NOTES
4020 ing, so it is conceivable that the reaction corresponds to the neutralization of a cation radical from IBVE and that the two hyperfine components arise from the latter species. However, since there is little information available on the hyperfine structure of authentic cation radicals derived from simple vinyl monomers, this assignment of the sharp doublet must be regarded as tentative. The radical present after photobleaching is almost surely of the isobutyl type, so it would appear that a hydrogen atom is eliminated from a -CH3 group in the processes occurring during y irradiation. The formation of a “propagating” free radical on warming the sample is somewhat unexpected in this system because IBVE does not ordinarily polymerize by a free radical mechanism. The most likely reaction which can be p ~ s t u l a t e d ’is~ the addition of the cation radical to the monomer according to eq 1.
(IBVE)+*
+ IBVE
--+
H
H
I
I I
+CCH2CH2C*
I OC4Hg
OCdHg
I
*
I
I
CHI
or
CHa
I
CH2=CH-O-CH-CH
I1
1
I CHa
Radical I would be expected to give more than three lines in the esr spectrum whereas I1 should give rise to a well-separated double doublet if the P-hydrogen makes a small dihedral angle with the axis of the p orbital containing the unpaired electron, as is usually the case for radicals of this general s t r u c t ~ r e . ~ It is likely that the formation of methyl radicals by uv photolysis of y-irradiated IBVE is caused by the intramolecular dissociation of a free radical, as in reaction 2, since methyl radicals were not produced *
CH2
\CH-CH~-O-CH=CHZ / CHa CH2=CH-CHz-O-CH=CHz The Journal of Physical Chemietry
Av
--t
+ CHa.
experimental facilities in their laboratories, and to Professor K. Hayashi and Dr. H. Yoshida for helpful discussions.
Temperature-Dependence Nuclear Magnetic Resonance Studies of a Very Strongly Coupled Spin System.
Ethylene Sulfite
by Harold Finegold
(1)
CHa
I
Acknozuledgment. Our thanks are due to Professor I. Sakurada and Professor S. Okamura for the use of
United States Department of Agriculture, Agricultural Research Service, Human Nutrition Research Division, Beltsville, Maryland 20706 (Received April 86, 1969)
The possibility of proton transfer processes from (IBVE)+’ to IBVE seems to be excluded in view of the failure to observe the anticipated free radical products
CH2=CH-O-CH2-C
from unirradiated IBVE under the same conditions of photolysis.
(2)
Studies of the temperature dependence of nuclear resonance parameters have heretofore emphasized those systems whose spins are weakly or moderately coupled, since instruments and accessories lacked sufficient stability to afford the high order of reliability demanded for the determination of multiple slight changes in the complex line patterns of strongly coupled systems. We now report sufficiently accurate temperature dependence measurements of all 24 lines in a very strongly coupled spin system, the AA’BB‘ spectrum of the cyclic glycol thioester, ethylene sulfite, to permit reliable calculations and definitive assessment of trends in the temperature dependence of all the spin coupling constants and the chemical shift difference. Comparison of these results with those recently reported for medium and dielectric constant dependence of the parameters in solution systems of this molecule’ permit further structure elucidation in terms of possible conformational mobility. Measurements were made from -70 to +200° using an externally locked Varian 60-1VIHz spectrometer with a variable-temperature accessory system especially modified to obtain both the long temperature time constant and the high resolution required at the very slow sweep rates necessarily employed for these experiments. Adequate long-term stability could not, however, be maintained at the lower temperatures even though there was sufficient short-term stability to give sharp high-resolution spectra and moderately precise, though probably inaccurate, data. Calculations from the spectral measurements at each temperature were made as described previously. Figure 1 illustrates the positive sign in the curve direction obtained for the temperature dependence of the chemical shift difference (1) H. Finegold, J.Phys. Chem., 72,3244 (1968).
402 1
NOTES
I
6.70 -
6.60 -
I“
: ; 6.506.40
-
6.30 -
I 5
20
60
100
140
I
180
Temp, “C.
Figure 2. The temperature dependence of all spin-coupling constants of OCHzCHzOSO at 60 MHz.
10
50
90 Temp,
170
130 OC.
Figure 1. - The temperature dependence of the proton chemical shift difference of OCH&HzOSO a t GO MHz. L-I
between the proton pairs on each side of the ring, and the solid curve in Figure 2 also shows a positive variation of the geminal spin coupling temperature dependence. The standard error (rms) of the spectral parameters from these curves is 10.03 Hz. Both can be compared with the similarly positive dependence on the reciprocal dielectric constant of these same parameters as determined from a study of the medium effects,l and both can thus be said to correspond in the sign of the first derivatives of their functions with respect to temperature and reciprocal dielectric constant. However, opposite signs obtain for the first derivatives involving the gauche vicinal coupling corresponding functions, as can be inferred from Figure 2 and the previously noted study of medium effects. The trans Table I: Curve Directions for Reciprocal Dielectric and Temperature Functions of Nuclear Parameters in Ethylene Sulfite Systems Dieleotrio function curvature
Temperature function curvature
+ + +
+ +-
(0)
-
vicinal coupling varies with temperature as does the gauche vicinal coupling, but significance cannot be attached to this in the present context since the first derivative of the trans vicinal coupling with respect t o reciprocal dielectric constant is very close to vanishing. (See Table I.) It is well established that a temperature dependence of the chemical shift and coupling parameters in the nuclear magnetic resonance spectrum can be a reflection of time-averaging changes due to temperature-sensitive conformational equilibria. It has also become widely recognized that changes in medium dielectric can similarly affect time-averaging of ~ p e c t r a . ~I,n~ the particular case of ethylene sulfite, cogent evidence has been found in closely related systems pertaining t o the probable conformational immobility of this and related structure^.^ On the other hand, thermochemical and kinetic studies of some other saturated, five-membered heterocyclics have indicated the existence of sufficient ring strain to give rise to considerable structural instability and a t least the possibility of temperature dependent conformational equilibration.6 If the presently determined temperature dependence of the spectral parameters is to be interpreted as proof of a conformational mobility in ethylene sulfite, then it should be possible to obtain a value for the conformational equilibrium constant which will not only be a “best fit” t o all the temperature dependent spectral functions, but which can also, with the appropriate (2) R. J. Abraham, L. Cavalli, and K. G. R. Pachler, Mol. Phys., 11, 471 (1967). (3) H. Finegold, J . Chem. Phys., 41, 1808 (1964). (4) J. Tillett, Quart. Rep. Sulfur Chem., 2,227 (1967). (6) F. H. Westheimer, Accounts Chem. Res., 1,70 (1968).
