Letter pubs.acs.org/JPCL
Crossed Molecular Beam Dynamics Studies of the O(3P) + Allene Reaction: Primary Products, Branching Ratios, and Dominant Role of Intersystem Crossing Francesca Leonori, Angela Occhiogrosso,† Nadia Balucani,* Alberto Bucci, Raffaele Petrucci, and Piergiorgio Casavecchia* Dipartimento di Chimica, Università degli Studi di Perugia, Perugia 06123, Italy ABSTRACT: We report on the determination of primary products and their branching ratios for the combustion relevant O(3P)+allene reaction by the crossed molecular beams method with soft electron-ionization mass-spectrometric detection at a collision energy of 39.3 kJ/mol. We have explored the reaction dynamics of the open channels leading to C2H4+CO, C2H2+H2CO, C2H3+HCO, CH2CCHO+H, and CH2CO+CH2. Because some of the observed products can only be formed via intersystem crossing (ISC) from triplet to singlet potential energy surfaces, from the product branching ratios we have inferred the extent of ISC. The conclusion is that the O(3P)+allene reaction proceeds mostly (>90%) via ISC. This observation poses the question of how important it is to consider nonadiabatic effects for this and other similar systems involved in combustion chemistry. Another important conclusion is that the interaction of atomic oxygen with allene breaks apart the three-carbon atom chain, mostly producing CO and ethylene. SECTION: Dynamics, Clusters, Excited States flight (TOF) analysis, based on “soft” ionization by low energy electrons4 or tunable VUV synchrotron radiation,5 have proved to be particularly suitable to identify unambiguously the primary reaction products of multichannel reactions. The implementation of the “soft” ionization technique has been crucial because it can mitigate or suppress dissociative ionization of products or interfering species, which has been the main limitation in typical CMB-MS experiments based on “hard” ionization (electron energy >60 eV). In any case, MS detection is advantageous for systems like these, where complex polyatomic radicals are produced and spectroscopic techniques are difficult to apply. In our laboratory, we have recently undertaken a systematic investigation of reactions of O(3P) with unsaturated hydrocarbons.6,7 For O(3P)+C2H2 and O(3P)+C2H4, by exploiting soft electron ionization (EI) detection, we have identified and characterized the dynamics of essentially all the open channels at one collision energy, Ec, corresponding to high temperatures.6,7 For the reaction O(3P)+C2H4, we have observed some primary reaction products that can only be formed after intersystem crossing (ISC) to the singlet potential energy surface (PES) takes place.6 Accurate theoretical treatments for this system would necessarily require the inclusion of nonadiabatic effects. Statistical calculations of product BRs have been performed by Nguyen et al.8 on the triplet and singlet adiabatic PESs. In their work, however, the extent of ISC
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complete comprehension of combustion processes, especially as far as the formation of pollutants is concerned, requires the knowledge on the molecular level of all of the important elementary reactions.1,2 Rate coefficients for most combustion reactions have been measured in kinetics experiments,3 but much less is known about the nature of the primary products and their branching ratios (BRs). Because the products of one elementary reaction become the reactants of a subsequent one in propagating or terminating chains, it is very important to determine the identity of the primary reaction products and their BRs. Only in this way might we be able to understand and, possibly, control the formation of combustion pollutants.1,2 Small unsaturated hydrocarbons are crucial intermediates in the combustion of most fuels1,2 and are involved in the generation of polycyclic aromatic hydrocarbons and soot. The prevalent consumption pathways of small unsaturated hydrocarbons are their reactions with ground-state 3 P oxygen atoms, for example, O+C2H2 and O+C2H4. Despite their apparent simplicity, these elementary reactions are actually multichannel reactions involving important nonadiabatic effects, and the product identities as well as their relative yields have not been easy to determine, especially under the conditions (particularly temperature) that resemble those of flames. The determination of the reaction products and their yields remain difficult to achieve in kinetics experiments, and a complementary method, the crossed molecular beam (CMB) technique in its ‘universal’ arrangement with mass spectrometric (MS) detection, represents an efficient experimental alternative to investigate polyatomic multichannel elementary reactions and provide useful additional information. In particular, CMB experiments with MS detection and time-of© 2011 American Chemical Society
