Combined Crossed Beam and Theoretical Studies of the N(2

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Combined Crossed Beam and Theoretical Studies of the N(2D) + C2H4 Reaction and Implications for Atmospheric Models of Titan Nadia Balucani,* Dimitrios Skouteris, Francesca Leonori, Raffaele Petrucci, Mathias Hamberg,† Wolf D. Geppert,† and Piergiorgio Casavecchia Dipartimento di Chimica, Università degli Studi di Perugia, 06123 Perugia, Italy

Marzio Rosi Dipartimento di Ingegneria Civile e Ambientale and ISTM-CNR, Università degli Studi di Perugia, 06123 Perugia, Italy ABSTRACT: The dynamics of the H displacement channels in the reaction N(2D) + C2H4 have been investigated by the crossed molecular beam technique with mass spectrometric detection and time-of-flight analysis at two different collision energies (17.2 and 28.2 kJ/mol). The interpretation of the scattering results is assisted by new electronic structure calculations of stationary points and product energetics for the C2H4N ground state doublet potential energy surface. RRKM statistical calculations have been performed to derive the product branching ratio under the conditions of the present experiments and of the atmosphere of Titan. Similarities and differences with respect to a recent study performed in crossed beam experiments coupled to ionization via tunable VUV synchrotron radiation are discussed (Lee, S.-H.; et al. Phys. Chem. Chem. Phys. 2011, 13, 8515−8525). Implications for the atmospheric chemistry of Titan are presented.

1. INTRODUCTION The reaction between atomic nitrogen in its first electronically excited state (2D3/2,5/2, a metastable state with an energy content of 230 kJ/mol with respect to the ground 4S3/2 state) with ethylene has raised some interest because it has been suggested to be the formation route of acetonitrile in the atmosphere of Titan.1 In the upper atmosphere of Titan, indeed, N(2D) is formed by several processes (such as EUV photodissociation of N2, electron impact induced dissociation of N2, and dissociative recombination of the ion N2+) with a comparable yield to N(4S).2 In the upper atmosphere the collision time is less than 1 s,3 and the very long radiative lifetime3 of N(2D), accompanied by the low efficiency of N2 as a physical quencher,4 implies that the main fate of N(2D) is the reaction with methane and minor constituents, more specifically the small hydrocarbons identified in the atmosphere of Titan originating from methane dissociation, i.e., ethane, ethylene, and acetylene.1,5 The importance of these reactive systems has triggered laboratory studies, at the level of both chemical reaction kinetics4 and dynamics.6−12 As for the title reaction, a kinetics investigation by Sato et al.13 has demonstrated that the reaction is relatively fast (k300 K = 4.3 × 10−11 cm3 s−1, activation energy ∼4 kJ/mol), while an early crossed molecular beam (CMB) study with mass spectrometric (MS) detection at a collision energy (Ec) of 33.3 kJ/mol demonstrated that reaction products with general formula C2H3N are formed,7 thus implying that N(2D) + C2H4 is an active route toward the formation of compounds with a novel C−N bond. These N-containing molecules can be among the basic building © 2012 American Chemical Society

blocks that lead to the formation of the N-rich organic macromolecules, which have been suggested to be the constituents of the haze aerosols of Titan.14−17 The importance of these reactions, as well as the reactions of nitrogen ions, has been recently assessed in a comprehensive review18 and book chapter.3 The title reaction is characterized by many possible product channels. Those which are energetically allowed and correlate with the reactant asymptote (directly or after isomerization of the initial addition intermediate) are shown in Chart 1 in order of decreasing exothermicity. The reported enthalpies of reactions are those obtained at the CCSD(T) level of calculations in the present work (see also ref 19). The large variety of products that can be generated is the result of the versatility of carbon and nitrogen in forming bonds, of the relatively large number of atoms involved, and of the high energy content of the reactants. Unfortunately, the first CMB study could not distinguish the isomeric forms produced in the H displacement channels (eqs 1c, 1f, 1g, 1h, 1i, and 1k in Chart 1) because their exothermicities were all compatible with the experimental product translational energy distribution, with the exception of channel 1k. Indeed, the sensitivity of CMB experiments is normally low in the high-energy cutoff of the product translational energy distribution,6,20 from which one can infer the total available Received: July 21, 2012 Revised: September 26, 2012 Published: September 27, 2012 10467

dx.doi.org/10.1021/jp3072316 | J. Phys. Chem. A 2012, 116, 10467−10479

The Journal of Physical Chemistry A

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spite of the increased complexity of the new PES with respect to the one derived by Takayanagi et al.,21 confirmed the dominant role of the 2H-azirine channel,11 as predicted in the old calculations. The RRKM estimates have guided the interpretation of the experimental distributions. Since RRKM estimates predict a negligible role of the H2 elimination channels 1a and 1e (Chart 1), the experimental distributions at m/z = 40 have been interpreted as originating from a large fraction of 2Hazirine decomposing into CH2CN + H before reaching the detector. To do so, 2H-azirine is first supposed to rearrange to acetonitrile (CH3CN) or ketenimine before losing an H atom (the loss of one H atom directly from 2H-azirine is an endothermic process, not compatible with the total energy of the experiment). The efficiency of this sequence of processes could have been sustained by RRKM calculations, but these have not been reported. The m/z = 41 distributions, instead, were associated with that part of 2H-azirine which does not undergo a secondary H loss. The best-fit functions for the two channels differ in the product translational distributions, while both channels show some preference for backward scattering. This is also at variance with the previous experiment by Balucani et al.,7 who derived at the higher Ec of their experiment a center-ofmass (CM) product angular distribution with some preference for forward scattering. In both cases of the old and new CMB experiments, however, the CM angular distributions were essentially isotropic. Notwithstanding the high quality of the new CMB data, it is the opinion of the present authors that the conclusions of the work of Lee et al. for the data at m/z = 40 are affected by the way the analysis of the experimental data has been performed. They considered two possibilities for the observation of reactive signal at m/z = 40: a three-body dissociation of the initial addition intermediate (or one of its isomer) leading to CH2CN + H + H or the fragmentation of the 2H-azirine product during its flight toward the detector. In both cases, the analysis of the laboratory data cannot be conducted as they did in this and other similar experiments,23 that is, by assuming that a product of mass 40 is scattered by an m/z = 2 coproduct. In the case of three-body dissociation channels, in fact, it is necessary to measure at least two cofragments to characterize the reactive event. In the second case, instead, as illustrated in many photodissociation experiments (see, for instance, the work by Wodtke and co-workers25 or Butler and co-workers26), the best (approximate) way to treat the process is to assume that the heavy coproduct remains unaltered by the loss of one light H atom during its flight toward the detector. The use of a mass combination where a product with m/z = 40 is scattered by a coproduct of m/z = 2 has no physical basis, unless an H2 elimination channel is considered. Lee et al. disregarded this possibility because a large exit barrier is predicted for the two H2 elimination channels, and this is not in line with their determination of the product translational energy distributions.11 In addition to that, the new CMB experiment of Lee et al.11 has left many questions unanswered. In particular, it is not clear which is the role played by an isomer of 2H-azirine, that is, the radical CH2NCH (channel 1i in Chart 1), which has an IE of 7.84 eV. Since the ionization of 2H-azirine has a threshold of 10.1 eV and the entire set of product TOF distributions at m/z = 41 have been measured at 9.6 eV, even if we admit some red-shift because of the internal energy content of the newly formed C2H3N products, it is difficult to conceive that reactive scattering distributions could be obtained with such an excellent signal-to-noise ratio at an IE which is lower by 0.5 eV

