Isomer-Specific Chemistry in the Propyne and Allene Reactions with

Letter pubs.acs.org/JPCL

Isomer-Specific Chemistry in the Propyne and Allene Reactions with Oxygen Atoms: CH3CH + CO versus CH2CH2 + CO Products Gianmarco Vanuzzo,† Nadia Balucani,† Francesca Leonori,† Domenico Stranges,†,§ Stefano Falcinelli,†,∥ Astrid Bergeat,†,⊥ Piergiorgio Casavecchia,*,† Ilaria Gimondi,‡ and Carlo Cavallotti*,‡ †

Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, 06123 Perugia, Italy Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, 20131 Milano, Italy

‡

S Supporting Information *

ABSTRACT: We report direct experimental and theoretical evidence that, under single-collision conditions, the dominant product channels of the O(3P) + propyne and O(3P) + allene isomeric reactions lead in both cases to CO formation, but the coproducts are singlet ethylidene (1CH3CH) and singlet ethylene (CH2CH2), respectively. These data, which settle a long-standing issue on whether ethylidene is actually formed in the O(3P) + propyne reaction, suggest that formation of CO + alkylidene biradicals may be a common mechanism in O(3P) + alkyne reactions, in contrast to formation of CO + alkene molecular products in the corresponding isomeric O(3P) + diene reactions, either in combustion or other gaseous environments. These findings are of fundamental relevance and may have implications for improved combustion models. Moreover, we predict that the so far neglected 1CH3CH + CO channel is among the main reaction routes also when the C3H4O singlet potential energy surface is accessed from the OH + C3H3 (propargyl) entrance channel, which are radical species playing a key role in many combustion systems.

A

are the same, cyclopropanone, methylketene, and acrolein (propenal), but the two C3H4 reactants experience different parts of the PES because they have a different structure. For both reactions, the CO-forming channel is one of the major channels6,8 and could occur in principle on both the triplet and singlet PES. The coproduct of CO can energetically be both ground-state (singlet) and excited-state (triplet) ethylene (CH2CH2 and 3CH2CH2) and ground-state (triplet) and excited-state (singlet) ethylidene (3CH3CH and 1CH3CH). However, pathways leading to triplet states, such as 3CH2CH2 (ΔH°0 = −52.0 kcal/mol) and 3CH3CH (ΔH°0 = −45.9 kcal/ mol) are characterized by high-lying barriers (see Supporting Information Figure S2 for O + propyne and ref 12 for O + allene) and are unlikely to be competitive with the singlet pathways involving ISC:

llene (CH2CCH2) and propyne (CH3CCH) constitute the smallest and thus simplest pair of stable structural isomers that allows for a detailed investigation of the influence of the chemical structure on the product distribution in, for instance, combustion-relevant reactions. Allene and propyne are, in fact, easily formed during the combustion of methane, larger aliphatics, and aromatics and are then involved in the generation of polycyclic aromatic hydrocarbons and soot. Their reactions with ground-state atomic oxygen, O(3P), are among their prevalent consumption pathways in oxygen-rich zones and represent an important step in their degradation to CO and subsequently CO2.1−4 Notably, these reactions are also of relevance in the combustion of biofuels, because C3H4 intermediates are easily formed during the combustion of butanol isomers (especially iso-butanol and tert-butanol) as well as esters (e.g., ethyl formate).5 The reactions of O(3P) with propyne and allene are characterized by a variety of exoergic product channels (with formation of H, H2, CH2, CH3, C2H3, C2H4, CH3CH, CO, etc.),6−8 some of which originate from the triplet potential energy surface (PES) while others from the singlet PES, which can be accessed after Intersystem crossing (ISC) takes place. Recall that for these reactions, as all those of O(3P) with unsaturated hydrocarbons, ISC from the entrance triplet PES to the underlying singlet PES is possible and this, in general, deeply affects the reaction outcome.9−11 Notably, the two O + C3H4 isomeric reactions occur on the same overall triplet/ singlet C3H4O PES; the main intermediates on the singlet PES © XXXX American Chemical Society

O(3P) + CH3CCH → 1CH3CH + CO ΔH0° = − 43.1 kcal/mol

(1a)

