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We develop a full-dimensional analytical potential energy surface (PES) for the F− + SiH3Cl reaction using Robosurfer for automatically sampling the configuration space, the robust [CCSD-F12b + BCCD(T) − BCCD]/aug-cc-pVTZ composite level of theory for computing the energy points, and the permutationally invariant polynomial method for fitting. Evolution of the fitting error and the percentage of the unphysical trajectories are monitored as a function of the iteration steps/number of energy points and polynomial order. Quasi-classical trajectory simulations on the new PES reveal rich dynamics resulting in high-probability SN2 (SiH3F + Cl−) and proton-transfer (SiH2Cl− + HF) products as well as several lower-probability channels, such as SiH2F− + HCl, SiH2FCl + H−, SiH2 + FHCl−, SiHFCl− + H2, SiHF + H2 + Cl−, and SiH2 + HF + Cl−. The Walden-inversion and front-side-attack-retention SN2 pathways are found to be competitive, producing nearly racemic products at high collision energies. The detailed atomic-level mechanisms of the various reaction pathways and channels as well as the accuracy of the analytical PES are analyzed along representative trajectories. [ABSTRACT FROM AUTHOR]

We report a detailed quasiclassical trajectory study on the F− + SiH3Cl multi-channel reaction using a full-dimensional ab initio analytical potential energy surface. Reaction probabilities, cross sections, initial attack and scattering angle distributions as well as product relative translational, internal, vibrational, and rotational energy distributions are obtained in the collision energy range of 1–40 kcal mol−1 for the following channels: SiH3F + Cl−, SiH2Cl− + HF, SiH2F− + HCl, SiH2FCl + H−, SiH2 + FHCl−, and SiHFCl− + H2. All the channels are translationally cold indicating indirect mechanisms, except proton transfer (SiH2Cl− + HF), which shows mixed direct–indirect character. The angular distributions vary depending on collision energy and inversion/retention for SiH3F + Cl−. In the case of SiH2Cl− + HF front-side/back-side attack backward–forward/forward scattering preference is found at low/high collision energy. SiH2F− + HCl is formed with isotropic scattering and the preferred angle of attack is similar to the SiH3F + Cl− channel. SiH2FCl + H−/SiH2 + FHCl− favors back-side attack and isotropic/backward scattering, whereas SiHFCl− + H2 does not show significant angular preference. [ABSTRACT FROM AUTHOR]

We determine the proton affinity (PA) and gas‐phase basicity (GB) of amino acid α‐alanine at a chemically accurate level by performing explicitly‐correlated CCSD(T)‐F12b/aug‐cc‐pVDZ geometry optimizations and normal mode vibrational frequency calculations as well as CCSD(T)‐F12b/aug‐cc‐pVTZ energy computations at the possible neutral and protonated geometries. Temperature effects at 298.15 K considering translational, rotational, and vibrational enthalpy and entropy corrections are obtained via standard statistical mechanics utilizing the molecular geometries and the harmonic vibrational energy levels. Both the amino nitrogen (N) and the carbonyl oxygen (O) atoms are proven to be potential protonation sites and a systematic conformational search reveals 3 N‐ and 9 O‐protonated conformers in the 0.00–7.88 and 25.43–30.43 kcal/mol energy ranges at 0 K, respectively. The final computed PA and GB values at (0)298.15 K in case of N‐protonation are (214.47)216.80 and 207.07 kcal/mol, respectively, whereas the corresponding values for O‐protonation are (189.04)190.63 and 182.31 kcal/mol. The results of the benchmark high‐level coupled‐cluster computations are utilized to assess the accuracy of several lower‐level cost‐effective methods such as MP2 and density functional theory with various functionals (SOGGA11‐X, M06‐2X, PBE0, B3LYP, M06, TPSS). [ABSTRACT FROM AUTHOR]

We report a detailed quasiclassical trajectory study on the F - + SiH 3 Cl multi-channel reaction using a full-dimensional ab initio analytical potential energy surface. Reaction probabilities, cross sections, initial attack and scattering angle distributions as well as product relative translational, internal, vibrational, and rotational energy distributions are obtained in the collision energy range of 1-40 kcal mol -1 for the following channels: SiH 3 F + Cl - , SiH 2 Cl - + HF, SiH 2 F - + HCl, SiH 2 FCl + H - , SiH 2 + FHCl - , and SiHFCl - + H 2 . All the channels are translationally cold indicating indirect mechanisms, except proton transfer (SiH 2 Cl - + HF), which shows mixed direct-indirect character. The angular distributions vary depending on collision energy and inversion/retention for SiH 3 F + Cl - . In the case of SiH 2 Cl - + HF front-side/back-side attack backward-forward/forward scattering preference is found at low/high collision energy. SiH 2 F - + HCl is formed with isotropic scattering and the preferred angle of attack is similar to the SiH 3 F + Cl - channel. SiH 2 FCl + H - /SiH 2 + FHCl - favors back-side attack and isotropic/backward scattering, whereas SiHFCl - + H 2 does not show significant angular preference.

We characterize the stationary points along the Walden inversion, front-side attack, and double-inversion pathways of the X - + CH 3 Y and X - + SiH 3 Y [X, Y = F, Cl, Br, I] S N 2 reactions using chemically accurate CCSD(T)-F12b/aug-cc-pV n Z [ n = D, T, Q] levels of theory. At the carbon center, Walden inversion dominates and proceeds via prereaction (X - ···H 3 CY) and postreaction (XCH 3 ···Y - ) ion-dipole wells separated by a usually submerged transition state (X-H 3 C-Y) - , front-side attack occurs over high barriers, double inversion is the lowest-energy retention pathway for X = F, and hydrogen- (F - ···HCH 2 Y) and halogen-bonded (X - ···YCH 3 ) complexes exist in the entrance channel. At the silicon center, Walden inversion proceeds through a single minimum (X-SiH 3 -Y) - , the front-side attack is competitive via a usually submerged transition state separating pre- and postreaction minima having X-Si-Y angles close to 90°, double inversion occurs over positive, often high barriers, and hydrogen- and halogen-bonded complexes are not found. In addition to the S N 2 channels (Y - + CH 3 X/SiH 3 X), we report reaction enthalpies for proton abstraction (HX + CH 2 Y - /SiH 2 Y - ), hydride substitution (H - + CH 2 XY/SiH 2 XY), XH···Y - complex formation (XH···Y - + 1 CH 2 / 1 SiH 2 ), and halogen abstraction (XY + CH 3 - /SiH 3 - and XY - + CH 3 /SiH 3 ).