Chart 1

energy and the exothermicity of the process. The most plausible C2H3N isomers produced in the reaction were suggested7 to be ketenimine (CH2CNH, channel 1g in Chart 1) and 2H-azirine (cyclic-CH(N)CH2, channel 1h in Chart 1) because they were both compatible with the best-fit product translational distribution and because available RRKM estimates on a computed potential energy surface (PES) indicated these two isomers as the main primary products.21 Finally, in the assumption that ketenimine and 2H-azirine are formed, it was suggested that a significant fraction of these molecules were formed with enough internal energy to spontaneously tautomerize to the most stable isomer acetonitrile.7 Based on these arguments, Wilson and Atreya1 included in their photochemical model of the atmosphere of Titan the reaction N(2D) + C2H4 as exclusively producing acetonitrile, which is, however, an oversimplified assumption. The reaction N(2D) + C2H4 has been very recently reinvestigated in a second CMB experiment at Ec= 18.0 kJ/mol by Lee et al.,11 who have used synchrotron VUV photoionization instead of electron impact in their mass spectrometer. This approach has been demonstrated to be very efficient in distinguishing isomeric forms produced in the photodissociation of molecules,22 in CMB experiments of bimolecular reactions,23 and also in kinetics studies where isomeric reaction products were identified.24 Nevertheless, in the case of this specific reactive system, the same degree of accuracy has not been achieved because several C2H3N isomers have similar ionization energies (IE) in the range between 8 and 10 eV, with the exception of acetonitrile and isoacetonitrile which have a significantly larger IE. In this new CMB experiment it was verified that the product time-of-flight (TOF) distributions at the mass-to-charge ratios (m/z) of 41 and 40 are different at a low energy of ionizing photons,11 while in the older CMB experiment the product angular distributions were estimated to be substantially similar for the two mass-to-charge ratios (with an electron energy of 100 eV).7 However, the signal at m/z = 41 was very small in the experiment by Balucani et al.,7 and this caused some uncertainty. The new data of Lee et al. were accompanied by new calculations of the underlying PES and RRKM estimates, which, in 10468



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6.5 mm from the nozzle. Starting from mixtures of N2 (2.5%) in He or Ne (30%)/He, a high degree of molecular dissociation (∼60%) was achieved. Atomic nitrogen was produced in a distribution of electronic states which has been characterized by Stern−Gerlach magnetic analysis:29 72% of the N atoms were found to be in the ground 4S state and 21% and 7% in the metastable excited 2D and 2P states, respectively. The presence of N(4S) does not represent a complication in the present experiments because the rate constant for the N(4S) reaction with ethylene is unmeasurably small.18 The N(2P) decay rate constant, instead, is smaller than that for N(2D) and is believed to be associated with physical quenching.4 In the experiment at the higher collision energy, the atomic nitrogen beam was generated by expanding the N2 (2.5%)/He mixture at 90 hPa and 250 W of RF power; the resulting beam has a peak velocity of 2316 m/s and a speed ratio of 5.3. In the experiment at the lower collision energy, the atomic nitrogen beam was generated by expanding the N2 (2.5%)/Ne (30%)/He mixture at 75 hPa and 260 W of RF power. In this case, peak velocity and speed ratio were 1747 m/s and 4.0, respectively. The beam of C2H4 was produced by supersonic expansion through a 100 μm stainless-steel nozzle of pure C2H4 with a stagnation pressure of 600 hPa and skimmed by a 1 mm diameter stainless-steel skimmer. The same beam was crossed with the two different atomic N beams. Peak velocity and speed ratio were 825 m/s and 5.2, respectively. In the case of the experiment using the N2 (2.5%)/He mixture, the resulting collision energy was 28.2 kJ/mol, while in the experiment using the N2 (2.5%)/Ne (30%)/He mixture the collision energy was 17.2 kJ/mol. The nominal angular resolution of the detector for a point collision zone is 1°. The secondary target beam (C2H4 beam) was modulated at 160 Hz with a tuning fork chopper for background subtraction. In the higher Ec experiment, laboratory angular distributions, N(Θ), at m/z = 40, 39, and 38 were obtained by taking at least 4 scans of 50 s counting time at each angle. Because of the much lower signal, instead, the angular distribution at m/z = 41 was obtained by taking at least 4 scans of 100 s counting time at each angle. In the lower Ec experiment, the signal was much lower because of the antiseeding effect of the 30% of Ne in the carrier gas. The laboratory angular distribution could be obtained only at m/z = 40 (4 scans with a counting time of 100 s). Velocity distributions of the products were obtained at selected different angles using the cross-correlation TOF technique with four 127-bit pseudorandom sequences. Hightime resolution was achieved by spinning the TOF disk, located at the entrance of the detector, at 328 Hz corresponding to a dwell time of 6 μs/channel. Counting times varied from 60 to 360 min depending upon signal intensity.

with respect to the 2H-azirine ionization threshold. In addition, in the assumption that the recorded signal at 9.6 eV is really due to internally hot 2H-azirine, the fraction of molecules produced with a low internal energy content (that is, those produced with a large amount of translational energy) would be inevitably lost in the 9.6 eV measurements. Incidentally, these molecules would be those which do not undergo a sequential H loss process exactly because they are internally cold. Because of these and other unresolved issues, we have repeated the experiment with our recently improved CMB apparatus.20,27 Compared to our previous experimental setup,28 the supersonic expansions of both beams occur much closer to the collision center, thus implying a gain of a factor 25 in the reactive signal. For this reason, we have decided to attempt to record the full distributions at m/z = 41 that we had failed to record 12 years ago. In addition to that, since in the work of Lee et al.11 not much information is given on the structures and energetics of the intermediates and transition states or on the RRKM results, we have also derived a new PES based on a larger basis set and performed RRKM estimates of the product branching ratios at the collision energies of our experiments (17.2 and 28.2 kJ/mol) and at the temperature of the stratosphere and surface of Titan (∼175 and 95 K, respectively, corresponding to a most probable energy of 1.5 and 0.8 kJ/mol).3 In addition, electronic structure and RRKM calculations have been performed explicitly to predict the destiny of the primary products af ter they are formed, in order to verify whether dissociation and/or isomerization of the primary products are feasible and fast, as previously suggested by Balucani et al.7 and Lee et al.11 The paper is organized as follows. In section 2 we describe the crossed beam experiments. In section 3 computational details on electronic structure as well as RRKM calculations are provided. The experimental results are presented in section 4. Theoretical results are presented in sections 5 and 6. A discussion and conclusion assessing the new physical insights we have gained from this study are presented in section 7, and the implications for the atmospheric chemistry of Titan are given in section 8.

