Unimolecular reactions - Book of abstracts

13-14 June 2022, London and Online Analytical Research Forum 2022 (ARF22)

22-24 June 2022, Oxford, UK and Online Unimolecular reactions Faraday Discussion #FDUnimolecular

Book of abstracts

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Introduction

Unimolecular reactions Faraday Discussion is organised by the Faraday Division of the Royal Society of Chemistry. This book contains abstracts of the 27 posters presented at Unimolecular reactions Faraday Discussion . All abstracts are produced directly from typescripts supplied by authors. Copyright reserved. Oral presentations and discussions All delegates at the meeting, not just speakers, have the opportunity to make comments, ask questions, or present complementary or contradictory measurements and calculations during the discussion. If it is relevant to the topic, you may give a 5-minute presentation of your own work during the discussion. These remarks are published alongside the papers in the final volume and are fully citable. If you would like to present slides during the discussion, please let the session chair know and load them onto the computer in the break before the start of the session. Faraday Discussion volume Copies of the discussion volume will be distributed approximately 6 months after the meeting. To expedite this, it is essential that summaries of contributions to the discussion are received no later than Friday 1 July 2022 for questions and comments and Friday 15 July 2022 for responses. Posters Posters have been numbered consecutively: P01-P27 The poster session will take place on Wednesday 22 June at 18:15. The posters will be available to view throughout the discussion by clicking on the link in the virtual lobby. During the dedicated poster sessions, the authors will be available to use the networking functions in the virtual lobby. Use the inbox in the top light blue bar of the virtual lobby screen to send the poster presenter a message or request a video call with them by clicking on their name in the networking section at the bottom of the screen. Poster prize The Faraday Discussions poster prize will be awarded to the best student poster as judged by the committee. Poster prize sponsored by:

Networking sessions There will be regular breaks throughout the meeting for socialising, networking and continuing discussions started during the scientific sessions. During the networking sessions, all delegates will have access to join online networking rooms and can set theses up from the virtual lobby.

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Scientific Committee

Invited Speakers

Struan Robertson (Chair) Dassault Systèmes Ltd., UK

Stephen J Klippenstein (Introductory lecture) Argonne National Laboratory, USA William H Green (Closing remarks lecture) MIT, USA

Paul Seakins University of Leeds, UK

Judit Zádor Sandia National Laboratories, USA

Mike Burke Columbia University, USA

Andrew Orr-Ewing University of Bristol, UK

Ahren W Jasper Argonne National Laboratory, USA Marsha Lester University of Pennsylvania, USA

György Lendvay Hungarian Academy of Sciences, Hungary

Claire Vallance University of Oxford, UK

Amy Mullin University of Maryland, USA

David Osborn Sandia National Laboratories, USA

Arthur Suits University of Missouri, USA Xiaoqing You Tsinghua University, China

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In order to record the discussion at the meeting, which forms part of the final published volume, your name and e-mail address will be stored in the Faraday Forum. This information is used for the collection of questions and responses communicated during each session. After each question or comment you will receive an e-mail which contains some keywords to remind you what you asked, and your password information for the forum. The e-mail is not a full record of your question. You need to complete your question in full on the forum . The deadline for completing questions and comments is Friday 1 July 2022.

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Poster presentations

P01

Isotope-labeled recombination pathways of ozone formation Dmitri Babikov Marquette University, USA Kinetic investigation of a promising synthetic route for several prebiotic molecules: the reaction between vinyl alcohol and radical cyanide Bernardo Ballotta Scuola Normale Superiore di Pisa, Italy Unimolecular chemistry of neutrals and cations by photoelectron photoion coincidence spectroscopy Andras Bodi Paul Scherrer Institute, Switzerland

P02

P03

P04

Direct study of peroxy radical unimolecular reactions Rabi Chhantyal Pun University of Nottingham, UK

P05

On the automation of VRC-TST simulations: strategies to determine wave function guesses, exploration of black box methodologies, and application to test systems Luigi Crisci Scuola Normale Superiore di Pisa/Universitá Degli Studi di Napoli Federico II, Italy Investigation of the reactivity on the C 4 H 6 O PES: methyl-vinyl ketone, 2- and 3-butenal decomposition Andrea Della Libera Politecnico di Milano, Italy Master equation modelling of the reactions of NH 2 with CH 2 O and NO Kevin Douglas University of Leeds, UK The formation of ethylidene in the thermal decomposition of ethane: a theoretical and experimental study Nadav Genossar Ben Gurion University of the Negev, Israel

P06

P07

P08

P09

Non-physical species in pressure-dependent networks: by the switch of an atom Alon Grinberg Dana Technion - Israel Institute of Technology, Israel Dramatic unimolecular decay of an unsaturated Criegee intermediate via allylic 1,6 H-atom transfer Anne S Hansen University of Pennsylvania, USA Unimolecular decomposition of decalin and methyldecalin Subharaj Hossain Indian Institute of Science, India Conformation-targeted dynamics of NO: alkane molecular complexes Nathanael Kidwell William & Mary, USA Novel OH roaming pathway in the unimolecular decay of alkyl- substituted Criegee intermediates Marsha Lester University of Pennsylvania, USA Photodissociation dynamics of N,N-dimethylformamide at 225 nm and 245 nm Dennis Milesevic University of Oxford, UK Combined crossed-beams and theoretical investigation of the O( 3 P) + acrylonitrile reaction: dominant formation of ketenimine (CH 2 CNH) via intersystem crossing Giacomo Pannacci Università degli Studi di Perugia, Italy Prompt carbon structure rearrangements enable rapid highly oxygenated organic molecule formation from a-pinene Matti Rissanen Tampere University, Finland Disentangling dication dissociation dynamics using covariance map imaging Patrick Robertson University of Oxford, UK

