Temperature and energy dependences of ion-molecule reactions: Studies inspired by Diethard Böhme

High-temperature reactivity of vanadium oxide clusters in methane activation: Vibrational degrees of freedom matter

Understanding the properties of small particles working under high-temperature conditions at the atomistic scale is imperative for exact control of related processes, but it is quite challenging to achieve experimentally. Herein, benefitting from state-of-the-art mass spectrometry and by using our newly designed high-temperature reactor, the activity of atomically precise particles of negatively charged vanadium oxide clusters toward hydrogen atom abstraction (HAA) from methane, the most stable alkane molecule, has been measured at elevated temperatures up to 873 K. We discovered the positive correlation between the reaction rate and cluster size that larger clusters possessing greater vibrational degrees of freedom can carry more vibrational energies to enhance the HAA reactivity at high temperature, in contrast with the electronic and geometric issues that control the activity at room temperature. This finding opens up a new dimension, vibrational degrees of freedom, for the simulation or design of particle reactions under high-temperature conditions.

Proton transfer-induced competing product channels of microsolvated Y-(H2O)n + CH3I (Y = F, Cl, Br, I) reactions

The potential energy profiles of three proton transfer-involved product channels for the reactions of Y^-(H₂O)_1,2 + CH₃I (Y = F, Cl, Br, I) were characterized using the B97-1/ECP/d method. These three channels include the (1) PT_CH3 product channel that transfers a proton from methyl to nucleophile, (2) HO^--induced nucleophilic substitution (HO^--S_N2) product channel, and (3) oxide ion substitution (OIS) product channel that gives CH₃O^- and HY products. The reaction enthalpies and barrier heights follow the order OIS > PT_CH3 > HO^--S_N2 > Y^--S_N2, and thus HO^--S_N2 can compete with the most favored Y^--S_N2 product channel under singly-/doubly-hydrated conditions, while the PT_CH3 channel only occurs under high collision energy and the OIS channel is the least probable. All product channels share the same pre-reaction complex, Y^-(H₂O)_n-CH₃I, in the entrance of the potential energy profile, signifying the importance of the pre-reaction complex. For HO^-/Y^--S_N2 channels, we considered front-side attack, back-side attack, and halogen-bonded complex mechanisms. Incremental hydration increases the barriers of both HO^-/Y^--S_N2 channels as well as their barrier difference, implying that the HO^--S_N2 channel becomes less important when further hydrated. Varying the nucleophile Y^- from F^- to I^- also increases the barrier heights and barrier difference, which correlates with the proton affinity of the nucleophiles. Energy decomposition analyses show that both the orbital interaction energy and structural deformation energy of the transition states determine the S_N2 barrier change trend with incremental hydration and varying Y^-. In brief, this work computes the comprehensive potential energy surfaces of the HO^--S_N2 and PT_CH3 channels and shows how proton transfer affects the microsolvated Y^-(H₂O)_1,2 + CH₃I reaction by competing with the traditional Y^--S_N2 channel.

Computational Studies of Coinage Metal Anion M- + CH3X (X = F, Cl, Br, I) Reactions in Gas Phase

We characterized the stationary points along the nucleophilic substitution (S_N2), oxidative insertion (OI), halogen abstraction (XA), and proton transfer (PT) product channels of M⁻ + CH₃X (M = Cu, Ag, Au; X = F, Cl, Br, I) reactions using the CCSD(T)/aug-cc-pVTZ level of theory. In general, the reaction energies follow the order of PT > XA > S_N2 > OI. The OI channel that results in oxidative insertion complex [CH₃–M–X]⁻ is most exothermic, and can be formed through a front-side attack of M on the C-X bond via a high transition state OxTS or through a S_N2-mediated halogen rearrangement path via a much lower transition state invTS. The order of OxTS > invTS is inverted when changing M⁻ to Pd, a d¹⁰ metal, because the symmetry of their HOMO orbital is different. The back-side attack S_N2 pathway proceeds via typical Walden-inversion transition state that connects to pre- and post-reaction complexes. For X = Cl/Br/I, the invS_N2-TS’s are, in general, submerged. The shape of this M⁻ + CH₃X S_N2 PES is flatter as compared to that of a main-group base like F⁻ + CH₃X, whose PES has a double-well shape. When X = Br/I, a linear halogen-bonded complex [CH₃−X∙··M]⁻ can be formed as an intermediate upon the front-side attachment of M on the halogen atom X, and it either dissociates to CH₃ + MX⁻ through halogen abstraction or bends the C-X-M angle to continue the back-side S_N2 path. Natural bond orbital analysis shows a polar covalent M−X bond is formed within oxidative insertion complex [CH₃–M–X]⁻, whereas a noncovalent M–X halogen-bond interaction exists for the [CH₃–X∙··M]⁻ complex. This work explores competing channels of the M⁻ + CH₃X reaction in the gas phase and the potential energy surface is useful in understanding the dynamic behavior of the title and analogous reactions.