Reaction Dynamics of O(3P) + Propyne: I. Primary Products

Publication Date (Web): April 5, 2016 .... A peculiar and very interesting characteristic of O(3P) reactions with unsaturated hydrocarbons is the ...
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Reaction Dynamics of O(P) + Propyne: I. Primary Products, Branching Ratios and Role of Intersystem Crossing from Crossed Molecular Beam Experiments Gianmarco Vanuzzo, Nadia Balucani, Francesca Leonori, Domenico Stranges, Vaclav Nevrly, Stefano Falcinelli, Astrid Bergeat, Piergiorgio Casavecchia, and Carlo Cavallotti J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b01563 • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 8, 2016

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Reaction Dynamics of O(3P) + Propyne: I. Primary Products, Branching Ratios and Role of Intersystem Crossing from Crossed Molecular Beam Experiments

Gianmarco Vanuzzo, Nadia Balucani, Francesca Leonori, Domenico Strangesa , Vaclav Nevrlyb, Stefano Falcinellic, Astrid Bergeatd, and Piergiorgio Casavecchia* Dipartimento di Chimica Biologia e Biotecnologie, Università degli Studi di Perugia, 06123 Perugia, Italy

Carlo Cavallotti Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica "Giulio Natta", 20131 Milano, Italy

*To whom correspondence should be addressed. E-mail: [email protected] Telephone: +39-075-5855514

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ABSTRACT We have performed synergic experimental/theoretical studies on the mechanism of the O(3P)+CH3CCH (propyne) reaction by combining crossed molecular beams (CMB) experiments with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy (Ec) with ab initio electronic structure calculations at a high-level of theory of the relevant triplet and singlet potential energy surfaces (PESs) and statistical calculations of branching ratios taking into account intersystem-crossing (ISC). In this paper (I) we report the results of the experimental investigation, while in the accompanying paper (II) those of the theoretical investigation together with comparison to experimental results. By exploiting soft electron ionization detection to suppress/mitigate the effects of the dissociative ionization of reactants, products and background gases, product angular and velocity distributions at different charge-to-mass ratios have been measured. From the laboratory data angular and translational energy distributions in the center-of-mass system have been obtained for the five competing most important product channels and product branching ratios (BRs) have been derived. The reactive interaction of O(3P) with propyne under single collision conditions is mainly leading to the rupture of the three-carbon atom chain, with production of the radical products methylketenyl+atomic hydrogen (CH3CCO+H) (BR=0.04), methyl+ketenyl (CH3+HCCO) (BR=0.10), vinyl+formyl (C2H3+HCO) (BR=0.11), and molecular products ethylidene/ethylene+carbon monoxide (CH3CH/C2H4+CO) (BR=0.74) and propandienal+molecular hydrogen (CH2CCO+H2) (BR=0.01). Because some of the products can only be formed via ISC from the entrance triplet to the low-lying singlet PES, we infer from their BRs an amount of ISC larger than 80%. This value is dramatically large when compared to the negligible ISC reported for the O(3P) reaction with the simplest alkyne, HCCH (acetylene). At the same time, it is much larger than that (about 20%) recently observed in the related reaction of the three-carbon atom alkene, O(3P)+CH3CHCH2 (propene) at a comparable Ec. This poses the question of how important it is to consider nonadiabatic effects and their dependence on molecular structure for this kind of combustion reactions. The prevalence of the CH3 over the H displacement channels is not explained by invoking a preference for the addition on the methyl substituted acetylenic carbon atom, but rather it is believed to be due to the different tendencies of the two addition triplet intermediates CH3CCHO (preferentially leading to H elimination) and CH3COCH (preferentially leading to CH3 elimination) to undergo ISC to the underlying singlet PES. It is concluded that the main co-product of the CO forming channel is singlet ethylidene (1CH3CH) rather than ground state ethylene (CH2CH2). By comparing the derived BRs with those very recently derived from kinetics studies at room temperature and 4 torr we have obtained information on how BRs vary with collision energy. The extent of ISC is estimated to remain essentially constant (about 85%) with increasing Ec from about 1 kcal/mol to about 10 kcal/mol. The present experimental results shed light on the mechanism of the title reaction at energies comparable to those involved in combustion and when compared with the results from the statistical (RRKM/Master Equation) calculations on the ab initio coupled PESs (see accompanying paper II) lead to an in depth understanding of the rather complex reaction mechanism of O+propyne. The overall results are expected to contribute to the development of more refined models of hydrocarbon combustion. 2 ACS Paragon Plus Environment