P10

P11

P12

P13

P14

P15

P16

P17

P18

Experimentally unravelling the ultrafast dynamics of thermal-energy chemical reactions Matthew Robinson CFEL / DESY/ Centre for Ultrafast Imaging, Hamburg, Germany A simplified multiple-well approach for the master equation modeling of blackbody infrared radiative dissociation (BIRD) of hydrated carbonate radical anions Magalena Salzburger Universitaet Innsbruck, Austria CH + H 2 : extracting the maximum rate coefficient information Paul Seakins University of Leeds, UK Boxed molecular dynamics as a tool for unimolecular rate theory Robin Shannon University Of Leeds, UK Modelling distonic radical ion kinetics: a systematic investigation of phenyl-type radical addition to unsaturated hydrocarbons Oisin Shiels University of Wollongong, Australia Master equation studies on unimolecular processes in the ozonolysis of isoprene Thomas Stephenson Swarthmore College, USA Coincidence ion pair production (cipp) spectroscopy of halogen molecules Balint Sztaray University of the Pacific, USA Mechanisms and dynamics of the thermal deazetization of 2,3-diazabicyclo[2.2.1]hept-2-ene Komal Yadav National Institute of Science Education and Research (NISER) Bhubaneswar, India

P19

P20

P21

P22

P23

P24

P25

P26

Machine learning the C 5 H 5 potential energy surface Yoona Yang Sandia National Laboratories, USA

P27

How the correlated quantum chemical calculation changes the uncertainty of theoretically predicted rate coefficients and branching ratios Feng Zhang University of Science and Technology of China, China

Isotope-labeled recombination pathways of ozone formation Elizaveta Grushnikova, Igor Gayday and Dmitri Babikov Marquette University, Chemistry Department, USA Theoretical approach is developed for the description of all possible recombination pathways for the Lindemann mechanism of ozone formation, without neglecting any process, and without decoupling the individual pathways one from another. These pathways become physically distinct when a rare isotope of oxygen is introduced, such as 18 O, which represents a sensitive probe of the reaction mechanism. Each isotopologue of ozonecontains two types of physically distinct entrance channels and two types of physically distinct product wells, creating four recombination pathways. Calculations are done for singly and doubly substituted isotopologues of ozone, eight rate coefficients total. Two pathways for the formation of asymmetric ozone isotopomer exhibit rather different rate coefficients, indicating large isotope effect driven by ΔZPE of the two channels. Rate coefficient for the formation of symmetric isotopomer of ozone (third pathway) is found to be in between of those two, while the rate of insertion pathway is smaller by two orders of magnitude. These trends are in good agreement with experiments, for both singly and doubly substituted ozone. The total formation rates for asymmetric isotopomers are found to be somewhat larger than those for symmetric isotopomers, but not as much as in the experiment. Overall, the distribution of lifetimes is found to be very similar for the metastable states in symmetric and asymmetric ozone 1. I. Gayday and D. Babikov, “Efficient method for an approximate treatment of Coriolis effect in the calculations of quantum dynamics and spectroscopy, with application to scattering resonances in ozone”, J. Phys. Chem. A 125, 5661–5669, 2021. 2. I. Gayday, E. Grushnikova and D. Babikov, "Influence of the Coriolis effect on the properties of scattering resonances in symmetric and asymmetric isotopomers of ozone", Phys. Chem. Chem. Phys. 22, 27560 - 27571, 2020. 3. I. Gayday, A. Teplukhin, B. Kendrick and D. Babikov, “On the role of rotation-vibration coupling in the spectra of ozone isotopomers” J. Chem. Phys. 152, 144104 (16 pages), 2020. isotopomers. References

P01

Kinetic investigation of a promising synthetic route for several prebiotic molecules: the reaction between vinyl alcohol and radical cyanide Bernardo Ballotta 1 , Surajit Nandi 2 , Sergio Rampino 1,3 ,Vincenzo Barone 1,3 1 SMART Laboratory, Scuola Normale Superiore, Italia, 2 Department of Energy Conversion and Storage, Technical University of Denmark, Denmark, 3 Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Italia Vinyl alcohol (Vy) and radical cyanide (CN) are two relatively abundant molecules present in the interstellar medium (ISM). In particular, Vy’s microwave transitions have been detected in emission toward the dense molecular cloud SagittariusB2 (N) which is one of the most studied by astronomers because it is rich in prebiotic interstellar complex organic molecules (iCOMs). Vy is the enolic tautomer of acetaldehyde, another very abundant species in the ISM [1] . It can exist in two rotameric forms, syn and anti , depending on the value assumed by the dihedral angle φ (C=C-O-H), which defines the position of the hydroxyl hydrogen with respect to the double bond [2] . To find out possible iCOMs formation pathways, investigations are ongoing in our group on the gas- phase reactions between Vy’s conformers and CN for which, to the best of our knowledge, no kinetic data are available in the literature. The reactions have been preliminarily characterized through a quantum mechanical approach based on the double hybrid functional DSDPBEP86-GD3BJ in conjunction with the jun-cc-pVTZ basis set. Preliminary results suggest that Vy’s conformers feature a completely different reactivity with CN. For the anti conformer, the hydrogen abstraction leading to radical Vy + HCN is strongly favored with respect to the association of CN to the double bond. For the syn conformer, the barrierless association reaction to the double bond shows a strong exothermicity leading to formation of an intermediate lying at -250.8 kJ mol -1 with respect to the reactant asymptote. Work is ongoing in our group in order to refine the potential energy surface through high- level ab initio composite schemes and to perform a kinetic analysis through StarRate [3] , a program specifically designed for astrochemical reactions, based on a master equation approach coupled to capture theory for the bimolecular association steps and to the Rice-Ramsperger-Kassel-Marcus for the unimolecular evolution of the intermediates. References 1. Turner, B. E. and Apponi, A. J. Microwave Detection of Interstellar Vinyl Alcohol, CH 2 =CHOH. The Astrophysical Journal Letters. 561, L207 . 2. Kleimeier, N. F. and Kaiser, R. I. Interstellar Enolization-Acetaldehyde (CH3CHO) Vinyl Alcohol (H2CCH(OH)) as a Case study. ChemPhysChem , 2021, 22, 1-9. 3. Nandi, S.; Ballotta, B.; Rampino, S.; Barone, V. A General User-Friendly Tool for Kinetic Calculations of Multi-Step Reactions within the Virtual Multifrequency Spectrometer Project. Applied Sciences . 2020, 10(5), 1872.