Received: November 17, 2011 Accepted: December 12, 2011 Published: December 12, 2011 75
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remained the subject of considerable uncertainty over the years. Channel 1e, leading to the molecular products C2H4+CO, has been recognized as the main reaction channel.12,16 Another kinetics study identified atomic hydrogen as a reaction product, thus indicating that channel 1a, channel 1b, or both are occurring.14 In a gas-phase investigation, even the adduct acrolein was identified as a minor product.12 Acrolein and its isomers cyclopropanone and allene oxide were detected by Pimentel and coworkers in a cryogenic matrix study, in which also ketene and formaldehyde were observed.17 With the exception of ketene, all other species observed in matrix require ISC to the singlet PES. Clearly, the reaction can proceed via ISC to the singlet PES, accessing a variety of products, such as CO+C2H4 and the stabilized singlet adducts observed in the matrix. ISC is certainly facilitated in matrix or under highmoderate pressure conditions, but it can also occur in isolated vibrationally excited intermediates, as observed in the reaction O(3P)+C2H4.6 Hence, one can reasonably expect that ISC is facile in the reaction O(3P)+allene, and the question remaining is: what is the extent of ISC under single collision conditions? A previous dynamic study of this reaction by the CMB-MS method was reported by Schmoltner et al.18 Besides identifying channel 1e detecting HCO at m/z = 29 and channel 1a detecting C3H3O at m/z = 53 (as C3HO+), the use of isotopically labeled 18O atoms permitted them to identify also channel 1f, detecting C18O at m/z = 30. The use of “hard” ionization detection in that study, however, prevented the observation of the other products and the estimate of BRs. Ab initio calculations of the stationary points and product energetics for the triplet and singlet PESs and statistical (RRKM) calculations of BRs have been carried out by Nguyen et al.11 also on this system. As already pointed out, these calculations cannot predict the extent of ISC and, in the absence of experimental information, the adiabatic results could not infer the extent of ISC. Nonetheless, those calculations concluded that if the reaction proceeds entirely on the triplet PES, then the main products are CH2CO+CH2 and CH2C− CHO+H, whereas if the reaction proceeds via ISC, then the dominant products are CO+C2H4.11 To identify unambiguously the primary products and their BRs, we have studied the O(3P)+allene reaction at Ec = 39.3 kJ/mol in CMB-MS experiments by exploiting soft EI detection to interrogate all channels. Operating with essentially zero background, no signal was detected at mass-to-charge ratios (m/z) of 15 and 17, which rules out (within our sensitivity, i.e., BR < 0.3%) channels 1h and 1i. From measurements at m/z = 55−52, we have ruled out also the occurrence of channel 1c, as the measured distributions were always superimposable and clearly attributable to the H-displacement channel 1a or 1b. Furthermore, reactive signal has been observed for all other competing channels. The most probable Newton diagram (showing the kinematics of the various channels) is reported in Figure 1. It can be easily appreciated that the H-displacement channel 1a is that with the most favorable kinematics: the heavy coproduct C3H3O is confined into a much smaller sphere compared with those associated with the C−C bond breaking channels, where two cofragments of comparable masses are produced. Because the solid angle subtended by the detector is always the same and so is the detector distance from the collision center, the signal recorded for the H-displacement channel will appear to be largely dominant in the laboratory (LAB) reference frame. In Figure 2 are reported the LAB angular distributions measured at m/z = 53, 29, 27, 26, and 14.
could be deduced only by comparing experimental information with the product yields from triplet and singlet PESs weighted by an adjustable parameter. The treatment of nonadiabatic effects in the dynamics of polyatomic reactive systems remains a challenging task, but high-level electronic structure calculations of triplet and singlet PESs, including nonadiabatic coupling terms, are becoming available for these systems.9,10 Schatz and coworkers9 have reported quasiclassical trajectory “surface-hopping” calculations for O(3P)+C2H4, whereas Rajak and Maiti10 have reported similar studies for O(3P)+C2H2, in both cases using ab initio coupled triplet and singlet PESs. After these first experimental and theoretical studies, the extent of ISC in O(3P) atom reactions with unsaturated hydrocarbons and its effect on the product yields have clearly emerged as one of the central issues in this class of combustion reactions. Because of the difficulty of correctly treating these effects in polyatomic systems (in both cases, the extent of ISC seems to be overestimated by theory9,10), detailed experimental results are required to validate theoretical predictions. In this Letter, we report on a detailed experimental investigation on the dynamics of the reaction of O(3P) with allene (propadiene) where ISC can be at play. Nine energetically allowed competing pathways are possible O + C3H4 → CH 2 = C − CHO + H Δr H °0K = − 58.6 kJ/mol
