2601
STRUCTURE OF A SULFUR SORPTION COMPLEX OF ZEOLITE4A of NHaZO and WHs.Hz021)are justified for two reasons. First, the oxygen atoms bear an approximate -0.23 formal charge, and most ammonia hydrogens, because the nitrogen is in the first coordination sphere of a cation, can be expected to be more acidic. Second, some of the ammonia to ammonia contacts can be viewed as intramolecular nonbonded approach distances within the aluminosilicate complex. The separations between atoms lying on or near to the threefold axis are given in Table IV. Three O(3) atoms of the six-oxygen ring are chosen as the reference lane. Na(1) is displaced into the large cavity by 0.6 from this plane. The K(1)-0(3) distance (3.0 A) indicates that three (or perhaps only two if "(1) is actually somewhat disordered with respect to the threefold axis as has been suggested above) hydrogen bonds can form simultaneously between the hydrogen atoms of the N(1) ammonia molecule and the O(3) atoms of the six-ring, lending stability to the out-of-plane Na(1) position. The ligand structure is very similar to that found for hydrated zeolites 4A15122and 5A.23 However, the long X(4)-K(4) distances (4.2 A) ensures that six edges of the pentagonal dodecahedron observed for hydrated structures are nonbonding here. This was expected because NHa has insufficient lone pairs (too many hydrogen atoms) to complete such a figure with
1
~
~
~~~~
Table IV : Deviationsoof Atoms from the 111 Plane at O ( 3 ) (in A)"
NU)
-1.9
W3) O(2)
0
NaU)
0.18 0.59
N(3)
2.8
A negative deviation indicates that the atom lies on the same side of the plane as the origin. Q
hydrogen bonds. This structure appears to support sec) dielectric relaxation timesz4obthe long served for ammonia in zeolite A.
Acknowledgment. This work was supported by the U. S. Army Research Office-Durham. We are also indebted to the NSF for their assistance (Grant No. GP-18213) in the purchase of the diffractometer. (20) I. Olovsson and D. H. Templeton, Acta Crystallogr., 12, 832 (1959). (21) I. Olovsson and D. H. Templeton, ibid., 12, 827 (1959). (22) K. Seff and D. P. Shoemaker, unpublished (1964). (23) K. Seff, Ph.D. Thesis, Massachusetts Institute of Technology, (1964), p 93. (24) B. Morris, "Molecular Sieves," Society of Chemical Industry, London, 1968, p 34.
The Crystal Structure of a Sulfur Sorption Complex of Zeolite 4 A by Karl Seff Chemistry Department, University of Hawaii, Honolulu, Hawaii 96828
(Received March 81, 1972)
Publication costs assisted by the A r m y Research Ofice (Durham)
The crystal structure of a sulfur sorption complex of zeolite A has been determined by single-crystal X-ray techniques. Fully vacuum-dehydrated zeolite A of approximate composition NallAlllSi13048 per unit cell was expose4 to dry sulfur for 240 hr at 270". The approximate cubic space group Pm3m was used with a = 12.289 (2) A. Two SS rings, each in a crown configuration, are found in the large cavity of each unit cell. The planes of the two molecules in a single cavity are parallel but are not in van der Waals contact. Four alternatin5 sulfur atoms in each ring are each 2.8 A from a threefold-axis sodium ion, and the other four are each 3.2 A from a framework oxygen atom. Threefold disorder exists in the sorbate structure; the normal to the planes of the pair of rings in each cavity can be [loo], [OlO], or [OOl].
Introduction Nearly rectangular reversible sorption isotherms for sulfur onto Ca(I1)-exchanged zeolite A and the sodium form of zeolite X have been determined.1 These in&cate that a single structure may describe each sorption complex over a wide range of conditions. The heats of sorption' (-25 and -31 kcal/mol, respectively) ah0
indicate that these complexes are relatively stable. Such systems afford a unique opportunity for investigating the configurations of physical sorption complexes crystallographically. Na(1)-zeolite A was selected as the host material and long exposure times were (1) R. M. Barrer and J. L. Whiteman, J. Chem. Soc. A , 13 (1967).
The Journal of Physical Chemistry, Vol. 76, N o . 18, 1971
2602 used in anticipation of slower sorption rates for these crystals, which are larger than those used previously1 and whose pores can be considered somewhat blocked by sodium ions.
