764
NOTES
system, and EL, the molar energy of the liquid adsorbate, have not been defined previously. The quantity ( ~ I ( s L ) - h ~ has ) thermodynamic sigriificance only if hI(sfL)= hL. 5. Heats of Immersion of High-Temperature Activated Solids.--Almost never are solids evacuated at elevated temperatures (300 to 500") prior to immersion on conventional apparatus protected from the influence of trace amounts of organic contaminants in the system. This statement applies to all references of this paper except 1 (in part), 3, 5 , and 7 although it is possible that in some (limited) instances precautions were taken but not reported. Heat values (ergs/cm.2) measured for the immersion of rutile evacuated at 450' to an ultimate vacuum of 10" mm. on a B.E.T.-type adsorption rig were much too erratic to be acceptable.3 The heat of immersion of rutile outgassed in this manner in water was found to be 708 i 20 ergs/ cm.2. Very divergent values were rejected on a statistical basis. If a liquid nitrogen trap separated the sample from the adsorption system during evacuation and further if dry oxygen was introduced in the sample system at the evacuation temperature, the heat of immersion of this rutile sample decreased to 588 i 9 ergs/cm.2. Reduction of the rutile surface is indicated; organics are present on the solid surface and (or) on the rig. Similar divergent heat values were found recently in this Laboratory for the immersion of protected and unprotected samples of alumina activated at 500'. In this instance, the use of the nitrogen trap alone was sufficient to prevent surface contamination; unlike the rutile sample its surface originally was free of organic contaminants. Footnote 10 of a recent publication which reported the heats of immersion, h I ( S I , ) , of a variety of rutile and anatase samples in water as a functiop of temperature of activation* reveals a belated recognition of this unsuspected, additional variable. Most articles, reporting immersion values for temperature-activated solids, suffer from this defect. The assumption made too frequently that polar solids such as anatase,Zd rutile,2aJh calcium or barium sulfate,2o and many others activated at temperatures near 500' only physically adsorb molecules of the wetting liquid can not be accepted without further detailed studies.a There is danger also in comparing ( h ~ ( s L )- ~ I ( L ) ) values with spreading pressures, p, correctly calculated to e's at the arbitrarily selected relative equilibrium pressure, both quantities being expressed and compared in units of ergs/cm.2.9 Free energies, enthalpies, and, particularly, entropies expressed in units of ergs/cm.2 or ergs/ cm.2/0C are much more difficult to interpret than those expressed in conventional units. A further need for converting cp and hnet values into conventional units is seen from inspection of eq. 2. Spreading pressure values are accurately comparable only to net integral heats when corrected by t'he RT In PIPo term; this was not done.g In addition, com(8) T. H. Wade and N. Hackerman, J . Phgs. Chem.. 66, 1861 (1961). (9) R. L. Every, W. H. Wade, and N. Hackermen, ibid., 66, 25 (1961).
VoI. 66
parison of integral heat, free energy, and entropy values at a relative pressure so high that multilayers form suffers because of the insensitivity of such values to surface-adsorbate effects so pronounced in the region zero to monolayer coverage. Capillary effects due to interparticle or particle pores and as a consequence the decrease in area of the solid sample offer further possible complications. Acknowledgment.-The author is grateful to the U. S. Air Force, Wright Air Development Division, Materials Center, Wright-Patterson Air Force Base, Ohio and the National Printing Ink Research Institute, Lehigh University, Bethlehem, Pennsylvania, for support in the development of this paper. FLUORINE N.M.R. SPECTROSCOPY. VI. FLUOROCARBON SULFIDES' BYGEORGEVAN DYKETIERS Contribution No. 214 from the Central Research Dept., Minnesota Mining & M f g . Co., St. Paul 19, Mznn. Received October 21, 1981