2. EXPERIMENTAL SECTION The scattering experiments were carried out using the improved CMB apparatus described in detail in refs 20 and 27. Briefly, two collimated (in angle and velocity) continuous supersonic beams of the reactants are crossed at 90° in the scattering chamber. During the experiments, the pressure is maintained in the 10−6 hPa range to ensure single-collision conditions. The detector is a tunable electron impact ionizer followed by a quadrupole mass filter and an off-axis electron multiplier. The ionizer is located in the innermost region of a triply differentially pumped ultrahigh-vacuum chamber which is maintained in the 10−11 hPa pressure range in operating conditions by extensive turbo pumping and liquid nitrogen cooling. The detector unit can be rotated in the collision plane around the axis passing through the collision center. Reactant and product velocities are derived from TOF measurements. Continuous supersonic beams of nitrogen atoms containing, in addition to the electronic ground state 4S, a sizable amount of the excited, metastable state 2D have been generated by the high-pressure radiofrequency discharge beam source successfully used in our laboratory to generate intense supersonic beams of transient species.29,30 In this series of experiments, we have used a 0.44 mm diameter quartz nozzle and a boron nitride skimmer (diameter 0.8 mm) located at a distance of

3. COMPUTATIONAL DETAILS AND RRKM CALCULATIONS The potential energy surface of the system N(2D) + C2H4 was investigated using a computational procedure previously described:31 the lowest stationary points were located at the B3LYP level of theory32,33 in conjunction with the correlation consistent valence polarized set aug-cc-pVTZ.34−36 At the same level of theory we have computed the harmonic vibrational frequencies in order to check the nature of the stationary points, i.e., minimum if all the frequencies are real, saddle point if there is one, and only one, imaginary frequency. The assignment of the saddle points was performed using intrinsic 10469



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reaction coordinate (IRC) calculations.37 The energy of all the stationary points was computed at the higher level of calculation CCSD(T)38−40 using the same basis set aug-cc-pVTZ. Both the B3LYP and the CCSD(T) energies were corrected to 0 K by adding the zero-point energy correction computed using the scaled harmonic vibrational frequencies evaluated at the B3LYP/aug-cc-pVTZ level. The energy of N(2D) was estimated by adding the experimental41 separation N(4S) − N(2D) of 230.0 kJ/mol to the energy of N(4S) at all levels of calculation. Thermochemical calculations were performed at the W1 level of theory42,43 for all the minima. Since the accuracy of our best computed values should not be smaller than the “chemical accuracy” of 1 kcal/mol, we rounded all the reported energies to 1 kJ/mol. All calculations were done using Gaussian 03,44 while the analysis of the vibrational frequencies was performed using Molekel.45 We have performed RRKM calculations for the N(2D) + C2H4 system using the code developed by us for this purpose.9,46 As usual, the microcanonical rate constant for each elementary step is calculated using the formula

Figure 1. Laboratory angular distribution recorded at m/z = 40 for the reaction N(2D) + C2H4 at Ec= 17.2 kJ/mol. Error bars, when visible outside the dots, represent ±1 standard deviation from the mean. The red continuous line represents the simulated angular distribution when using the best-fit CM functions reported in Figure 6.

k(E) = N (E)/hρ(E)

where N(E) denotes the sum of states in the transition state at energy E, ρ(E) is the reactant density of states at energy E, and h is Planck’s constant. N(E) is obtained by integrating the relevant density of states up to energy E, and the rigid rotor/ harmonic oscillator model is assumed. Where possible, tunneling (as well as quantum reflection) has been taken into account by using the corresponding imaginary frequency of the transition state and calculating the tunneling probability for the corresponding Eckart barrier. For the cases where no obvious transition state barrier exists (essentially for dissociation steps), the corresponding microcanonical rate constants have been obtained in a variational way, evaluating k(E) at various points along the reaction coordinate and choosing the point which minimizes the rate constant. For dissociation steps, the products at infinite separation have also been taken into account as a possible “transition state”. After all microcanonical rate constants have been calculated, a Markov (stochastic) matrix is set up for all intermediate and final channels in the reaction containing all branching ratios derived from the rate constants. This Markov matrix is subsequently raised to a high enough power to achieve convergence. In this way, the branching ratios for all product channels are calculated.

Figure 2. Time-of-flight distributions of the products (open circles) detected at m/z = 40 for the reaction N(2D) + C2H4 at Ec= 17.2 kJ/ mol at the indicated LAB angles. The red continuous lines represent the TOF distributions calculated from the best-fit CM functions reported in Figure 6.

where the number density of the N atoms is lower by a factor of 3 because of the presence of the antiseeding Ne gas). In the high-Ec experiment, while the distributions at m/z = 40, 39, and 38 were totally superimposable, the angular distribution obtained at m/z = 41 is clearly different (see Figure 3), even considering the error bars (representing ±1 standard deviation). The m/z = 41 angular distribution is wider, with a rounded peak and some preference for forward scattering. The m/z = 40 angular distribution is more bell-shaped, and the peak of the distribution is centered over the center-of-mass position angle (ΘCM). The TOF spectra are also different, as is well visible by comparing those recorded for both masses at Θ = 22°, 34°, and 46° (see Figure 4 and 5). Those at m/z = 41 are slightly faster. The m/z = 40 angular and TOF distributions at the low Ec are quite in line with the m/z = 40 distributions at high Ec. The scattering measurements have been carried out in the laboratory (LAB) system of coordinates, while for the physical interpretation of the scattering process it is necessary to transform the data (angular, N(Θ), and time-of-flight, N(Θ, t) distributions) to a coordinate system which moves with the centerof-mass of the colliding system. Because of the finite resolution of the experimental conditions, i.e., finite angular and velocity spread of the reactant beams and angular resolution of the detector, the LAB-CM transformation is not single-valued, and, therefore, analysis of the laboratory data is carried out by the usual forward convolution procedure. Trial CM angular, T(θ), and translational energy, P(E′T), distributions are assumed, averaged, and transformed to the LAB frame for comparison

4. RESULTS AND ANALYSIS OF REACTIVE SCATTERING EXPERIMENTS In the high-Ec experiment, reaction products could be detected at m/z = 41, 40, 39, and 38 corresponding respectively to the ions C2H3N+ (associated with the parent ion of products with gross formula C2H3N), C2H2N+ (which can be associated with the parent ion of products with gross formula C2H2N or to the daughter ion of C2H3N), and the daughter ions C2HN+ and C2N+. We have tried to reduce the energy of ionizing electrons to suppress or reduce the dissociative ionization of the parent molecules, but we did not note any significant difference down to 17 eV. Therefore, all the measurements have been performed using an electron energy of 60 eV. In the low-Ec experiment, laboratory angular and time-offlight distributions could only be measured at m/z = 40 (see Figures 1 and 2) because of the much lower reactive signal (this is due to the use of the N2 (2.5%)/Ne (30%)/He mixture 10470



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Figure 5. Time-of-flight distributions of the products (open circles) detected at m/z = 41 for the reaction N(2D) + C2H4 at Ec= 28.2 kJ/mol at the indicated LAB angles. Solid lines represent the TOF distributions calculated from the best-fit CM functions reported in Figures 7 and 8 (red line and blue line as in Figure 3).

Figure 3. Laboratory angular distributions recorded at m/z = 40 (top panel) and 41 (bottom panel) for the reaction N(2D) + C2H4 at Ec= 28.2 kJ/mol. In the top panel, the red line represents the simulated angular distribution when using the best-fit functions reported in Figure 7. In the bottom panel, the two separated contributions are also indicated. The red line represents the same contribution that accounts for the distributions at m/z = 40, 39, and 38. The blue line represents the contribution of the additional channel observed at this mass. The best-fit functions relative to the additional contribution are reported in Figure 8.

ionization to C2H2N+ + H in the electron impact ionizer. The bestfit CM distributions (with error bars) are reported in Figures 6 and 7. A slight dependence of the P(E′T) on the scattering angle has been observed (see bottom panels of Figures 6 and 7). The same

Figure 4. Time-of-flight distributions of the products (open circles) detected at m/z = 40 for the reaction N(2D) + C2H4 at Ec= 28.2 kJ/mol at the indicated LAB angles. Solid lines represent the TOF distributions calculated from the best-fit CM functions reported in Figure 7.