P02

Unimolecular chemistry of neutrals and cations by photoelectron photoion coincidence spectroscopy Andras Bodi 1 , Patrick Hemberger 1 , Bálint Sztáray 2 , Jordy Bouwman 3 and Xiangkun Wu 1 1 Paul Scherrer Institute, Switzerland, 2 University of the Pacific, United States, 3 University of Colorado, United States Threshold photoionization has been established as a versatile tool to understand the unimolecular fragmentation of internal energy selected ions 1 as well as a universal, sensitive and specific tool to detect elusive intermediates in gaseous reactive environments. 2, 3 Here, we review how PEPICO detection and statistical modeling revealed six fragmentation channels leading to only three fragment ion mass channels and their kinetics in the 1,3-dioxolane cation. 4 Neutral detection can also offer insights into competing reaction steps. For instance, several product channels are energetically allowed in the barrierless association reaction of benzyne + allyl. The observed branching ratios of the intermediate, derived based on photoion mass-selected threshold photoelectron spectra, as well as the unimolecular stabilization of various isomerization products by hydrogen atom loss, observed in a flash vacuum pyrolysis experiment, can be understood in terms of the reactor environment and unimolecular kinetics. 5 Detection minutiae in probing pyrolysis experiments, such as sampling effects and temperatures, will also be addressed. 6 References 1. T. Baer and R. P. Tuckett, Phys Chem Chem Phys , 2017, 19 , 9698-9723. 2. P. Hemberger, J. A. van Bokhoven, J. Pérez-Ramírez and A. Bodi, Catal Sci Technol , 2020, 10 , 1975-1990. 3. P. Hemberger, A. Bodi, T. Bierkandt, M. Köhler, D. Kaczmarek and T. Kasper, Energy Fuels , 2021, 35 , 16265-16302. 4. P. Weidner, K. Voronova, A. Bodi and B. Sztaray, J. Mass Spectrom. , 2020, 55 , e4522. 5. M. N. McCabe, P. Hemberger, E. Reusch, A. Bodi and J. Bouwman, J Phys Chem Lett , 2020, 11 , 2859-2863. 6. P. Hemberger, X. Wu, Z. Pan and A. Bodi, J Phys Chem A , 2022, DOI: 10.1021/acs.jpca.2c00766

P03

Direct study of peroxy radical unimolecular reactions Rabi Chhantyal-Pun University of Nottingham, UK

Biogenic volatile organic compounds such as isoprene and pinenes emitted by plants play an important role in the atmosphere by setting the background chemistry. These compounds predominantly undergo OH radical initiated oxidation in the troposphere to produce peroxy radicals which are key intermediates. Peroxy radicals react with nitric oxide to form nitrogen dioxide and eventually ozone, which lead to reduction in air quality. Autoxidation has been suggested to be the dominant peroxy radical reaction pathway in the low nitric oxide environments.[1] Recently, autoxidation of peroxy radicals has been shown to initiate formation of highly oxygenated molecules which provide a purely biogenic pathway for nucleation and aerosol formation.[2] Such pathwaysare critical to our understanding of pre-industrial, present and future environments. Autoxidation involves intramolecular H- atom abstraction by the peroxy radical terminal oxygen atom, followed by addition of oxygen molecule to form another peroxy radial. The intramolecular H- atom transfer reaction is expected to be highly structure dependent. Near infrared cavity ring-down spectroscopy (CRDS) of peroxy radicals has been shown to provide structure specific probe and is ideal for kinetic measurements.[3] A new CRDS apparatus being developed at Nottingham to provide direct time resolved measurement of the unimolecular H- atom transfer reaction of peroxy radical will be presented. References 1. Crounse et al. J. Phys. Chem. Lett. 2013, 4, 3513 2. Kirkby et al. Nature 2016, 533, 521 3. Chhantyal-Pun et al. J. Phys. Chem. Lett. 2010, 1, 1846

P04

On the automation of VRC-TST simulations: strategies to determine wave function guesses, exploration of black box methodologies, and application to test systems Luigi Crisci 1,2 , Andrea Della Libera 3 , Carlo Cavallotti 3 , Nadia Rega 2,4,5 and Vincenzo Barone 1,6 1 Scuola Normale Superiore, Italy; 2 Dipartimento di Scienze Chimiche, Universitá di Napoli Federico II, Italy; 3 Dipartimento di Chimica, Italy; 4 Scuola superiore Meridionale, Italy; 5 Centro di Ricerca Interdipartimentale sui Biomateriali (CRIB), Italy; 6 Istituto Nazionale di Fisica Nucleare (INFN) sezione di Pisa, Italy Theoretical research on the reactivity of singlet potential energy surfaces (PES) is critical for the entire gas- phase kinetics community. It is useful in combustion to forecast fragmentation patterns of molecules present in biomass components, as well as in atmospheric chemistry and astrochemistry to examine reaction pathways that can be accessible following the recombination of two radicals. Its correct description, however, is a difficult undertaking since the quickest exit (or entry) channels on singlet PESs are often barrierless reactions. In these cases, determining rate constants can be quite difficult because it either necessitates computationally expensive trajectory calculations involving high quality multidimensional analytic PESs, or it requires the use of a variational form of transition state theory (TST). The latter technique, whose most advanced form is Variable Reaction Coordinate Transition State Theory (VRC-TST), [1,2] necessitates the stochastic sampling of a 6-Dimensional PES using multireference ab initio methods if accuracy is demanded. Neither VRC-TST nor multireference ab initio computational techniques have black box implementations at the moment. The current work focuses on the creation of techniques for making VRC-TST theory more accessible by enhancing the stability of the stochastic multidimensional sampling step. We concentrate on two distinct characteristics of VRC-TST simulations. VRC- TST computations are done on the PES utilizing multireference simulations in its most common implementation. One feature of this technique that hinders implementation is the requirement that each single point energy (SPE) estimation conducted on the PES uses the same active space in order to be consistent (AS). However, developing an approach that ensures the selection of the same active area for the enormous number of SPE determinations required by VRC-TST theory, frequently in the tens of thousands, is problematic. In this paper, we describe a method we devised to ensure that electrical structure computations always converge to the same AS. To reach these findings, we link VRC-TST computations with EStokTP [3] to create a CASPT2 input in which the structure is translated to internal coordinates so that it may use a reference guess wave function to systematically converge to the required electronic structure. The second way we suggested to improve VRC-TST robustness is to use Density Functional Theory (DFT) for sampling the PES, again utilizing reference wavefunctions as guesses. When applied to barrierless processes, correction potentials based on multireference computations are used to improve DFT predictive capabilities. In this paper, we discuss the implications of using alternative functionals to estimate rate constants for a variety of systems. References 1. L. B. Harding, Y. Georgievskii, S. J. Klippenstein, J. Phys. Chem. A , 109, 4646-4656 (2005). 2. Y. Georgievskii, L. B. Harding, and S. J. Klippenstein, VaReCoF 2016.3.23. 3. C. Cavallotti, M. Pelucchi, Y. Georgievskii, S.J. Klippenstein, J.Chem.Theory Comput. 4. 15, 1122–1145 (2018).