(1a)
O + C3H4 → CH 2 = CH − CO + H Δr H °0K = − 129.7 kJ/mol
(1b)
O + C3H4 → CH 2 = C = C = O + H 2 Δr H °0K = − 320 kJ/mol
(1c)
O + C3H4 → CH 2CO + CH 2 Δr H °0K = − 102.1 kJ/mol
(1d)
O + C3H4 → C2H3 + HCO Δr H °0K = − 100.4 kJ/mol O + C3H4 → C2H4 + CO
(1e)
Δr H °0K = − 500.4 kJ/mol (1f)
O + C3H4 → C2H 2 + H 2CO Δr H °0K = − 324.7 kJ/mol
(1g)
O + C3H4 → CH3 + CHCO Δr H °0K = − 119.7 kJ/mol O + C3H4 → C3H3 + OH
(1h)
Δr H °0K = − 54.0 kJ/mol (1i)
The reaction enthalpies of channels 1a, 1b, and 1c are those derived by electronic structure calculations by Nguyen et al.11 The title reaction has been investigated in kinetics experiments, employing a variety of techniques and under different conditions of pressure and temperature (300−800 K).12−14 The suggested rate coefficient at room temperature is 1.4 × 10−12 cm3/(molec s).15 In the kinetics experiments only one or a few of the possible products have been identified, and the identity of the primary reaction products and their BR has 76
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Figure 1. Newton diagram showing the circles that delimit the angular range and center-of-mass speed for the products of the various channels of the reaction O(3P)+allene at Ec = 39.3 kJ/mol. Black continuous line: Newton circle for the H-displacement channel leading to CH2C−CHO (C3H3O in the Figure). Cyan continuous line: Newton circle for CH2; dashed line: Newton circle for CH2CO, both produced in channel 1d. Red continuous line: Newton circle for C2H3; dashed line: Newton circle for HCO, both produced in channel 1e. Green continuous line: Newton circle for H2CO; dashed line: Newton circle for C2H2, both produced in channel 1g. Blue line: Newton circles for both CO and C2H4, which are produced in channel 1f; the two circles are identical because CO and C2H4 have the same mass.
The TOF distributions recorded at the center-of-mass angle, ΘCM, for the five masses are reported in Figure 3. The distributions recorded at m/z = 55−52 were obtained by using an ionizing electron energy of 60 eV, as no significant improvement in the signal-to-noise ratio (S/N) was obtained by reducing it. Because of the best S/N all final measurements were carried out at m/z = 53. On the contrary, to measure product angular and TOF distributions at m/z of 29, 27, 26 and 14, we took full advantage of the soft EI technique, as these distributions could be measured only by reducing the background signal and interferences from dissociative ionization by employing a 17 eV electron energy. As is well-visible, even at this low electron energy in the angular and TOF distributions at m/z of 29, 27, and 26, there is a significant contribution arising from the dissociative ionization of the CH2C−CHO product, the channel which appears the dominant one in the LAB reference frame. Nevertheless, product intensity is clearly visible at LAB angles that are outside the angular range amenable to channels 1a/1b. The shapes of the additional contributions at the m/z = 29, 27, 26, and 14 distributions are quite different, thus implying different signal sources. Additional information on the contributions to the observed signal is given by the TOF spectra. The m/z = 53 TOF spectrum is clearly unimodal, whereas the other spectra have a pronounced structure with two or three peaks. Quantitative information is obtained by moving from the 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 factorized. (The best-fit CM functions are actually derived by a forward convolution fit of the product LAB angular and TOF distributions.) The solid lines superimposed on the experimental results in Figures 2 and 3 are the calculated curves when using the best-fit CM functions reported in Figures 4 and 5. To reach a good fit of the data at m/z = 29, 27, 26, and 14, it was necessary to consider multiple contributions associated
Figure 2. LAB angular distributions at m/z = 53, 29, 27, 26, 14. The solid black curves represent the calculated distributions when using the best-fit CM functions of Figure 4. The separate contributions to the calculated global LAB angular distributions are also shown. Light black line: H-displacement channel leading to CH2C−CHO. Cyan continuous line: CH2; cyan dashed line: CH2CO, both produced in channel 1d. Red continuous line: C2H3; dashed line: HCO, both produced in channel 1e. Green continuous line: H2CO; dashed line: C2H2, both produced in channel 1g. Blue line: C2H4, produced in channel 1f.
