Positron annihilation studies in solid substituted aromatic compounds

Positronium formation was investigated in benzene and naphthalene compounds with electron donating (NH2 and OH) and electron withdrawing (CN and NO2) substituents. The results exhibit an increase in the positronium formation yield whenever donating groups are bound to the ring and a decrease with withdrawing groups. These results can be attributed to the π-system electronic density variation in the aromatic ring. The amount of positronium obtained, I3 parameter, has been correlated with the Hammett (σ) and Brown–Okamoto ( σ p + ) constants and adjusted through the modified Hammett equation, which employs the ratio I3/ I 3 ϕ , yielding a satisfactory fit.