→ CH 2CH 2 + CO ΔH0° = − 118.0 kcal/mol

(1b)

O(3P) + CH 2CCH 2 → 1CH3CH + CO ΔH0° = − 44.7 kcal/mol

(2a)

→ CH 2CH 2 + CO ΔH0° = − 119.6 kcal/mol (2b)

The reaction kinetics of allene and propyne with O(3P) have been investigated since the 1960s.13−17 From end-product Received: February 5, 2016 Accepted: March 1, 2016

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Figure 1. TOF distributions for m/z = 26 and 27 at ΘCM (= 37°) for the reactions O(3P) + propyne (a) and O(3P) + allene (b). Panel c shows the m/z = 27 TOF spectrum for O + propyne amplified by a factor of 3. The solid black curves represent the calculated distributions when using the best-fit CM functions (see Supporting Information). The separate contributions to the calculated global TOF distributions are indicated with the formula of the corresponding product.

formation at 293 K with a CO laser resonant absorption and a discharge-flow gas chromatographic sampling method.18 Notably, the CO formed from the allene reaction was found to be vibrationally much hotter than that from the propyne reaction, and this18 was considered to show unambiguously that CH2CH2 was produced in the allene reaction (ΔH°0 = −119.6 kcal/mol) while 1CH3CH (lying 67 kcal/mol above CH2CH2) in the propyne reaction (ΔH0° = −43.1 kcal/mol). However, recent experimental studies of CO vibrational distributions in O(3P) + propyne by step-scan time-resolved Fourier transform infrared emission spectroscopy in a flow chamber, accompanied by theoretical calculations of the triplet and singlet PESs,19 have suggested that also the O(3P) + propyne reaction produces predominantly CH2CH2 + CO. Notably, in these PESs, the 1 CH3CH + CO channel was not contemplated. Recent state-ofthe art kinetic investigations at 298 K and 4 Torr using timeresolved multiplexed photoionization mass spectrometry and tunable vacuum−ultraviolet synchrotron radiation8 determined the branching ratios for all relevant channels, and the COforming channel(s) was found to be the most important one with a total branching fraction of 0.56, of which 0.37 ± 0.09 was attributed to C2H4 + CO and 0.19 ± 0.04 to C2H2 + CO + H2. This study did not rule out that ethylidene could actually initially be formed as a primary product that rapidly isomerizes to excited ethylene and/or dissociates to C2H2 + H2. Clearly, additional experiments as well as more detailed theoretical work

analysis, these reactions were assumed to proceed largely by ISC to several primary products, among which the predominant products are CO and singlet ethylene (CH2CH2) via a cyclopropanone intermediate in the case of allene,14 and CO and singlet ethylidene ( 1CH 3 CH) via a methylketene intermediate in the case of propyne.15−17 However, while CO from O + allene is now accepted to be formed along with the coproduct CH2CH2 in its ground singlet electronic state (see below), the nature of the C2H4 product in the O + propyne reaction has not been fully established. Regarding O + allene, high-level theoretical calculations of the triplet and singlet PES and statistical calculations of branching ratios12 led to the conclusion that the main product on the singlet PES is CH2CH2 + CO (although the extent of ISC was not evaluated). Recent detailed crossed molecular beam (CMB) studies have characterized the dynamics of the five most important product channels and established that the CH2CH2 + CO channel accounts for about 75% of the total yield at the collision energy Ec = 9.4 kcal/mol.6 The conclusions were based on the shape of the product translational energy distribution that clearly reflected a large exit potential barrier, consistent with the theoretical C3H4O PES.12 Regarding O + propyne, the suggestion from the early kinetic works15−17 that O(3P) + propyne leads to 1CH3CH + CO was shortly afterward supported by a comparative study of the O(3P) reaction with allene and propyne leading to CO 1011