Experimental Section More than 100 g of single crystals of zeolite A was prepared using Charnell's2 method, modified to include a second crystallization using seed crystals from the first preparation. Crystals formed nicely as cubes from 30 to 70 p on an edge. Elemental analysis indicated that the framework formula for these preparations is best represented as AlllSi1304811-and that the number of sodium ions per unit cell is therefore approximately 11. A sample of zeolite A, approximately 100 mg, was dehydrated at 350" and Torr for 24 hr. Similarly, about 10 g of finely divided sublimed sulfur powder was dried at 105" and Torr for 36 hr. The two samples \yere mixed, under vacuum, and maintained at 300" for 100 hr, then at 270" for 240 hr, and finally at 115" for 90 hr as the sulfur either distilled away or formed separate crystals mixed with those of the zeolite in a loose brown powder. The product did not change in color during this last 90-hr process. IvIicroscopic examination of the zeolite crystals revealed that they were clear, bright yellow, and entirely uninjured. The crystals have not become clouded even upon prolonged exposure to the local (humid) atmosphere, indicating that they are hydrophobic. The density, measured by flotation in aqueous Zn12 solution, is 1.99 (2) g ~ m and - ~agrees adequately with the calculated value of 1.97 g ~ m for - ~16 sulfur atoms per unit cell. A single crystal, a cube 65 I.L on an edge, was selected for X-ray investigation. A Syntex four-circle computer-controlled diffractometer with graphite-monochromatized &Io Ka! radiation (GI, X 0.70926 8;K a z , X 0.71354 8) and a pulseheight analyzer was used throughout for preliminary experiments and for the collection of diffraction intensities. The cubic cell constant (12.289 (2) A at Z O O ) was determined by a least-squares refinement of 15 intense reflections with 20 values up to 23.6". The approximate space group l"3m (no systematic absences) was used instead of Fm% because the stoichiometry of our sample ensures a disorder which cannot be better described by the latter space group, because Gramlich and Meier3have shown that deviations from the former space group are small, and because a brief check of Gramlich and hleier's most intense "b" reflections indicated that they were absent here. The 0-20scan technique was employed at a constant scan rate of 0.5"/min (in 20). The scan range varied from 2.0" at 20 = 3" to 2.5" at 20 = 70". All 888 unique reciprocal lattice points for which 28 < 70" were examined. A time equal to half of the scan time for each reflection was spent counting background a t each end of the scan The Journal of Physical Chemistry, Vol. 76, No. 18, 1072
KARLSEFF range, Three check reflections which were measured periodically during data collection showed no significant trend in intensity. Standard deviations were assigned according to the formula
~ ( 1= ) [CT
+ 0.26(tc/tb)2(B1+ Bz) + ( p 1 ) 2 ] l " ~
where CT is the total integrated count obtained in a scan time of t,, B1 and Bz are the background counts each obtained in time t b j and I = CT - 0.5(tc/tb)(B1 B2). A value of 0.02 was assigned to the empirical parameter p to account for instrument instability. The net counts were then corrected for Lorentz and polarization effects. An absorption correction (pR = 0.03) was unnecessary. The 184 reflections for which the net count exceeded three times its standard deviation were used throughout.
+
Structure Determination Initial full-matrix least-squares refinement using framework and threefold axis cation positions found for the 32 ammonia complex* of zeolite 4A converged ~-
Table I : Observed and Calculated Structure Factors" 01 KI
He
1 659 3 1@14 4 247 5 1056 6 1888 8 1071 9 650 10 I146 11 1124 16 669 17 376 HE
424H-
0, K =
1
444 426
-774
1
9 10
2 -351H4 1100 -1026 11 6 314 289 I b 7 662 -131 9 252 -286 H* 188 1 11 251 3 12 303 - 2 8 4 4 15 305 -258
5 2 7 -679 8 530 -376 10 4 628 -608 7 277 -293 H. 8 ~3~ -484 2 9 *66 -484 3 5 0, 7 8 3 418 -368 9 5 380 -311 6 231 192 7 284 260H8 344 -348 5 9 399 -468 7 297 8 10 300 11 314 31)
0, K =
H=
2 3
638
".
