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STUDIES OF FLUOROCARBOX-HYDROCARBON MIXTURES
Nuclear Spin-Lattice Relaxation and Chemical Shift Studies of Fluorocarbon-Hydrocarbon Mixtures by Charles L. Watkins and Wallace S. Brey, Jr. Department of Chemistry, University of Florida, Gainesville, Florida (Received M a y 10,1980)
896’01
The concentration dependence of the 19F spin-lattice relaxation times and the proton chemical shifts has been studied for four hexafluorobenzene-hydrocarbon mixtures. These physical measurements provide no indication of the existence of specific fluorocarbon-hydrocarbon interactions in the hexafluorobenzene-cyclohexane and hexafluorobenzene-benzene systems. The results confirm the existence of a weak interaction in the hexafluorobenzene-mesitylene system. Complex formation is detected in the hexafluorobensene-dimethylformamide system. The experimental results do not offer any evidence for charge-transfer interactions. A discontinuity is observed in the viscosity-corrected TI plot as a function of fluorocarbon concentration in the 2,2,2-trifluoroethanol-ethanol system indicating strong mixed hydrogen bonding involving the hydroxyl groups.
Introduction The properties of perfluorocarbon-hydrocarbon mixtures have generated a considerable amount of interest in the past few years.’ Various thermodynamic and spectroscopic studies have been conducted, chiefly to explain the unusual solubility relations encountered. The chief approach to the analysis of the thermodynamic properties of these systems has been the solubility parameter theory2first proposed by Scatchard and Hildebrand. Experimental data accumulated after the first attempt by Scott3 to apply the regular solution theory to fluorocarbon-hydrocarbon mixtures were in disagreement with that Various attempts have been made to explain this unusual solution behavior. One proposal was that the hydrocarbon molecules interpenetrated in a special way,6s12and Hildebrand suggested that the solubility parameters for the hydrocarbons should be taken from the solubility behavior rather than from the heats of vap~rization.’~Attempts have been made to apply the “corresponding states” treatment of Scott,14 but the geometric mean law fails to predict the correct magnitude of unlike interactions. Reid16 proposed a correction based on the estimated ionization potentials of the solution constituents, but the most nearly successful treatment seems to be that of Munn16 who used an equation involving directly the interaction energies as predicted from molecula- polarizabilities of the components. Recent studies of hexafluorobenzene with various hydrocarbon solvents indicate the possibility of chargetransfer complex formation in aromatic perfluorocarbon-aromatic hydrocarbon systems.‘7-’9 Most of the systems form a 1:1 solid complex. X-Ray and wideline nuclear magnetic resonance studies indicate that the solid complexes exist in alternate stacked planes.20j21
This stacking arrangement is also thought to prevail to some extent in the liquid state. Strong n-electron donors give rise to a new absorption band in the ultraviolet spectrum when mixed with hexafluorobenzene. However, no absorption attributed to charge-transfer complex formation has been reported for n-electron donor^.^^-^^ A slight shift in the absorption tail of the n-electron donor was noted by Hammond for some (1) R. L. Scott, J . Phys. Chem., 62, 136 (1958). (2) J. H. Hildebrand and R. L. Scott, “Solubility of Nonelectrolytes,” 3rd ed, Reinhold Publishing Corp., New York, N. Y., 1950. (3) R. L. Scott, J . Amer. Chem. Soc., 70,4090 (1948). (4) J. H. Simons and J. W. Mausteller, J. Chem. Phys., 20, 1516 (1952). (5) J. H. Sirnons and R. D. Dunlap, ibid., 18,335 (1950). (6) J. H. Hildebrand, B. B. Fisher, and H. A. Benesi, J . Amer. Chem. Soc., 72, 4348 (1950). (7) G. J. Rotariu, R. J. Hanrahan, and R. E. Fruin, ibid., 76, 3752 (1954). (8) N. Thorp and R. L. Scott, J . Phys. Chem., 60, 670 (1956). (9) N. Thorp and R. L. Scott, ibid., 60,1441 (1956). (10) I. M. Croll and R. L. Scott, ibid., 62, 954 (1958). (11) I. M. Croll and R. L. Scott, ibid., 68,3853 (1964). (12) R. D. Dunlap, J . Chem. Phys., 21, 1293 (1953). (13) J. H . Hildebrand, ibid., 18, 1337 (1950). (14) R. L. Scott, ibid., 25, 193 (1956). (15) T. M.Reid, 111,J . Phys. Chem., 59,425 (1955). (16) R. J. Munn, Trans. Faraday Soc,, 57, 187 (1961). (17) D. V. Fenby, I. A. McLure, and R. L. Scott, J . Phys. Chem., 70, 602 (1966). (18) G. S. Prosser and C. P. Patrick, ‘Vaature, 187, 1021 (1960). (19) W. A. Duncan and F. L. Swinton, Trans. Faraday SOC.,62, 1082 (1966). (20) J. C. A. Boeyens and F. H. Herbstein, J . Phys. Chem., 69, 2153 (1965). (21) D. F. R. Gilson and C. A. McDowell, Can. J . Chem., 44, 945 (1966). (22) R. Foster and C. A. Fyfe, Chem. Commun., 642 (1965). (23) P. R. Hzbmmond, J. Chem. SOC.,A , 145 (1968). (24) T. G. Beaumont and K. M. C. Davis, ibid., B , 1131 (1967). Volume 7.4, hTumber 9 January 89, 1970
CHARLES L. WATKINSAND WALLACE S. BREY,JR.
