Nonlinear dielectric spectroscopy is used to study the self-association of 2-pyrrolidinone in benzene solutions up to 0.9 mol/L at 25 °C. The nonlinear dielectric spectra below 100 MHz are recorded using an LC-resonant circuit, while above 100 MHz a partial coaxial resonant cavity is used. Both the kinetic and thermodynamic data are very well described by a dimerization mechanism with an equilibrium constant for dimerization K(ε) = 2676 exp[−17.1(ε − 1)/(2ε + 1)] M-1, depending on the permittivity of the solution according to the reaction field theory. At infinite dilution in benzene, K = 53 M-1 and the rate of association is diffusion-controlled with k 1 = 0.87 × 1010 M-1 s-1. The description proposed in the present work is also confirmed by the analysis of (static) linear dielectric data up to 0.9 mol/L and qualitatively predicts the behavior at higher concentrations. The present study has shown that even for high concentrations there is no need to introduce oligomers such as trimers or tetramers in the association model. Both linear and nonlinear dielectric data lead to a dipole moment value of 3.96 D for the monomer and 2.30 D for the dimer. The latter is believed to undergo a rapid interconversion between a closed and a (partially) open form explaining the relatively high dipole moment. As an overall conclusion, it can be stated that the dynamic measurements have proven to be a much more sensitive tool to discriminate between different association models than thermodynamic results.

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We present complete nonlinear dielectric spectra for a rigid dipolar molecule in dilute solution. The spectra are described theoretically by dispersions of the form as predicted by Coffey and Paranjape ͓Proc. R. Ir. Acad. 78, 17 ͑1978͔͒ and by Alexiewicz and Kasprowicz-Kielich ͓Modern Nonlinear Optics, edited by M. Evans and S. Kielich ͑Wiley, New York, 1993͒, Vol. 85, pt. 1, p. 1͔ and are consistent with linear dielectric measurements. ͓S1063-651X͑98͒09602-0͔