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Reaction Dynamics of O(P) Propyne: II. Primary Products, Branching Ratios and Role of Intersystem Crossing From Ab Initio Coupled Triplet/Singlet Potential Energy Surfaces and Statistical Calculations Ilaria Gimondi, Carlo Cavallotti, Gianmarco Vanuzzo, Nadia Balucani, and Piergiorgio Casavecchia J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b01564 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on March 24, 2016
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The Journal of Physical Chemistry
Reaction Dynamics of O(3P) + Propyne: II. Primary Products, Branching Ratios and Role of Intersystem Crossing from Ab Initio Coupled Triplet/Singlet Potential Energy Surfaces and Statistical Calculations
Ilaria Gimondi and Carlo Cavallotti* Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica "Giulio Natta", 20131 Milano, Italy
Gianmarco Vanuzzo, Nadia Balucani, and Piergiorgio Casavecchia Dipartimento di Chimica Biologia e Biotecnologie, Università degli Studi di Perugia, 06123 Perugia, Italy
*To whom correspondence should be addressed. E-mail:
[email protected] Telephone: +39-02-23993176
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ABSTRACT The
mechanism
of
the
O(3P)+CH3CCH
reaction
was
investigated
using
a
combined
experimental/theoretical approach. Experimentally the reaction dynamics was studied using crossed molecular beams (CMB) with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy. Theoretically master equation simulations were performed on a potential energy surface (PES) determined using high-level ab initio electronic structure calculations. In this paper (II) the theoretical results are described and compared with experiments, while in paper (I) are reported and discussed the results of the experimental study. The PES was investigated by determining structures and vibrational frequencies of wells and transition states at the CASPT2/aug-cc-pVTZ level using a minimal active space. Energies were then determined at the CASPT2 level increasing systematically the active space and at the CCSD(T) level extrapolating to the complete basis set limit. Two separate portions of the triplet PES were investigated, as O(3P) can add either on the terminal or the central carbon of the unsaturated propyne bond. Minimum energy crossing points (MECPs) between the triplet and singlet PESs were searched at the CASPT2 level. The calculated spin-orbit coupling constants between the T1 and S0 electronic surfaces were about 25 cm-1 for both PESs. The portions of the singlet PES that can be accessed from the MECPs were investigated at the same level of theory. The system reactivity was predicted integrating stochastically the 1D master equation (ME) using RRKM theory to determine rate constants on the full T1/S0 PESs, accounting explicitly for intersystem crossing (ISC) using the Landau Zener model. The computational results are compared both with the branching ratios (BRs) determined experimentally in the companion paper (I) and with those estimated in a recent kinetic study at 298 K. The ME results allow to interpret the main system reactivity: CH3CCO+H and CH3+HCCO are the major channels active on the triplet PES and are formed from the wells accessed after O addition to the terminal and central C, respectively; 1CH3CH+CO and C2H3+HCO are the major channels active on the singlet PES and are formed from the methylketene and acrolein wells after ISC. However, also a large number of minor channels (about 15) are active, so that the system reactivity is quite complicated. The comparison between computational and experimental BRs is quite good for the kinetic study, while some discrepancy with the CMB estimations suggests that dynamic non ergodic effects may influence the system reactivity. Channel specific rate constants are calculated in the 300-2250 K and 1-30 bar temperature and pressure ranges. It is found that as the temperature increases the H abstraction reaction, whose contribution is negligible in the experimental conditions, increases of relevance and the extent of ISC decreases from about 80% at 300K to less than 2% at 2250K.