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1. INTRODUCTION Small unsaturated hydrocarbons (alkenes and alkynes) are of paramount importance in combustion chemistry. In fact, they can be easily formed during the combustion of a large variety of hydrocarbons, from simple methane to large aliphatics and aromatics, and play a role in the formation of polycyclic aromatic hydrocarbons and hence soot.1,2 Their reactions with atomic oxygen in its ground state, O(3P), are among their prevalent consumption pathways in oxygen-rich zones.2,3,4,5 In order to improve existing combustion models, detailed information on the overall rate coefficients of these reactions, on the identity of the primary products and on their branching ratios at combustion temperatures are needed. The rate coefficients for most O(3P) reactions with alkenes and alkynes have been determined in kinetics experiments at room temperature or, better, at temperatures of relevance in combustion processes.6 Much less is known, instead, on the chemical identity of the primary products and their branching ratios (BRs). This piece of information is particularly relevant, since the products of one elementary reaction are the reactants of a subsequent one that may be chain propagating or chain terminating and may lead to the formation of different pollutants with very different efficiencies. In the case of relatively complex, multichannel reactions, as those of O(3P) with unsaturated hydrocarbons certainly are, the primary products are not easy to predict. A peculiar and very interesting characteristic of O(3P) reactions with unsaturated hydrocarbons is the possible occurrence of intersystem crossing (ISC) from the entrance triplet to the underlying singlet potential energy surface (PES), a nonadiabatic process that can deeply affect the reaction outcome.7,8 Notable examples are the reactions O(3P)+C2H4 and O(3P)+CH2CCH2 where ISC accounts for ca. 50% and more than 90%, respectively.7,9,10 The extent of ISC is somewhat unpredictable relying on simple chemical intuition. For instance, the reaction of O(3P) with the simplest alkyne, ethyne (C2H2), is not characterized by a significant ISC11,12 while there has been already experimental evidence13,14,15 that the title reaction involving the next member in the series, propyne, is actually characterized by a very significant amount of ISC (more than 80% at 298 K16), because it makes accessible several reaction channels which would be otherwise precluded, were the reaction proceeding only on the triplet PES.15,16 A detailed study of the O+propyne reaction under single collision conditions at translational energies higher than those corresponding to room temperature can allow us to tackle the questions: How do the distribution of primary products, their branching ratios and the extent of ISC vary when an H atom is replaced with a methyl group in the alkyne series and how all this vary with the translational energy (temperature) of the reactants? We note that the reaction of O(3P) with propyne (CH3CCH) is also of relevance in the combustion of biofuels (as butanol and esters).1 The reaction O(3P)+CH3CCH is much more complex than O(3P)+HCCH; as a matter of fact, it exhibits the following large number of exothermic channels (those which can only originate from the singlet PES via ISC are marked with an S):

O + CH3CCH

→

H + CH3CCO (methylketenyl)

∆H°0 = –17.4 kcal/mol

(1) 3

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→

H + CH2CCHO (acrolein-2-yl)

∆H°0 = –10.4 kcal/mol

(2)

→

H + CH2CHCO (propenoyl)

∆H° = –27.6 kcal/mol

(3)

→

CH3 + HCCO

∆H°0 = –26.7 kcal/mol

(4)

→

C2H3 + HCO

∆H°0 = –23.1 kcal/mol

(5)

