The reaction of ortho-positronium with nitroaromatics via complex formation

A study was made to ascertain the mechanism by which ortho-positronium reacts with nitroaromatics in solution. These reactions are two to three orders of magnitude faster than expected by simple pickoff. Conjugation of the nitro group with the aromatic ring was found to be a necessary condition for this behavior. The rate constant for each of these reactions was measured in suitable solvents over a wide temperature range, 200-500°K. Distinct departure from the Arrhenius law was found. In general, the rate constant increased linearly with increasing temperature up to a point, and then decreased linearly as the temperature was raised further. The following mechanism is postulated: Ps + M [stacked right and left arrows with k₁ above and k₋₁ below] PsM [right arrow with k₂ above] M + 2γ. Ortho-Positronium reacts with the nitroaromatic, M, to form a complex in a reversible step. The complex may either decompose into a positronium atom and nitroaromatic molecule or it may go on to annihilate the positron. A steady-state concentration ot the complex is assumed and the observed rate constant for the process is K_obs = k₁k₂ /(k₋₁+ k₂) The two limiting cases are: (1) k₂ >> k₋₁ causing k_obs = k₁ . Typical Arrhenius behavior is expected and observed in this region, and (2) k₋₁ >> k₂ causing k_obs = K_EQk₂ . In this region H_EQ + E_a(2) <O and the observed rate constant decreases with increasing temperature. This corresponds to a stable but temperature sensitive PsM complex. An approximate molecular orbital study has been made to study the possible existence of positron and positronium complexes with a select number of organic molecules. The CND0/2 approximation is used for all electronic integrals and for electron positron Coulomb integrals. The core Hamiltonian matrix elements for the positron were estimated by combining the Wolfsberg-Helmholtz and Cusachs-Cusachs approximations. This approach shows that the positron should become bound to the molecules. Using the same criteria, the formation of stable positronium complexes is more improbable. The calculated binding energies are strongly dependent on the value of the Wolfsberg-Helmholtz proportionality constant for positrons.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

doctoral

Department dc:contributor.department

Chemistry

Grantor dc:publisher

Virginia Polytechnic Institute and State University

Year dc:date.issued

1975

Author dc:creator

• Madia, William Juul

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Handle dc:identifier.uri

http://hdl.handle.net/10919/88682

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/88682