Figure 6. Best-fit CM distributions obtained for the LAB distributions recorded at m/z = 40 in the reaction N(2D) + C2H4 at Ec= 17.2 kJ/mol. Top panel: CM product angular distribution (the gray area delimits the range of product angular distributions which still furnish an acceptable fit of the LAB data). Central panel: the blue line represents the angle-dependent P(E′T, θ)s associated with the angular ranges 0°−20° and 160°−180°, the red line represents the P(E′T, θ)s associated with the angular ranges 20°− 40° and 140°−160°. Bottom panel, the P(E′T, θ) associated with the angular range 40°−140° is reported together with the error bounds (gray area) defining the range of P(E′T)s which still furnish an acceptable fit of the data. Similar error bounds can be derived also for the other P(E′T, θ)s.

with the experimental data until the best fit of the LAB distributions is achieved. The m/z = 40 LAB angular and TOF distributions were easily fit using a single contribution, but considering that this mass was that of the daughter ion of one of the C2H3N products associated with the H displacement channel. Since we cannot control the dissociative ionization of C2H3N products, we are not able to distinguish whether the large signal recorded at m/z = 40 is due to the fragmentation of C2H3N products during their flight toward the detector as suggested by Lee et al.11 or to dissociative

functions also nicely reproduce the distributions recorded at m/z = 39 and 38. 10471



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Figure 8. Best-fit CM product (a) angular and (b) translational energy distributions obtained for the additional channel necessary to fit the laboratory distributions at m/z = 41 in the reaction N(2D) + C2H4 at Ec= 28.2 kJ/mol (the gray area delimits the range of product angular distributions which still furnish an acceptable fit of the LAB data).

product(s) occurs extensively also at small ionization energies (as low as 17 eV). Once all the distributions recorded also at m/z = 38 and 39 are considered, the yield of the second channel visible only at m/z = 41 is 3−5% of the total reactive signal. This also justifies the fact that there is no trace of this second component in the other m/z distributions because it can only make a contribution too small compared to the dominant one. In conclusion, from the high-Ec experiments we can state that at least two product channels are open: a largely dominant one which mostly undergoes dissociative ionization (or secondary fragmentation before reaching the detector) and a very small one, which becomes visible only at the parent mass of 41 where the dominant product makes a very small contribution. A comment on the attribution of the m/z = 40 signal to the H displacement channels rather than to the H2 elimination channel is in order. According to the previous and present electronic structure calculations, both H2 elimination channels require to surmount a huge exit barrier. That would imply a neat dynamical feature, that is, a product translational energy distribution peaked at large values of E′T. This has not been observed either by us or in previous CMB experiments,7,11 and we can safely assume that only H displacement channels are producing signal in this range of masses. The only alternative explanation would be a roaming mechanism, where the H atom which is about to leave from the dissociating intermediate interacts with another H atom of the molecular coproduct, thus forming vibrationally excited H2. Similar mechanisms have been observed in photodissociation studies and invoked in bimolecular reactions.47−50 In the assumption that the roaming mechanism plays no role in this reactive system, the question that remains open is which of the six C2H3N isomers is actually formed, since the maximum energy released as product translational energy is not clear-cut. Lee et al., by comparing the appearance energy of their detected ions, could clearly establish that, under their conditions, CH3CN and CH3NC are not significantly formed. However, the appearance of their m/z = 41 ion signal is compatible with CH2NCH (1i),

Figure 7. Best-fit CM distributions obtained for the LAB distributions recorded at m/z = 40 in the reaction N(2D) + C2H4 at Ec= 28.2 kJ/mol. Top panel: CM product angular distribution (the gray area delimits the range of product angular distributions which still furnish an acceptable fit of the LAB data). Central panel: the blue line represents the angledependent P(E′T, θ)s associated with the angular ranges 0°−20° and 160°−180°, the red line represents the P(E′T, θ)s associated with the angular ranges 20°−40° and 140°−160°; the green line represents the P(E′T, θ)s associated with the angular ranges 40°−60° and 120°− 140°. Bottom panel: the P(E′T, θ) associated with the angular range 60°−120° is reported together with the error bounds (gray area) defining the range of P(E′T)s which still furnish an acceptable fit of the data. Similar error bounds can be derived also for the other P(E′T, θ)s.

On the contrary, the m/z = 41 distributions required a twocomponent fit. One of the components is characterized by the same CM functions derived at m/z = 40. An additional contribution is, however, necessary to fit the m/z = 41 data. In particular, the larger width of the angular distribution and the rounded, slightly forward peak, as well as the fast component in the TOF spectra, were a stringent test to derive the additional contribution. The best-fit LAB simulations are reported in Figures 3 and 5 while the CM best-fit functions (with error bars) associated with the second contribution are reported in Figure 8. It must be noted that the counting rate at m/z = 41 is much smaller (40 c/s at the peak of the angular distribution) than that at m/z = 40 (265 c/s at the peak of the angular distribution). In the m/z = 41 data analysis, it has been possible to distinguish the importance of the additional contribution (roughly 60%), so that it follows that it accounts for only ∼25 c/s, to be compared with the much larger value associated with the dominant C2H3N product(s) which undergoes substantial dissociative ionization. As already mentioned, an attempt to reduce the amount of dissociative ionization of the dominant product by lowering the energy of the ionizing electrons was not successful, indicating that dissociative ionization of the dominant C2H3N 10472



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CH2CNH (1g), c-CH2(N)CH (1h), and c-CH(NH)CH (1k) (see Table 2 of ref 11). Among these candidates, the radical CH2NCH has the right ionization threshold to explain the trend of the m/z = 41 ion signal as a function of the ionizing photon energy. The best-fit functions we have derived for the m/z = 40 distributions are substantially similar to those derived by Lee et al., especially those at the very similar Ec of 17.2 kJ/mol There is, however, an important difference: since we have used a different kinematics (associated with a product of mass 41 scattered by H atom, rather than 40 scattered by H2) the E′T axis of our product translational distributions is multiplied by a factor of 2 with respect to the one derived by Lee et al. Our best-fit CM angular distribution is isotropic at both Ec's for this contribution, not very different from the slightly backward functions of Lee et al. However, the use of a slightly backward CM angular distribution could nicely reproduce our TOF spectra, but not the bell-shaped angular distribution, which is clearly symmetric around ΘCM. As illustrated by the error bounds in Figures 6 and 7, some preference for sideways scattering is, instead, possible at both Ec’s. For the low-Ec experiment, the average amount of energy released as product translational energy, ⟨E′T⟩, varies from 86 kJ/mol for the angular ranges Δθ = 0°−20° and 160°−180° to 83 kJ/mol for the angular ranges 20°−40° and 140°−160° and 77 kJ/mol for the other angles. For the highenergy experiment, these values become 87 kJ/mol for 0°−20° and 160°−180°, 83 kJ/mol for 20°−40° and 140°−160°, 77 kJ/mol for 40°−60° and 120°−140°, and 73 kJ/mol for θ = 60° to θ = 120°. The m/z = 41 functions, instead, are clearly different with respect to those derived by Lee et al., and we believe they originate from a different reaction mechanism which opens up only at higher Ec (see Discussion and Main Conclusions). The CM angular distribution is quite polarized with a clear preference for forward scattering, with a T(180°)/T(0°) = 0.6. The average fraction ⟨E′T⟩ is much larger than in the case of the other channel, being 200 kJ/mol. Such a large value might well reflect the presence of a significant exit barrier along the minimum pathway leading to this channel.

