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Crossed Beams Study on the Dynamics of the F-Atom Reaction with Ammonia Chongfa Xiao, Guanlin Shen, Xiuyan Wang, Hongjun Fan,* and Xueming Yang* State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China ReceiVed: January 16, 2010; ReVised Manuscript ReceiVed: February 22, 2010
The F-atom reaction with NH3 and ND3 has been studied using the universal crossed beams technique. Angular resolved time-of-flight spectra were measured for the HF and DF reaction products. Product angular distribution and product kinetic energy distribution in the center-of-mass frame were determined from the experimental TOF spectra. Experimental results show that the HF and DF products are largely forward-scattered relative to the F-atom beam direction with a considerable amount of product at sideway and backward scattering directions. High-level ab initio calculation on the reaction energy pathway suggests that the forward-scattered products are mainly produced via a direct abstraction mechanism at large impact parameters, whereas sidewayand backward-scattered products are likely due to a long-lived complex formation mechanism. 1. Introduction In the past few decades, the F-atom reactions with different organic molecules have been studied extensively. It was found that the dynamics of the F-atom reactions with different species vary substantially. In the F-atom reaction with hydrogen, a near colinear abstraction mechanism is responsible. At energy below the reaction barrier, reaction resonances are very important.1-5 In the F atom reaction with methane,6-9 the very interesting state-to-state correlated dynamical picture has been observed in which the methyl radical products at different umbrella mode states correlate with dramatically different scattering pictures of the HF products at various vibrational states.10,11 In the F-atom reaction with silane, the HF formation channel mainly occurs through abstraction at large impact parameters, producing forward scattering products. In the F-atom reaction with ethylene, however, the HF reaction channel mainly goes through a long-lived complex formation process through the F atom addition to the double bond, as suggested from previous experimental results.12,13 Clear site specific dynamics has been observed in the F-atom reactions with propyne14 and propene15 in which the abstraction of the H-atoms by the F atom is clearly dependent on specific sites. The F atom reaction with aromatic and heterocyclic molecules as well as chloroethylenes have also been investigated previously using the crossed molecular beam method.16-18 The different dynamics of these F-atom reactions present a colorful picture of how chemical reactions occur for this important atom with various species. The reaction of F-atom with NH3 and ND3 has been previously investigated using the infrared chemiluminescence method.19,20 The experimental results show that a much larger fraction of the available energy is deposited into the DF product vibration in the F + ND3 reaction than the analogous NH3 reaction into the HF vibration. This result is interpreted by a mechanism involving competition between direct abstraction and formation of an intermediate complex. However, the detailed physical picture of how these two mechanisms occur exactly remains unclear. In this work, we would like to present the * To whom correspondence should be addressed. E-mail:
[email protected] (H.F.);
[email protected] (X.Y.).
experimental result of a crossed beam study on the F-atom reaction with NH3 and ND3. In this work, crossed molecular beam studies of the F + NH3 and F + ND3 reactions have been carried out using the universal crossed molecular beam technique. By careful measurements and detailed analysis of the time-of-flight (TOF) spectra and angular distributions of different products from the F + NH3(ND3) reaction, a single reaction channel has been observed. The detailed dynamics of this channel has also been analyzed. This article is organized in the following way: a brief description of the experimental method used in this study, the experimental result presentation and discussion, and a short conclusion. 2. Experimental Method The reactions of the F atom with NH3 and ND3 have been studied recently in our laboratory using the crossed molecular beam technique based on electron impact ionization detection. The apparatus used in this experiment is a universal crossed molecular beam machine, which has been described in detail elsewhere.21 In brief, the F-atom beam, generated using the pulsed dc discharge of F2 in He, was crossed with another skimmed beam at a fixed angle of 90°. The 5% F2 in He sample was expanded through a commercial pulsed valve (General Valves) with a gas pulse width of about 150 µs, going through a pulsed discharge region, which is a 2 mm long channel before the discharged products re-expand out to vacuum. The highvoltage discharge pulse is ∼2 µs long, and it is normally set to discharge on the peak of the gas pulse. From the time-of-flight measurement of the beam, the pulse width of the F-atom beam pulse is determined to be ∼10 µs. The expanded F-atom beam was skimmed by a skimmer (Beam Dynamics) with 2 mm diameter orifice. The other molecular beam was generated by expanding a neat NH3 or ND3 sample at ∼3 atm stagnation pressure through a carefully adjusted pulsed valve (General Valve) with a rise time of ∼50 µs and a pulse width of ∼100 µs. The expansion was then skimmed by a 1.5 mm orifice skimmer before entering the main chamber. The F-atom beam, the other beam, and the detection axis are all in the same plane. The speed of the F-atom beam is ∼1600 m/s with a speed ratio (ν/4ν) of ∼8. The speed of the other beam is ∼1200 or 850 m/s with a speed ratio of ∼8 and an angular divergence of about
10.1021/jp100435q 2010 American Chemical Society Published on Web 03/15/2010
Dynamics of the F-Atom Reaction with Ammonia
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Figure 2. Time-of-flight spectra measured at mass 20 (HF+) at six different laboratory angles. The open circles are the experimental data, whereas the solid lines are the simulated results. The experimental results were taken by averaging over 75 k pulses with discharge on and off. Figure 1. Reaction Energetics of NH3 + F f NH2 + HF.