0, K = 4 1061 5 504 6 230 7 452 9 637 12 336 15 346
H=
0,
~i
I(=
17" 9
;:$
452
4r K i 3 3 4 ti6 371 406 H= 1, K i 6 8 1527 -1534 12 299 756 6 2 4 8 -213 9 415 -462 473 7 248 -253 8 383 400 13 438 -44nH- 4, K = 6 12 3.15 -353 14 418 -467 11 307 641 19 403 -352 H= 5 . K = 344H- 1 1 K = 8 5 914 444 9 279 260 H- 3, K 4 6 770 714 I t 310 369 4 691 721 7 475 5 191 -159 376 H= 2 . K. 2 6 328 344 8 405 0, K = 7 3 1171 - 1 0 4 2 7 581 521 10 248 481 -505 4 552 543 12 324 3 3 1 11 603 16 436 265 -221 5 682 674 6 358 -322 H= 3. K5 5 1 K0 1 K= 11 7 229 -131 6 415 388H6 631 745 148 8 WO -322 8 371 -311 7 308 429 372 9 531 526 9 386 -347 8 248 11 296 -290 11 452 1, K I 1 12 282 2 1 5 HC 3 , K = 6 907 -1307 15 352 256 6 348 321 8 2 6 8 -250H- 5 . K= 1236 1201 9 292 420 -391H- 2 1 K3 378 -296 3 1278 -1174 HI 3 r 7 507 -459 6 355 -309 7 622 b39H.1 5 1 K; 339 323 7 207 150 8 234 -94 11 283 541 -527 8 07n - - a 5 4 9 383 358 51 Kr 11 257 -213 10 338 368H9 284 1. K= 2 731 5 9 1 H= 2, K = 4 n- 3 , K- 8 51 867 846 4 206 165 8 1021 -1053 ti. 332 251 5 297 -262 13 448 - 5 5 0 11 556 270 235 T 426 396 14 463 -507 HI 6, K r 206 334 8 245 352 b 261 395 H= 4 , K4 341 9 367 359 4 321 319 7 332 I r K. 3 H. 2 , K. 5 5 775 -754 11 308 282 408 -321 5 220 175 6 208 -244 I2 8 712 603 740 741 b r K* 271 271Hi 2. K6 9 642 70ZH. 302 6 404 -377 11 464 - 4 2 0 11 4 3 3 366 7 283 -248 Hm 7, K = HI 4 , K r 5 7 317 1, Ke 4 M. 2 , K7 5 397 -407 9 288 496 -549 9 296 373 6 271 -220 7 6 5 4 -664 6711 -68b 441 441 H I 2 9 K. 8 12 282 -263 I+* 8. K * 8 825 239 -156 8 728 -692 13 455 289 -318 12 285 214H* 4 1 K= 6 1 373 -361 2 1 5 -283 HI 2 , K. 9 17 3 4 8 -145H- 1 1 1 K11 440 1. K i 5 12 337 333 H- 4, K- 7 -:i:Hs 3 , K. 3 7 397 372 303 -325 3 646 -616 8 243 321 4 311 377 11 2 1 4 -264
0 10 372 5 2 4 11 772 1290 16 525 373 LO51 HI 0, K = 1896 6 713 9n3 8 364 659 I D 411 1167 11 720 1206 I6 376 689
4 1032H.
-::: -490 -586
5 6
8
IO
-369 11 -299 12 5
M=
-:::
265 9 -284 5
935 788 -472 446 199 653 362 6 605
-289 251 522
7 -300 8
261 9 -291 11 553
6
171 -352 299 -172 11 380
7 359 319 8
-924 -399 11 415
:i:
1658
-470
$::
8
9
a The running index is I; values of h and IC for each group immediately precede that group. The central column is lop,; the right-hand column is IOF,.
(2) J. F.Charnell, J. Crystal Growth, 8 , 291 (1971). (3) V. Gramlich and W. M. Meier, 2. Kristallogr., 133, 134 (1971). (4) R. Y. Yanagida and K. Seff, J. Phys. Chem., 76, 2597 (1972).
