NOTES varies from substance to substance, still affording support to hypothesis that plastic sulfolane can dissolve solutes. Since an ideal solution is defined as one in which the activities of the components are equal to their mole fractions, their freezing points can be calculated by replacing the activity of the solvent by its mole fraction in Lewis and Randall’s equation.” In the case of solutions in sulfolane, thermal data on molal heat of solidification and on heat capacities of the solid and liquid phases are lacking. Anequation8 = 65m - 17.9m2,valid in .the range 0-0.2 m, can be obtained if we put (8/m)m=o= 65 and we disregard the small difference between the heat capacities of liquid and plastic sulfolane. As can be seen from Figure 3, the experimental points, in the case of solutions of diphenglmethane and naphthalene in sulfolane, lie close to the dotted line calculated by this equation; n-heptane and benzoic acid are indeed some examples of nonideal solutes, as might be expected. The initial freezing points of solutions, having
1071 molality higher than the transition value, may be represented well by the expression 17.07 - 7.57m (dotted line in Figure 2), where 17.07 is the virtual fusion temperature of the crystalline nonrotational phase I1 and 7.57 may be reasonably indicated as the cryoscopic constant of sulfolane in equilibrium with the phase 11. Starting from these values, an entropy change associated with the transition phase I1 + liquid can be calculated as 9.1 eu, whereas the corresponding value for the fusion of phase I does not exceed 1.1eu. Therefore, the entropy that characterizes the transition I1 -t I amounts to 8 eu. Both transition and fusion entropies are in good agreement with the figures referred to by Staveley12 and Guthrie and McCullough4 for some substances forming plastic crystals. (11) S. N. Lewis and M. Randall, “Thermodynamics and Free Energy of Chemical Substances,” MoGraw-Hill Book Co., Ino., New York, N. Y., 1923, p 283, eq 17. (12) L . A . K. Staveley, Ann. Rev. Phys. Chem., 13, 351 (1962).
NOTES
On the Reaction of D2+ with Cyclohexane by Jean H. Futrell,la Fred P. Abramson,lb and Thomas 0. Tiernan Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 46433 (Received September 22, 1967)
In a recent communication, Wolfgang and Menzinger2 presented a preliminary account of an ion-impact experiment in which cyclohexane depositied on a surface was bombarded with Tz+and T + ions as a function of ion kinetic energy in order to evaluate the threshold energy and energy dependence of a typical hot-atom reaction. The initial stage of the reaction could involve charge exchange and/or various ion-atom interchange processes--?‘+ transfer, H atom abstraction, and T stripping. To aid in the interpretation of this initial stage of the reaction, we have performed the related gas-phase experiment of impacting D2+on cyclohexane a t various ion energies and examining the ionic products of this reaction. The experiments which we report here were carried out using the ARL tandem mass spectrometer described in detail el~ewhere.~In the present experiments, Dz+
ions were formed by electron impact on D2 in the ion source of the first-stage mass spectrometer, accelerated, both energy and mass selected, decelerated to terminal energy, and impacted on cyclohexane vapor in a differentially pumped collision chamber. The energy spread in the ion beam, determined by the defining slit of the electrostatic analyzer, was 0.5 eV in these experiments. The lowest energy investigated approached thermal velocities (ion energy 0-0.5 eV) and is reported in Table I. Ions monochromatic in energy to this degree were then impacted on cyclohexane a t various energies up to 20-eV nominal ion kinetic energy. Cyclohexane pressure in the collision chamber was maintained at 0.005 torr, and the chamber temperature was ca. 230’ ; under these conditions secondary reactions of the ions initially produced are negligible. The distribution of cyclohexane ions from the reaction of 04.5-eV Dz+ ions is reported in Table I. At this energy no signal was detected a t m / e 2 (D+) or a t (1) (a) Department of Chemistry, University of Utah, Salt Lake City, Utah 84112. (b) Consolidated Electrodynamics Corp., Monrovia, Calif. 91017. (2) R. Wolfgang and M. Menzinger, J. Am. Chem. SOC.,89, 5992 (1967). (3) J. H. Futrell and C. D. Miller, Rev. Sci. Instr., 37, 1521 (1966).
Volume 79, Number 3 March 1968
NOTES
1072 Table I: Products from the Reaction of Low-Energy DQ+Ions with Cyclohexane Relative intensity,
dissociation and reduce the reaction to simple charge exchange. Evidence for stabilization in the gas phase of excited cyclohexane ions produced by electron impact has been presented elsewhere.6
%
27 28 41 42 43 54 55 56 57 69 83 84
0.4 0.8 12.5 8.1 4.4 0.9 19.4 30.8 1.6 3.5 4.5 13.1
m/e 5 (D2H+). At 2 eV, a D+ signal is barely discernible above the noise level and appears to increase slightly with increasing energy. This collision-induced dissociation (stripping) reaction is
