Automated full-dimensional potential energy surface development and quasi-classical dynamics for the HI(X 1 Σ + ) + C 2 H 5 → I( 2 P 3/2 ) + C 2 H 6 reaction

We develop a high-level spin–orbit-corrected analytical ab initio potential energy surface and perform quasi-classical trajectory simulations to study the dynamics of the 9-atomic HI + C 2 H 5 → I + C 2 H 6 reaction in full (21) dimensions

Revealing a double-inversion mechanism for the F- + CH3Cl S(N)2 reaction

Stereo-specific reaction mechanisms play a fundamental role in chemistry. The back-side attack inversion and front-side attack retention pathways of the bimolecular nucleophilic substitution (S(N)2) reactions are the textbook examples for stereo-specific chemical processes. Here, we report an accurate global analytic potential energy surface (PES) for the F- + CH3Cl S(N)2 reaction, which describes both the back-side and front-side attack substitution pathways as well as the proton-abstraction channel. Moreover, reaction dynamics simulations on this surface reveal a novel double-inversion mechanism, in which an abstraction-induced inversion via a FH center dot center dot center dot CH2Cl- transition state is followed by a second inversion via the usual [F center dot center dot center dot CH3 center dot center dot center dot Cl](-) saddle point, thereby opening a lower energy reaction path for retention than the front-side attack. Quasi-classical trajectory computations for the F- + CH3Cl(upsilon(1) = 0, 1) reactions show that the front-side attack is a fast direct, whereas the double inversion is a slow indirect process

Dynamics of the O(3P) + CHD3(vCH = 0,1) reactions on an accurate ab initio potential energy surface

Recent experimental and theoretical studies on the dynamics of the reactions of methane with F and Cl atoms have modified our understanding of mode-selective chemical reactivity. The O + methane reaction is also an important candidate to extend our knowledge on the rules of reactivity. Here, we report a unique full-dimensional ab initio potential energy surface for the O((3)P) + methane reaction, which opens the door for accurate dynamics calculations using this surface. Quasiclassical trajectory calculations of the angular and vibrational distributions for the ground state and CH stretching excited O + CHD(3)(v(1) = 0,1) → OH + CD(3) reactions are in excellent agreement with the experiment. Our theory confirms what was proposed experimentally: The mechanistic origin of the vibrational enhancement is that the CH-stretching excitation enlarges the reactive cone of acceptance

Rotational Mode-Specificity in the Cl + C2H6 -> HCl + C2H5 Reaction

We perform rotational mode-specific quasi-classical trajectory simulations using a high-quality ab initio analytical potential energy surface for the Cl(P-2(3/2)) + C2H6 -> HCl + C2H5 reaction. As ethane, being a prolate-type symmetric top, can be characterized by the J and K rotational quantum numbers, the excitation of two rotational modes, the tumbling (J, K = 0) and spinning (J, K = J) rotations of the reactant is carried out with J = 10, 20, 30, and 40 at a wide range of collision energies. The impacts of rotational excitation on the reactivity, the mechanism, and the post-reaction distribution of energy are investigated: (1) exciting both rotational modes enhances the reactivity with the spinning rotation being more effective due to its coupling to the C-H stretching vibrational normal modes (C-H bond elongating effect) and larger rotational energies, (2) rotational excitation increases the dominance of direct rebound over the stripping mechanism, while collision energy favors the latter, (3) investing energy in tumbling rotation excites the translational motion of the products, while the excess spinning rotational energy readily flows into the internal degrees of freedom of the ethyl radical or, less significantly, into the HCl vibration, probably due to the pronounced rovibrational coupling in this case. We also study the relative efficiency of vibrational and rotational excitation on the reactivity of the barrierless and thus translationally hindered title reaction

Benchmark ab initio potential energy surface mapping of the F + CH 3 NH 2 reaction

Ab initio computations reveal several stationary points and their relative energies at the relativistic full-configuration-interaction complete-basis-set limit along the abstraction and substitution pathways of the F + CH 3 NH 2 reaction

Vibrational mode-specific dynamics of the F- + CH3CH2Cl multi-channel reaction

We investigate the mode-specific dynamics of the ground-state, C-Cl stretching (v(10)), CH2 wagging (v(7)), sym-CH2 stretching (v(1)), and sym-CH3 stretching (v(3)) excited F- + CH3CH2Cl(v(k) = 0, 1) [k = 10, 7, 1, 3] -> Cl- + CH3CH2F (S(N)2), HF + CH3CHCl-, FHMIDLINE HORIZONTAL ELLIPSISCl- + C2H4, and Cl- + HF + C2H4 (E2) reactions using a full-dimensional high-level analytical global potential energy surface and the quasi-classical trajectory method. Excitation of the C-Cl stretching, CH2 stretching, and CH2/CH3 stretching modes enhances the S(N)2, proton abstraction, and FHMIDLINE HORIZONTAL ELLIPSISCl- and E2 channels, respectively. Anti-E2 dominates over syn-E2 (kinetic anti-E2 preference) and the thermodynamically-favored S(N)2 (wider reactive anti-E2 attack angle range). The direct (a) S(N)2, (b) proton abstraction, (c) FHMIDLINE HORIZONTAL ELLIPSISCl- + C2H4, (d) syn-E2, and (e) anti-E2 channels proceed with (a) back-side/backward, (b) isotropic/forward, (c) side-on/forward, (d) front-side/forward, and (e) back-side/forward attack/scattering, respectively. The HF products are vibrationally cold, especially for proton abstraction, and their rotational excitation increases for proton abstraction, anti-E2, and syn-E2, in order. Product internal-energy and mode-specific vibrational distributions show that CH3CH2F is internally hot with significant C-F stretching and CH2 wagging excitations, whereas C2H4 is colder. One-dimensional Gaussian binning technique is proved to solve the normal mode analysis failure caused by methyl internal rotation

Benchmark Ab Initio Determination of the Conformers, Proton Affinities, and Gas-Phase Basicities of Cysteine

A systematic conformational mapping combined with literature data leads to 85 stable neutral cysteine conformers. The implementation of the same mapping process for the protonated counterparts reveals 21 N-(amino-), 64 O-(carbonyl-), and 37 S-(thiol-)protonated cysteine conformers. Their relative energies and harmonic vibrational frequencies are given at the MP2/aug-cc-pVDZ level of theory. Further benchmark ab initio computations are performed for the 10 lowest-lying neutral and protonated amino acid conformers (for each type) such as CCSD(T)-F12a/cc-pVDZ-F12 geometry optimizations (and frequency computations for cysteine) as well as auxiliary correction computations of the basis set effects up to CCSD(T)-F12b/cc-pVQZ-F12, electron correlation effects up to CCSDT(Q), core correlation effects, second-order Douglass-Kroll relativistic effects, and zero-point energy contributions. Boltzmann-averaged 0 (298.15) K proton affinity and [298.15 K gas-phase basicity] values of cysteine are predicted to be 214.96 (216.39) [208.21], 201.83 (203.55) [194.16], and 193.31 (194.74) [186.40] kcal/mol for N-, O-, and S-protonation, respectively, also considering the previously described auxiliary corrections