with each product contributing at the signal for that m/z. According to our analysis at m/z = 29, we have a contribution from H2CO (which is known to dissociate strongly into the daughter ion HCO+) from channel 1g and from the formyl radical produced in channel 1e. The analysis at m/z = 27, instead, leads to the attribution of two products, that is, C2H4 (via dissociative ionization to C2H3+) and C2H3 radical (parent ion) produced in the channels 1f and 1e, respectively. Both products C2H4 and C2H3 give a contribution also at m/z = 26, where the additional contribution from acetylene (channel 1g) is also visible. Finally, at m/z = 14 (CH2+), we have measured together the CH2 parent ion and the CH2CO daughter ion originating from the same channel 1d. (The appearance energy of CH2+ from CH2CO is only 13.8 eV, and, among the various 77
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Figure 4. Best-fit CM angular distributions for the products CH2 C−CHO (black line), C2H4 (blue line), C2H3 (red continuous line), HCO (red dashed line), C2H2 (green continuous line), H2CO (green dashed line), CH2 (cyan continuous line), and CH2CO (cyan dashed line).
Figure 3. Time-of-flight distributions at m/z = 53, 29, 27, 26, and 14 recorded at ΘCM(= 37°). Symbols as in Figure 2.
molecular products formed in the reactions, only ketene can produce CH2+ at 17 eV.) The TOF peak of the contribution associated with CH2 is much faster than that associated with CH2CO because the lighter coproduct is produced with a much higher speed. (See the Newton diagram of Figure 1.) The best-fit CM functions associated with the various channels are shown in Figures 4 and 5. The T(θ) associated with C3H3O is substantially isotropic with a small preferences for forward scattering. The other T(θ) functions are instead quite polarized, especially those associated with the C2H4+CO and C2H2+H2CO channels. As expected, the slight asymmetry with respect to θ = 90° visible in some T(θ) functions is specular for each pair of cofragments. In all cases, the intensity distributed in the entire angular range is consistent with the formation of one or more long-lived complexes (i.e., living several rotational periods), as predicted by Nguyen et al.11 The product translational energy distributions, P(E′T), exhibit a quite different shape for each channel, so implying that they experience different parts of the PES. In the Hdisplacement channel where the structureless H atom is formed, the average fraction of available energy released as product translational energy, , is 0.42 if we consider the energetics of channel 1a. We could not distinguish among the two isomers CH2C−CHO and CH2CH−CO. Channel 1b is more exothermic, and, therefore, it might seem to be favored.
Nevertheless, the features of the PES are strongly in favor of channel 1a,11 which is produced by the direct fission of one of the addition intermediates, and, in fact, the predicted RRKM yield of channel 1b is almost negligible.11 The average fraction has a value quite typical for reactions of this kind and in line with the previous determination.18 On the contrary, the C−C bond breaking channels are all characterized by a much smaller . (See Figure 5.) This clearly indicates that when two molecular fragments are produced, the fraction of energy channelled into their internal degrees of freedom is much larger. For each channel, the P(E′T) is unique for the two cofragments. The only exception regards the channel CH2+CH2CO, where two slightly different P(E′T)s have been used to fit the data. This is probably due to the fact that the molecules of CH2CO that preferentially undergo dissociative ionization to CH2+ are those with a higher content of internal energy (and therefore a smaller content of translational energy). The present experimental results are in line with the description of the reaction mechanism obtained by the ab initio and RRKM calculations of Nguyen et al.11 According to them, the O addition to the π system of allene can occur in two ways, that is, O can add to one of the terminal carbon atoms or 78
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81.5% channel 1f; 7.0% channel 1e; and 9.6% channel 1g. The estimated yield uncertainty is about ±20% for each channel. Whereas channels 1a and 1d take place on the triplet PES, channels 1e, 1f, and 1g can be rationalized only by invoking ISC from the triplet to the singlet PES. Therefore, our study indicates that the O(3P)+CH2CCH2 reaction is largely dominated (>90%) by ISC. Quite interestingly, the main reaction channel is a C−C bond breaking channel, and that means that the three-carbon chain of allene is not maintained when attacked by atomic oxygen. We are planning to extend this kind of investigation to other combustion relevant reactions, such as those with the isomer of allene, methylacetylene, and larger dienes. The aim is not only to characterize combustion elementary reactions but also to provide benchmark results for dynamical calculations (at the QCT level) taking into account nonadiabatic effects. These calculations are becoming available, also at the level of differential cross section predictions (Bowman, J. M., private communication).
