Ultrasonic and spectroscopic investigations of charge-transfer complexes in ternary liquid mixtures

Charge-transfer complexes formed between iodine (σ-acceptor) and pyrrole, thiophene and pyridine (π as well as n-electron donors) have been studied spectrophotometric and ultrasonic techniques at 303.15 K in dimethyl sulphoxide medium. The measured values of ultrasonic velocity, density and viscosity and the derived acoustical parameters such as adiabatic compressibility (κ), free length (Lf), internal pressure (πi), molar volume (Vm) and available volume (Va) have been used to study the molecular interactions and the complexes of heterocyclic aromatic compounds. The excess thermodynamic parameters like excess velocity (uE), excess adiabatic compressibility (κE), excess free length (LfE), excess molar volume (VmE) and excess available volume (VaE) have been computed and the sign and magnitude of these parameters indicate the strength of interactions and the formation of charge-transfer complex. The formation constants of the charge-transfer complexes were determined by the Benesi-Hildebrand equations (optical method) and compared with those obtained from Kannappan method (ultrasonic method). The formation constants of complexes of iodine with aromatic compounds depend on the structure of the donor and acceptor molecules.