P05

Investigation of the reactivity on the C 4 H 6 O PES: methyl-vinyl ketone, 2- and 3-butenal decomposition Andrea Della Libera and Carlo Cavallotti Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Italy The theoretical investigation of the reactivity on singlet potential energy surfaces (PES) is a challenging task, since it requires characterization of all the isomers that can be reached from each reacting well under investigation, the saddle points by which they are connected, and the exit channels. A further obstacle to the exploration of singlet PESs is the possible diradical nature of saddle points, which is badly described by black box single reference ab initio methods routinely adopted to investigate PESs, such as density functional theory. Finally, the fastest exit channels are usually barrierless, which require sophisticated methods to determine their reaction rates, such as Variable Reaction Coordinate Transition State Theory (VRC-TST) together with a multidimensional sampling of the PES with multireference ab initio techniques. In this work, we used the EStokTP software [1] to investigate the reactivity on the C 4 H 6 O singlet PES using automated or partially automated approaches to overcome the afore-mentioned hurdles. The interest in this system is determined by the fact that the reactivity of the main C 4 H 6 O isomers, methyl-vinyl ketone (MVK), 2-butenal (2-BUT), and 3-butenal (3-BUT), which are important intermediates in combustion and atmospheric chemistry, is only partially understood. The approach we followed consisted in the study of isomerization and decomposition pathways of MVK, 2-BUT, and 3-BUT. Saddle points connecting the isomers were identified. Geometries and frequencies were determined at the UωB97XD/ aug-cc-pVTZ level, while energies at the CCSD(T) level with extrapolation to the complete basis set and accounting for the correlation of core electrons. Diradical singlet transition states’ energies were obtained with the multireference method CASPT2. Energy barriers leading to bimolecular exit channels, namely CO and C 3 H 6 , ethene and ketene, and H 2 + CH 2 CHCHCO, were computed. The lowest energy barrier, 43.2 kcal/mol, was identified for the decomposition of 3-BUT to CO and C 3 H 6 . Rate constants of seven barrierless reactions (and corresponding recombinations) were determined using our recent partially automated VRC- TST protocol, which relies on EstokTP [1] for the preparation of the input files and VaReCoF [2] for VRC-TST calculations. All multireference calculations were performed with the Molpro suite of packages [3] at the CASPT2 level of theory. Master equation simulations were run with MESS [4] to determine the rates of all the reactive channels. The results of the simulations were compared with available experimental data, giving new insights into the decomposition pathways of these molecules. At low pressure, the main products are CO and C 3 H 6 . When pressure and temperature increase, the barrierless channel from 3-BUT to HCO + C 3 H 5 becomes dominant. In conclusion, we determined the temperature dependence of seven recombination reaction rates and analyzed in depth the reactivity of the principal isomers of the C 4 H 6 O PES, which is an important step forward in the comprehension of this system. References 1. C. Cavallotti, M. Pelucchi, Y. Georgievskii, S. J. Klippenstein, J. Chem. Theory Comput., 2019, 15 , 1122. 2. L. B. Harding, Y. Georgievskii, S. J. Klippenstein, J. Phys. Chem. A, 2005, 109 , 4646-4656. 3. H.-J. Werner, P. J. Knowles, F. R. Manby, et al. J. Chem. Phys., 2020, 152 , 144107. 4. Y. Georgievskii, J. A. Miller, M. P. Burke, S. J. Klippenstein, J. Phys. Chem. A, 2013, 117 , 12146-12154.

P06

Master equation modelling of the reactions of NH 2 with CH 2 O and NO Kevin M. Douglas 1 , Daniel Lucas 1 , Catherine Walsh 2 , Niclas A. West 1 , Mark A. Blitz 1 and Dwayne E. Heard 1 1 School of Chemistry, University of Leeds, UK, 2 School of Physics, University of Leeds, UK The reaction of NH 2 with CH 2 O has been suggested as a source of formamide (NH 2 CHO) in interstellar environments. To investigate this, we have used the Master Equation Solver for Multi-Energy well Reactions (MESMER) 1 to predict temperature and pressure dependent rate coefficients and branching ratios (BRs) for the reaction. The potential energy surface of NH 2 + CH 2 O system was calculated at the CCSD(T)//M062X-aug- cc-pVTZ level of theory using the Gaussian 09 suit of programs.The reaction may proceed via the formation of one of two pre-reaction complexes (PRCs), from which there are two exothermic product channels; a hydrogen- abstraction channel in which the NH 2 abstracts an H atom from formaldehyde to produce ammonia, NH 3 , and the formyl radical, CHO (R1a), and an addition-elimination channel in which the NH 2 first attacks the C of the formaldehyde to form a bound adduct, which then goes on to eliminate an H atom and produce formamide + H (R1b): NH 2 + CH 2 O → NH 3 + CH 2 O (R1a) - 79 kJ mol -1 → NH 2 CHO + H (R1b) - 45 kJ mol -1 MESMER modelling of this reaction indicates that at the low temperatures of the interstellar medium, the lifetimes of the weakly bound PRCs are extended, and as such unimolecular decay back to reactants competes with reactive removal (either over a barrier or through it via tunnelling) to products. Stabilization of the PRCs can also occur via collisions with a third body; as such, the reaction exhibits a strong pressure dependence. We have also investigated the reaction between NH 2 and NO with MESMER, using a PES surface calculated at the B3LYP/6-311G(d,p) level of theory. The NH 2 + NO PES is complex, with many deep wells and large barriers, and three possible product channels: NH 2 + NO → N 2 + H 2 O (R2a) - 468 kJ mol -1 → N 2 O + H 2 (R2b) - 192 kJ mol -1 → HN 2 + OH (R2c) + 7.5 kJ mol -1 The reaction is initiated by the barrierless addition of the NH 2 to the NO to form the adduct H 2 NNO. This adduct undergoes an H-atom shift followed by a series of cis-trans isomerizations, giving rise to four distinct HNNOH isomers. All four of these isomers may undergo unimolecular decay to form the products HN 2 + OH (R2a), while only one of the isomers is configured correctly to form the N 2 + H 2 O products (R2c) via a four membered ring transition state. Stabilisation into the deep wells requires very high pressures (> 1 × 10 22 molecules cm -3 ), and as such the predicted rate coefficients and BRs are effectively pressure independent at atmospheric pressure and below. The BRs do however heavily depend on the reverse inverse Laplace transform parameters (for the unimolecular decay of the HNNOH isomers) used in MESMER; at higher temperatures, this unimolecular decay is favourable and as such the endothermic HN 2 + OH channel is dominant, while at low temperatures, the low energy route to N 2 + H 2 O is favoured. References 1. Glowacki, D. R.; Liang, C.-H.; Morley, C.; Pilling, M. J.; Robertson, S. H., MESMER: An Open-Source Master Equation Solver for Multi-Energy Well Reactions. The Journal of Physical Chemistry A 2012, 116 (38), 9545-9560.

