Discrepancy Between Experimental and Theoretical Predictions of the Adiabaticity of Ti⁺+CH₃OH

Abstract

The reaction between Ti and methanol (CH OH) is a model system for competition between activation of C-O, C-H, and O-H bonds and of the role of excited electronic pathways in catalytic processes. Herein, we use experimental kinetics, quantum chemical calculations, and statistical modeling to identify the critical features of the reaction's potential energy surface. Experimental kinetics data between 300 and 600 K shows the reaction largely proceeds through C-O bond activation, yielding TiOH and TiO . Products of the O-H activation pathway, TiOCH and TiOCH are minor, whereas C-H bond activation is not observed at thermal energies. Statistical modeling well-reproduces the experimental results and offers insight into the reaction mechanism. Notably, efficient spin-crossing along the C-O activation pathway is required to produce the observed product distribution, in contrast to a previous estimate of inefficient crossing based on calculation of a small spin-orbit coupling constant. This discrepancy highlights a potential limitation of simple models within the Landau-Zener framework, which are commonly used to calculate surface-crossing probabilities in reactive systems.

Old School Techniques with Modern Capabilities: Kinetics Determination of Dynamical Information Such as Barriers, Multiple Entrance Channel Complexes, Product States, Spin Crossings, and Size Effects in Metallic Ion–Molecule Reactions

“…However, temperature-dependent kinetics of the system are instead consistent with formation of ground state TiO + , and the complicated temperature-dependent product branching is consistent with a high crossing probability (Figure 7). 118 The failure to predict the crossing probability could be due to the consideration of the crossing at a single point, instead of along the entirety of the crossing seam, or more likely, the difficulties in the Landau−Zener framework of determining molecular velocity or multiple crossing attempts due to finite lifetimes. The latter refers to the fact that in a long-lived well, the ability to cross does not involve a single approach but many.…”

Thermal activation of methane by MgO+: temperature dependent kinetics, reactive molecular dynamics simulations and statistical modeling

“…The calculated energies and vibrational and rotational frequencies along the reaction coordinate were inputs to statistical modeling of the reaction, described in detail elsewhere. 36 Briefly, formation of an initial intermediate is determined using capture theory and the simplified statistical adiabatic channel model (SSACM), 37,38 with reactant internal and collision energies varied over thermal distributions in a stochastic manner. Intermediates are assumed to be sufficiently long-lived that the fundamental statistical assumption of energy redistribution is met, and the fate of the intermediate determined by calculated unimolecular rate curves as a function of both energy and angular momentum, (E, J), for all exit channels.…”

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

“…The temperature dependences of these competing processes are reflected in the energy dependences of the other processes occurring in the well, while the ISC occurs with an energy independent rate. Theoretical approaches to predicting the likelihood of spin crossings in reactions such as these are computationally taxing, and simplified predictions based on spin‐orbit coupling or a Landau‐Zener approach can be misleading (Schultz & Armentrout, 1987; Sweeny et al, 2017). Our approach is only semi‐quantitative, yet our aim is to apply this to a broad enough range of systems across the periodic table, a la Diethard, to gain a deeper understanding at a reasonable cost.…”