Pressure effect on the eda complexes in solution

Abstract

The intermolecular charge-transfer spectra of the complexes between tetracyanoethylene (TCNE) and methyl substituted benzenes in various solvents have been studied under high pressure at 25℃. Both the equilibrium constant K and the molar absorption coefficient a increased with increasing pressure. The volume change of the complex formation in carbon tetrachloride was in the range of -3.4～-14.1cm^3/mol for the various donors, benzene, toluene, mesitylene and hexamethylbenzene (HMB), and these negative values could be mainly interpreted by the contraction of the distance between the donor and the acceptor (TCNE) on the complex formation. The enhancement of ε by pressure ε_max(p)/ε_max(1) was ca 1.2 at 1500kg/cm^2 for most EDA complexes, and it was discussed by taking into account both the solute-solvent interaction and the decrease of the donor-acceptor distance by pressure. Nearly linear red shift of absorption maximum λ_max with pressure was observed for benzene, toluene and mesitylene up to 1500kg/cm^2. However, λ_max of HMB complex shifted to lower frequency with pressure at first and then turned to higher frequency. The turning pressure was solvent dependent: around 800kg/cm^2 in carbon tetrachloride and 4500kg/cm^2 in n-pentane. The spectral shift was considered from the viewpoint of not only the effect of solvent property but also the change of resonance interaction between the donor and acceptor caused by compression.