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The Journal of Physical Chemistry Letters are needed to explore this possibility and try to settle the issue whether ethylidene is actually formed as primary product in the O(3P) + propyne reaction. In this Letter we present a combined experimental and theoretical study of the O(3P) + propyne reaction using the CMB method with “universal” soft-electron ionization mass spectrometric detection and time-of-flight (TOF) analysis and high-level electronic structure calculations of the triplet and singlet PESs and their couplings with statistical RRKM/ME (Rice-Ramsperger-Kassel-Marcus/Master Equation) calculations of the CO forming channel. The results, when compared with those recently reported for the related O(3P) + allene reaction both experimentally (using the same technique6) and theoretically,12 reveal that while O(3P) + allene leads to CO and only CH2CH2, O(3P) + propyne leads to CO and predominantly 1CH3CH. These findings establish conclusively that very pronounced structural effects on the reaction dynamics and product distribution for the two isomeric reactions occur and corroborate the early suggestion18 that ethylidene is actually a primary product of the O(3P) + propyne reaction. Moreover, they provide insights into the OH + C3H3 (propargyl) reaction channels, because the C3H4O PES can be accessed also from these reactants, which are radicals playing a key role in many combustion systems. Experimentally, we have investigated the O + propyne reaction at Ec= 9.2 kcal/mol by the CMB method with mass spectrometric detection and TOF analysis, exploiting the capability of performing soft electron ionization of the reaction products which permits us to suppress (or mitigate) the serious problem of dissociative ionization. The basics of our CMB apparatus have been described elsewhere,20 while details of the experiments are given in the Supporting Information (see also ref 7). We have probed the CH2CH2/CH3CH + CO channels by measuring the angular and TOF distributions of the CH2CH2/CH3CH products at the daughter ions m/z = 27 and 26. Panels a and b of Figure 1 show comparatively the TOF spectra at m/z = 26 (top) and 27 (bottom) at the CM angle for the O + propyne and O + allene reactions, respectively; Figure 1c is an enlargement of the m/z = 27 TOF spectrum at Θ = 37° for O + propyne. In this figure, the fast peak in the m/z = 27 and 26 spectra can only originate, on the basis of energy and linear momentum conservation, from the CH2CH2/CH3CH products of channels 1(a,b) (for O + propyne) and channels 2(a,b) (for O + allene). Quantitative information is obtained by moving from the laboratory (LAB) coordinate system to the center-of-mass (CM) one and analyzing the product angular, T(θ), and translational energy, P(E′T), distributions into which the CM product flux can be factorized20,21 (the best-fit CM functions are actually derived by a forward convolution fit of the product LAB angular and TOF distributions, see Supporting Information). While the T(θ) of the CH2CH2/ CH3CH products are backward−forward symmetric and polarized for both systems (Figure S1), the P(E′T) distributions are dramatically different (Figure 2). As can be seen, while the P(E′T) for O + allene peaks at about 22 kcal/mol and extends up to about 70 kcal/mol, that for O + propyne peaks at about 12 kcal/mol and extends up to only about 40 kcal/mol. It should be noted that the CMB data are particularly sensitive to the rise and peaking of the P(E′T) distributions, while somewhat less sensitive to their cutoff, as the error bounds in Figure 2 clearly indicate. If formation of the products from the two isomeric reactions occurs via decomposition of the same intermediate, the main features of the two P(E′T)s should be

Figure 2. Best-fit CM product translational energy distributions for the CO + 1CH3CH/CH2CH2 channels for the O(3P) + propyne (blue) and O(3P) + allene (red) reactions. Arrows mark the total available energy for channels (1a, 2a) and (1b, 2b) of the two isomeric systems. Shaded areas indicate the limits of the error bars for the P(E′T) functions.

