Benchmark ab initio characterization of the complex potential energy surfaces of the HOO- + CH3Y [Y = F, Cl, Br, I] reactions.

The α-effect is a well-known phenomenon in organic chemistry, and is related to the enhanced reactivity of nucleophiles involving one or more lone-pair electrons adjacent to the nucleophilic center. The gas-phase bimolecular nucleophilic substitution (SN2) reactions of α-nucleophile HOO- with methyl halides have been thoroughly investigated experimentally and theoretically; however, these investigations have mainly focused on identifying and characterizing the α-effect of HOO-. Here, we perform the first comprehensive high-level ab initio mapping for the HOO- + CH3Y [Y = F, Cl, Br and I] reactions utilizing the modern explicitly-correlated CCSD(T)-F12b method with the aug-cc-pVnZ [n = 2-4] basis sets. The present ab initio characterization considers five distinct product channels of SN2: (CH3OOH + Y-), proton abstraction (CH2Y- + H2O2), peroxide ion substitution (CH3OO- + HY), SN2-induced elimination (CH2O + HY + HO-) and SN2-induced rearrangement (CH2(OH)O- + HY). Moreover, besides the traditional back-side attack Walden inversion, the pathways of front-side attack, double inversion and halogen-bond complex formation have also been explored for SN2. With regard to the Walden inversion of HOO- + CH3Cl, the previously unaddressed discrepancies concerning the geometry of the corresponding transition state are clarified. For the HOO- + CH3F reaction, the recently identified SN2-induced elimination is found to be more exothermic than the SN2 channel, submerged by ∼36 kcal mol-1. The accuracy of our high-level ab initio calculations performed in the present study is validated by the fact that our new benchmark 0 K reaction enthalpies show excellent agreement with the experimental data in nearly all cases.

First-principles mode-specific reaction dynamics.

Controlling the outcome of chemical reactions by exciting specific vibrational and/or rotational modes of the reactants is one of the major goals of modern reaction dynamics studies. In the present Perspective, we focus on first-principles vibrational and rotational mode-specific dynamics computations on reactions of neutral and anionic systems beyond six atoms such as X + C2H6 [X = F, Cl, OH], HX + C2H5 [X = Br, I], OH- + CH3I, and F- + CH3CH2Cl. The dynamics simulations utilize high-level ab initio analytical potential energy surfaces and the quasi-classical trajectory method. Besides initial state specificity and the validity of the Polanyi rules, mode-specific vibrational-state assignment for polyatomic product species using normal-mode analysis and Gaussian binning is also discussed and compared with experiment.

Vibrational mode-specificity in the dynamics of the OH- + CH3I multi-channel reaction.

We report a comprehensive characterization of the vibrational mode-specific dynamics of the OH- + CH3I reaction. Quasi-classical trajectory simulations are performed at four different collision energies on our previously-developed full-dimensional high-level ab initio potential energy surface in order to examine the impact of four different normal-mode excitations in the reactants. Considering the 11 possible pathways of OH- + CH3I, pronounced mode-specificity is observed in reactivity: In general, the excitations of the OH- stretching and CH stretching exert the greatest influence on the channels. For the SN2 and proton-abstraction products, the reactant initial attack angle and the product scattering angle distributions do not show major mode-specific features, except for SN2 at higher collision energies, where forward scattering is promoted by the CI stretching and CH stretching excitations. The post-reaction energy flow is also examined for SN2 and proton abstraction, and it is unveiled that the excess vibrational excitation energies rather transfer into the product vibrational energy because the translational and rotational energy distributions of the products do not represent significant mode-specificity. Moreover, in the course of proton abstraction, the surplus vibrational energy in the OH- reactant mostly remains in the H2O product owing to the prevailing dominance of the direct stripping mechanism.

Quasi-classical trajectory study of the OH- + CH3I reaction: theory meets experiment.

Regarding OH- + CH3I, several studies have focused on the dynamics of the reaction. Here, high-level quasi-classical trajectory simulations are carried out at four different collision energies on our recently developed potential energy surface. In all, more than half a million trajectories are performed, and for the first time, the detailed quasi-classical trajectory results are compared with the reanalysed crossed-beam ion imaging experiments. Concerning the previously reported direct dynamics study of OH- + CH3I, a better agreement can be obtained between the revised experiment and our novel theoretical results. Furthermore, in the present work, the benchmark geometries, frequencies and relative energies of the stationary points are also determined for the OH- + CH3I proton-abstraction channel along with the earlier characterized SN2 channel.