→

3

CH3CH + CO

∆H°0 = –45.9 kcal/mol

(6)

→

C2H4(3A) + CO

∆H°0 = –52.0 kcal/mol

(7)

→

3

CH2 + CH2CO

∆H°0 = –22.6 kcal/mol

(8)

→

CH2CCH + OH

∆H°0 = –11.7 kcal/mol

(9)

→

H2 + CH2=C=C=O

∆H° = –66.5 kcal/mol

(S10)

→

C2H2 + H2CO

∆H°0 = –75.5 kcal/mol

(S11)

→

C2H2 + H2 + CO

∆H°0 = –77.9 kcal/mol

(S12)

→

1

CH3CH + CO

∆H°0 = –43.1 kcal/mol

(S13)

→

1

C2H4 + CO

∆H°0 = –118.0 kcal/mol

(S14)

The reaction enthalpies are those derived by the electronic structure calculations described in paper II17, with the exception of channel S10 (in this case we have reported the value obtained by Zhao et al.15). The global reaction rate coefficient (pressure independent in the 100-700 mbar range) for O(3P)+propyne has been determined to be k(300-1350 K) = 2.9 × 10-11 exp(-1134 K/T) cm3 molecule-1 s-1.18 As for the product BRs, much uncertainty has existed for decades until when, very recently, kinetic studies at 298 K and 4 Torr using photoionization mass spectrometry and VUV synchrotron radiation have reported the first attempt to quantify the BRs for all channels16 (see Table 1). However, some uncertainties still remain and in particular BRs are not yet available at combustion temperatures. Furthermore, theoretical predictions of BRs with which to compare experimental results have not been available up to now. On the other hand, statistical calculations of BRs are still a challenge for this kind of systems because they require accurate ab initio PESs including nonadiabatic effects (i.e., triplet/singlet ISC). We remind that the knowledge of primary products and BRs is an extremely valuable and much needed information for the modeling of combustion systems, as the products of one elementary reaction become the reactants of a subsequent one5,19 in the intricate networks of reactions that account for the global transformation. Some implications of the title reaction are also expected in the modeling of hydrocarbon-rich planetary atmospheres, such as that of Titan, where a little

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Table 1. History of the branching ratios of the O(3P) + CH3CCH reaction.

Reaction channel

Kanofsky et al. 13 (1974)

Lin et al.25,26 (1976)

Blumenberg et al.24 (1977)

Xing et al.14 (1996)

Zhao et al.15 (2009)

Savee et al.16 (2015) 300 K

This work CMB Ec=9.2 kcal/mol

H + CH3CCO

major

-

-

∼1/3 of CO

-

0.05 ± 0.03

0.04 ± 0.02

CH3 + HCCO

major

-

-

-

-

0.10 ± 0.03

0.10 ± 0.05

C2H3 + HCO

major

-

-

-

-

0.28 ± 0.05

0.11 ± 0.04

0.95

0

-

major

CO* (up to v=5)

(CO* up to v=5)

0.20 ± 0.07 (037± 0.09)

-

0.36 ± 0.11

1

CH3CH + CO

C2H4 + CO

large

-

0.74 ± 0.25

C2H2 + CO + H2

major or minor

-

minor

-

-

H2 + CH2CCO

-

-

minor

-

-

< 0.01

0.01 ± 0.005

C3H3 + OH

minor

-

-

-

-

0.01 ± 0.007

< 0.02

CH2CO + CH2

major

-

-

-

-

-

< 0.02

(0.19± 0.04)