5. COMPUTATIONAL RESULTS The lowest minima localized on the new PES have been reported in Figure 9, where the main geometrical parameters (in Å and deg) are shown together with the energies computed at the B3LYP/aug-cc-pVTZ, CCSD(T)/aug-cc-pVTZ, and W1 levels, relative to that of CH3CHN which is the most stable isomer at all levels of calculation. Notably, the most stable CH3CHN isomer (the absolute minimum of the PES) was not identified in the previously calculated PESs.11,21 The enthalpy changes and barrier heights computed at 0 K with inclusion of the zero-point energy correction for the main isomerization and dissociation processes are reported in Table 1. As expected, CCSD(T) relative energies are in better agreement with the W1 values than the B3LYP ones; for this reason we will consider only CCSD(T) values in the following discussion. A schematic representation of the potential energy surface of the system N(2D) + C2H4 is shown in Figure 10. For the sake of clarity, in Figure 10a we have reported only the isomerization processes, while in Figure 10b we have shown the main dissociation processes. From Figure 10 we can see that the interaction of N(2D) with C2H4 gives rise to the species CH2(N)CH2 (1) in agreement with the previous investigations of Takayanagi et al.21 and Lee et al.11 1 is more stable than the

Figure 9. B3LYP optimized geometries (Å and deg) and relative energies (kJ/mol) at 0 K of minima, localized on the PES of N(2D) + C2H4; CCSD(T) and W1 relative energies are reported in parentheses.

reactants by 429 kJ/mol and is formed without any barrier, at least at the B3LYP level. The cyclic species 1 can isomerize to the other cyclic isomer CH2(NH)CH (3′) through the transfer of a hydrogen atom from carbon to nitrogen with a barrier of 189 kJ/mol or to the open structure CH2NCH2 (2) with a barrier of 117 kJ/mol, due to the opening of the ring through 10473



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Table 1. Enthalpy Changes and Barrier Heights (kJ/mol, 0 K) Computed at the B3LYP/aug-cc-pVTZ and CCSD(T)/aug-ccpVTZ Levels of Theory for Selected Dissociation and Isomerization Processes for the System N(2D) + C2H4a ΔH00 B3LYP CCSD(T) N(2D) + C2H4 → CH2(N)CH2 (1) CH2(N)CH2 (1) → CH2(NH)CH (3′) CH2(N)CH2 (1) → CH2NCH2 (2) CH2(N)CH2 (1) → CH3NCH (7′) CH2(N)CH2 (1) → CH2(N)CH + H CH2(N)CH2 (1) → 3 CH2(N)CH + H CH2(NH)CH (3) → CH2(NH)CH (3′) CH2(NH)CH (3) → CH2CHNH (4) CH2(NH)CH (3′) → CH2CHNH (4′) CH2(NH)CH (3) → CH2(N)CH + H CH2(NH)CH (3) → 3 CH2(N)CH + H CH2(NH)CH (3′) → CH2(N)CH + H CH2(NH)CH (3′) → 3 CH2(N)CH + H CH2(NH)CH (3) → CH (NH)CH + H CH2(NH)CH (3′) → CH(NH)CH + H CH2(NH)CH (3) → 3 CH(NH)CH + H CH2(NH)CH (3′) → 3 CH(NH)CH + H CH2NCH2 (2) → CH2NCH + H CH2NCH2 (2)→ CH2NC + H2 CH2NCH2 (2) → 3 CH2NCH + H CH2NCH2 (2) → 3CH2 + CH2N CH2CHNH (4) → CH2CHNH (4′) CH2CHNH (4) → CH3CNH (5) CH2CHNH (4′) → CH3CNH (5′) CH2CHNH (4) → CH2CN + H2 CH2CHNH (4) → CH2CNH + H CH2CHNH (4′) → CH2CNH + H CH2CHNH (4) → 3 CH2CNH (trans) + H CH2CHNH (4′) → 3 CH2CNH (cis) + H CH2CHNH (4) → C2H3 + 3NH

a

ΔH00

barrier height W1

B3LYP CCSD(T)

−463

−429

−447

52

53

50

195

189

−78

−66

−71

109

117

−22

−14

−17

205

207

198

185

184

207

199

471

477

483

4

3

4

39

46

−148

−140

−142

78

86

−149

−141

−143

80

87

149

136

137

155

149

422

428

436

146

132

134

419

424

433

285

272

273

285

272

281

269

269

281

269

419

427

431

415

424

428

286

289

293

286

289

118

117

123

320

343

399

406

412

383

377

390

3

2

2

67

66

16

13

15

208

218

30

31

32

221

233

43

44

45

397

403

189

190

192

205

213

186

188

190

202

211

393

402

408

408

419

425

431

425

440

152

B3LYP CCSD(T) CH2CHNH (4′) → C2H3 + 3NH CH2CHNH (4) → 3CH2 + CHNH CH2CHNH (4′) → 3 CH2 + CHNH CH3CNH (5) → CH3CNH (5′) CH3CNH (5) → CH3CHN (6) CH3CNH (5′) → CH3CN + H CH3CNH (5) → CH3CN + H CH3CNH (5) → 3 CH3CN + H CH3CNH (5′) → 3 CH3CN + H CH3CNH (5) → CH2CNH + H CH3CNH (5′) → CH2CNH + H CH3CNH (5) → 3 CH2CNH (trans) + H CH3CNH (5′) → 3 CH2CNH (cis) + H CH3CNH (5′) → CH3 + HNC CH3CNH (5) → CH3 + HNC CH3CHN (6) → CH3CN + H CH3CHN (6) → 3 CH3CN + H CH3CHN (6) → CH3 + HCN CH3CHN (6) → CH2(N)CH + H CH3NCH (7′) → CH3NCH (7) CH3NCH (7) → CH2NCH + H CH3NCH (7) → CH2NHCH (8) CH3NCH (7′) → CH2NHCH (8′) CH3NCH (7′) → CH3 + HCN CH3NCH (7) → CH3 + HCN CH3NCH (7′) → CH3NC + H CH3NCH (7) → CH3NC + H CH2NHCH (8) → CH2NHCH (8′) CH2NHCH (8) → CH2NCH + H CH2NHCH (8′) → CH2NCH + H

146

barrier height W1

B3LYP CCSD(T)

