2417
C O M M U N I C A T I O N S TO THE EDITOR
The Thermodynamics of the System Pentafluorobenzene-Benzene
Sir: Much interest has recently been shown in the thermodynamic properties of binary systems of the type aromatic fluorocarbon-aromatic hydrocarbon. Hexafluorobenzene has been shown to form solid equimolar coniplexes with most aromatic hydrocarbons, and this is believed to be clue to charge-transfer interaction with the fluorocarbon acting as the electron acceptor. From the experimental excess volumes of mixing,a it appears that the extent of complexing increases as the electron-donating power of the hydrocarbon is increased. The heats of mixing of the two systems hexafluorobenzene-benzene and pentafluorobenzene-benzene have been measured recently by Fenby, blclure, and Scott,' who show that a reasonable interpretation of the results is that the specific interactions are much weaker in the system containing pentafluorobenzene. Our results presented here confirm that, in the solid state, this interpretation is correct. The experimental phase diagram for the system pentafluorobenzene-benzene is shown in Figure 1 and was obtained using an apparatus previously describedV2 The system is seen to form two weak compounds s2
~
~~
~~~
~
(1) C.R.Patrick and G . S. Prosser, hiature, 187,1021 (1960). (2) W. A. Duncan and F. L. Swinton, Trans. Faraday Soc., 62, 1082 (1966). (3) W. A. Duncan, J. P. Sheridan, and F. L. Swinton, ibid., 62, 1090 (1966). (4) D.V. Fenby, I. A. McLure, and R. L. Scott, J . Phys. Chem., 70,602 (1966).
DEPARTMENT OF PUREAND APPLIED CHEMISTRY OF STRATHCLYDE UNIVERSITY GLASGOW C.l., SCOTLAND
WILLIAM A. DUNCAN FINDLAY L. SWINTON
RECEIVED APRIL18, 1966
Isomeric Transition Induced Reactions of Iodine-130 in Cyclohexane Sir: Discovery of the I3OIrn (I.T.) nuclear reaction (TI,*9.2 min)' has made it possible to compare isomeric transition induced reactions with reactions effected by other nuclear transformations in the iodine system. Several processes were reported to produce the same effects in several liquid alkyl halide^.^^^ I n the cyclohexane-iodine system, where iodine is not contained in the target molecule, the (n, 2n) reaction was reported to cause higher organic yields than the (n, y) reaction due to its higher recoil energy.4 Since isomeric transition activated I3OI would be expected to have less kinetic energy than (n, ?)-activated lZsI,an isotope effect might be expected. None was found in the nhexane-iodine system' within f5%. Using the freeze-thaw technique developed in the study of *2Brm(I.T.) s2Br reaction^,^ we obtained the
10
0
-20
9 h-
-40
-60
with incongruent melting points, the mole ratios of fluorocarbon to hydrocarbon being 1 : l and 3:2. The occurrence of an intermolecular complex of a molar ratio 3:2 appears to be unique among binary systems of organic substances. Our results do not agree with those of Patrick and Prosser,' who found no evidence for compound formation in this system. I n contrast, hexafluorobenzene-benzene forms a strong 1 :1 complex melting some 20" higher than the melting point of either pure component.
t 0
Figure 1. T h e phase diagram of the system pen tafluorobeneene-benzene.
1.0
(1) D.D.Wilkey and J. E. Willard, J . Chem. Phys., 44,970 (1966). (2) R.H.Schuler, ibid., 22,2026 (1954). (3) G.Levey and J. E. Willard, J . Am. Chem. Soc., 74,6161 (1952). (4) R. H. Schuler and C . E. McCauley, J . Chem. Phys., 25, 1080 (1956). (5) J. A. Merrigan, W. K. Ellgren, and E. P. Rack, ibid., 44, 174 (1966).
