points considering additive contributions of the CCSD(T)/complete-basis-set limit, core and post-CCSD(T) correlation, scalar relativistic and spin-orbit effects, and zero-point energy corrections. The importance and magnitude of the different energy terms are discussed using examples of atom/ion + molecule reactions, such as X + CH 4 /C 2 H 6 and X - + CH 3 Y/CH 3 CH 2 Cl [X, Y = F, Cl, Br, I, OH, etc.]. We test the performance of various ab initio levels and recommend the modern explicitly-correlated CCSD(T)-F12 methods for potential energy surface (PES) developments. We show that the choice of the level of electronic structure theory may significantly affect the reaction dynamics and the CCSD(T)-F12/double-zeta PESs provide nearly converged cross sections. Trajectory orthogonal projection and an Eckart-transformation-based stationary-point assignment technique are proposed to provide dynamical characterization of the stationary points, thereby revealing front-side complex formation in S N 2 reactions and transition probabilities between different stationary-point regions.

We report a story where CCSD(T) breaks down at certain geometries of the potential energy surface (PES) of the OH- + CH3I reaction. To solve this problem, we combine CCSD-F12b and Brueckner-type BCCD(T) methods to develop a full-dimensional analytical PES providing method- and basis-converged statistically-accurate SN2 and proton-transfer cross sections.

The construction of high-dimensional global potential energy surfaces (PESs) from ab initio data has been a major challenge for decades. Advances in computer hardware, electronic structure theory, and PES fitting methods have greatly alleviated many challenges in PES construction, but building fitting sets has remained a bottleneck so far. We present the robosurfer program system that completely automates the generation of new geometries, performs ab initio computations, and iteratively improves the PES under development. Unlike previous efforts to automate PES development, robosurfer does not require any uncertainty estimate from the PES fitting method and thus it is compatible with the permutationally invariant polynomial (PIP) method. As a demonstration we have developed five related but different global reactive PIP PESs for the CH 3 Br + F - system and used them to perform quasiclassical trajectory (QCT) reaction dynamics simulations over a wide range of collision energies. The automatically developed PESs show good to excellent accuracy at known stationary points without any manual sampling, and QCT results indicate the lack of unphysical minima on the fitted surfaces. We also present evidence suggesting that the breakdown of single reference electronic structure theory may contribute significantly to the fitting errors of global reactive PESs.

Accuracy of the different levels of electronic structure theory is frequently studied for stationary-point properties; however, little is known about the effects of the electronic structure methods and basis sets on the dynamics of chemical reactions. Here we report such an investigation for the F - + CH 3 I S N 2 and proton-transfer reactions by developing 20 different analytical potential energy surfaces (PESs) obtained at the HF/DZ, HF/TZ, HF-D3(BJ)/DZ, HF-D3(BJ)/TZ, MP2/DZ, MP2/TZ, MP2-F12/DZ, MP2-F12/TZ, CCSD/DZ, CCSD-F12b/DZ, CCSD(T)/DZ, CCSD(T)-F12b/DZ, OQVCCD(T)/DZ, B97-1/TZ, PBE0/TZ, PBE0-D3(BJ)/TZ, M06-2X/TZ, M06-2X-D3(0)/TZ, B2PLYP/TZ, and B2PLYP-D3(BJ)/TZ levels of theory, where DZ and TZ denote the aug-cc-pVDZ and aug-cc-pVTZ basis sets with a relativistic effective core potential and the corresponding bases for iodine. Millions of quasiclassical trajectories on these PESs reveal that (a) in the case of standard methods, increasing the basis from DZ to TZ decreases the S N 2 cross sections by 20-30%; (b) the explicitly correlated F12 reactivity is converged with a DZ basis;, ((c) the quasi-variational OQVCCD(T) and the CCSD(T) methods provide virtually the same cross sections; (d) the above DFT functionals give significantly larger S2 cross sections than the ab initio methods; (e) retention S2 cross sections show striking method and basis dependence and double inversion is substantially enhanced with a TZ basis or F12 methods; (f) the TZ basis doubles the DZ proton-transfer reactivity; (g) at a high collision energy ab initio methods show dominance of backward scattering, in agreement with experiment, whereas most DFT functionals provide slight forward preference; and (h) at high energy the ab initio correlation (DFT) methods slightly underestimate (overestimate) the CHF internal energy excitations, whereas the broad experimental distribution is qualitatively reproduced.)