identical for the two isomeric systems. Therefore, the experimental results clearly indicate that in the case of the propyne reaction the channel originates from a different intermediate than in the case of allene and follows a different dynamics, and the results strongly suggest that the product detected at m/z = 27 and 26 as fast component in the TOF spectra is actually 1CH3CH, which corresponds to the much less exothermic channel 1(a). This in turn suggests that this other intermediate, different than cyclopropanone, may well be methylketene, as suggested in early work.15−18 To support this interpretation and rationalize the mechanisms leading to CO formation in the O(3P) + propyne and allene reactions, we have investigated the triplet and singlet C3H4O PESs for O + propyne. The PES investigation relied on the initial screening performed by Zhao et al.,19 which was extended to search whether additional reaction channels are possible. Structures and vibrational frequencies of all the relevant stationary points were determined at the CASTP2/ aug-cc-pVTZ level.22,23 Energies were computed at the CCSD(T)/CBS level24 and, for energy barriers, at the CASPT2 level when T1 diagnostics were larger than 0.02. In the latter case, the CASPT2 active spaces were systematically increased to improve the accuracy of the estimations. The portions of the PESs that are relevant to the analysis of the CMB results are shown in Figure 3a together with the corresponding singlet portion of the PES for O + allene (Figure 3b) adapted from the work of Nguyen et al.,12 while a more detailed representation of all the relevant reaction channels is reported as Supporting Information. RRKM rates were computed for all the investigated channels. The analysis of the reaction mechanism in the two isomeric systems is performed comparing similarities and differences between pathways and energy barriers. The C3H4O triplet PESs (Figures S2 and S3, and ref 12) can be accessed through the addition of O either to the central or terminal C of the triple (double) bond of propyne (allene). As can be seen in Figure 3a, following O attack to the (most favored, see Supporting Information) terminal C atom of propyne, ISC occurs readily, leading to the very stable methylketene intermediate, which can either dissociate to CH2CH2 + CO via the tight TS16 or, preferably, barrierless to 1CH3CH + CO, with isomerization to cyclopropanone being unfavored because of a very high barrier 1012

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channel, that is, formation of ethylidene + CO is dominant (Table S23). In contrast, cyclopropanone, which is the prevailing intermediate in the O + allene reaction and whose formation is prevented in the O + propyne reaction by the large energy barriers of TS34 (73.6 kcal/mol) and TS35 (84.0 kcal/ mol) (Figure 3a), has been shown12 to decompose dominantly to CH2CH2 + CO via the loose TS33. The RRKM rate constants for the 1CH3CH + CO, CH2CH2 + CO, TS34 (ratecontrolling for isomerization of cyclopropanone), C2H3 + HCO, and C2H2 + CO + H2 channels are compared in Figure 4

Figure 4. RRKM rate constants for the reactions of decomposition of acrolein and methyl-ketene to 1CH3CH + CO, C2H4 + CO, C2H3 + HCO, and C2H2 + CO + H2 and for the isomerization of methylketene to cyclo-propanone. The dotted vertical line corresponds to the average energy of the CMB experiments (1 kcal/mol = 349.757 cm−1). Figure 3. Potential energy diagrams (schematic) illustrating reactants, stationary points, and main products on the singlet C3H4O PESs: (a) O + propyne and (b) O + allene. Red, solid lines mark the preferential pathways to 1CH3CH and C2H4 products from methylketene and cyclopropanone, respectively, in the two isomeric cases. The experimental collision energy, Ec, is indicated.

and clearly evidence the dominance of the ethylidene channel. These theoretical results are in complete agreement with the experimental evidence of both the present CMB results and those of early work by Lin et al.18 Notably, they are also consistent with the results of the most recent studies.8,19 However, both in the work of Zhao et al.,19 where the vibrational distribution of CO was found to be as hot as in the early studies,18 and in the kinetic work of Savee et al.,8 where the C2H4 product was detected mass spectrometrically and assumed to be in the ground electronic state, because of the lack of kinematic constraints as you can have in CMB experiments, the theoretical treatment used to discuss the reaction mechanism was not sufficiently detailed to predict product branching ratios that would highlight the primary role of the 1CH3CH + CO channel. In fact, the singlet ethylidene channel was not taken into account in ref 19, where only the triplet ethylene (3C2H4) and triplet ethylidene (3CH3CH) channels were considered and characterized to have high-lying barriers that render them unlikely to be accessible. Because the accuracy of the C3H4O PES has been verified by comparisons with experimental results on the O(3P) + propyne reaction, we can extend the use of this PES to investigate the combustion important reaction between OH and propargyl (see sections S5, S11−S13). The ME simulations performed at temperatures comprised between 1400 and 2500 K reveal that three competitive channels are possible:

