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Acetylene-vinylidene isomerization dynamics and influence ...

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ACETYLENE-VINYLIDENE ISOMERIZATION DYNAMICS AND INFLUENCE ON ENERGETICS AND COLLISIONAL ENERGY TRANSFER. Mark A. Fennimore (undergraduate) and Jonathan M. Smith Department of Chemistry Temple University
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Page 1: Acetylene-vinylidene isomerization dynamics and influence ...

ACETYLENE-VINYLIDENE ISOMERIZATIONDYNAMICS AND INFLUENCE ON ENERGETICS AND COLLISIONAL ENERGY TRANSFER.

Mark A. Fennimore (undergraduate) and Jonathan M. SmithDepartment of ChemistryTemple University

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Abstract

Acetylene has proven an interesting case study, both for its ubiquity in nature as well as its implementation in industry. It is readily used in a variety of commercial applications and scientific inquiries, and as such, knowledge of its chemical and physical properties is indispensible. Furthermore, its prevalence as an intermediate in combustion reactions makes the study of highly vibrationally excited acetylene a potentially lucrative endeavor, and perhaps holds the promise of a new and exciting field of study.

This promise has compelled us—and other researchers—to probe the dynamics of energized acetylene, through its photodissociation with vinylhalides. This photodissociation is accomplished via vacuum UV at 193nm, which results in rovibrationally excited hydrogen halide (HX) and vibrationally excited acetylene; this excited acetylene is subsequently tracked via time resolved Fourier transform infrared emission spectroscopy; the data obtained in this manner is used in conjunction with molecular dynamics in order to explore the underlying mechanisms that dictate vibrationally excited acetylene’s behavior.

With the use of Born-Oppenheimer molecular dynamics (BOMD), we are attempting to explore the behavior of highly excited acetylene and perhaps more importantly, its unstable isomer counterpart, vinylidene. Vinylidene is the bi-radical transient of acetylene and exists only on the order of the pico-second. The highly anharmonic potential surface of the isomerization between acetylene and vinylidene, the high barrier height towards the transition state, and the shallow potential well of the transient make an equilibrium between acetylene and vinylidene seemingly unlikely, but in fact such an equilibrium is observed both theoretically and experimentally given the appropriate detection method. It has further been demonstrated that by neglecting the isomerization equilibrium described above, the kinetics fail and only by its inclusion does theory match observation.

We present computational results using Born-Oppenheimer-molecular-dynamics (BOMD) to explore the creation of vibrationally excited acetylene/vinylidene via the photolysis of vinylbromide and vinylfluoride. Calculations on the former resulted in a 3-centered (a-a) dissociation of HBr , while the later produced both a 3 (a-a) and 4-centered (a-b) dissociation pathway for HF. Furthermore, because acetylene in a know intermediate in combustion reactions, we will explore various free-radical and noble­gas collision trajectories in order to determine the resulting vibrational energeticsof acetylene and its isomer. We will also present the ground-state energies of relevant species at a refined level of theory.

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IntroductionVinylidene has been the subject of great interest in recent years. Its participation in

combustion reactions has made it a logical pursuit for researchers due to the pervasiveness of fossil fuel consumption. By understanding the mechanisms that govern its existence, we would gain not only a better understanding of combustion reactions but of other fundamental reactions as well.

Vinylidene has proven hard to charecterize experimentally due to its transient nature and rapid isomerization to acetylene. In a classic experiment, Lineberger and coworkers observe the vinylidene vibrational signature initiated from generation of vinyl anions, followed by electron removal via photo-detachment [1]. Carter and coworkers carried out extensive ab initio Born-Oppenheimer molecular dynamics to examine the isomerization process at energies relevant to Lineberger’s experiments [2]. They posit a revisiting of vinylidene conformations on short time scales reflecting a greater significance for vinylidene structures in the properties and reactivity of energized acetylene. However, this system may be probed using a different and perhaps somewhat more simplified approach. We generate vinylidene using vacuum UV, where photolysis of vinylhalide (e.g. vinylbromide) initiates a free radical reaction resulting, ultimately, in the dissociation of HX, and these results may then be compared with experimental time resolved IR emissions data [3].

Figure 1 illustrates the general mechanism for the creation of vinylidene via photolysis of vinylbromide and the subsequent isomerization of the transient.

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Methods

To develop the static ground-state energies for each species of interest, we used Gaussian 09 with the following theory and basis set: B3LYP/aug-cc-pVTZ.

Furthermore, to reproduce the experimental parameters, thusly discussed, we have employed several methods to replicate the various potions of the experiment. To create the dissociation dynamics for the Vinylhalides (See figure 2 and 6), we have chosen to implement the NSample computational technique available in G09 via the Born-Oppenheimer approximation. This method allows for the allocation of vibrational energy into particular mode(s) of interest towards potential transition vectors. Because our experimental parameters included the use of vacuum UV in order to initiate the dissociation of X—and subsequently HX in vinylhalides—it was determined to place the requisite energy(l=193 or 148 kcal/mole) into various normal modes in order to determine where and to what extent photodissociation (PD) occurs.

To produce the collision dynamics, meant to replicate acetylene’s interactions with other radicals or inert gases, several methods were applied in sequence. First, it was necessary to generate the vibrational excitations of the system; for this, we used RandomVelocity generation. This computational method results in x,y,z velocity vectors for each atom at the appropriate temperature thanks to thermal sampling criteria. These generated velocities were layered onto a second dynamics model: Read mass weighted velocity (ReadMWVelocity). This method allows one to track the energy exchange inherent in any collisional process. Note: all dynamics were accomplished with Density Functional Theory (DFT) using B3LYP and a 6-31G(d,p) basis set on Gaussian 09.