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hexafluorobenzene-aromatic hydrocarbon systems.23 However, the donor absorption prevents any observation of a charge-transfer maximum. The lack of observation of a charge-transfer absorption may be attributed to the small overlap of the a-donor orbitals with the weak acceptor orbitals of hexafluorobenzene. Preliminary nuclear magnetic resonance investigations of the hexafluorobenzene-hexamethylbenzene-carbon tetrachloride system showed a concentration-independent fluorine chemical shift.22 Phase diagrams, volume changes on mixing, and dipole moments have been reported by Swintonl9t26 for six hexafluorobenzene-hydrocarbon systems. He concludes that the existence of a compound in the solid state does not necessarily imply any specific interaction in the liquid state. Scottz6 divides any interactions which might occur in a system composed of an aromatic fluorocarbon and a hydrocarbon into three categories : physical interactions such as dispersion forces, specific chemical interactions such as charge transfer, and specific interactions between aromatic rings such as quadrupolar-quadrupolar and bond dipolar interactions. A fluorocarbon-hydrocarbon system in which there is a known specific chemical interaction is the 2,2,2trifluoroethanol-ethanol system. Ethanol and 2,2,2trifluoroethanol (TFE) resemble each other in molecular structure, boiling point, and dielectric constant, but differ considerably in acidity, basicity, and solvating ability.27 Since the two liquids are miscible in all proportions, the TFE-ethanol system is of interest in studying hydrogen bonding and relative solvating ability between the two components and in comparing with the hexafluorobenzene-hydrocarbon systems. The above hydrocarbon-fluorocarbon mixtures have been examined by measurement of the concentration dependence of the l9F spin-lattice relaxation time and of the proton chemical shift to determine what types of solute-solvent interactions are occurring. The Bloembergen, Purcell, and Pound theory2* gives the dependence of TI on the temperature and viscosity for liquids. l/Tl, the inverse of the spin-lattice time, is proportional t o the ratio of the viscosity to the absolute temperature, ? / T , provided the correlation time describing the motion is much less than the Larmor frequency. For solutions in which molecular association occurs, the BPP bheory does not hold since the correlation time for the motion considered is usually much longer than for unassociated liquids, and T I usually possesses a minimum. Giulotto29 and Murthy30 have both observed minima in TI where strong association occurs. Brownstein31 has studied by spin-lattice relaxation various weakly interacting systems which do not show a chemical shift change with change in concentration. Experimental Section The
l9F
spin-lattice relaxation times were measured
The Journal of Physical Chemistry
by the method of adiabatic fast passage at 56.4 MHz. The linear sweep unit of a Varian DP-60 spectrometer was replaced by a Wavetek function generator which provided a triangular sweep which was fed directly to the sweep coils. The sweep was displayed on a Hewlett-Packard Model 120B oscilloscope. The field position and period were adjusted until there was a null point on the return trace. At least five independent measurements were made on each solution. For most of the measurements, the average deviation was 0.100.15 sec. The temperature was regulated by the flow rate of dry nitrogen through a Varian V4340 variable temperature probe assembly. The temperature was determined by a copper-constantan thermocouple placed within the Dewar insert, accurate to 1". All of the T1 measurements were taken a t 30" except for the hexafluorobenzene-mesitylene mixtures which were taken a t 40°1 The removal of molecular oxygen is very important since paramagnetic species can provide a dominant relaxation m e c h a n i ~ m . ~ *The ' ~ ~ pump-freeze-thaw method was used to remove any oxygen present. The cycle was repeated at least ten times for each sample. The relative viscosity measurements were made using an Ostwald viscometer; 4 ml of solution were used in each case. Each measurement was repeated five times, using a water bath in which the temperature could be regulated to 0.1". Results of the viscosity measurements are given in Table I. Table I: Relative Viscosities of Mixtures at 30'
Xfa
0.90 0.80 0.70 0.60 0.50 0.40 0.30
0.20 0.10
CaFsCsHiz
CeFeCsHe
COFE- CEFE- CFaCHz- CFaCHzCeHa- HCONOHOH(CHs)ab (CI