428

423

438

437

436

448

434

434

446

18

20

19

63

68

−21

−26

−24

157

150

64

43

45

83

80

82

63

64

101

99

513

507

519

496

488

500

173

177

177

174

177

156

158

158

156

158

377

389

392

378

389

393

93

84

89

119

114

111

103

108

136

134

103

88

88

125

117

535

533

543

75

69

71

111

109

303

288

288

355

354

−18

−20

−19

29

36

247

256

257

248

256

138

142

144

302

319

123

124

126

307

326

−8

−20

−17

62

62

10

−1

2

80

82

118

99

103

119

103

135

119

122

136

123

2

1

2

62

77

109

114

114

166

179

107

113

112

164

178

Enthalpy changes have been computed also at the W1 level for comparison purposes.

of a C−H bond in 1 with formation of 2H-azirine, CH2(N)CH, is an endothermic reaction with a ΔH00 of 185 kJ/mol and a barrier of 199 kJ/mol. The formation of the corresponding

the breaking of the C−C bond. The transfer of a hydrogen from one carbon atom to the other implies the isomerization of 1 to CH3NCH (7′) with a barrier of 207 kJ/mol. The breaking 10474



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Figure 10. Schematic representation of the N(2D) + C2H4 potential energy surface. For simplicity, only the CCSD(T) relative energies (kJ/mol) are reported. (a) Isomerization reactions; (b) main dissociation processes.

triplet product 3CH2(N)CH is a barrierless process, but it is much more endothermic, ΔH00 being 477 kJ/mol. The breaking of a C−H bond in 2 implies the formation of CH2NCH + H in a reaction endothermic by 289 kJ/mol; this reaction shows a barrier almost equal to the endothermicity of the reaction at the B3LYP level which disappears at the CCSD(T) level. For simplicity this barrier has not been reported in Figure 10b. The formation of the corresponding triplet product 3CH2NCH is barrierless, but much more endothermic, ΔH00 being 406 kJ/mol.

2 can also dissociate to CH2NC + H2 in a reaction endothermic by only 117 kJ/mol, but with a barrier as high as 343 kJ/mol. The dissociation of 2 into CH2N + 3CH2 is a barrierless reaction endothermic by 377 kJ/mol. We could not find any pathway toward the formation of CHNCH + H2 from 2, in contrast with Takayanagi et al.21 The species CH2(NH)CH (3′) can isomerize to the similar species 3 through a very low barrier of 42 kJ/mol or to the open species cis-CH2CHNH (4′) through a barrier of 87 kJ/mol. This last reaction is exothermic by 141 kJ/mol. The cyclic 10475



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species 3 can isomerize to the more stable open species transCH2CHNH (4) through a barrier of 86 kJ/mol. 3 can also lose a hydrogen giving rise to the 2H-azirine CH2(N)CH with a barrier of 149 kJ/mol or to the 1H-azirine CH(NH)CH; for this last reaction we found a very small barrier at the B3LYP level almost equal to the endothermicity of the dissociation, which disappears at the CCSD(T) level. Also in this case for simplicity we have not reported this barrier in Figure 10b. The dissociations of 3 toward the corresponding triplet products 3 CH2(N)CH or 3CH(NH)CH are very endothermic barrierless reactions (428 and 427 kJ/mol, respectively). Also 3′ can dissociate in barrierless reactions to the triplet products 3CH2(N)CH or 3CH(NH)CH + H; however, both these reactions are strongly endothermic having ΔH00 = 424 kJ/mol. We were not able to find transition states for the dissociation toward the corresponding singlet products. However, the variational calculations show the presence of a barrier for these reactions, but any attempts to localize these transition states lead to the corresponding barriers of species 3; this means that these transition states should be very similar. For this reason we consider for both isomers the same transition states for the corresponding reactions. cis-CH2CHNH (4′) can isomerize to trans-CH2NHCH (4) through a low barrier of 64 kJ/mol. cis-CH2CHNH (4′) can also isomerize to species CH3CNH (5′), but the barrier is 233 kJ/mol. trans-CH2CHNH (4) isomerizes to the corresponding species CH3CNH (5) with a barrier of 218 kJ/mol. transCH2CHNH (4) can dissociate into CH2CN + H2; however, the barrier for this reaction is as high as 403 kJ/mol. Also the dissociation of trans-CH2CHNH (4) into ketenimine CH2CNH + H shows a barrier which is however much lower (213 kJ/mol). We were not able to find a barrier for the corresponding dissociation reaction of the cis isomer. However, a barrier is present also for this reaction as it is suggested by the variational calculations. Since any attempts to localize this barrier on the potential energy surface leads to the same barrier found for species 4, we consider the same barrier also for the dissociation of 4′. The dissociation reactions of both the trans-CH2CHNH (4) and cis-CH2CHNH (4′) toward a triplet product are all barrierless. In Table 1 we have reported the dissociation reactions into 3 CH2CNH + H, C2H3 + 3NH, and 3CH2 + CHNH. The species CH3CNH (5′) can isomerize to CH3CNH (5) with a small barrier of 48 kJ/mol. CH3CNH (5) can also isomerize to species CH3CHN (6) which is the most stable isomer localized on the PES of N(2D) + C2H4; this isomerization shows a barrier of 150 kJ/mol. (5′) can dissociate into acetonitrile CH3CN + H with a barrier of 50 kJ/mol and into CH3 + HNC with a barrier of 114 kJ/mol. We consider the same saddle points for the corresponding reactions of 5, since we were not able to localize these saddle points, which, however, should exist as it is suggested by the variational calculations. 5 can dissociate into ketenimine CH2CNH + H with a very small barrier which disappears at the CCSD(T) level. The same saddle point has also been considered for the corresponding reaction of 5′. However, for simplicity, these saddle points are not shown in Figure 10b. The dissociations of both 5 and 5′ into 3CH3CN + H and 3CH2CNH + H are all barrierless. Species 6 can dissociate into 2H-azirine + H with a barrier of 354 kJ/mol, into acetonitrile CH3CN + H with a barrier of 117 kJ/mol, and into CH3 + HCN with a barrier of 109 kJ/mol. The dissociation of 6 into 3CH3CN + H is barrierless and very endothermic (533 kJ/mol). Species CH3NCH (7′) can isomerize to CH3NCH (7) with a very low barrier of 36 kJ/mol or to the less stable species CH2NHCH (8′) with a very high barrier of 326 kJ/mol. Also species 7 can isomerize to the corresponding

species CH2NHCH (8) with a still high barrier of 319 kJ/mol. 8′ can isomerize to 8 with a small barrier of 75 kJ/mol. Since both 8′ and 8 show very high barriers for their formation, we will not further consider these isomers. Species CH3NCH (7′) can dissociate into CH3 + HCN with a barrier of 62 kJ/mol and into CH3NC + H with a barrier of 103 kJ/mol. We consider also in this case the same saddle points for the corresponding reactions of 7. Species CH3NCH (7) can dissociate into CH2NCH + H with a very small barrier which disappears at the CCSD(T) level. For this reason, this saddle point has not been reported in Figure 10b. Also in this case we considered the same saddle point for the corresponding reaction of species 7′. For 8 and 8′ we have considered only the dissociation into CH2NCH + H, shown in Table 1, for the relevance of this product. The dissociation of CH2NHCH (8) into CH2NCH + H is an endothermic process with a relatively high barrier. For the corresponding dissociation of 8′ we were not able to find a barrier; however, since the presence of a barrier is suggested by the variational calculations, we considered the same transition state of the dissociation reactions of 8. For clarity, we have not reported these reactions in Figure 10b. In Table 2 we have reported the variation of enthalpy and barrier height for some isomerization processes starting from Table 2. Enthalpy Changes and Barrier Heights (kJ/mol, 0 K) Computed at the B3LYP/aug-cc-pVTZ and CCSD(T)/ aug-cc-pVTZ Levels of Theory for Selected Dissociation and Isomerization Processes of Acetonitrilea ΔH00 B3LYP CH3CN → CH2(N) CH CH3CN → CH2CNH CH3CN → CH2CN +H CH2CNH → CH2CN +H CH2CNH → CHCNH + H CH2(N)CH → CH(N)CH + H CH2(N)CH → CH2(N)C + H CH2NCH → CH3NC

