Maleic anhydride–hexamethylbenzene mixtures in methylcyclohexane solution and in the solid state. Part I. Physical properties
Maleic anhydride and hexamethylbenzene in a solution in methylcyclohexane from a charge-transfer complex with the absorption maximum at 340 mµ. Spectral data appear to indicate the absence of stable complexes of other than the 1 : 1 stoicheiometry. Formation constants of the complex at 25, 40, 50, and 60°, and its bonding enthalpy were calculated (Kc= 1·5 ± 0·2M–1 at 25°; ΔH=–4·0 kcal.mole–1). From the latter value the contribution of the dative structure was obtained. It amounts to 2·3% in the ground state, and in the excited state to about 90%, depending on the assumption made as to the value of the overlap integral. From the integrated absorption intensity, the difference between the dipole moments of the dative and the no-bond structures was calculated and compared with the value obtained by addition of dipole moments expected to characterise the model structure of the complex.
Attempts to prepare a crystalline charge-transfer complex were unsuccessful. The phase diagram of maleic anhydride and hexamethylbenzene indicated no evidence for the presence of stable complexes in the solid state.