amount of oxygen compounds are present20, as well as in the modelling of oxygen-rich regions of the interstellar medium.21 A summary of the previous experimental and theoretical studies on the title reaction is useful to put into context the present work. The reaction kinetics of propyne with O(3P) have been investigated since the 1960s.13,22,23,24,25,26 From end-product analysis the reaction was assumed to proceed largely by ISC to several primary products, among which, predominantly, CO and singlet ethylidene (1CH3CH) via a methylketene intermediate; notably, detection of small amounts of 2-butene was attributed to the self reaction of ethylidene.22,23 The nature of the detected C2H4 product, whether CH3CH or CH2CH2, has been a topic of debate since the early studies. In the 1970s, with progress in time-resolved detection techniques a limited number of the many possible reaction products started to be detected directly. In 1974, pioneering crossed effusive molecular beam experiments by Kanofsky et al.13 using photoionization mass-spectrometric detection with atomic resonance lamps permitted the observation of a variety of primary products from this 5 ACS Paragon Plus Environment

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reaction. In particular, CH2, CH3, C2H4, HCO, C2H2O and C3H3O were clearly identified as major products, while a minor contribution was made also by C2H2 and C3H3 (see Table 1). Notably, in that study the fact that the product at m/z=28 (C2H4+) was detected below its ionization potential was interpreted as either the CH3CH radical or internally excited C2H4 was actually formed; in addition, the fact that also C2H2 was detected lead to suggest that the initially formed ethylidene radical (CH3CH) could rearrange to yield excited ethylene or decompose to yield acetylene and H2. Later studies in 1976 by Blumenberg et al.24 in low pressure, crossed jets and Laval nozzle reactors, employing mass spectrometric detection with tunable energy electrons (from 4.5 to 70 eV) confirmed that the CH3CH (C2H4)+CO channel(s) are dominant (contributing for 95% to the overall reactive signal) (see Table 1); minor detected products were also H2 and C2H2. Absolute branching ratios, however, could not be obtained from these studies13,24, whose average collision energy corresponds to about 1 kcal/mol. The suggestion from the early kinetic works22,23 and the studies of Kanofsky et al.13 and Blumenberg et al.24 that O(3P)+propyne leads to 1CH3CH+CO was shortly afterwards supported by a comparative study by Lin et al.25 of the O(3P) reaction with allene and propyne leading to CO formation at 293 K with a CO laser resonant absorption and a discharge-flow gas-chromatographic sampling method.25,26 An investigation by VUV laser induced fluorescence (LIF) by Xing et al.13 provided information on H, H2 and CO channels by determining the translational energy distribution of the H product and the internal energy of H2 and CO. It was concluded that the CO channel should provide a large contribution to the overall reaction, with the H channel being also quite significant (Table 1). In particular, these authors supported the early conclusion of Lin et al.25,26 that together with CO ethylidene was formed, and part of it dissociates producing H2 either along with vinylidene or isomerizes to C2H4 which can partly dissociate to H2 and C2H2 (see ref. 27). However, more recent experimental studies by Zhao et al.15 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, have suggested that the O(3P)+propyne reaction actually produces predominantly ground state C2H4+CO, similarly to the O(3P)+allene reaction.10,28 This result is in contrast with the conclusions of Lin et al.25 , even though the CO vibrational distributions was found as hot as in Lin's studies. The possibility of ISC was theoretically explored by Zhao et al.15 and it was found to be a competitive reaction pathway as the minimum energy crossing point between the triplet and singlet PESs, which can be considered as a transition state for the nonadiabatic process, is lower in energy than any other isomerization or fragmentation route originating from the addition intermediate which is formed when atomic oxygen adds to the terminal carbon (C1) on the unsaturated bond. Very recently, state-of-the-art kinetic investigations at 298 K and 4 torr by Savee et al.16 have led to a quantitative determination of the BRs of the five major product channels of the title reaction (see Table 1). Minor production of OH+C3H3 and H2+CH2CCO was also observed. It was concluded that 84%±14% of the products are coming via ISC from the entrance triplet PES to the lower-lying singlet PES. Overall the main channel(s) was found to be the CO forming channel(s); however, under their experimental conditions a BR of 0.20±0.07 was attributed to C2H4+CO (channel S14) and a BR of 0.36±0.11 to formation 6 ACS Paragon Plus Environment

