Dynamics of Chemical Reactions

We have studied the details of organic chemiionization reactions. This is done experimentally using crossed molecular beams and teheoretically using computer simulations of the reactions. We have found four types of reactions:

1. SbF₅+RX -> SbF₅X^-+R⁺
2. SnCl₄+B -> SnCl₄^-+B⁺
3. HI+B -> I^-+HB⁺
4. RI+NR₃ -> I^-+NR4⁺

X is a halogen, and B is a base, typically an amine. In solution, reaction (1) is a Lewis acid-Base reaction, and reaction (2) is a redox reaction. Reaction (3) is the reaction of an acid with a base, and reaction (4) is the SN2 reaction.

A supersonic nozzle beam is made for each reactant by expanding a mixture of 1% of the reactant in 99% of carrier gas (H₂, He, or a mixture of H₂ and He). The translational and rotational distributions are cooled to a few Kelvin, and the reactants are accelerated up to energies of a few eV. We can change the translational energy and the vibrational temperatures of the reactants by changing the nozzle temperature and the type of carrier gas. The detector can be rotated about the beam center. It operates in two modes. If the exit grid is at the same electrical potential as the grid cage, the product ions are formed in an equipotential region, and only those going in the direction of the detector are seen. We can then measure the distributions in product scattering angle and translational energy. In the second mode the ions are extracted by an electric field, we can measure the reactant cross section vs. energy.

The reactions can be modeled on a computer. We note first that each reaction requires at least two different-potential energy surfaces: one covalent dissociating to the neutral reactants, and the other ionic dissociating to the ionic products. The reaction is modeled as a three-body system: A+BC -> AB⁺ + B^-. Using a random number generator the initial conditions are chosed to mimic the reaction studied in the lab. The classical trajectory is then followed using Hamilton's equations until the dividing seam between the two surfaces is reached. At this point the program calculates the probability that the reactants will jump to the ionic surface. We can then either choose which surface the reactions continues on by using the random number generator, or we can follow both trajectories but weight them accordingly. After a few hundred trajectories we plot the distributions of the products for comparison with the experimental results.

A summary of recent work can be found in:

• Crossed-Beam Studies of the Dynamics of Chemiionization Reactions, R. J. Cross and M. Saunders, Accts. Chem. Res. 24, 104 (1991).
• Crossed-Beam Studies of the SN2 Reaction, Y. F. Yen, R. J. Cross, and M. Saunders, J. Am. Chem. Soc. 113, 5563 (1991).
• Molecular-Beam Study of the Reactions of HI and Amines, Y. F. Yen, Y. D. Huh, R. J. Cross, and M. Saunders, J. Phys. Chem. 95, 8753 (1991).
• Trajectory Modeling of Organic Chemiionization Reactions, Y. F. Yen and R. J. Cross, J. Chem. Phys. 96, 1904 (1992).
• Photon-Promoted Chemiionization Reactions, C. X. Wang, Y. F. Yen, R. J. Cross, and M. Saunders, J. Am. Chem. Soc. 114, 4423 (1992).
• Dynamics of the Gas-Phase Acid-Base Reaction, R. Shimshi, X. Wang, R. J. Cross, and M. Saunders, J. Am. Chem. Soc. 117, 9756 (1995).

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