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EXPERIMENTAL METHOD The basics of our CMB apparatus have been described elsewhere,19 whereas some recent improvements have been detailed in review papers.4,20 In brief, two supersonic beams of the reactants are crossed at a fixed angle of 90° under single collision conditions in a large scattering chamber kept in a pressure range of 10−6 hPa under operating conditions. The angular and velocity distributions of the reaction products are recorded by a triply differentially pumped, ultrahigh-vacuum (10−11 hPa) detector equipped with a tunable electron impact ionizer, followed by a quadrupole mass filter. The whole detector unit can be rotated in the plane of the two beams around their intersection axis (Θ = 0° represents the direction of the atomic oxygen beam). The velocity of reactants and products is derived using TOF analysis. An intense and continuous beam of atomic oxygen was obtained by means of the radio frequency (RF) discharge beam source,21 widely used in our laboratory to produce supersonic beams of transient species starting from a suitable precursor.22 The atomic oxygen beam employed in the present study was produced by discharging 200 hPa of a 5% O2/He gas mixture through a 0.28 mm diameter quartz nozzle at 300 W of RF power; peak velocity and speed ratio were 2514 m/s and 5.8, respectively. The very small percentage of O(1D) also present in the beam is expected to contribute negligibly to the results because the reactivity of O(3P) (dominant in the beam) is comparable to that of O(1D) at this high collision energy. The supersonic beam of allene was generated by expanding through a 100 μm diameter stainless-steel nozzle kept at room temperature 400 hPa of neat allene. Beam peak velocity and speed ratio were 750 m/s and 4.7, respectively.
Figure 5. Best-fit CM product translational energy distributions. Symbols as in Figure 4. The total energy available to the products is indicated by an arrow. The average fraction of energy released as product translational energy, , is also reported.
to the central one. Nguyen et al.11 have also predicted the possibility of H-atom abstraction at high collision energies, but this channel is negligible under the present experimental conditions. The addition to the central C atom (characterized by a barrier height of 3.8 kJ/mol) leads to the formation of CH2COCH2 diradical (oxyallyl) that, accordingly to RRKM calculations, preferentially dissociates into CH2+CH2CO if the system remains in the triplet PES. Triplet oxyallyl can undergo ISC to the singlet PES, and singlet oxyallyl easily isomerizes to cyclopropanone. Because of the high internal energy content with which cyclopropanone is formed, under collision-free conditions it undergoes fragmentation, preferentially to the products CO+C2H4.11 Once passed to the singlet PES, other products such as H2CO+C2H2 can be formed. The addition to one of the terminal carbon atoms, instead, leads to the formation of a much less stable intermediate, O−CH2−C−CH2 (barrier height 5.6 kJ/mol), which can easily fragment into CH2 −C−CHO+H or, after some rearrangements, into C2H3+HCO. The two initial addition intermediates can isomerize to each other and the complete scheme of the PES is quite complex.11 From our experimental data, once the origin of the various ion signals is sorted out, the relative yield of each product can be derived from the estimated ionization cross section and the measured total ion yield for a specific product. We have obtained the following BRs: 1.6% channel 1a; 0.3% channel 1d;
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AUTHOR INFORMATION
Corresponding Author
*Tel: +39 075 5855507; E-mail:
[email protected] (N.B.).Tel: +39 075 5855514; E-mail:
[email protected] (P.C.) Present Address †
A. O.: Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom. 79
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(19) Alagia, M.; Balucani, N.; Casavecchia, P.; Stranges, D.; Volpi, G. G. Reactive Scattering of Atoms and Radicals. J. Chem. Soc., Faraday Trans. 1995, 91, 575−596. (20) Balucani, N.; Capozza, G.; Leonori, F.; Segoloni, E.; Casavecchia, P. Crossed Molecular Beam Reactive Scattering: From Simple Triatomic to Complex Polyatomic Reactions. Int. Rev. Phys. Chem. 2006, 25, 109−163. (21) Alagia, M.; Aquilanti, V.; Ascenzi, D.; Balucani, N.; Cappelletti, D.; Cartechini, L.; Casavecchia, P.; Pirani, F.; Sanchini, G.; Volpi, G. G. Magnetic Analysis of Supersonic Beams of Atomic Oxygen, Nitrogen and Chlorine Generated from a Radio-Frequency Discharge. Israel J. Chem. 1997, 37, 329−342. (22) Leonori, F.; Hickson, K. H.; Le Picard, S.; Wang, X.; Petrucci, R.; Foggi, P.; Balucani, N.; Casavecchia, P. Crossed-Beam UniversalDetection Reactive Scattering of Radical Beams Characterized by Laser-Induced-Fluorescence: The Case of C2 and CN. Mol. Phys. 2010, 108, 1097−1113.
ACKNOWLEDGMENTS This work was supported by MIUR (PRIN 2007) and EC COST Action CM0901 - Detailed Chemical Models for Cleaner Combustion.
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REFERENCES
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