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of C2H2+H2+CO (channel S12) (total CO BR=0.56), where C2H2 was attributed to dissociation (within a time shorter than the time scale of their experiment) of internally hot C2H4. Interestingly, using the information from the studies of Lin et al.24,25 that the ratio of acetylene to ethylene (NC2H2/NC2H4) is ∼ 0.5 at 600 Torr, they estimated that 37% of the reactive flux leads to C2H4+CO and 19% leads to C2H2 + H2 + CO (see Table 1). It should be noted that Savee et al.16 relied on the theoretical calculations of the triplet/singlet PESs of Zhao et al.15 for the interpretation and discussion of their experimental results. With the aim of achieving a detailed understanding of the O(3P) + propyne non-adiabatic multichannel reaction, in the present paper (I) and the accompanying paper (II)17 we report a combined experimental/theoretical investigation of its dynamics and kinetics. The experimental approach is based on the crossed molecular beam (CMB) scattering technique coupled to mass spectrometric (MS) detection and time-of-flight (TOF) analysis, empowered with soft electron ionization, which has been successfully used for a number of years in our laboratory to investigate the reactions O(3P) + C2H2,12,29 O(3P) + C2H4,7,9,30,31,32 O(3P) + CH2CCH2,10 and O(3P) + CH3CHCH2.33,34 We remind that the CMB technique guaranties singlecollision conditions and therefore avoids any complication that can arise in experiments under multiplecollision conditions. Furthermore, the technique can exploit the conservation laws of energy and linear momentum to unambiguously identify the primary products and can assess the presence or not of exit potential barriers from the shape of the product translational energy distribution. All this information permits to elucidate and rationalize the dynamics of the various reaction channels in terms of the relevant potential energy surfaces. In the theoretical approach (paper II) ab initio electronic structure calculations at a high level of theory are performed on the triplet/singlet PESs (in particular barrier heights, stationary points, isomerization paths, seams of triplet-singlet intersystem crossing, spin-orbit couplings). On these coupled PESs, statistical Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) calculations, with inclusion of intersystem crossing, are carried out to predict product branching ratios, as done in our recent study on the reaction O(3P)+propene.34 The aims are: (i) to determine BRs at translational energies of the reactants higher than room temperature and closer to combustion conditions; (ii) address the issue whether singlet ethylidene is formed or not in the reaction; (iii) to calculate ab initio the triplet and singlet PESs and their couplings at the highest possible level of theory for this kind of systems; (iv) to perform statistical calculations of BRs taking into account ISC; (v) to compare the statistical predictions with experimental results, from both the present CMB experiments at Ec=9.2 kcal/mol and the kinetics ones at 298 K and 4 Torr. These detailed comparisons will be the subject of the accompanying paper II. We emphasize that the complexity of the problem calls for a synergic experimental/theoretical approach. Specifically, a detailed theoretical treatment is required to assist the interpretation of the experimental findings and achieve a detailed understanding of the mechanism of the title reaction. An in-depth comprehension of this and other relatively simple reactions may facilitate the elucidation of the mechanisms also in reactions involving larger alkynes and alkenes (having four or more carbon atoms). It should be noted that, notwithstanding the remarkable progresses made by a recent experimental method employing tunable VUV synchrotron radiation (see, for instance, ref. 16 and 35) and the novel CP7 ACS Paragon Plus Environment