While nuclear magnetic resonance (n.m.r.) data have been reported for several fluorocarbon derivatives of sulfur hexafluoride12~3 and for certain unusual fluorocarbon s~lfides,*~6 such data for simple fluorocarbon mono- or polysulfides have been lacking. The n.m.r. measurements here presented fill this gap; and owing to their high precision several significant generalizations concerning fluorine n.m.r. spectroscopy also emerge from this work. Experimental The syntheses and physical properties of the fluorocarbon sulfides have been reported,e-B save for the ultraviolet spectrum and certain constants of 1,4-dithiaperfluorocy~10hexane. This compound, b.p. 81.5", f.p. -6.5", was studied a t a concentration of 1 pl./ml. in isooctane, and had X., = 2232 A. and ernax. = 132. From its refractive index, 12% 1.3585, (lit. 1.35688) and density, d26 1.693, there is calculated M R = 34.28, from which the atomic refractivity 7.40 is obtained for sulfur, in excellent agreement with the value previously given for fluorinated thmethers.? The n.m.r. equipment and techniques have been described.s In Table I are presented numerical shielding values, on.the +scale,g for all fluorine positions in each compound studied. Procedures for the approximate conversion of older and less precise numerical dataa-6 into +values have been given.s
Discussion ShieIding values for the -CF2S- group are seen to fall generally in the region 80-95 +, about midway between +-values for the isoelectronic P and C1 compounds.l0 Much narrower limits are appropriate when similar structures are compared. (I) Part I11 of the latter series; presented a t the 138th National A.C.S. Meeting, New York, 1960. (2) N. Muller, P.C. Lauterbur and G. F, Svatos. J . A m . Chem. 8 0 C . , 79, 1043 (1957). (3) R. P. Dresdner and J. A. Young, ibid.. 81,574 (1959). (4) M. Hauptsohein and M. Braid, %bid., 80,853 (1958). ( 5 ) C. G.Krespan, ibid., 83, 3434 (1961). (6) G.V. D. Tiers, J . Org. Chem., 26, 3515 (1961). (7) G.V. D. Tiers, ibid., 26, 2538 (1961). (8) C. G. Krespan, U.S. Pat. 2,931,803(April 5, 1960). (9) G.Filipovich and G. V. D. Tiers, J. Phys. Chem., 63,761 (1959). (10) E. Pitcher, A. D. Buokingham and F. G. A. Stone, J. Chem. Phys., 3 6 , 124 (1962).
NOTES
April, 1962
765
133.25 411 and for perfluorocyclobutane, 134.94 4, TABLE I N.M.R.SHIELDING VALUES FOR FLUOROCARBON SULFIDES as compared with the typical mid-chain value, +*-Valuesb and multiplicitiesc for
Compoynd (n-C:Fv)zS
C0nc.a 10.0
(cF~---cF~---(cF~)s--CF~-CF~)ZS~
...
m(sV
126.5
122.5
119.7
...
...
..
...
... ...
89.79 b; 26 87.04
Ld
(n-C9r)zSz
12.7
(n-CsF?)zSr
10.0
(n-C?Fls)&
16.0
80.88 t* 80.80 tf 81.54 to
(CFe)rS l,&(CFe)tSz
6.0
20.0
... .. ,
... ... ...
80.58
,
124.22 b ; 8.2 124.21 b ; 4.5 124.22 b; 5 3
131.91 th
.. .
83.81 b ; 17 90.80 91.82
d th
91.08’
bi
Volume per cent. in CChF (wt./vol. for perfluoroheptyl disulfide). b See ref. 9; standard deviation less than dzO.01 for all values given to two decimals, and f0.15 for others. Symbols for multiplicity: s, singlet; t, triplet; q, quadruplet; m, multiplet resolved but not analyzed; b, brori;d and unresolved peak, width a t half-height, WI/Z, being given in c./s. d J(CF8-GCF2) = 9.5 f 0.2 c./s. “ J (CFa-C-CFz) =: 9.2 i 0.1 c./s. f J ( C F r G C F 2 ) = 9.10 i 0.05 c./s. 0 J(CF3-C-CF2) = 9.40 It: 0.05 c./s.; each 2.5 c./s. component is rtn indistinct triplet having J J = 4.4 f 0.1 c./s.; each component appears to be a triplet having J = 2.9 i 0.5 c./s. i Line width is temp. dependent. At 0% concn., 4 = 91.05. a
-
For example, the range 89-91 d, includes the nperfluoroalkyl disulfides having three or more carbons in the chain. The presence of an extra sulfur atom, in the trisulfide, causes only a small increase in shielding; but when both perfluoroalkyl groups are attached to the same sulfur atom the -CF2S- peak is displaced downfield by about 7 p.p.m. One might be tempted to attribute this shift to electron withdrawal from sulfur by the perfluoroalkyl group, an effect already apparent in the shift to short wave lengths of the ultraviolet absorption maximum.6.7 The inadequacy of this explanation is evident, however, in the pronounced upjield displacements, about 10 p.p.m., resulting from fluorination of sulfur to the hexavalent state2V3; a similar