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 pi-system electronic density variation in the aromatic ring. The amount of positronium obtained, I-3 parameter, has been correlated with the Hammett (sigma) and Brown-Okamoto (sigma(+)(p)) constants and adjusted through the modified Hammett equation, which employs the ratio I-3/I-3(phi), yielding a satisfactory fit. (C) 2011 Elsevier B.V. All rights reserved.