Kinetics and thermodynamics of the formation of the donor-acceptor complex of tetracyanoethylene with hexamethylbenzene in solution, by a microwave temperature-jump method

Abstract

The rate of formation of the donor-acceptor complex between tetracyanoethylene and hexamethylbenzene, in l-chlorobutane as solvent, has been measud at -83°C by a ternlnjratmjump method using microwave heating. Equilibrium constants have also been determined over a range of temperature. The rate constant is (1.45f0.3) x lo8 1. mol-1 s-1 (the uncertainty referring to the estimated limits of error). This is smaller by an order of magnitude than the calculated value for a diffusionantrolted reaction. The reason for this is discussed and it is concluded that there is possibly an energy barrier due to solvation changes. The rate constant in a solvent consisting of chlorobenzene+n-heptane (40-60 v-v) has also been determined, allowance being made for the parallel complex-formation between chlorobenzene and tetracyanoethylene ; at -60°C, it was found to be (8 f 3) x lo8 1. mol-I s-l.Complexes formed in solution between electron-acceptors such as trinitrobenzene, iodine or tetracyanoethylene on the one hand and electron-donors such as aliphatic amines or aromatic hydrocarbons on the other have been studied in recent years '* from various points of view-spectroscopic, structural, and thermodynamic. Kinetic investigation has lagged behind, because the reactions are extremely fast ; exploratory investigations by the ultrasonic-absorption method and by the stopped flow method at -120°C were unsuccessful, and evidence from studies of dielectric relaxation 5 * ti indicates that the rates are very high. In the most favourable cases, however, and at low temperatures, they are within the range of the temperature-jump technique, and we have used this method to determine the rate of complex-formation between tetracyanoethylene (TCNE) and hexamethylbenzene (HMB), for which the equilibrium constant and enthalpy of reaction are favourable : kf TCNE + HMB +complex.

The kinetics of formation of electron donor–acceptor complexes. A temperature-jump study

“…It is generally considered that the forward and back reactions of the complex components are diffusion controlled (1,3) or nearly so (4)(5)(6). However, actual measurements of these rates have been few.…”