P07

The formation of ethylidene in the thermal decomposition of ethane: a theoretical and experimental study Nadav Genossara b , Sharona Atlasa a,c , Dana Breskera a , Shani Har-Lavana a , Uri Zamira a , Patrick Hembergerd d , Thanh Lam Nguyene e , Joshua H. Barabana a a Department of Chemistry, Ben-Gurion University of the Negev, Israel, b IAEC, Tel Aviv, Israel, c NRCN, Beer Sheva, Israel, d Laboratory for Synchrotron Radiation and Femtochemistry, Switzerland, e Quantum Theory Project, Department of Chemistry, USA The unimolecular decomposition of ethane remains incompletely characterized, despite being a fundamental reaction in various thermal processes. Experimental data based on direct observations of the various decomposition products such as ethyl radical or ethylidene is lacking, in particular. We present experimental results from a microreactor study performed at the Swiss Light Source, including isotopically labeled experiments. Using a coincidence detection setup, we manage to observe the formation of the various isotopologues of ethylene, methyl radicals, and ethyl radicals, the latter directly observed in this reaction system for the first time, to the best of our knowledge. Along with CFD simulations coupled to high-accuracy ab initio kinetic analyses based on semi-classical transition state theory, we infer that the mechanism for the formation ethane occurs through two subsequent H atom loss steps, and recover hints for the formation of the elusive high-energy isomer of ethylene, ethylidene, during the reaction.

P08

Non-physical species in pressure-dependent networks: by the switch of an atom Sharon Haba, Nelly Mitnik, Mark E. Fuller, Alon Grinberg Dana Department of Chemical Engineering, Technion – Israel Institute of Technology, Israel Large-scale application of connectivity-based reaction templates, as used in automatically generated chemical kinetic models, 1 may result in non-physical species either in the model itself or in pressure-dependent networks used to compute rate coefficients. This problem, which has not received enough attention, results in ill-formulated networks and may introduce considerable errors into computed pressure-dependent rate coefficients, eventually propagating into the predicted observables. We begin by consider two structural isomers, ⋅ OONNH and ⋅ NNOOH , and show using several established electronic structure methods that the former corresponds to an energetic well while the latter is non-physical (Figure 1) at various optimization methods (DFT and CCSD(T)-F12 2 ). We use multi-dimensional scans of internal coordinates (bond distances and dihedral angles) to explain this difference. Next, we explore a pressure-dependent network on the N 2 O 2 H potential energy surface (PES). We show that an automatically generated PES may contain non-physical species and requires additional treatment. We identify all non-physical species in the network, correct the network by properly connecting physical isomers, e.g., using a neural network based method, 3 and show the effect of this modification on the computed well-skipping pressure- dependent reaction rate coefficients. Finally, we give recommendations on how to generalize and automate the approach to treat such cases when considering large numbers of PES networks.

Figure 1: ( A ) ⋅ OONNH and ( B ) ⋅ NNOOH optimized at the ωB97XD/Def2TZVP level of theory 4,5 starting from reasonable energetic well conformers embedded using force fields. 6 While structure A was found to represent an energetic well, structure B was found to break into smaller fragments when optimized. References 1. M. Liu, A. Grinberg Dana, M.S. Johnson, M.J. Goldman, A. Jocher, A.M. Payne, C.A. Grambow, K. Han, N.W. Yee, E.J. Mazeau, K. Blondal, R.H. West, C.F. Goldsmith, W.H. Green, Reaction Mechanism Generator v3.0: Advances in Automatic Mechanism Generation, J. Chem. Inf. Model. 2021, 61(6), 2686-2696, https://doi.org/10.1021/acs.jcim.0c01480 2. T.B. Adler, G. Knizia, H.-J. Werner, A simple and efficient CCSD(T)-F12 approximation, J. Chem. Phys. 2007, 127, 221106, https://doi.org/10.1063/1.2817618 3. L. Pattanaik, J.B. Ingraham, C.A. Grambow, W.H. Green, Generating transition states of isomerization reactions with deep learning, Phys. Chem. Chem. Phys. 2020, 22, 23618-23626, https://doi.org/10.1039/D0CP04670A 4. J.-D. Chai, M. Head-Gordon, Long-range corrected hybrid density functionals with damped atom–atom dispersion corrections, Phys. Chem. Chem. Phys. 2008, 10, 6615-6620, https://doi.org/10.1039/B810189B 5. F. Weigend, R. Ahlrichs, Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy, Phys. Chem. Chem. Phys. 2005, 7, 3297-3305, https://doi.org/10.1039/B508541A 6. A. Grinberg Dana, D. Ranasinghe, H. Wu, C. Grambow, X. Dong, M. Johnson, M. Goldman, M. Liu, W.H. Green, "ARC - Automated Rate Calculator", version 1.1.0, https://github.com/ReactionMechanismGenerator/ARC, DOI: 10.5281/ zenodo.3356849