Volume 79, Number 11
November 1969
COMMUNICATIONS TO THE EDITOR
4022 TdH/dT correction factors,2 be fitted into the previously determined dielectric constant dependent functions,l should these changes also originate in conformational equilibration. However, the “best fit” value of the temperature dependent functions as evaluated by a least-squares method cannot be fitted to the dielectric-dependent functions because of the opposite signs of the curve directions discussed above. This method, therefore, affords a new criterion for the validity of free energy values determined by computer fittings to temperature-dependent functions, since the corresponding curve directions for all spectral functions in the temperature dependent vs. dielectric constant dependent functions must be related, pairwise, in sign if the spectral changes are due primarily to changes in the conforma,tional free energy. It has not been widely appreciated, though it is very well established, that temperature dependence in spectral changes of the magnitude described above can be exclusively ascribed to vibrational eff ects.‘jJ Since
the curves in Figures 1 and 2 cannot be accounted for in terms of an equilibrium constant compatible with the corresponding dielectric functions, it is probable that the effect of temperature on the population of methylene group torsional vibration levels makes the major contribution to the spectral changes observed here. The implications regarding conformational stability in ethylene sulfite would thus be consistent with the closely related observations that each of the separable geometrical isomers of a simple 4-substituted methyl analog of ethylene sulfite shows no chromatographically detectable inversion over long periods of time, and that the methyl and methylene resonance lines of the gemdimethyl analog, which are excellent probes of time averaging, indicate no conformational mobility.s (6) L. Petrakis and C. H. Sederholm, J . Chem. Phys., 35,1174 (1961). (7) K.C. Ramey and W. S.Brey, Jr., ibid., 40,2349 (1964). (8) J. G. Pritchard and P. C. Lauterbur, J . Amer. Chem. SOC.,83, 2105 (1961).
C O M M U N I C A T I O N S T O THE E D I T O R C o m m e n t s on the Paper “Activity Coefficients for Ionic Me1ts”l
Sir: Haase2 has proposed that an ideal ionic melt be defined as one in which the chemical potential of any ion j is taken as I.L$
= PO^
+ R T In
xj
(1)
where is the chemical potential of the pure liquid ion and xj is the ion fraction defined as the number of ions of j divided by the total number of species in the melt. For an ideal melt all of these species would be fully ionized. The proposal was made without reference to any statistical model, but in fact it corresponds to one for which in an ideal melt all ions mix randomly with each other regardless of charge. This can be shown from Haase’s eq 4, here slightly rearranged as Xu -2.0
Vl(1 Vl(1 - X)
2)
+
v22
(2)
I n a mixture of salts A and B of stoichiometric mole fractions (1 - x) and x, a is an ion found only in A. vl and v2 are the numbers of particles produced by the complete ionization of 1 formula weight of A and B; if A is Ce2(SO,),, VI is 5 N . The denominator thus conThe Journal of Physical Chemistry
tains all the particles in the melt. 22,the fraction of a in pure A, would be va/vll vu being the number of a ions in 1 formula weight of A. (22is used to normalize xu which would otherwise not equal 1 at II: = 0.) Ion fractions as in (2) will be referred to as the “total ion fraction” in this paper. The ion fractions for the other ions are defined in a similar fashion. Haase then derives expressions for the ideal activities of the component salts which are the products of the total ion fractions of the constituent ions. Basing the ion activities on the total ion fractions seems t o have been proposed first by Herasymenko and Speight in 1950,3 and then again by Bradley in 1962.4 While such a treatment is quite reasonable for a mixture of uncharged species it does not take into account the very strong Coulombic forces that tend to alternate ions of opposite charge. The Coulombic ordering has been observed experimentallys and predicted from a general theoretical modeL8 In 1945, long before this (1) Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation. (2) R. Haase, J . Phys. Chem., 73, 1160 (1969). (3) P. Herasymenko and G. E. Speight, J.Iron Steel Inst. (London), 166, 169 (1950).
(4) R. 5. Bradley, Amer. J. Sci., 260, 374 (1962). (5) See, for example, M. Blander, Ed., “Molten Salt Chemistry,” Interscience Publishers, New York, N. Y.,1964, Chapter 2. (6) Reference 6, Chapter 3.