(TS34). In contrast, as can be seen in Figure 3b, following O attack to the (most favored)12 central C atom of allene, triplet oxyallyl is formed which readily undergoes ISC to singlet oxyallyl (the triplet−singlet energy splitting is only 1 kcal/ mol)12 that in turn rapidly isomerizes to cyclopropanone almost exclusively; its isomerization to methylketene is highly unfavored because of a high energy barrier. Then cyclopropanone, because of its high energy content, readily undergoes ring-opening, H migration, and C−C bond rupture through a loose TS to CH2CH2 + CO, as discussed by Nguyen et al.12 It should be observed that the barrierless 1CH3CH + CO channel was not present in the PES used by Zhao et al.19 to interpret their experimental results. Master equation simulations25 were performed on the singlet PES to evaluate the relative rates of decomposition of methylketene to CH2CH2 + CO and 1CH3CH + CO for the O + propyne reaction. We simulated the CMB experiments, thus adding the energy of the colliding reactants to the exothermicity of the entrance well, which was assumed to be the minimum energy crossing points (MECP). It was thus found that CO is mainly formed through the 1CH3CH + CO 1013

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The Journal of Physical Chemistry Letters ΔH0° = − 11.4 kcal/mol

OH + CH 2CCH → C2H3 + HCO

Present Addresses

(3a)

§

D.S.: Dipartimento di Chimica, Università degli Studi La Sapienza, 00185 Roma, Italy. ∥ S.F.: Dipartimento di Ingegneria Civile ed Ambientale, Università degli Studi di Perugia, 06125 Perugia, Italy. ⊥ A.B.: Institut des Sciences Moléculaires, Université de Bordeaux, UMR 5255 CNRS, 33400 Talence, France.

→ C2H 2 + CO + H 2 ΔH0° = − 66.2 kcal/mol (3b) → 1CH3CH + CO

ΔH0° = − 31.4 kcal/mol

(3c)

In particular, at temperatures between 1400 and 1600 K and high pressures (10−30 bar), the branching ratio of channel 3c is between 0.1 and 0.3 and the rate constant is relatively high. For instance, it is larger than that of the self-reaction of propargyl radicals.26 Because both C3H3 and OH are present in most combustion systems, it can be reasonably suggested that the ethylidene reaction channel may be common in combustion. The high-pressure rate constants for the reactions reported in Figure 4 were fitted to the modified Arrhenius form and are reported in Table 1.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Italian “Ministero Istruzione, Università e Ricerca − MIUR − (PRIN 2010-2011, Grant 2010ERFKXL_002)” and in part from the “Fondazione Cassa Risparmio Perugia” (Project codes: 2014.0253.021 and 2015.0331.021 Scientific and Technological Research) and the University of Perugia (“Fondo Ricerca di Base 2014”).

Table 1. High-Pressure Rate Constants (s−1) of the Reactions Reported in Figure 4 Fitted between 500 and 2500 K as k = ATα exp(−Ea/RT) reaction

log10 A

α

Ea (cal/mol)

C2H3HCO → C2H3 + HCO C2H3HCO → C2H2 + CO + H2 CH3CHCO → CH3CH + CO CH3CHCO → C2H4 + CO CH3CHCO → cyclopropanone

18.4 11.9 15.9 8.14 9.85

−0.359 1.08 −0.235 1.61 0.936

96 400 85 700 74 700 71 800 72 300

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The two important conclusions of this study are the following: First, it is confirmed that the interaction of atomic oxygen with both propyne and allene breaks apart the carbon atom chain, but with propyne mostly producing CO and ethylidene and with allene6,12 mostly producing CO and ethylene. Second, it is suggested that the 1CH3CH + CO channel is active also when the C3H4O PES is accessed from the OH + CH2CCH entrance channel. We remark that the reactivity of 1CH3CH in combustion systems is something unexplored to date. In particular, internally excited 1CH3CH (see ref 27) may rapidly dissociate to HCCH + H2 (acetylene has been indeed detected in the very recent kinetic work8) or isomerize to highly vibrationally excited CH2CH2, which can in turn dissociate to CCH2 + H2. Alternatively, 1CH3CH, overcoming easily even significant energy barriers, may have a rich bimolecular kinetics. These results may have significant implications for improved combustion models.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00262. Experimental details, computational methods, triplet PES, singlet PES, minimum energy crossing points, active spaces, CCSD(T)/CBS energies and T1 diagnostic, CASPT energy barriers, PESs of variational transition states, high-pressure rate constants, ranching ratios, channel-specific rate constants for C3H3 + OH, and structures and vibrational frequencies of wells and saddle points (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 1014

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