Positronium complex formation: mechanisms, solvent effects and its application to the study of molecular phenomena

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Virginia Polytechnic Institute and State University

One of the most widely studied “exotic atoms" in Chemistry and Physics is positronium. Positronium (Ps) is the bound state of an electron and positron. It is formed in two ground states, para-positronium (p-Ps) and ortho-positronium (o-Ps) whose lifetimes in free space are 0.125 nsec and 140 nsec respectively.

When thermalized o-Ps forms a complex with a molecule, the electron density at the position of the positron will be drastically increased thus shortening its lifetime.

A study was performed to evaluate what role the solvent plays in stabilizing the complexes formed between Ps and nitroaromatics in solution and the various thermodynamic variables associated with these processes. This study also included the evaluation of the effects displayed by several different solvents on complexes formed between Ps and CuCl₂. The interpretation of the solvent effects of Ps reactions in terms of the "bubble shrinkage model" was tested using the results obtained in the reactions of Ps with various inorganic and organic molecules in different solvents. It was found that this model is definitely not applicable below a certain critical viscosity of the solvent and appears to be restricted to diffusion controlled reactions.

The fact that the reactivity of Ps towards nitroaromatics is drastically reduced when the latter molecules are already complexed with other conventional molecules can be utilized to determine the molecular formation constant, Kc, for inclusion Compounds. A study was made using Ps as a probe to provide valuable information about the influence of steric factors on the ability of guest molecules to bind in the cavity of cyclohexaamylose,