2603
STRUCTURE OF A SULFUR SORPTION COMPLEX OF ZEOLITE 4A Table I1 : Positional, Thermal, and Occupancy Parametersa Atom
B , i z , or and biz
bii
21
2
0 0 0 0.112 (1) 0.209 (2)
0.1824 (4) 0.219 (2) 0.295 (1) 0.112 (1) 0.209 (2)
0.3706 (3)
0 0.293 (13) 0.246 (7)
0.440 (12) 0.358 (8) 0.307 (7)
Position
2
1.20 (6) 2 . 6 (4) 2.0 (3) 2 . 8 (3) 0.014 (2); 0 021 (4) 19 (1) 30 (7) 20 (3)
‘/z
0.295 (1) 0.339 (1) 0.209 (2)
Occupancy factor
1 1 1 1 1
I
0.440 (12) 0.358 (8) ‘/z
“4
‘/3
1/3
See Figures 1 and 2 for a Standard deviations are in the units of the least significant digit given for the corresponding parameter. kZ 1%) - blz(hk hl kl)]. the identities of the atoms. For Na(l), the anisotropic temperature factor = e x p [ - h ( h 2
+ +
1.
ZEOLITE riR
.
+ +
Y
ZEOLITE UR
258
Figure 1. A stereodrawing of the large cavity in zeolite A containing two SSrings. Ellipsoids of 50% probability are used.
.
258
The unit cell is shown.
= (Zw(F, - IF,/)Z/2~F,2)i’2) is 0.094. Calculated and observed structure factors are presented in Table I and the final structural parameters are presented in Table 11. The goodness of fit ( ( Z w ( F 0 - IFc))2/(m- 8 ) ) ” ’ ) is 1.29; m is the number of observations (184) and s (22) is the number of variables in least squares. Anisotropic refinement of the two sulfur atoms allowed RI to fall to 0.089 and R2 to 0.091, but very elongated ellipsoids with large standard deviations appeared. The largest anisotropic thermal parameter by a factor of 3.5 for S(l) (bll = 0.15) corresponds to motion normal to the plane of the ssring, and the largest by a factor of 2.5 for S(2) ( b 2 2 = 0.075) describes a motion in the plane of the ring and tangent to it. The results of these anisotropic refinements for positions of fractional occupancy may not correspond to real thermal motions, because of the disorder present, and are not included in subsequent tables and figures. Alternate cycles of occupancy and thermal parameter refinement for the sulfur atoms converged at 1.6 (3) Ss rings per unit cell and R1 and E , decreased to 0.087 and 0.092, respectively. Owing to expected high least-squares correlations, these results are also not
R2 index (Rz
OCTRSULFUR ON THE HRLF CELL
Figure 2. A view of a complexed S8ring in one-half of a unit cell. Ellipsoids of 30% probability are used.
quickly to an R1 index (R1 = ( B / F o - iFeiI)/ZFo) of 0.150. Correct sulfur positions were guessed and refined by full-matrix least-squares methods to RI = 0.115. A subsequent Fourier synthesis prepared without sulfur atoms included in the calculation of phases corroborated this result. The principal sodium ion position was allowed to refine anisotropically in the final cycles of least squares, resulting in a final RI index of 0.093. The corresponding generalized weighted
The Journal of Physical Chemistry, Vol. 76, iVo. 18,1972
KARLSEFF
2604
Table 111: Interatomic Distances and Anglesa (Si, A1)-O(1) (Si, A1)-0(2) (Si, A1)-0(3) Na(1)-O(3) Na( 1)-O(2) Na(2)-0 (2) Na(2)-O (1) O(1)-(Si, A1)-O(2) O(1)-(Si, A1)-O(3) O(2)-(Si, A1)-O(3) O(3)-(Si, A1)-0(3) (Si, A1)-O(1)-(Si, Al) (Si, A1)-0(2)-(Si, Al) (Si, A1)-0(3)-(Si, Al)
S(1)-W)
1.653 (7) 1,663 (6) 1.678 ( 5 ) 2.32 (1) 2.97 (2) 2 . 5 (2) 2 . 8 (1)
S( 1)-"a( 1) 8(2)-0 (1)
108.2 (7) 114.4 (6) 107.4 (8) 110.8 (9) 148.4 (6) 158.0 (12) 143.0 (9)
S(l)-S(2)-S(l) S(2)-S (1)-S(2)
128 (9) 119 (8)
S(2)-S(l)-Xa(l) S(l)-Na(l)-O(3) S(l)-Xa(1)-0(3)' O(l)--S(2)-S(l) 8(2)-0(1)-(Si, Al)
105 (3) 94 (2) 115 (3) 113 (4) 95 (2)
a Standard deviations are in the units of the least significant digit given for the corresponding parameter. angles in degrees.
