Nader Anz and Dr. Jonathan SmithDepartment of Chemistry

Temple University

Vinylidene is a bi-radical that is very high in energy relative to acetylene y y g gy ywith a 45.91 kcal/mole energy difference and a low barrier to isomerization1. Thus, vinylidene isomerizes to acetylene. To develop a better understanding of the isomerization mechanism of vinylidene to acetylene, the dynamics were studied using computational methods. First, the experimental energy transfer amounts were obtained from the examination of the time-resolved IR emission and using the Lennard-Jones

lli i b i d b A l i f h dcollision rate parameters obtained by our group. Analysis of the data showed a loss of energy resulting from the collisions of different inert gas atoms with acetylene. Acetylene showed tremendous enhancement of energy transfer per collision rate when its vibrational energy was higherenergy transfer per collision rate when its vibrational energy was higher than 1.5x 104 cm-1. This energy is near the barrier of its isomerization to vinylidene. The enhancement of the energy transfer rate is accredited to the presence of the highly energized vinylidenepresence of the highly energized vinylidene.

The formation of vinylidene was ycarried by subjecting vinylbromideto vacuum UV radiation of a wavelength of 193 nm as shown in the reaction presented in Figure 1. The process led to the formation of vinylidene with HBr as a b d E h f h i dbyproduct. Each of these excited molecules experienced energy transfer that resulted in a loss of energy as well as emission of IRenergy as well as emission of IR which was measured in the experiment.

Rare gas Rare gas –– AcetyleneAcetyleneI t l l P t ti lI t l l P t ti lIntermolecular PotentialIntermolecular Potential

Lennard-Jones

Buckingham

Use Classical trajectory calculations (Venus 96) to simulate collisions. AverageUse Classical trajectory calculations (Venus 96) to simulate collisions. Average 10,000 trajectories with sampled impact parameter. The energy transfer rates obtained by the Buckingham equation were more accurate than the energy transfer rates generated by the Lennard-Jones equation; the Buckingham equation is more fl ibl i h hil h L d J i lflexible as it uses three parameters while the Lennard-Jones equation uses only two parameters and significantly overestimates the average energy transferred per collision.

Experimental and Computational Energy Experimental and Computational Energy T f R t f T f R t f AA U i th U i th Transfer Rates for Transfer Rates for ArAr Using the Using the

Buckingham EquationBuckingham Equation

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e) Computational energy transfer rate using the Lennard-Jones equation for Ar as a collider

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Experimental energy transfer rate for Ar as a collider

Computational energy transfer rate

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Vibrational energy of Acetylene

Computational energy transfer rate using the adjusted A parameter for the Buckingham equation for Ar as a collider

Our computational simulation showed a near linear increase in energy transfer per collision as a function of acetylene excess energy. Thus, the enhancement of the energy transfer rate, observed at an approximate energy of 15,000 cm-1, in the experimental plot is caused by theobserved at an approximate energy of 15,000 cm , in the experimental plot is caused by the influence of the vinylidene molecules that are high in energy.

Computational Energy Transfer Rates for Computational Energy Transfer Rates for AA d N U i th B ki h E tid N U i th B ki h E tiArAr and Ne Using the Buckingham Equationand Ne Using the Buckingham Equation

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Ar as a collider

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Ne as a collider

The energy transfer rates were found to be higher for Ne than for Ar due to Ne

Vibrational energy of Acetylene (cm-1)

being lighter and smaller with lower dipole polarizability than Ar.

VinylideneVinylidene to Acetylene Reaction Pathto Acetylene Reaction Path

The vinylidene to acetylene reaction path can be usefully partitioned into three regions: the rotation of a vinylidene hydrogenrotation of a vinylidene hydrogen until the C1C2H angle is reduced to p/2, the linear transit of this hydrogen parallel to the C=C bond until the C2C1H angle has increased to p/2, and finally the rotation of this hydrogen about C2to form acetyleneto form acetylene.George Petersen, personal communication

Angular dependence of the Energy Angular dependence of the Energy T f R t IT f R t ITransfer Rate ITransfer Rate I

Angular dependence of the Energy Transfer Angular dependence of the Energy Transfer R t IIR t IIRate IIRate II

The model shows the collision angle responsible for the most efficient energy transfer rate. The di tidirecting arrows are associated with the lowest computational vibrationalmode and they show themode and they show the path that the wandering hydrogen follows as vinylidene isomerizes tovinylidene isomerizes to acetylene after the energy loss due to the collision with the Ar atom

Angular dependence of the Energy Transfer Angular dependence of the Energy Transfer R t IIIR t IIIRate IIIRate III

The electron density yclouds presented in the vinylidene model show the angular dependence g pof the energy transfer rate as a result of the collision. Collisions that occur at the node depicted by a green electron cloud lead to the highest energy transfer rate.

A new computational program is being developed that will be A new computational program is being developed that will be used instead of Venus 96. The program will provide the same applications as Venus 96 but it will be easier to use as we are d l i it b d th i t f hdeveloping it based on the requirements of our research.

Newer and more accurate dynamic models shall be presented in the near future.