Themed collection Unimolecular reactions

Dr Struan H. Robertson introduces the Faraday Discussion on unimolecular reactions, held in Oxford during June 2022, 100 years after the publication of the Faraday Discussion where Lindemann proposed collisions as the mechanistic basis behind unimolecular reactions.

100 years after Lindemann, advances in prediction and measurement of reactions are summarized. Needed next steps, including extensions to liquid phase, are highlighted.

We develop a microcanonical version of instanton theory for studying deep tunnelling reactions under the statistical assumptions of RRKM. The new theory provides a correction to the theory of Chapman, Garrett and Miller for non-separable systems.

The historical and continuing advances in our understanding of unimolecular reaction dynamics have arisen from the synergy between improvements in experimental measurements and in theoretical methodologies.

We demonstrate a significant mechanism for pressure dependence of bimolecular reactions that has not historically been considered: high-energy reactants are depleted by unimolecular dissociation and not available to undergo bimolecular reaction.

Rotational energy transfer of optically centrifuged CO is investigated with high-resolution transient IR absorption probing and master equation modeling. Observed rates are smaller than simulated rates, highlighting the role of angular momentum in collisional relaxation.

New quantum chemical and statistical rate theory calculations predict branching fractions for the acetylperoxy + HO₂ reaction in fair to good agreement with recent experiment.

The unimolecular decomposition of 2,4-dimethyloxetane peroxy radicals is a competition between conventional and ring opening pathways controlled by stereochemistry.

Electron ionisation is a fundamental ionisation process that often leads to unimolecular dissociation. Velocity-map and covariance-map imaging experiments provide detailed insight into the often complex dissociation dynamics.

Lennard-Jones self-collision diameters of benzene and PAHs derived from different methods.

We measure second-order rate coefficients for a suite of radical-ion reactions involving unsaturated hydrocarbons (C₂H₂ and C₂H₄), and report our efforts to develop an accurate modelling framework using a Rice–Ramsperger–Kassel–Marcus theory Master Equation approach.

The channel branching between the unimolecular decomposition steps of dimethoxymethane is analyzed with a multichannel master equation.

The O(³P) + 1,3 C₄H₆ reaction is studied through non adiabatic AITSTME simulations and CMB experiments. The main reaction channels are HCO + C₃H₅, CO + C₃H₆, H₂CO + C₃H₄, and H + C₄H₅O. Temperature dependent rates are then theoretically determined.

In this work we benchmark the performance of various methods for obtaining pressure-dependent phenomenological rate coefficients against direct numerical simulations and propose a new method called simulation least-squares.

The unimolecular isomerisation of the propargyl + propargyl “head-to-head” adduct, 1,5-hexadiyne to fulvene and benzene via 3,4-dimethylenecyclobut-1-ene (all C₆H₆) was studied in the high-pressure limit by threshold photoelectron spectroscopy.

Modelling case study on the role of pressure dependence in single event kinetic modelling for steam cracking of both ethane and propane. Results are validated with in-house generated experimental data.

Product HCO rovibrational levels that are near-coincident with prepared rovibrational levels in H₂CO mediate roaming resonances that impact the roaming, tight-TS and radical yield.

The photoinduced unimolecular decay of the electronically excited HCO(òA′′) is investigated in a combined experimental–theoretical study.

A priori theory quantitatively predicts pressure-dependent kinetics for polyatomic and diatomic bath gases.

A combination of high-level coupled cluster theory, Active Thermochemical Tables, and master-equation simulations is used to study the reversible reactions: C₂H₃ + H₂ ⇌ C₂H₄ + H ⇌ C₂H₅.

The phenomenological descriptions of complicated R + O₂ ↔ RO₂ → products reaction systems simplify significantly under pre-equilibrium conditions.

We investigated the role of second-order saddle points on the dynamics of the thermal denitrogenation of 1-pyrazoline using ab initio classical trajectory simulations at the CASSCF(4,4)/6-31+G* level of theory.

Unimolecular decay of infrared activated hydroperoxyalkyl radicals (˙QOOH) observed via time-resolved appearance of OH radical products.

A statistical treatment of the reaction N + OH → NO + H is developed to characterize the redissociation processes by a state-specific criterion.

Reaction-rate analysis of thermal ring cleavage in sooting flames shows that it can be comparable in rate to oxyradical decomposition and that fast internal ring radical migration is comparable in frequency to reaction events of aromatic growth.

We show that, in the very low pressure regime of a stellar outflow, molecules can exhibit significant vibrational disequilibrium because optical transitions occur on a faster timescale than collisions; this profoundly affects their reaction kinetics.

Combined advanced ab initio treatments and experiments reveal that the pyruvic acid cation decomposes to mostly form HOCO, in contrast to the photodynamics of electronically excited pyruvic acid being dominated by decarboxylation.

A new extension of the TUMME master-equation program is used to explore the time evolutions of the concentrations of the OH radical and the reaction complex under pseudo-first-order conditions.