CCSD(T)

barrier height W1

B3LYP

CCSD(T)

201

200

200

397

380

118 381

115 396

112 398

369

377

289

281

285

408

417

418

414

424

428

402

408

414

−112

−138

−135

310

284

a

Enthalpy changes have been computed also at the W1 level for comparison purposes.

acetonitrile. The isomerization of acetonitrile to 2H-azirine has a barrier of 380 kJ/mol, while that of acetonitrile to ketenimine has a comparable barrier of 377 kJ/mol. Both these reactions are endothermic, which means that the transformation of the possible primary products 2H-azirine and ketenimine into acetonitrile are strongly exothermic processes characterized by barriers of 180 and 262 kJ/mol, respectively. As for the radical CH2NCH (another possible primary product from channel 1i in Chart 1), a transition state could only be found for the isomerization to CH3NC; the process is exothermic by 138 kJ/mol and has a barrier of 284 kJ/mol. In addition, in Table 2 are also reported the dissociation energies of various C2H3N isomers associated with the least strongly bound H atom. The values reported in Table 2, together with variational calculations for several bond fissions, have been used to quantify via RRKM calculations the possible tautomerization of the C2H3N isomers and/or their fragmentation under the 10476



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conditions of our and previous CMB experiments, as well as in the upper atmosphere of Titan.

Table 3. Branching Ratios for the Formation of Each of the Possible Final Products, for Each of the Four Energies Considered in the Article

6. RRKM RESULTS As illustrated in the previous section, once the addition CH2(N)CH2 cyclic intermediate is formed, it can undergo either direct dissociation to 2H-azirine + H (channel 1h in Chart 1) or isomerization. According to the present RRKM calculations based on the results of our electronic structure calculations, the four most important processes are (listed in order of decreasing importance): (1) rearrangement to noncyclic CH2NCH2 with the fission of the C−C bond of the strained ring of the addition intermediate cyclic-CH2(N)CH2 (the calculated constant for this process varies from 1.6 × 1012 to 2.0 × 1012 s−1 in the range of the four collision energies considered); (2) loss of an H atom to form the product 2H-azirine, CH2(N)CH (from 3.5 × 1011 to 6.0 × 1011 s−1); (3) isomerization to cyclic-CH2(NH)CH (from 1.3 × 1011 to 1.9 × 1011 s−1) ; (4) isomerization to CH3NCH (from 3.7 × 1010 to 6.2 × 1010 s−1). Therefore, the most efficient process is isomerization to the open-CH2NCH2 intermediate. For this species, in turn, the most probable process is loss of an H atom to give CH2NCH + H (channel 1i in Chart 1) with a rate constant that varies from 2.5 × 1011 to 3.8 × 1011 s−1. The second most probable fate is back-rearrangement to the original CH2(N)CH2 intermediate. Nonetheless, according to the present RRKM calculations and at variance with those by Lee et al.,11 the most abundant product at all four energies considered is CH2NCH + H (with a branching ratio ranging from 0.668 at the lowest energy up to 0.596 at the highest one). The second most abundant product is 2H-azirine, with a branching ratio ranging from 0.230 at the lowest energy up to 0.286 at the highest one. The reason for the increase in the branching ratio with the available energy is the fact that the rate constant of its production from the original intermediate CH2(N)CH2 rises much more steeply than the one for rearrangement into CH2NCH2. Note that the branching ratios for the two most abundant products reflect closely the ratio of the corresponding rate constants from the original intermediate. The third most abundant product is CH2CNH (ketenimine) + H (channel 1g in Chart 1), with a branching ratio of 4.5−5% at all energies. This is related to the third most important fate of the original intermediate, i.e., rearrangement to cyclic-CH2(NH)CH. This latter intermediate can also lose an H atom to give 2H-azirine (with a rate constant of k = 8.6× 1011−1.4× 1012 s−1), but its most probable fate is rearrangement to CH2CHNH (k = 1.7−2.0× 1012 s−1). The only decay channel of any importance for CH2CHNH is CH2CNH + H (rate constant k = 7.4× 1011−1.1× 1012 s−1). Other minor channels include CH3 + HCN and CH3NC + H, both with branching ratios very close to 1% at all energies. Both channels are the results of the decay of the CH3NCH intermediate, which is the least probable product of arrangement of the original intermediate (k = 3.7−6.2 × 1010 s−1). The calculated branching ratios for all the channels under the conditions relevant to Titan and those of the present CMB experiments are reported in Table 3. Because of the suggestions of Balucani et al.7 on the possible isomerization of primary products to acetonitrile and of Lee et al.11 on the isomerization of 2H-azirine followed by a sequential H atom loss, we have performed RRKM calculations to assess the final fate of the molecular primary products, taking into account the endothermicity of the further fragmentation channels and the amount of internal energy of the products,

CH2CN + H2 CH3 + HCN CH3CN + H CH3 + HNC CH2NC + H2 CH3NC + H CH2CNH + H CH2(N)CH + H 3 CH2(N)CH + H CH2NCH + H 3 CH2NCH + H CH2N + 3CH2 CH(NH)CH + H 3 CH2CNH + H C2H3 + 3NH CHNH + 3CH2 3 CH3CN + H

0.8 kJ/mol

1.5 kJ/mol

17.2 kJ/mol

28.2 kJ/mol

0.00% 1.18% 0.06% 0.22% 0.84% 1.11% 5.06% 23.05% 0.00% 66.76% 0.69% 0.49% 0.16% 0.20% 0.01% 0.17% 0.00%

0.00% 1.18% 0.06% 0.22% 0.85% 1.11% 5.04% 23.19% 0.00% 66.58% 0.70% 0.50% 0.17% 0.20% 0.01% 0.18% 0.00%

0.00% 1.26% 0.06% 0.21% 1.24% 1.25% 4.73% 26.39% 0.00% 62.46% 0.94% 0.64% 0.32% 0.25% 0.02% 0.24% 0.00%

0.00% 1.31% 0.05% 0.20% 1.56% 1.34% 4.45% 28.59% 0.00% 59.57% 1.12% 0.75% 0.47% 0.29% 0.03% 0.28% .

