Thermodynamic constants for some π-acceptor–methylbenzene charge-transfer complexes from nuclear magnetic resonance shift measurements

Abstract

The enthalpies (ΔH) and entropies (ΔS) of formation of a number of charge-transfer complexes between methylbenzenes and π-acceptors in solution have been determined from chemical-shift measurements at various temperatures. In general regular, though non-linear, relationships are observed between ΔH and ΔS, and between ΔH and the ionisation potential of the donor, for series of complexes involving a common acceptor. In changing from carbon tetrachloride to chloroform or 1,2-dichloroethane as the diluting solvent, the variation of ΔH for the series of complexes of a given acceptor with the methylbenzenes is considerably reduced. Proton chemical shifts in both the pure acceptor and the complex show only slight changes with temperature. Such changes are more marked for ¹⁹F magnetic resonances.