(2.5°. The reaction collision energy in this experiment is ∼4.5 kcal/mol. To measure the reactive scattering signals from the F atom reaction, signals were measured normally with the discharge pulse on and off so that background subtraction can be done properly. Reaction products from the crossed region are detected by an ultrahigh vacuum mass selective, electron impact ionization detector. The whole experiment was carried out in a pulsed mode. Time zero (τ0) was defined as the time when the two beams were crossed. After flying about 25 cm from the crossed region, the neutral reaction products were then ionized by Brink-type electron impact ionizer with electron energy of ∼60 eV. The product ions were mass filtered by a quadrupole mass filter and counted by a Daly ion detector. All TOF spectra were taken at 4 µs/channel during the experiment. The product angular distribution can also be measured by rotating the detector. The experimental angular distribution is measured by taking the TOF spectra at different laboratory angles back and forth many times to reduce the measurement errors. In typical, the error bars for the experimental angular distribution should be about (5%. During the experiments, the vacuum in the detector ionization region was maintained at 1 × 10-12 torr. The time-of-flight spectra and angular distributions of the neutral products measured in the laboratory frame were converted to the kinetic energy distributions and the angular distributions in the center-of-mass (CM) frame by using a forward convolution program. 3. Reaction Energetics and Pathways Theoretical studies were carried out to investigate the reaction profile of NH3 + F f NH2 + HF. All calculations have been done on UCCSD(T)/aug-cc-PVTZ22-24 level of theory using the MOLPRO package.25 The reaction begins with adduct formation between NH3 and F. The most stable adduct is structure A, as shown in Figure 1, characterized by tetrahedral nitrogen center (distorted) and weak N-F bond. We located another isomer (B) by inversion of the nitrogen center of isomer A. B is less stable than A by 0.93 kcal/mol. The transition state (TS1) associating the two isomers is only 0.03 kcal/mol higher in energy than B, and we have carefully confirmed that TS1 is indeed a transition state. Adduct B is responsible for the hydrogen atom transfer from nitrogen to fluorine to form
NH2 · HF adduct, with the barrier of only 1.16 kcal/mol and an early transition state, as shown in Figure 1 (TS2). The hydrogen transfer step is exothermic by 23.43 kcal/mol. NH2 · HF adduct is a planar complex bearing strong N · · · HF hydrogen bond, and it takes 9.94 kcal/mol to break it to form isolate NH2 and HF. In the summary, reaction NH3 + F f NH2 + HF is very facile, without any overall reaction barrier and with a shallow potential well. The total reaction energy is -25.77 kcal/mol. With the collision energy of 4.5 kcal/mol in this experiment, the total available energy of the reaction is ∼30.2 kcal/mol. The calculated energetics and transition state pathways presented above should be helpful to the understanding of the dynamics of the title reaction. 4. Experimental Results and Discussions Product at mass 20 from the F + NH3 reaction has been detected and can be clearly assigned to the HF + NH2 channel. Figure 2 shows the TOF spectra at mass 20 at six laboratory angles. In the laboratory frame, the F-atom beam is defined at 0°, whereas the NH3 molecular beam is at 90°. To obtain the product kinetic energy distribution and angular distribution in the CM frame for the observed channels, a WINDOWS-based forward convolution program is used in the simulation. After simulations to these TOF spectra, CM kinetic energy distributions as well as product angular distribution have been determined for the HF formation process. The simulated TOF spectra for signals at mass 20 are shown together with the measured TOF spectra in Figure 2. The overall agreement between the experimental and the simulated TOF spectra is quite good in both shapes and intensities. Because the kinetic energy release is normally angular dependent, four different kinetic energy distributions at four CM angles were used in fitting the HF product signal in the TOF spectra at mass 20. Figure 3A shows the four kinetic energy distributions at CM angles of 0, 30, 60, and 180° used in modeling the HF formation channel, whereas Figure 3B shows the CM product angular distribution used in the simulation. From these distributions, a HF product flux contour diagram can be constructed and is shown in Figure 4. From the product flux diagram and the CM product angular distribution, it is quite clear that the HF product in the HF + NH2 channel is mainly forward-scattered relative to the F-atom beam direction. From the kinetic energy distributions obtained experimentally, the backward scattering products are slightly different from the forward and sideways scattering products.