which is obtained by the difference with respect to the average amount of product translational energy, as determined in the CMB experiments. The results are as follows. 2H-Azirine can isomerize to acetonitrile, and once equilibrium between the two species is achieved, the percentage of 2H-azirine is only 0.5− 1.0% at all four energies considered. However, the process is extremely slow at all energies, and the time needed to achieve equilibrium is practically infinite at 0.8 kJ/mol, 4.3× 1013 s at 1.5 kJ/mol, 7× 107 s at 17.2 kJ/mol, and 7.1× 103 s at 28.2 kJ/mol. Therefore, neither in the time scale (several hundreds of microseconds) of our CMB experiments nor of those by Lee et al.11 can this isomerization be significant. It will also be null under the condition of the stratosphere of Titan, where the collision time is less than 1 s. CH2CNH can also isomerize to acetonitrile. The efficiency of this process is not as high (at the equilibrium the percentage of CH2CNH at all energies remains 15− 16%), but the process is now much faster. At 0.8 kJ/mol the time needed is 41 ms, dropping to 36 ms at 1.5 kJ/mol, to 1.9 ms at 17.2 kJ/mol, and to 0.27 ms at 28.2 kJ/mol. At the highest collision energy considered, therefore, the time scale is comparable to that of our CMB experiments, while under the temperature of the atmosphere of Titan it competes with bimolecular collisions. Notably, CH2CNH can directly lose an H atom forming CH2CN + H. This process essentially does not occur at 0.8 and 1.5 kJ/mol, whereas its lifetime is 2 ns at 17.2 kJ/mol and 320 ps at 28.2 kJ/mol. In this case, therefore, the fragmentation of the main primary product is very fast and largely occurs in our CMB experiments and in those of Lee et al.11 Acetonitrile also has the possibility to lose another H atom forming CH2CN + H, with lifetimes comparable to CH2C NH: 4 ns at 17.2 kJ/mol and 660 ps at 28.2 kJ/mol. Finally, according to our data, the conversion of CH2NCH to isoacetonitrile is slow enough to be neglected. In addition, isoacetonitrile does not easily lose one hydrogen atom.

7. DISCUSSION AND MAIN CONCLUSIONS In this section, we would like to comment essentially on two points: the difference in the RRKM predictions with respect to 10477



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the work by Lee et al.11 and the possible explanation of the two contributions observed in our new experiment at the higher Ec. The origin of the large difference between the present and previous RRKM calculations is unknown. The PES of Takayanagi et al.21 was rather incomplete, as already commented on by Lee et al.11 For instance the pathways originating from the intermediate CH2NCH2 were not considered. Unfortunately, in the paper by Lee et al. very few details on the theoretical calculations are given. Essentially, the main results are summarized in the schematic of the PES, while a more detailed account is given for the pathway leading to ketenimine, which is minor both in present calculations and in the calculations by Lee et al. The energy values reported in Figure 9 of ref 11 are substantially in line with those obtained in the present work. The most relevant difference is related to the absence of the absolute minimum (CH3CHN) in the PES by Lee et al. The structures of bound intermediates and transition states, however, have not been reported, so it is very difficult to understand why the results of RRKM are so different in the two cases. The only comment we can make is that RRKM estimates are very sensitive not only to the energy value associated with the species involved but also to the “floppiness” of the motions involved, both in the transition state and in the reactants. Thus, for example, even though the TS for conversion of the original intermediate to azirine + H is slightly higher than the one for conversion to CH2(NH)CH, the corresponding rate is actually higher, due to the presence of lowfrequency vibrations in the transition state associated with the outgoing hydrogen atom. It is for the same reason that CH2NCH2 preferentially loses an H atom to give CH2NCH + H rather than isomerize back to CH2(N)CH2, even though the relevant transition state is substantially lower in energyas before, the lowfrequency motions of the outgoing H atom lower the entropy of activation enough to favor this channel. Clearly, some differences in the calculated properties of the transition states are responsible for the observed discrepancies between our RRKM estimates and those by Lee et al.11 It is interesting to note that, in the light of our new RRKM estimates, the experimental data of Lee et al. at m/z = 41 appear to be more consistent with the formation of the CH2NCH radical rather than 2H-azirine because of the much lower ionization threshold of the former with respect to the latter. As for the data recorded at m/z = 40, the suggested isomerization to ketenimine or acetonitrile followed by a secondary H loss is ruled out by our RRKM calculations. As commented in the previous section, only ketenimine and acetonitrile can lose an H atom in the time scale of the experiment. Therefore, there are only two possibilities left to account for the very large signal recorded at m/z = 40 with IE ∼ 10 eV: either the m/z = 40 distributions are due to a significant fragmentation of the parent species already at such a low ionization energy (again the best candidate in this case would be CH2NCH, as the IE is much lower, and it is reasonable to expect some dissociative ionization at 10 eV) or a real H2 elimination channel is open. In this second case, a roaming mechanism would be much more in line with the experimental observations of Lee et al. of a small amount of energy released as product translational energy. Our CMB data cannot resolve these issues. The most probable interpretation of our data is that both 2H-azirine and CH2NCH are formed with a similar yield and undergo significant dissociative ionization under the conditions of our experiments. We can infer that because we saw that part of the signal of the parent ion at m/z = 41 is characterized by the same CM functions of the distributions at m/z = 40. Since the

two reaction channels 1h and 1i (Chart 1) will probably undergo a similar reaction dynamics (because of the deep potential wells associated with the bound intermediates of the C2H4N PES), we are unable to distinguish them. Of course, we cannot rule out that CH2NCH or CH2(N)CH lose one H atom before reaching the detector. In this case, however, we would have some evidence that the secondary H loss does not alter the scattering distributions of the parent ion (m/z = 41). Furthermore, we have observed an additional channel leading to a product of m/z = 41 at the higher Ec investigated. As already commented on, the yield of this channel is extremely small, compared to the dominant ones. The fact that it is visible only at m/z = 41 makes us think that it is associated with a parent species which does not undergo significant dissociative ionization upon electron impact. As a matter of fact, two candidates have this characteristic: acetonitrile and isoacetonitrile. In both cases, the parent peak at mass 41 is by far the most intense one in the mass spectrum at 70 eV reported in the NIST database.51 RRKM estimates predict a very small fraction of isoacetonitrile formation. The yield of this channel was also found to increase with the available collision energy. Our suggestion, therefore, is that, at the higher Ec investigated, channel 1f (Chart 1) opens up. This is also corroborated by the very large fraction of energy released as product translational energy in this case.

8. IMPLICATIONS FOR THE CHEMISTRY OF THE ATMOSPHERE OF TITAN According to the present study, where electronic structure calculations and RRKM estimates were performed with the highest accuracy, the suggestion that 2H-azirine or any other molecular product of reaction 1 can isomerize to acetonitrile is not validated. The time required to achieve such a conversion is much longer than the collision time even in the rarefied upper atmosphere of Titan. Nevertheless, we suggest that the molecules identified by electronic structure and RRKM calculations under the conditions (temperature) of the atmosphere of Titan could significantly contribute to the growing of the N-rich organic molecules which appear to be the main constituent of the orange haze that covers completely this exotic moon of Saturn.3,16−18,52 The potential importance of these N-containing species in prebiotic chemistry has been widely discussed.14−17,53

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +39 075 585 5507. Fax: +39 075 585 5606. Notes

The authors declare no competing financial interest. † Visiting Scientists from Molecular Physics Division, Department of Physics, Alba Nova, University of Stockholm, S-10691 Stockholm, Sweden.

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ACKNOWLEDGMENTS We acknowledge financial support from the Italian MIUR (Ministero Istruzione Università Ricerca) under project PRIN 2009SLKFEX_004 and COST Action CM0805 The Chemical Cosmos: Understanding Chemistry in Astronomical Environments. N.B. thanks Ralf I. Kaiser for his hospitality at the Department of Chemistry of the University of Hawaii at Manoa, where the writing of this paper was completed. 10478



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