taken t o be sufficient cause for modifying the structural model. The standard deviation of the electron density on Fourier functions was approximately 0.2 e/83, and the largest peak on the final differences synthesis was 0.8 e/A3 in height and appeared at an unlikely position. The full-matrix least-squares program used5 minimizes Z W ( A ~ F ~the ) ~weights ; were the reciprocal squares of Q, the standard deviation for each observation. Atomic scattering factors6 for Si1.5+,AllJ+, Na+, 0-, and So were used. In the last cycle of least-squares refinement, all shifts were less than 2% of their corresponding esd's. A stereoview7 of the structure is shown in Figure 1.
Discussion Two equivalent S8 rings in the crown configuration lie parallel to each other in the large cavity of the zeolite A structure. The molecular symmetry of the octasulfur ring is 4mm (C40), substantially greater than 2(C2) as is in orthorhombic sulfur, but less than the ideal symmetry foundlo in the vapor phase, s2m (Ddd). Threefold disorder exists : the molecular planes of the two octasulfur molecules coexisting in the same large cavity are both normal t o [OOl], [OlO], or [loo]. After one set of disordered positions is selected for one ring, however, packing considerations allow only the parallel position for the second molecule. The closest approach between the two rings is 4.96 8, much more than the sum (4.0 A) of the van der Waals radii for sulfur. Two nonequivalent sulfur atoms, S(1) and S(2), alternate t o form the s8 ring. Each atom in the sulfur ring makes only one important approach to a nonsulfur atom (see Figure 2 and Table 111). The S(1)-Xa(1) distance, 2.80 (15) 8, is the same as the sum of the corretponding van der Waals and ionic radii,ll 1.85 0.95 A. I n complexes12between alkali metal ions and cyclic polyethers (crown ethers), the Na+ to 0 distances average 2.70 8, 0.35 8 more than the sum of the
+
The Journal of Phusical Chemistry, Vol. 76, N o . 18, 1972
1.94 (8) 2.80 (15) 3.21 (9)
Distances in Lngstroms,
+
corresponding van der Waals and ionic radii (1.40 0.951 A). Accordingly, a significant jon-induced dipole interaction is indicated. The S(2)-0(1) distance, 3.21 (9) 8, is close t o the sum of the corresponding van der Waals radii, 3.25 A. I n the SO2 complex13 of zeolite 5A, the sulfur aiom makes only a single significant approach of 2.95 A to 0(1),the same framework oxygen atom as in the present structure. I t is reasonable t o expect that a symmetric electric octupole is induced in the plane of the sulfur ring, and that a dipole is induced normal to that plane, by the host lattice. The heats of sorption indicate that the enthalpy of sorption for octasulfur is approximately -200 to -250 kcal mol-l. s8 rings, then, effectively seal the 8-oxygen windows at environmental temperatures. The large thermal motions of the octasulfur ring obscure the observations of significant structural differences with respect to previous findings. The S(1)S(2) bond length, 1.94 (8) 8, is insignificantlyoshorter than that found in rhombic sulfur, 2.048 A. The S-S-S angles, averaging 123 (6)") are again insignificantly greater than that found in rhombic sulfur, 107.9". The dihedral angle has decreased from 98" to 72") and ( 5 ) P. K. Gantzel, R. A. Sparks, and K. N. Trueblood, UCLALS4, American Crystallographic Association Program Library (old), Xo. 317, modified. (6) "International Tables for X-Ray Crystallography," Vol. 111, Kynoch Press, Birmingham, England, 1962, p 202. (7) C. K. Johnson, "ORTEP," Report ORNL-3794, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1965. (8) (a) S. C. Abrahams, Acta Crystallogr., 8 , 661 (1955); (b) ibid., 14, 311 (1961); (c) A. S. Cooper, W. L. Bond, and S. C. Abrahams, ibid., 14, 1008 (1961); (d) S. C. Abrahams, ibid., 18, 566 (1965). (9) (a) A. Caron and J. Donohue, ibid., 14, 548 (1961) ; (b) A. Caron and J. Donohue, ibid., 18, 562 (1965). (10) C. S. Lu and J. Donohue, J . Amer. Chem. Soc., 66, 818 (1944). (11) L. Pauling, "The Nature of the Chemical Bond," 3rd ed, Cornel1 University Press, Ithaca, N. Y., 1960. (12) 11. A. Bush and M.R. Truter, J . Chem. SOC. B , 1440 (1971). (13) K. Seff, Ph.D. Thesis, Massachusetts Institute of Technology, 1964, p 175.