P09

Dramatic unimolecular decay of an unsaturated Criegee intermediate via allylic 1,6 H-atom transfer Anne S. Hansen 1 , Yujie Qian 1 , Stephen J. Klippenstein 2 and Marsha I. Lester 1 1 Department of Chemistry, University of Pennsylvania, USA, 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, USA Very rapid unimolecular decay of an unsaturated Criegee intermediate is shown to proceed via a novel allylic 1,6 H-atom transfer mechanism, which is orders of magnitude faster than typical decay via 1,4 H-atom transfer. A new four-carbon Criegee intermediate with extended conjugation across the vinyl and carbonyl oxide groups, 2-butenal oxide (CH 3 CH=CHCHOO), is generated and shown to facilitate rapid allylic 1,6 H-atom transfer, resulting in hydroxyl (OH) radical products. 1 A low-energy reaction pathway involving isomerization of 2-butenal oxide from a lower energy ( tZZ ) conformer to a higher energy ( cZZ ) conformer followed by 1,6 hydrogen transfer via a 7-membered ring transition state is predicted theoretically and shown experimentally to yield OH products. The low-lying ( tZZ ) conformer of 2-butenal oxide is identified based on computed anharmonic frequencies and intensities of its eight conformers. Experimental IR action spectra recorded in the fundamental CH stretch region with OH product detection by UV laser-induced fluorescence reveal a distinctive IR transition of the low-lying ( tZZ ) conformer at 2996 cm -1 that results in rapid unimolecular decay to OH products. Statistical RRKM calculations involving a combination of conformational isomerization and unimolecular decay via 1,6 H-transfer yield an effective decay rate k eff ( E ) on the order of 10 8 s -1 at ca. 3000 cm -1 in good accord with experiment. Unimolecular decay proceeds with significant enhancement due to quantum mechanical tunneling. A rapid thermal decay rate of ca. 10 6 s -1 is predicted by master-equation modeling of 2-butenal oxide at 298 K, 1 bar. This novel unimolecular decay pathway is expected to increase the non-photolytic production of OH radicals upon alkene ozonolysis in the troposphere. References 1. A. S. Hansen, Y. Qian, C. A. Sojdak, M. C. Kozlowski, V. J. Esposito, J. S. Francisco, S. J. Klippenstein, and M. I. Lester, Rapid allylic 1,6 H-atom transfer in an unsaturated Criegee intermediate, J. Am. Chem. Soc. , in press (2022).

P10

Unimolecular decomposition of decalin and methyldecalin Subharaj Hossain 1,2 , Jagadeesh Gopalan 2 and Elangannan Arunan 1 * 1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, India, 2 Department of Aerospace Engineering, Indian Institute of Science, India Decalin is an important component in the aviation/diesel fuel 1 . Trans-decalin has been selected as the surrogate component for investigation of transportation fuel 2 . Initial unimolecular decomposition reaction of cycloalkanes is a significant step in pyrolysis/combustion/oxidation of cycloalkanes. Initial reaction of alkylcyclohexane may proceed via the dissociation of C-H bond, elimination of side-chain alkyl group, or isomerization via ring-opening leading to alkene. Understanding of initial unimolecular reaction is important to develop complete pyrolysis/oxidation kinetic models of cycloalkane. Moreover, the competition between ring-opening isomerization and CH 3 elimination channel has a significant effect on aromatic formation. Several Studies on unimolecular reaction of cyclohexane are available in the literature and there is disagreement between these studies about the initial process (proposed initial process:- Tsang 3 : cC6H12=1-hexene, Aribike 4 et al.: cC 6 H 12 =3C 2 H 4 , cC 6 H 12 =2C 3 H 6 , cC 6 H 12 =C 4 H 6 +C 2 H 4 +H 2 ; Voisin 5 et al.: cC 6 H 12 =cC 6 H 11 +H). Finally, Kiefer et al. 6 theoretically calculated the reaction pathways for cyclohexane dissociation and confirmed that the initial decomposition of cyclohexane produces 1-hexene. Similar disagreement was observed for methylcyclohexane 7 as well. This discrepancy for the initial reaction of cyclohexane/methylcyclohexane motivate us to theoretically study the initial unimolecular decomposition of decalin and methyldecalin. Scheme1 shows all possible unimolecular reactions. Ring-opening isomerization of decalin/methyldecalin occurs in two steps.We used multi-reference calculation (CASSCF/MRCI) as these reactions proceeds through biradical intermediates. Figure1 shows the PES of C1-C2, C1-H bond dissociation of decalin, and C1-C2, C1-CH 3 bond dissociation of methyldecalin. Our preliminary result shows that for decalin, ring-opening isomerization channel is energetically favorable compared to the C-H dissociation channel and that can be the initial process. On the other hand, for methyldecalin, energy difference between CH 3 elimination channel and ring-opening isomerization channel is very small. We can see that the C-H dissociation and CH 3 elimination channel are barrier-less processes (without a saddle point) whereas ring-opening isomerization channel has a saddle point along the reaction coordinate. If we compare the ring- opening dissociation channel between decalin and methyldecalin, C-C bond(next to methyl group) dissociation energy decreases (~ 3 kcal/mol) due to the presence of CH 3 group in methyldecalin. We calculated the rate parameters for all the channels (using Transition state/variational transition state theory). The pressure-dependent rate constants for all the channels have been calculated using the RRKM/ME method. Results from these investigations will be presented in the meeting.

Scheme1 Decomposition and isomerization pathways of (a) decalin and (d) methyldecalin.

Figure1 PES of C1-H, C1-C2 bond of decalin and C1-CH3, C1-C2 bond of methyldecalin. Level of theory: CASSCF(2,2)/6-31+g(d,p).

References 1. Yu et al., Fuel , 2018, 212 , 41–48.

