2528
ELFREDA T. CHANG AND EDGAR F. WESTRUM, JR.
cation radicals of naphthalene and anthracene,6bthere appears to be no major distortion or redistribution of 7r electrons occurring on dimerization, with the possibility that the two hydrocarbon fragments lie in parallel planes. The dianion radical of I can assume two probable conformations in which the vinylidene parts of the molecules are in parallel planes. The one conformation in which the entire 7r electron system is overlapped is not the only conformer, since all of the coupling constants of the dimer anion radical are not approximately one-half those of the corresponding coupling constants of the monomer anion radical. I n contrast to the behavior of the other monomers, the systems 11-DRTE-RI (RI = Li, Na, K) do not dimerize or polymerize, but undergo preferential disproportionation (eq 6). The stability of XI. - and/ or its steric hindrance towards dimerization would explain these results. Both the change in spin con-
centration and the hfs line width upon addition of metal t o I1 indicate the hfs lines of 11.- are broadened by RI and AI2-. I n summary, the reduction of substituted benzylidene malononitrile monomers can be represented by eq 1 to 6, with the relative importance and direction of these reactions varying as a function of metal, solvent, and particular malonitrile derivative.
Acknowledgment. Gerald Stevenson is grateful for a NASA fellowship. We are indebted to Professor AI. Szwarc and Dr. R. A. Rembaum for helpful discussion. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We acknowledge partial support of this work by the Research Council of Texas A and 11University. The esr spectrometer was made available by the National Science Foundation Grant GP-3767.
Heat Capacities and Thermodynamic Properties of Globular Molecules.
XV.
The Binary System Tetramethylmethane-Tetrachloromethane' by Elfreda T. Chang and Edgar F. Westrum, Jr.2 Department of Chemistry, University of ikfichigan, Ann Arbor, Michigan
(Received December 16, 1969)
Heat capacities and derived thermal properties on five compositions of the system tetramethylmethanetetrachloromethane (mole fraction CC14 = 0.200, 0.334, 0.501, 0.666, and 0.826) were determined by adiabatic calorimetry from 5 to 300'K. The temperature, enthalpy, and entropy increments of each of the transitions, including fusion, in each composition were determined. The results indicated the effects on the reorientationalrotation transitions of the pure components due to the change of molecular environment and provided a tentative partial phase diagram for this system below room temperature.
Introduction The study of mixtures provides insight into the physical forces acting between molecules of different species, as well as phenomenological information concerning various types of phase equilibria resulting from variation of the proportions of the components. Moreover, examination of heat capacity, dielectric constant, and nuclear magnetic resonance of mixed crystals yields valuable information about the nature of the transitions occurring in their pure components. Since one of the objectives of this investigation was to experimentally test some theories of solution thermodynamics in the solid phase, a binary system satisfying certain criteria was sought. It was (1) to be a system The Journal of Physical Chemistry, Vol. Y+$,N o . 12, 19YO
for which impeccable mixing enthalpies were available for the liquid phase, (2) to have molecules of sufficient similarity in shape and size to permit the existence of a continuous range of solid solutions, yet have sufficient difference between the molecules to occasion deviation from ideal behavior, and (3) be a system forming a plastically crystalline phase so that thermal equilibration might be rapid enough to make measurements practicable. (1) Based upon a dissertation submitted to the Horace H. Raclcham School of Graduate Studies of the University of Michigan by E.T.C. in partial fulfillment of the requirements for the degree of Doctor of Philosophy. This investigation was supported in part by the Division of Research of the U. S. Atomic Energy Commission. (2) To whom correspondence concerning this work should be submitted.
THEBINARY SYSTEM TETRAMETHYLMETHANE-TETRACHLOROMETHANE The system tetramethylmethane-tetrachloromethane [C(CH,)&C14] seemed to most satisfactorily meet the chosen criteria. Rloreover, the nearly isostructural, isotropic lattices of the components in the region just below the melting point made it seem probable that these two substances would form a continuous series of solid solutions. Since both components form plastic c r y s t a l ~ their , ~ molecules have the ability to reorient themselves in the solid state and hence assume apparent spherical symmetry so that the intermolecular forces may be considered central. I n addition, heat capacities of the pure tetrameth~lmethane~ and tetrachloromethane5 have been reported. RIoreover, enthalpy of mixing data in the liquid phase6 are also available for this system. I n order to ascertain the temperature dependence of the enthalpy of mixing and to acquire data on the transitions and thermodynamics of globular molecules, the heat capacities from 5 to 3OO0I