TRANSFORMATION BEHAVIOR IN ~-AZOXYANISOLE the distance between the plane of the four S(l) atoms of a single ring t n d that of the four S(2)'s decreased from 0.99 to 0.6 A. These changes, although marginally significant, are qualitatively consistent with the expected effects of the assumed induced electric octupole. The aluminosilicate framework differs insignificantly from those of the 32-ammonia complex4 of zeolite 4A or of the hydrated sodium3 or thallium(I)I4 forms of zeolite A. This can be considered the relaxed configuration, the geometry adopted by zeolite A at its synthesis. The thermal parameters of the aluminosilicate structure are insignificantly larger, even for 0(1), the oxygen atom involved in the close S(2) contact, indicating that the tetragonal distortions expected by the sorption of sulfur are small. The sodium ions at Na(1) are found out of the plane of their netrest oxygen neighbors (Table IV), but less so by 0.15 A than was found in the ammonia complex. The anisotropic thermal paramthe eters for Na(l)' demonstrating motion threefold axes, are identical in these two structures.
2605
+
+
The average S(l)-Na(l)-0(3) angle is (94 94 115)/3 = 101 (2)", close to tetrahedral, and the O(1)S(2)-S(1) angle, 113 (4)O, is tetrahedral.
Table IV: Deviationsa (A) of Atoms from the 111Plane a t O ( 3 ) (23) O(2)
Na(1) SO)
0 0.18
0.45 3.0
a A negative deviation indicates that the atom lies on the same side of the plane as the origin.
Acknowledgment. This work was supported by the U. S. Army Research Office-Durham. We are also indebted to the NSF for assistance (Grant No. GP-18213) in the purchase of the diffractometer, and to the University of Hawaii Computation Center. (14) P. E. Riley, K. Seff, and D. P. Shoemaker, 2593 (1972).
Transitions in Mesophase Forming Systems.
IV.
J. Phys, Chem., 76,
Transformation
Behavior and Pretransition Effects in p-Azoxyanisole'
by Fraser P. Price* and Joachim H. Wendorff Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts (Received December IS, 1971)
01002
Publication costs assisted by the National Institutes of Health
The equilibrium density-temperature behavior of p-azoxyanisole has been studied over the temperature range 25-145O. Equilibrium transition temperatures of 117.0 and 134.2' were found for the solid-nematic and the nematic-isotropic phases, respectively. Linear reversible density temperature behavior was observed in all phases except on the low temperature side of each transition. Here marked downward curvature, attributable to increasingly significant disordering, was observed. Thermal expansion coefficients of 4.1 X lo-*, 9.4 X lod4,and 8.4 X were calculated, respectively, for the crystalline, the nematic, and the isotropic phases, while latent volumes of transitions of 6.9 and 0.34% were measured for the solid-nematic and the nematic-isotropic transition. Transformation rates for the isotropic-nematic transformation were at all supercoolings too rapid to measure. The nematic-solid transformation required supercoolings of greater than 20' at which temperature the transformation rates were immeasurably fast.
We are engaged in a systematic study of the equilibrium density-temperature behavior and the interphase transformation kinetics of the cholesteryl esters. Thus far we have studied cholesteryl myristate,2a aceand n ~ n a n o a t e . ~These substances form stable mesophases whose order is intermediate between that Of crystals and isotropic liquids* The cholesteryl esters exhibit smectic and cholesteric mesophases but never
a nematic phase. It is desirable to establish whether mesophase forming substances which do form nematic phases behave like the cholesteryl esters. Accordingly (1) This work was supported by Grant No. HE13188 from the
rnstitutes
Of
(2) (a) F. P. Price and J. H. Wendorff, J . Phys. Chem., 75, 2839 (1971); (b) F. P. Price and J. H. Wendorff. ibid., 75, 2849 (1971). (3) F. P. Price and J. H. Wendorff, ibid., 76, 276 (1972).
The Journal of Physical Chemistry, Vol. 76, No. 18, 1979