Table 1 shows the ground state energies of all three molecules of interest. It is important to note that the energy equivalent to the ground state of hydrogen was added to both the vinylidene and acetylene to make the relative energetics of all three species comparible. Note: all static computations were done with B3LYP/aug-cc-pVTZ

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hn

For vinylbromide. Nsampling of normal modes 1, 3, 5, and 7 (148kcal/mol) showed 3-centered (a-a) dissociation pathways of HBr. Each of these (a-a) mechanisms resulted in vinylidene which rapidly isomerized to acetylene and back.

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Figure 2 (above) shows the formation of vinylidene via BOMD; the computation was accomplished using the Nsample option, where 148kcal/mole (l=193nm) was allocated into vibrational mode 1. Also, note that the final frame shows vinylidene at the transition state[2]

Figure 3 (left) displays vinylbromide’sdissociation and the resulting vinylidene/acetylene isomerizationtrajectory energetics. The initial increase in the system’s potential energy is attributed to the injection of 148kcal/mol into mode 1. Notice how the system exchanges potential energy for kinetic near vinylidene’s zero-point (see Table 1)

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Figure 4 displays two independent dissociation pathways for vinylfluoride. The 4-centered (a-b) pathway was created by allocating 193nm into vibrational mode 5. This resulted in vibrationally excited acetylene with no vinylidene isomerization dynamics, which is clear by observing the final frame of the upper portion of the diagram. The 3-centered (a-a) pathway—on the other hand—behaved in a manner consistent with the PD of vinylbromide, where the nascent product was vinylidene, which subsequently isomerized with acetylene and so on. Also note, that the final frame displayed on the bottom portion of the figure illiustrates the transition-state of the acetylene/vinylidene system.

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Figure 5 (upper-left) illustrates the difference in the kinetic energy between the 3-centered and 4-centered photodissociation pathway inherent to vinylfluoride. The 3-centered mechanism results in an acetylene/vinylideneisomerization, where the 4-centered does not. The 3-centered mechanism was replicated by allocating 193nm (148kcal/mole) into vibrational mode 7; whereas the 4-centered mechanism was generated via 148kcal/mole in mode 5. Other modes were tested as well; however, none produced dissociation dynamics of any kind. Furthermore, the baseline for vinylfluoride is provided and is a model by thermal sampling each vibrational mode at 300K.

Table 2: (bottom-right) the relative energy allocation for each vibrational mode is displayed for the three systems pertinent to Figure 6. The thermal sampling was generated via a separate computation, which provided the standard baseline for the dissociation models. These computations raise important questions as to what vibrational mode energy distributions give rise to these types of photodissociations, besides those thusly displayed; for vibrational mode coupling too likely plays a key factor in such mechanistic pathways and as such, must be accounted for.

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T-V energy dynamics

Figure 6 (left) illustrates hot hydrogen’s trajectory as a function of two different angles with respect to acetylene’s center of mass. Cosine sampling is used to introduce a bias into the experimental parameters.

Figure 7 (right) shows the energy exchange between translationally hot hydrogen and acetylene.

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Our BOMD trajectories followed a 3-center HBr elimination path yielding vinylidene which rapidly isomerizes to excited acetylene as shown in snapshots (Figure 2). This trajectory follows the expected preferred 3-center mechanism [3]. Moreover, both the 3 and 4-centered pathways were replicated for vinylfluoride, which resulted in an isomerization mechanism in the former and simply vibrationally excited acetylene for the latter.

Our trajectories also show a rapidly isomerizing vinylidene system which visits energized acetylene structures and revisits vinylidene on a ~50 femtosecond timescale similar to the results of Carter and cowrkers [2]. Our static calculations [Table 1] of minima on the acetylene/vinylidene/vinyl surface provide agreement with previous calculations [4] and show the ZPE of vinylidene to be 41.1 kcal/mole above acetylene. The (BOMD) dynamics produce HBr with 21.5 kcal/mole rotational energy and 11.1 kcal/mole vibrational energy compared with 13.8 kcal/mole and 17.1 kcal/mole nascent energy observed experimentally [3]. This puts vinylidene ~16 kcal/mole above ZPE and >10 kcal/mole above TS to acetylene.

One goal of this work is to use an equilibrated but energized acetylene/vinylidene system and examine V-RT energy transfer by rare gases and correlate with our experiments. Figure 6 and 7 illustrate similar calculations on hot hygrogen atom collisional trajectories with room temperature acetylene. This protocol successfully models T-VR energy transfer in this systems and will be applied to this related process.

Discussion

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Conclusion

Our initial computations have probed the energetics and dynamics of the photolysis of vinylbromide leading to HBr + vinylidene via a 3-center mechanism and also that of the 3 and 4-centered pathways for vinylfluoride. The vinylidene that is produced from these photolysis processes has been shown to isomerize to acetylene but revisit vinylidenestructures in the 3-centered (a-a) mechanisms but not the 4-centered (a-

b). We plan to examine the V-T energy transfer between this isomerizingsystem and rare gases comparing to our experimentally observed vinylidene enhanced energy transfer efficiency.

References

1. Burnett, S. M.; Stevens, A. E.; Feigerle, C. S.; Lineberger, W. C., Chemical Physics Letters 1983, 100 (2), 124-128.2. Hayes, R. L.; Fattal, E.; Govind, N.; Carter, E. A., Journal of the American Chemical Society 2001, 123 (4), 641-657.3.Liu, D.-K.; Letendre, L. T.; Dai, H.-L., J. Chem. Phys. 2001, 115 (4), 1734-1741.4.Osamura, Y.; Schaefer, H. F.; Gray, S. K.; Miller, W. H., Journal of the American Chemical Society 1981, 103 (8), 1904-1907.

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Contact information

Temple University

Mark Fennimore

[email protected]

Dr. Jonathan Smith

[email protected]


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