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The Journal of Physical Chemistry
1. INTRODUCTION The reactions of O(3P) with unsaturated hydrocarbons are of considerable interest in combustion science and atmospheric chemistry. The reason is that the ground state of atomic oxygen is among the chemical species that can be easily formed in virtually all the reactive environments in which molecular oxygen is present, so that its reactivity is included in most kinetic mechanisms. This class of reactions has been the subject of considerable experimental and theoretical work for more than 50 years, since the first papers of Cvetanovic were published.1,2 One of the first pieces of evidence of these studies was that these reactive systems are extremely complicated, with a large variety of products that can be formed through a sequence of steps taking place both on the singlet and the triplet potential energy surfaces (PESs). In particular, the experimental evidence that some major products could be formed only after inter system crossing (ISC) from the triplet to the singlet PES was quite interesting from a fundamental standpoint, as the transition between two different PESs is controlled by quantum mechanical rules. The reactions where ISC is active are known as spin forbidden, since they involve a change of the total spin quantum number and thus formally break the spin selection rule. The inherent complexity of the O(3P) + alkene/alkynes reactions is such that only recently, with the advent of experimental techniques that are able to identify unambiguously the reaction products, such as multiplexed photoionization mass spectrometry using tunable VUV synchrotron radiation3 or crossed molecular beams (CMB) with mass spectrometric (MS) detection and time-of-flight (TOF) analysis,4,5 it has been possible to determine with accuracy the nature and relative amount of the reaction products. Among these reactions, the most investigated ones are the reactions of O(3P) with ethylene6-10 and acetylene,11-14 both for the relative simplicity (smallest alkene and alkyne) of the reactants and for their high relative concentration in many reaction environments due to their thermodynamic stability (the cited references are a small selection of recent works, where additional references to this vast literature can be found). While the reactivity of O(3P) with ethylene and acetylene can be considered to be mostly understood, this is not the case for their higher homologues. More recently several studies have thus been dedicated to investigate the reactivity of O(3P) with propene3,4,15 and propyne,16-18 which are respectively the higher homologues of ethylene and acetylene. In particular, we have investigated the reaction of O(3P) with propene both from the experimental and the theoretical standpoints.4,15 On the experimental side, branching ratios (BR) were measured under single collision conditions using the CMB method coupled with MS detection and TOF analysis. Theoretically, the reactivity on the C3H6O PES was investigated combining a high level determination of energy and structure of its stationary points performed at the CCSD(T) and CASPT2 levels with master equation simulations. In the present work, we extend the same combined experimental-theoretical approach used to investigate O(3P) + C3H6 to study the reactivity of O(3P) with propyne. The results are reported in two
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interrelated papers, the first19 focused on experimental results and the present one on theoretical aspects. A short letter by the same authors, focused on the study of the relevance of a specific reaction channel for the title reaction, has been recently submitted for publication.20 The calculations reported in ref. 20 are based on the same triplet and singlet PES, but do not include ISC as a competitive pathway and thus do not report channel specific rate constants for the full reactive system. Several experimental11,16,17,21-24 and a few theoretical18 studies have been dedicated to the investigation of the reaction between O(3P) and propyne. Before considering what is known about this specific reaction, it is useful to review the literature about the reaction of O(3P) with acetylene, which has been studied much more extensively. Both experimental and theoretical studies agree that the two most important reaction channels are O(3P) + C2H2
→ H + HCCO
∆H°0 = –19.7 kcal/mol
→ 3CH2 + CO
∆H°0 = –47.5 kcal/mol
Both of them are accessed on the triplet PES, with the first reaction channel being predominant over the second. There is no experimental evidence of 1CH2 formation, which indicates that the role of ISC is minor. Nguyen et al. recently studied the reactivity on the triplet PES for this reaction.25 Similarly to what anticipated by Harding and Wagner26 and successively found by Sabbah et al. also for propene,27 it was found that O(3P) addition to C2H2 can take place through two transition states (TS), the first leading to the formation of a ground state 3A'' adduct and the second to the 3A' excited state. The excited transition state has an energy barrier about 2.6 kcal/mol higher than the ground TS and was estimated to contribute for about 16% to the overall system reactivity in the simulation of a CMB experiment performed with a collision energy of 9.5 kcal/mol.28 The reactivity on the 3A'' adduct is rather simple and involves cis-trans isomerization of the adduct, H transfer of the aldehydic hydrogen followed by decomposition into CH2 and CO, and loss of the aldehydic H. Only two reaction routes were found for the 3A' adduct: decomposition to reactants or, following cis-trans isomerization, loss of the aldehydic H. Notably, a quasiclassical trajectory surface hopping (QCT-SH) study by Rajak and Maiti on coupled triplet-singlet C2H2O PESs concluded that a small amount of ISC (