Volume 70,Number 7 Julg 1966
COMMUNICATIONS TO THE EDITOR
2418
organic yields due to laOIrn (I.T.)lYI processes in liquid result from a similarity of environmental activation cyclohexane within i0.8%. Detailed experimental and/or decomposition. These similarities were also procedures and equipment used were described p r e found between laoI,lZ8I,and *OBr formed by (I.T.), viou~ly.~~~ (n, r),and (1.T.) processes, respectively, in n-hexane The organic yield of 1301 following neutron irradiation by Wilkey and Willard.' The Auger electron reaction hypothesis9 may explain these phenomena. The and decay of 1301rn in the solid state was 2.8% in the fact that radiative neutron capture induced yields of 7.8 X mole fraction 12sIs system. This low yield bromine were lower than (1.T.)-produced yields of was attributed to a phase separation between iodine 82Brwas found universal for the liquid C6 alkanes.l0 and the cyclohexane (clumping) upon freezing, similar This may indicate that processes additional to Auger to that found in Brz-containing systems.6J*8 When radiation induced reactions occur in iodine and (1.T.)an identical sample was melted 20 sec after a 30-sec activated 82Br, or possibly that internal conversion solid-state irradiation and 1301rn allowed to decay in of (n, 7)-activated bromine occurs before the recoil the liquid system, the yield was 21.701,. Allowing for energy has been dissipated; thus it would be carried the fraction of 1301 born by (I.T.) reactions (0.61),l away from the center of the pocket of fragments caused and the fraction of lmImwhich decayed while the by radiolytic effects. The freeze-thaw t e ~ h n i q u e , ~ sample was solid, the organic yield of laoIdue to (I.T.) which lends itself to a high degree of precision, is a was 1/(0.956 X 0.61) times the 1301yield observed after very valuable tool for studying isomeric transition the lmIm decayed out in the liquid, minus 1.9%, Le., 35.37& The 1.9% is a correction for the organic- induced reactions. ally combined 1301 as a result of solid-state reactions Acknowledgment. This is A.E.C. Document No. induced by (n, y) processes and the fraction of laOIm COO-1617-1. which decayed while the sample was solid. By resolidifying samples containing laoIrn, the high (6) J. A. Merrigan and E. P. Rack, J. Phys. Chem., 69,2795(1965). rate of increase in organic yield due to (I.T.) could be (7) R.M. A. Hahne and J. E. Willard, ibid., 68,2582(1964). halted by the clumping of 12911301m. Thus, growth in 80,5592(1958). (8) M. Milman, J. Am. Chem. SOC., (9) P.R.Geissler and J. E. Willard, J. Phys. Chem., 67, 1675 (1963). 1301 yield due to (I.T.) in the liquid state could be de(10) J. A. Merrigan, J. B. Nicholas, and E. P. Rack, Radiochim. scribed by the e q ~ a t i o n Rt , ~ - RO = R" - RO(l Acta, in press. e-"), where R" is the yield due to (I.T.) after all DEPARTMENT OF CHEMISTRY J. A. MERRIQAN lmIm had decayed, Rt is the yield due to (I.T.) at any UNIVERSITY OF NEBRASKA J. B. NICHOLAS time t, RO is the yield contribution of (n, y) and the R. M. LAMBRECHT LINCOLN, NEBRASKA 68508 part of the (I.T.) processes occurring in the solid state, N. J. PARKS and X is the decay constant for lmIrn.By analyzing E. P. RACK a plot of 1301 organic yield due to (1.T.) 8s. time after RECEIVED MAY6, 1966 irradiation, RS was done in the bromine system,6 a 1mIm half-life of 8.9 i 0.3 min was found. The values of organic yields of 1301 due to lwIrn (I.T.) laoIprocesses process and those of lzsI produced by the lZ7I(n, y) lZEI Anomalous Effect of Pressure on the Protolytic are compared with those due to 82Brm(I.T.) Be2 and Dissociation of Excited States of Nitrophenols 7'3Br(n, y) @Brmin Table I. Table I: Organic Yields of (1.T.)-Produced 1s0I and 82Br and (n, r)-Produced lz*I and NBrm in Cyclohexane at Room Temperature Mole faction of 1 2 7 . 8 x 10-3 5 . 2 x 10-3
lroI
1281
82Br
35.3 39.6
36.1 40.5
35.3 39.2
*Brm 22.5 25.4
The similarity between la01 and lzSIyields may indicate a similarity of reaction mechanisms independent of the init,ial kinetic energy of the activated atom. Their likeness to 82Bryields would suggest that activated halogens may trace chemical processes which The Journal of Physical Chsmistry
Sir: I n general, the ionic dissociation of a weak electrolyte in water involves a substantial contraction, and as a corollary the process is favored by an increase of hydrostatic pressure. The contraction arises because the free ions exert a powerful electrostatic attraction on the surrounding solvent and compress it to a greater density than normal. The phenomenon and its cause have been known for many years and were the subject of a review by the writer' in 1963. At that time, some 30 weak acids and bases had been examined under (1) S. D. Hamann in "High Pressure Physics and Chemistry," R. S. Bradley, Ed., Academic Press Inc., New York, N. Y., 1963,Vol. 2,pp
146-156.