shift is seen in n-CsF1$302F, for which the a-CFZ shielding value is 108.56 +* (10 vol. % concn.). These oxidized sulfur atoms must be considered as much more strongly electron-withdrawing than those of the sulfides. The very large downfield shifts observed for perfluoroalkyl halides and similar compounds have been attributed to “mixing” of low-lying excited states with the ground state in the magnetic field.1° While this treatment accounts for the gross position of the -CRS- resonance, it appears to predict the reverse of the observed correlation of ultraviolet maxima with +-values. For the proposed explanationlo to be correct one must assume that the particular “low-lying excited states” responsible for the observed ultraviolet maxima have the wrong symmetry properties and so fail to affect the n.m.r. spectra. The effect of a single ring closure upon CF2 groups is, generally, to shift their n.m.r. peaks as much as 10 p.p.m. to higher values. Such a displacement is seen here in the comparison of the +values for the C-CFz-C groupings of bis-(perfluoropropyl) sulfide, or of bis-(perfluoroheptyl) disulfide, with that of thiaperfluorocyclopentane. The high shieldings for perfluorocyclohexane,
122.5 cp, found for long n-perfluoroalkyl groups,12 illustrate this effect. The smaller effect of sulfur upon more distant fluorine atoms (p and y ) can be recognized most readily by comparison of +values with those for a related fluorocarbon group. Thus, in bis-(perfluoroheptyl) disulfide the +-value to sulfur is 119.7, or 6.9 p.p.m. lower than the usual value, 126.64 d,*, found for CF2 groups adjacent to CF312; the bis-(perfluoropropyl) sulfides show a remarkably constant shift of - 6.99 p.p.m. relative to the CF2 in CaFs, 131.21 +. It would appear that only the nearest sulfur atom causes the shift. At %he y position the shift is only -2.0 p.p.m. (relative to C3Fg)for the CF, group in the perfluoropropyl sulfides, and apparently somewhat less in the long-chain disulfide. From the numerical values cited for the ,3- and y-positions in perfluoropropyl chloride, 125.2 cp* and 80.7 $*,lo it is apparent that very similar p (and 7)shifts are caused by the isoelectronic chlorine atom; such might be attributed either to the similar size or to the similar polarizability of the S and C1 atoms. It now is well known that the coupling constant J(CF3-CF2) is very small within perfluoroethyl groups,13 while in longer chains the coupling of CF3to the second CF2group, Le., J(CF3-C-CF2), is ca. 8 to 12 C . / S . ~ ~ - ’ ~ J ~In the present work the latter coupling constant is 9.1 to 9.5 c./s., and, in the long-chain disulfide, coupling to the third CF2 group is about 2.5 c./s. It is evident that some further coupling occurs across one and two, but not three, sulfur atoms, for the expected relatively simple CF2S quadruplet, slightly broadened due to splitting by the adjacent CF2 group, is seen only in the latter case. Similar coupling across a nitrogen atom has been reported.16 In the cyclic sulfide (CE’&S, couplings between adjacent and between separated CF2 groups are seen. The coupling constants are about 4.4 and 2.9 c./s., (exact analysis would be exceedingly laborious), but owing to the symmetry of the compound it cannot be determined which is which. The single broadened line found for 1,4-dithiaperfluorocyclohexane has a temperature-dependent width which undoubtedly results from a slow chairchair interconversion resembling that of perfluorocyclohexane.11 Further studies in this area will be reported elsewhere. Acknowledgment.-I thank Donald R. Hotchkiss for careful maintenance and operation of the i1.m.r. spectrometer, and Jane E. H. Tiers for measurement of the spectra.
(11) G. V. D. Tiers, Proc. Chem. Soc., 389 (1960). (12) G.V. D.Tiers, to be submitted for publication. (13) C . A. Reilly, J. Chem. Phus., 25, 604 (1956). (14) N. Muller, P. C. Lauterbur and G. F. Svatos, J . Am. Chem. Soc., 79, 1807 (1957). (15) G. V. D. Tiers and J. M. MoCrea, paper presented a t the N.M.R. Symposium, 132nd National Meeting of the American Chemical Sooiety, New York, 1957. (16) A. Saika and H. S. Gutowsky, J. Am. Chem. Soc., 78, 4818 (1 956).