2. Mueller et al., Energy & Fuels , 2012, 26 , 3284–3303. 3. W. Tsang, Int. J. Chem. Kinet. , 1978, 10 , 1119–1138. 4. Aribike et al., Thermochim. Acta , 1981, 47 , 1–14. 5. Voisin et al., Combust. Sci. Technol. , 1998, 138 , 137–158. 6. Kiefer et al., J. Phys. Chem. A , 2009, 113 , 13570–13583. 7. Zhang et al., Energy & fuels , 2013, 27 , 1679–1687.

P11

Conformation-targeted dynamics of NO: alkane molecular complexes Nathanael M. Kidwell William & Mary, USA The photochemistry of flexible molecular complexes, such as a nitric oxide:methane (NO:CH 4 ), are defined by their intermolecular interactions and thus the subtle features that arise on reactive and nonreactive potential energy surfaces. Indeed, the photochemical outcomes sensitively depend on the chemical functionality and relative spatial orientations of the accessible conformational isomers. Leveraging a synergy of conformation- specific spectroscopy and dynamics experimental techniques, we aim to obtain a molecular-level understanding of the intermolecular interactions and reaction mechanism outcomes adopted by NO:CH 4 and NO:alkane complexes more broadly. Ultimately, we seek to gain insights into the conformation- and mode-specific energy transfer pathways following fragmentation of NO:alkane molecular complex isomers. Furthermore, our experimental results will be compared to several theoretical approaches to reveal multifaceted signatures of dynamical events using spectroscopy probes.

P12

Novel OH roaming pathway in the unimolecular decay of alkyl- substituted criegee intermediates T. Liu 1 , M. Zou 1 , M. F. Vansco 2 , S. N. Elliot 2 , C. R. Markus 3,4 , R. Almeida 5 , K. Au 5 , L. Sheps 5 , D. L. Osborn 5,6 , C. J. Percival 3 , C. A. Taatjes 5 , S. J. Klippenstein 2 , R. L. Caravan 2 , and Marsha I. Lester 1 1 Department of Chemistry, University of Pennsylvania, USA, 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, USA, 3 NASA Jet Propulsion Laboratory, California Institute of Technology, USA, 4 Division of Chemistry and Chemical Engineering, California Institute of Technology, USA, 5 Combustion Research Facility, Sandia National Laboratories, USA, 6 Department of Chemical Engineering, University of California, USA Anthropogenic and biogenic alkenes, abundant volatile organic compounds emitted into the atmosphere, react with ozone to generate short-lived, highly reactive carbonyl oxide intermediates (R 1 R 2 C=O + O − ), known as Criegee intermediates. Unimolecular decay of alkyl-substituted Criegee intermediates generally proceeds via intramolecular 1,4 H-atom transfer from an alkyl group (R 1 , R 2 ) to the terminal O-atom, transiently forming a vinyl hydroperoxide, followed by O-O bond fission that releases hydroxyl (OH) radicals.While unimolecular decay of Criegee intermediates to OH products can occur promptly or following thermalization, recent experimental and theoretical studies suggest that the separating OH radical can also reorient, roam, and add to the vinyl group, resulting in roaming-induced isomerization to hydroxycarbonyl products. 1-3 The present work identifies stable hydroxybutanone products arising from OH roaming in the unimolecular decay of the methyl-ethyl substituted Criegee intermediate (CH 3 )(CH 3 CH 2 )C=O + O − , along with products derived from C-C fragmentation. Kinetic time profiles, exact masses, and photoionization spectra enable identification of roaming products generated under thermal conditions. The experiments utilize multiplexed photoionization mass spectrometry with tunable vacuum ultraviolet radiation at the Advanced Light Source (Lawrence Berkeley National Laboratory). Complementary theoretical calculations validate the OH roaming pathway leading to hydroxybutanone and other products. References 1. Taatjes, C. A.; Liu, F.; Rotavera, B.; Kumar, M.; Caravan, R.; Osborn, D. L.; Thompson, W. H.; Lester, M. I., Hydroxyacetone Production From C3 Criegee Intermediates. J. Phys. Chem. A 2017, 121 , 16-23. 2. Kuwata, K. T.; Luu, L.; Weberg, A. B.; Huang, K.; Parsons, A. J.; Peebles, L. A.; Rackstraw, N. B.; Kim, M. J., Quantum Chemical and Statistical Rate Theory Studies of the Vinyl Hydroperoxides Formed in trans-2-Butene and 2,3-Dimethyl-2- butene Ozonolysis. J. Phys. Chem. A 2018, 122 , 2485-2502. 3. Barber, V. P.; Pandit, S.; Green, A. M.; Trongsiriwat, N.; Walsh, P. J.; Klippenstein, S. J.; Lester, M. I., Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production. J. Am. Chem. Soc. 2018, 140 , 10866-10880.

P13

Photodissociation dynamics of N,N-dimethylformamide at 225 nm and 245 nm

Dennis Milesevic, Patrick Robertson, Divya Popat, Claire Vallance Department of Chemistry, Chemistry Research Laboratory, United Kingdom