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FTMW (chirped-pulse Fourier transform microwave)/pulsed uniform flow technique36, the characterization of the primary products and product branching ratios remains in general arduous in kinetics investigations. The universal CMB technique based on mass spectrometric detection and time-of-flight (TOF) analysis represents an efficient complementary approach.8,31,37 In particular, when empowered with soft ionization by tunable low energy electrons or VUV synchrotron radiation38, this technique has proved to be suitable to identify unambiguously all the primary reaction products of multichannel reactions and to determine the product BRs under single-collision conditions.8-10,12,32-34 We remind that the pioneering work on the dynamics of O(3P) reactions with unsaturated hydrocarbons, including aromatics, using the CMB technique with universal detection was performed in the 1980s-1990s by Lee and coworkers.39,40,41,42,43 A preliminary account on the relative importance of the H and CH3 forming channels in the title reaction has been reported previously.44 Some selected results of the present joint experimental/theoretical study, which highlight the importance of the 1CH3CH+CO forming channel in the O(3P)+propyne reaction, in contrast to the importance of the CH2CH2+CO channel in the related isomeric O+allene reaction, have been very recently published in a Letter.45 The present work and that in the accompanying paper (II)17 extend and complement the previous partial reports.44,45 This paper (I) is organized as follows. Section 2 presents the experimental details. The experimental results are reported and analyzed in Section 3. The results are then discussed in Section 4 with reference to the theoretical PESs and compared with the room temperature branching ratios from kinetics studies. A summary and conclusions follow in Section 5.

2. EXPERIMENTAL The O(3P)+propyne reaction dynamics has been investigated using the CMB technique with soft electron ionization mass spectrometric detection and TOF analysis. The apparatus and methodology is the same used since a number of years for studying the dynamics of a variety of reactions of O(3P) with unsaturated hydrocarbons.8,31,46,47 As in other similar reactive systems, critical has been the capability of performing soft electron ionization detection, which permits to suppress or mitigate the dissociative ionization of interfering species (reactants, products and background gases) that would otherwise make extremely difficult and often impossible to perform this kind of studies. In general, only the atomic and molecular hydrogen elimination channels, in which the O atom replaces an H atom or two H atoms of the hydrocarbon, can be easily studied without resorting to soft ionization detection. In contrast, for a multichannel reaction such as O+propyne, detection of all other products that arise from rupture of the carbon chain of the initial O-hydrocarbon intermediate complex, are usually plagued by interferences coming from dissociative ionization processes. The setting of the CMB apparatus used for this study is similar to that recently used for the investigation of the O(3P)+propene reaction dynamics.34 So, here we only give the details of the reactant beam characteristics relevant to the present reactive system. The O(3P) atom supersonic beam was generated from a radio-frequency (RF) discharge source48,49,50 by discharging a 5% O2/He gas mixture at 190 mbar and 8 ACS Paragon Plus Environment

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300 W of RF power through a 0.28 mm diameter quartz nozzle; the O beam peak velocity was 2490 m/s and its speed ratio 5.8. A supersonic beam of propyne was obtained by expanding 400 mbar of propyne through a stainless-steel nozzle of 0.1 mm diameter kept at 300 K; the resulting beam peak velocity was 760 m/s and the speed ratio 4.6. Because the two reactant beams are crossed at an angle of 90° the collision energy is 9.2 kcal/mol. Counting times for the laboratory (LAB) product angular distributions, N(Θ), and the TOF distributions, N(Θ, t), (Θ being the LAB scattering angle and t the time) were similar to those used in the study of O(3P)+propene, namely, 50 or 100 s for the angular distributions and one to four hours for the TOF distributions. The psudorandom TOF spectra of the products were recorded using 6 µs/channel dwell time. The dynamical information from CMB experiments is contained in the product flux in the center-ofmass (CM) frame, ICM(θ,u) (where θ and u are the CM scattering angle and velocity, respectively). We recall that the transformation of the CM flux to LAB number density N(Θ) is given by the relation: N(Θ) = (v/u2)ICM(θ,u)

(15)

which indicates that slow products in the CM frame (i.e., products having small u) are amplified in the LAB frame and are therefore easier to detect.51 The ICM(θ,u) is treated, for each channel, as being separable into an angular part, T(θ), and a velocity part, P(u): ICM(θ,u) = T(θ)×P(u), with the latter usually expressed as a function of the product recoil energy distribution, P(E'T), i.e., ICM(θ,E'T) = T(θ)×P(E'T). In order to reproduce quantitatively the shape of the angular and TOF distributions measured at the various mass-to-charge (m/z) ratios, a weighted total CM differential cross section, ICM(θ,E'T)total, reflecting the various possible contributions to that m/z signal, was used in the data analysis: ICM(θ,E'T)total = ∑i wi × [T(θ)i × P(E'T) i]