Unconventional SN2 retention pathways induced by complex formation: High-level dynamics investigation of the NH2 - + CH3I polyatomic reaction.

Investigations on the dynamics of chemical reactions have been a hot topic for experimental and theoretical studies over the last few decades. Here, we carry out the first high-level dynamical characterization for the polyatom-polyatom reaction between NH2 - and CH3I. A global analytical potential energy surface is developed to describe the possible pathways with the quasi-classical trajectory method at several collision energies. In addition to SN2 and proton abstraction, a significant iodine abstraction is identified, leading to the CH3 + [NH2⋯I]- products. For SN2, our computations reveal an indirect character as well, promoting the formation of [CH3⋯NH2] complexes. Two novel dominant SN2 retention pathways are uncovered induced by the rotation of the CH3 fragment in these latter [CH3⋯NH2] complexes. Moreover, these uncommon routes turn out to be the most dominant retention paths for the NH2 - + CH3I SN2 reaction.

SN2 Reactions with an Ambident Nucleophile: A Benchmark Ab Initio Study of the CN- + CH3Y [Y = F, Cl, Br, and I] Systems.

We characterize the Walden-inversion, front-side attack, and double-inversion SN2 pathways leading to Y- + CH3CN/CH3NC and the product channels of proton abstraction (HCN/HNC + CH2Y-), hydride-ion substitution (H- + YH2CCN/YH2CNC), halogen abstraction (YCN-/YNC- + CH3 and YCN/YNC + CH3-), and YHCN-/YHNC- complex formation (YHCN-/YHNC- + 1CH2) of the CN- + CH3Y [Y = F, Cl, Br, and I] reactions. Benchmark structures and frequencies are computed at the CCSD(T)-F12b/aug-cc-pVTZ level of theory, and a composite approach is employed to obtain relative energies with sub-chemical accuracy considering (a) basis-set effects up to aug-cc-pVQZ, (b) post-CCSD(T) correlation up to CCSDT(Q), (c) core correlation, (d) relativistic effects, and (e) zero-point energy corrections. C-C bond formation is both thermodynamically and kinetically more preferred than N-C bond formation, though the kinetic preference is less significant. Walden inversion proceeds via low or submerged barriers (12.1/17.9(F), 0.0/4.3(Cl), -3.9/0.1(Br), and -5.8/-1.8(I) kcal/mol for C-C/N-C bond formation), front-side attack and double inversion have high barriers (30-64 kcal/mol), the latter is the lower-energy retention pathway, and the non-SN2 electronic ground-state product channels are endothermic (ΔH0 = 31-92 kcal/mol).

Uncovering an oxide ion substitution for the OH- + CH3F reaction.

Theoretical investigations on chemical reactions allow us to understand the dynamics of the possible pathways and identify new unexpected routes. Here, we develop a global analytical potential energy surface (PES) for the OH- + CH3F reaction in order to perform high-level dynamics simulations. Besides bimolecular nucleophilic substitution (SN2) and proton abstraction, our quasi-classical trajectory computations reveal a novel oxide ion substitution leading to the HF + CH3O- products. This exothermic reaction pathway occurs via the CH3OH⋯F- deep potential well of the SN2 product channel as a result of a proton abstraction from the hydroxyl group by the fluoride ion. The present detailed dynamics study of the OH- + CH3F reaction focusing on the surprising oxide ion substitution demonstrates how incomplete our knowledge is of fundamental chemical reactions.

A benchmark ab initio study of the complex potential energy surfaces of the OH- + CH3CH2Y [Y = F, Cl, Br, I] reactions.

We provide the first benchmark characterization of the OH- + CH3CH2Y [Y = F, Cl, Br, I] reactions utilizing the high-level explicitly-correlated CCSD(T)-F12b method with the aug-cc-pVnZ [n = 2(D), 3(T), 4(Q)] basis sets. We explore and analyze the stationary points of the elimination (E2) and substitution (SN2) reactions, including anti-E2, syn-E2, back-side attack, front-side attack, and double inversion. In all cases, SN2 is thermodynamically more preferred than E2. In the entrance channel of SN2 a significant front-side complex formation is revealed, and in the product channel the global minimum of the title reactions is obtained at the hydrogen-bonded CH3CH2OHY- complex. Similar to the OH- + CH3Y reactions, double inversion can proceed via a notably lower-energy pathway than front-side attack, moreover, for Y = I double inversion becomes barrier-less. For the transition state of the anti-E2, a prominent ZPE effect emerges, giving an opportunity for a kinetically more favored pathway than back-side attack. In addition to SN2 and E2, other possible product channels are considered, and in most cases, the benchmark reaction enthalpies are in excellent agreement with the experimental data.