N,N -dimethylformamide is the simplest tertiary amide and a model compound for investigating the photofragmentation of peptide bonds. [1-7] We report the photodissociation dynamics following excitation at 225 nm and 245 nm using velocity-map imaging. [8-9] Excitation at 225 nm leads to excitation to the 2 1 A'' Rydberg state, following a parallel transition. This state exhibits a barrier along the N-CO "peptide" bond leading to strong correlations between the anistropy parameter β , the product velocities, and the internal energy of the products, as excited molecules try to circumvent the barrier via out-of-plane motions. The oscillator strength between the two lowest lying excited singlet surfaces 1 1 A'' and 2 1 A'' is high as products are formed in their electronic ground-state. Excitation at 245 nm leads to direct population of the two low-lying triplet states viaπ-π * and n-π * transitions to the 1 3 A' and 1 3 A'' states, respectively. The low dissociation barrier of the former leads to high kinetic energy releases following fast dissociation. The high dissociation barrier of the 1 3 A'' state results in lower kinetic energy releases. References 1. N. R. Forde, T. L. Myers, and L. J. Butler, “Chemical reaction dynamics when the Born-Oppenheimer approximation fails: Understanding which changes in the electronic wavefunction might be restricted,” Faraday Discuss. 108, 221–242 (1997). 2. N. R. Forde and L. J. Butler, “Electronic accessibility of dissociation channels in an amide: N,N-dimethylformamide photodissociation at 193 nm,” J. Chem. Phys. 110, 8954–8968 (1999). 3. S. Shin, A. Kurawaki, Y. Hamada, K. Shinya, K. Ohno, A. Tohara, and M. Sato, “Conformational behavior of N-methylformamide in the gas, matrix, and solutionstates as revealed by IR and NMR spectroscopic measurements and by theoretical calculations,” J. Mol. Struct. 791, 30–40 (2006). 4. M. Ruzi and D. T. Anderson, “Photodissociation of N-methylformamide isolated in solid parahydrogen,” J. Chem. Phys. 137, 194313 (2012). 5. X. Qiu, Z. Ding, Y. Xu, Y. Wang, and B. Zhang, “Ultrafast excited-state dynamics in a prototype of the peptide bond: Internal conversion of the isolated N,Ndimethylformamide,” Phys. Rev. A - At. Mol. Opt. Phys. 89, 033405 (2014). 6. P. Salén, V. Yatsyna, L. Schio, R. Feifel, R. Richter, M. Alagia, S. Stranges, and V. Zhaunerchyk, “NEXAFS spectroscopy and site-specific fragmentation of N -methylformamide, N,N -dimethylformamide, and N,N -dimethylacetamide,” J. Chem. Phys. 144, 244310 (2016). 7. M. L. Lipciuc, S. H. Gardiner, T. N. Karsili, J. W. Lee, D. Heathcote, M. N. Ashfold, and C. Vallance, “Photofragmentation dynamics of N,N-dimethylformamide following excitation at 193 nm,” J. Chem. Phys. 147, 013941 (2017). 8. D. W. Chandler and P. L. Houston, “Two-dimensional imaging of state-selected photodissociation products detected by multiphoton ionization,” J. Chem. Phys. 87, 1445–1447 (1987). 9. A. T. Eppink and D. H. Parker, “Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen,” Rev. Sci. Instrum. 68, 3477–3484 (1997).

P14

Combined crossed-beams and theoretical investigation of the O( 3 P) + acrylonitrile reaction: Dominant formation of ketenimine (CH 2 CNH) via intersystem crossing Giacomo Pannacci 1 , Luca Mancini 1 , Pengxiao Liang 1 , Gianmarco Vanuzzo 1 , Demian Marchione 1 , Pedro Recio 1 , Marzio Rosi 2 , Dimitrios Skouteris 3 , Piergiorgio Casavecchia 1 and Nadia Balucani 1 1 Dept. of Chemistry, Biology and Biotechnology, Italy, 2 Dept. of Civil and Environmental Engineering, University of Perugia, Italy, 3 Master-Tec Srl, Italy The relevance of the oxidation of acrylonitrile, CH 2 CHCN (also termed cyanoethene or vinylcyanide or 2-propenenitrile), a widespread nitrile compound, covers areas ranging from combustion to astrochemistry. As a matter of fact, thermal combustion and selective catalytic combustion are the main treatments to reduce acrylonitrile emissions into the atmosphere following its industrial uses, being CH 2 CHCNa volatile compound with toxic/carcinogenic properties. Most notably, CH 2 CHCN was the first molecule with a C=C double bond to be detected in the Interstellar Medium (ISM) and the inclusion in models of its oxidation (O is the third most abundant element in the ISM) could open new insights on the formation/destruction processes of biologically relevant species. In this context, we have investigated the O( 3 P)+CH 2 CHCN reaction by combining synergistically crossed molecular beam (CMB) experiments and theoretical calculations. This combined effort is needed because the reactions of O( 3 P) with unsaturated hydrocarbons are multichannel reactions where, after the initial attack of the O-atom to the electron rich unsaturated bond(s), the triplet diradical intermediate can undergo a unimolecular decomposition adiabatically on the triplet Potential Energy Surface (PES), or nonadiabatically on the singlet PES via intersystem crossing (ISC), and many different products can be formed. The CMB technique with mass spectrometric detection and time-of-flight analysis, coupled to theoretical calculations of the relevant triplet/singlet PESs and statistical (RRKM/Master Equation) estimates of product branching fractions (BFs) with inclusion of ISC, is the best resource to unveil the micro-mechanism of O( 3 P) reactions and to identify the primary products and their relative yields (BFs), as reported in numerous publications concerning the reactions between O( 3 P) and unsaturated 1,2 as well as aromatic hydrocarbons. 3 By applying a similar combined CMB/theoretical approach for the study of the O( 3 P)+acrylonitrile reaction, we have found that the reactive flux is dominated by ISC from the triplet to the singlet PES following a barrierless addition of O( 3 P) to the methylenic carbon of acrylonitrile, with CO+CH 2 CNH (ketenimine) being the most favored product channel (BF ∼ 0.9). The reaction leads also, to a minor extent, to H+HCOCHCN adiabatically on the triplet PES (BF ∼ 0.1). 4 Because the title reaction represents a possible destruction pathway of CH 2 CHCN and a possible formation route of ketenimine, a key intermediate towards the formation of biologically relevant molecules, also at the typical temperature of SgrB2(N) hot cores, our results are expected to contribute to the improvement of not only current combustion models, but also current astrochemical models. Acknowledgments : This work was supported by MUR and University of Perugia (Department of Excellence- 2018-2022-Project AMIS) and MUR (PRIN 2017, MAGIC DUST, Prot. 2017PJ5XXX). P.L. acknowledges support from the Marie Sklodowska-Curie project "Astro-Chemical Origins" (Grant No. 811312). D.M. thanks ASI (DC- VUM-2017-034, Grant No. 2019-3 U.O Life in Space). References 1. C. Cavallotti, et al., Faraday Discuss."Unimolecular Reactions" (2022), in press (DOI: 10.1039/D2FD00037G).A. Caracciolo, et al., J. Phys. Chem. A 123 , 9934 (2019). 2. G. Vanuzzo, et al., J. Phys. Chem. A 125 , 8434 (2021).G. Pannacci, et al., Phys. Chem. Chem. Phys. , in preparation.

P15

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