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Figure 3. (A) Product CM kinetic energy distributions at four CM angles for the HF product from the HF formation channel obtained from the simulations. Kinetic energy distributions at other CM angles are obtained from the linear interpolations of these four distributions. (B) Product CM angular distribution for the HF product from the HF formation channel obtained from the simulations.
Figure 4. Three-dimensional product contour diagram for the HF + NH2 channel in the F + NH3 reaction.
The F + ND3 reaction has also been studied for detecting the effect of isotope substitution on the dynamics of this reaction. Reaction product at mass 21 (DF+) has been detected and was assigned to the DF product from the DF + ND2 channel. TOF spectra at mass 21 at six laboratory angles have been measured. Similar to the F + NH3 experiment, the F-atom beam is defined at 0° whereas the ND3 molecular beam is at 90° in the laboratory frame. After simulating these TOF spectra using the same procedure described above, the CM product angular distributions as well as kinetic distributions have been determined for the DF formation channel. Figure 5A shows the kinetic energy distributions obtained from simulating the TOF signals, whereas Figure 5B shows the CM product angular distribution obtained in the same simulation. From these results, it seems that the total kinetic energy distributions for the DF product channel from the F + ND3 reaction is very similar to that for the HF product channel from the F + NH3 reaction. From the CM
Xiao et al.
Figure 5. (A) Product CM kinetic energy distributions at four CM angles for the DF product from the F + ND3 reaction obtained from the simulation. Kinetic energy distributions at other CM angles are obtained from the linear interpolations of these four distributions. (B) Product CM angular distribution for the DF formation channel obtained from the simulation.
product angular distribution in Figure 5B, the DF product channel in the DF + ND2 channel is also mainly forward scattered relative to the F-atom beam direction, similar to the HF product channel in the HF + NH2 channel. The similarity between the kinetic energy distributions and the angular distributions for the two reactions suggests that the dynamics of the two reactions are essentially the same. From this point of view, it is not clear how to account for the dynamical difference observed in the IR chemiluminescence study. From the angular distributions, it is obvious that product distribution is mainly forward scattered. The forward scattered products are clearly due to the direct abstraction channel at large impact parameters. This is consistent with the theoretical picture presented in Figure 1. Clearly, the most likely pathway in this picture is the large impact parameter abstraction at the TS1 and TS2 geometry. This pathway will very likely generate forward scattering HF products. In addition to the forward-scattered products, however, it seems that there is a significant component that the product angular distribution is more or less isotropic. Such product distribution is likely produced via a long-lived complex formation pathway. Indeed, looking at the reaction pathway in Figure 1, there is also an energy minimum with a depth of 35.7 kcal/mol in the reaction path near the exit channel. A long-lived complex can certainly be formed at this energy minimum, therefore producing forward-backward scattering products. From the product kinetic energy distributions of the F + NH3 and F + ND3 reactions, it seems that the forward scattering products have a higher kinetic energy distribution than that at other scattering directions. This result supports the above conclusion that the forward-scattered products are produced via a different mechanism from products scattered in other directions. It is, however, difficult to separate the contributions of the two different mechanisms.
Dynamics of the F-Atom Reaction with Ammonia From the angular distributions (Figure 3B and Figure 5B) for the F + NH3 and F + ND3 reactions, it seems that the forward peak is slightly larger relative to the isotropic component in the F + NH3 reaction than that in the F + ND3 reaction. This suggests that direct abstraction channel in the F + NH3 reaction might be more important relative to the long-lived complex formation channel than that in the F + ND3 reaction. This could be due to the fact that H-atom abstraction is faster than the D-atom abstraction in the F atom reaction. This might be the reason that the dynamics looks different in the infrared chemiluminescence study. 5. Conclusions In this report, the dynamics of the F + NH3 and ND3 reaction have been studied using the universal crossed molecular beam method. Angular resolved time-of-flight spectra have been measured for the HF and DF products from the F + NH3 and F + ND3 reactions, respectively. Product angular distributions as well as energy distributions were determined for its product channel. Experimental results show that the HF and DF products are mainly forward scattered, relative the F atom beam direction, with certain amount of products distributed isotropically. Theoretical calculations were performed on the reaction energy pathway. The forward-scattered component can be attributed to a direct abstraction reaction channel at large impact parameters, whereas the isotropic component is assigned to a longlived complex formation pathway. These dynamical features can be explained on the basis of the calculated reaction pathway. No large isotope effect is found in the title reaction. However, some small differences are observed in the angular distributions between the F + NH3 and F + ND3 reactions and might be the cause of the dynamical difference observed in the previous infrared chemiluminescence study. More studies, however, are needed to confirm this dynamic isotope effect. Acknowledgment. This work is supported by the Chinese Academy of Sciences, the National Science Foundation of China, and the Ministry of Science and Technology. References and Notes (1) Skodje, R. T.; Skoouteris, D.; Manolopoulos, D. E.; Lee, S.-H.; Dong, F.; Liu, K. J. Chem. Phys. 2000, 112, 4536.
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