(16)

where the wi parameters represent the relative contribution of the integral cross section of the ith channel and are treated as best-fit parameters.31,47 The best-fit ICM(θ,E'T)i are retrieved using a forward convolution fit of the LAB N(Θ) and N(Θ, t) distributions measured at the various m/z. Simulated LAB densities are calculated taking into account the averaging over experimental parameters (angular and velocity divergence of the two reactant beams, finite ionizer length, and detector acceptance angle). The CM functions, T(θ) and P(E'T), for each channel so determined are the best-fit functions for that given product channel.

3. RESULTS AND ANALYSIS Figure 1 shows the velocity vector (also called Newton) diagram of the reaction; this diagram depicts the kinematics of the various possible reaction channels. The Newton circles are drawn by assuming that all the available energy is channelled into product translational energy and, therefore, delimit the angular ranges within which the indicated products can be scattered on the basis of energy and momentum conservation. As can be seen the channels (1-3) and (S10), corresponding to the H elimination and H2 elimination channels, 9 ACS Paragon Plus Environment

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Figure 1. Newton (velocity vector) diagram with superimposed the circles which delimit the center-of-mass speed (and angular range in the LAB frame) of the various indicated products of the O(3P)+propyne reaction at Ec=9.2 kcal/mol. Black continuous line: Newton circles for the H-elimination channel which leads to CH3CCO (the circles corresponding to the isomeric products CH2CCHO/CH2CHCO would be indistinguishable in the compressed scale of the figure - see text). Orange solid line: CH2CCO (from H2 channel). Red dashed line: HCO. Red solid line: C2H3. Magenta solid line: CH3CH/CO. Violet solid line: CH3. Blue solid line: C2H4/CO.

have a favorable kinematics because the corresponding co-products C3H3O and C3H2O, being very heavy (with respect to H and H2), are allowed to be scattered only within small Newton circles. In contrast, coproducts of channels associated to breaking of the C-C bond exhibit more similar masses and therefore, on the basis of momentum conservation, are scattered over Newton circles which are considerably wider. At the same time, these products are faster both in the CM and LAB frames. For instance, being the C3H3O product very heavy with respect to its co-product H, it can only have low CM velocities and therefore (see eq. (15)) its intensity in the LAB system will be confined in a small angular range and hence amplified with respect to the intensity of a product coming from a C-C bond breaking channel (under the assumption that the reaction cross sections are comparable in the two cases), which is spread over a much wider angular range (see below). 3.1 Product angular and velocity distributions in the LAB frame. Reactive signals were detected at m/z=55 (C3H3O+), 54 (C3H2O+), 53 (C3HO+), 29 (HCO+), 27 (C2H3+), 26 (C2H2+), 15 (CH3+) and 14 (CH2+) using 17 eV electrons; however, measurements at m/z=55-53 could also be performed at 60 eV because of the lack of dissociative ionization problems for the H and H2/O exchange channels. No reactive signal was detected at m/z=30 (H2CO+) or at m/z=17 (OH+) which rules out (within our sensitivity) the occurrence of 10 ACS Paragon Plus Environment

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The Journal of Physical Chemistry

channel S11 and 9, respectively. It should be noted that the abstraction pathway (9) which leads to formation of OH, although exothermic by 11.9 kcal/mol, is characterized by a high entrance barrier (of 7.5 kcal/mol)17 above the reactant asymptote and therefore is not expected to be open under our experimental conditions. The signal at m/z=14 was indeed very small, thus suggesting that channel 8 is open but with a very small yield (below our sensitivity of measuring angular and TOF distributions, i.e., B.R.