First-Principles Reaction Dynamics beyond Six-Atom Systems.

Moving beyond the six-atomic benchmark systems, we discuss the new age and future of first-principles reaction dynamics, which investigates complex, multichannel chemical reactions. We describe the methodology starting from the benchmark ab initio characterization of the stationary points, followed by full-dimensional potential energy surface (PES) developments and reaction dynamics computations. We highlight our composite ab initio approach providing benchmark stationary-point properties with subchemical accuracy, the Robosurfer program system enabling automatic PES development, and applications for the Cl + C2H6, F + C2H6, and OH- + CH3I post-six-atom reactions focusing on ab initio issues and their solutions as well as showing the excellent agreement between theory and experiment.

On the development of a gold-standard potential energy surface for the OH- + CH3I reaction.

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.

Benchmark ab initio and dynamical characterization of the stationary points of reactive atom + alkane and SN2 potential energy surfaces.

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.

Rethinking the X- + CH3Y [X = OH, SH, CN, NH2, PH2; Y = F, Cl, Br, I] SN2 reactions.

Moving beyond the textbook mechanisms of bimolecular nucleophilic substitution (SN2) reactions, we characterize several novel stationary points and pathways for the reactions of X- [X = OH, SH, CN, NH2, PH2] nucleophiles with CH3Y [Y = F, Cl, Br, I] molecules using the high-level explicitly-correlated CCSD(T)-F12b method with the aug-cc-pVnZ(-PP) [n = D, T, Q] basis sets. Besides the not-always-existing traditional pre- and post-reaction ion-dipole complexes, X-H3CY and XCH3Y-, and the Walden-inversion transition state, [X-CH3-Y]-, we find hydrogen-bonded X-HCH2Y (X = OH, CN, NH2; Y ≠ F) and front-side H3CYX- (Y ≠ F) complexes in the entrance and hydrogen-bonded XH2CHY- (X = SH, CN, PH2) and H3CXY- (X = OH, SH, NH2) complexes in the exit channels depending on the nucleophile and leaving group as indicated in parentheses. Retention pathways via either a high-energy front-side attack barrier, XYCH3-, or a novel double-inversion transition state, XHCH2Y-, having lower energy for X = OH, CN, and NH2 and becoming submerged (barrier-less) for X = OH and Y = I as well as X = NH2 and Y = Cl, Br, and I, are also investigated.

Benchmark ab Initio Characterization of the Inversion and Retention Pathways of the OH- + CH3Y [Y = F, Cl, Br, I] SN2 Reactions.

We study the Walden-inversion, front-side attack retention, and double-inversion retention pathways of the OH- + CH3Y [Y = F, Cl, Br, I] SN2 reactions using high-level ab initio methods. Benchmark stationary-point structures and frequencies are computed at the CCSD(T)-F12b/aug-cc-pVTZ level of theory and the best technically feasible relative energies are determined on the basis of CCSD(T)-F12b/aug-cc-pVQZ computations complemented with post-CCSD(T) correlation effects at the CCSDT(Q)/aug-cc-pVDZ level, core correlation corrections at the CCSD(T)/aug-cc-pwCVTZ level, scalar relativistic effects using effective core potentials for Br and I, and zero-point energy corrections at the CCSD(T)-F12b/aug-cc-pVTZ level. Walden inversion proceeds via hydrogen-bonded HO-···HCH2Y (Cl, Br, I) complex → hydrogen-bonded HO-···HCH2Y (Cl, Br, I) transition state → ion-dipole HO-···H3CY (F, Cl, Br) complex → Walden-inversion [HO-CH3-Y]- (F, Cl, Br) transition state → hydrogen-bonded CH3OH···Y- (F, Cl, Br, I) complex, where the Y-dependent existence of the submerged stationary points is indicated in parentheses. Front-side HO-···YCH3 (Cl, Br, I) complexes are also found and HO-···ICH3 is a deeper minimum than HO-···HCH2I. Front-side attacks go over high barriers of 42.8 (F), 28.7 (Cl), 22.4 (Br), and 17.2 (I) kcal/mol, well above the double-inversion barrier heights of 16.7 (F), 3.4 (Cl), 1.1 (Br), and -3.7 (I) kcal/mol.