Solvent Effects on the Complex Formation of Benzophenone Ketyl Radical and Triethylamine

Abstract

The 1:1 complex formation between benzophenone ketyl radical (BPK) and triethylamine in various solvents was studied by means of nanosecond laser flash photolysis. The complex shows an absorption band slightly red-shifted from that of uncoupled BPK. The formation constants ( K f 's) were determined from the absorbance change of the complex against amine concentration. The K f values are in the range 60−103 M -1 depending on the dielectric constant of solvent ( ε r = 10.7−1.89). Enthalpy and entropy changes in the complex formation reactions were determined from van't Hoff plots around room temperature. The measured enthalpy change of −2.0 to −7.5 kcal mol -1 depends on ε r, which is explained with an Onsager's reaction field theory. The measured entropy change varies from −16.5 cal K mol -1 to nearly zero, and the dependence on the solvent is discussed in terms of the solvation effect.