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for interactions between solute molecules in these systems. Type @ aromatics are those molecules which have one dimension corresponding to the length of normal alkane molecule but the other dimension too large to substitute one molecule and too small to substitute two or more molecules. From the concentration dependence of the absorption spectra of these solutions, two important concentration intervals can be distinguished. Starting from concentrations as low as M, the M and up to about absorption spectrum consists of quasi-lines superimposed on broad bands. Transferring energy to “red” impurity has been observed from centers responsible for these broad bands. At higher concentrations up to about 5 X M, a pseudocrystalline (PC) spectrum is observed along with the quasi-line (QL) absorption spectrum. The relative intensity of the PC spectrum with respect to the QL spectrum does not change in a wide concentration range. At all concentrations, an anomalous intensity distribution was found for QL spectra (hyper- or hypochromy), and therefore, molecules associated with quasi-line spectra cannot be considered as strictly isolated.
On the basis of these spectroscopic features and on solutesolvent incomplete geometrical suitability, we postulate that molecules of the @-typeform segregations in the nonequilibrium n-alkane solid solutions and that molecules responsible for quasi-line spectra belong to these segregations. Upon electronic excitations, these molecules do not participate in the formation of excitonic states (localized excitations). Therefore, only second-order intensity effects (hypo- and hyperchromy) are observed in the quasi-line spectra. Other molecules in the segregation (at high concentrations) do participate in collective excitations (exciton-like states). These molecules give rise to pseudocrystalline spectra. When the concentration is increased, the structure of the segregation does not change essentially but the number of segregations increases. This accounts for the constant intensity ratio of pseudocrystalline absorption to quasi-line absorption.
Acknowledgment. L.A.N. is indebted to Colgate University and to Trinity College for partial support of this research. Registry No. Phenanthrene, 85-01-8; dibenzofuran, 132-64-9.
Deuterium Nuclear Magnetic Resonance Study of Several Cyclophosphazene Inclusion Compounds: Guest Conformation, Dynamics, and Intrachannel Orientation Eva Meirovitch* and Igal Belsky Isotope Department, The Weizmann Institute of Science, 76100 Rehovot, Israel (Received: September 19, 1983; In Final Form: March 2, 1984)
We report on a 2H NMR study of polycrystalline powders of several cyclophosphazeneinclusion compounds between 370 and 150 K. With p- and o-xylene we encounter rapid internal methyl reorientation. The aromatic rings orient, on the average, in the upright position within the inclusion channels and reorient about their C2 symmetry axis. With p-xylene this mode is rapid above room temperature, slows down at lower temperatures, and consists of threefold jumps about the channel axis d. With o-xylene we find that although the rate of this motion stays high throughout the temperature range investigated, , at roughly 308 K. Cyclohexaneexecutes simultaneouslyrapid ring inversion its symmetry decreases from C,, (or higher) to C and fast reorientation about the molecular symmetry axis z’, whereas dioxane only spins about the latter. Dramatic temperature-induced spectral modifications are interpreted in terms of the extent of z’ ordering along d or in terms of a temperature-dependenttilt of z’with respect to d. We find that tetrahydrofuran assumes a C2 envelope conformation, orients in an upright position within the channels, and spins rapidly about the channel axis. The spectral consequences of varying the temperature are interpreted in terms of changes in the molecular geometry. The acetone deuterons experience rapid methyl rotation with the internal diffusion axis fluctuating about the a mean orientation. It is suggested that two types of methyl groups, differing in the extent of this wobbling motion, prevail at higher temperatures and that the maximum amplitude of these fluctuations is temperature dependent.
I. Introduction Cyclophosphazenes (CPZ) are phosphonitrile derivatives. A variety of small molecules readily form channel-type crystalline inclusion compounds with CPZ, the latter serving as host and the former as guests.’ With most solid adducts knwon today, guest escape results in the collapse of the host structure. However, with CPZ inclusions, guests can be interchanged without destroying the host lattice. Many physical techniques, including x-ray crystallography, had been used in the past to investigate solid inclusion c o m p o ~ n d s . ~ , ~ The data accumulated can, in most general terms, be classified as related to static structural properties and to dynamic characteristics. The former aspect has been evaluated exhaustively; the latter, however, is quite obscure. Thus, x-ray crystallography (1) (a) Frank, S. G. J . Pharm. Sci. 1975,64, 1585. (b) McNicol, D. D.; McKendrick, J. J.; Wilson, D. R. Chem. SOC.Reu. 1978, 7, 65. (2) Parsonage, N. G.; Stabeley, A. K.“Disorder in Crystals”; Rowlinson, J. S., Baldwin, J. E. Eds.; Calrendon Press: Oxford, 1978. (3) Chang, H. C.; Tang, C. P.; Popovitz-Biro, R.;Lahav, M.; Leiserowitz, L. Nuou. J . Chim. 1981, 5, 475 and references cited therein.
cannot, due to its extremely short time scale, differentiate between static and dynamic disorder, when motions occur at rates slower than approximately 101L1013s-l.* Other methods such as nuclear quadrupole resonance, broad-line NMR, dielectric relaxation, etc. can provide qualitative information only due to low spectral sensitivity and/or inadequate experimental time ~ c a l e s . ~The ?~ majority of these techniques inform, thus, at most on the order of magnitude of motional rates, whereas the intricate details of the motion cannot be specified. From these and other studies it turned out, however, that a variety of dynamic processes of unexplored nature, likely to induce conformational changes in the mobile molecules, do often take place in inclusion crystals at rates of the order of 1-lo6 Hz, i.e. are just of the right order of magnitude to affect solid-state N M R spectra. However, the conventional N M R techniques-broad-line N M R and relaxation measurements-are not very informative, the former due to the complexity of the spectrum and the latter because it is only ap(4) Clement, R.; Mazieres, C.; Gourdji, M.; Guibe, L. J . Chern. Phys. 1977, 7, 5381 and references cited therein.
0022-3654/84/2088-4308$01 .50/0 0 1984 American Chemical Society
Cyclophosphazene Inclusion Compounds plicable in the limit of rapid motions, whereas spectral sensitivity to intricate details of structure and dynamics are fully borne out by, and therefore only obtainable with, slow-motional spectra, wherein features belonging to individual molecular orientations had not yet been averaged out c ~ m p l e t e l y . ~ The recent development of experimental solid-state N M R methodologies,6 advances in theoretical formulations of dynamic effects on N M R line shape^,^ and progress in the chemistry of selective isotope labeling have led to a third approach, namely careful line-shape analysis of high-resolution dynamic N M R spectra.8 incorporating all the indispensable aspects the technique discussed above is deprived of. We have examined the deuterium N M R spectra of p- and o-xylene-dlo, cy~lohexane-d,~, dioxane-d8, tetrahydrofuran-d8, and acetone-d6 as guests. Above approximately 310 K the xylene spectra report on rapid spinning about the molecular C2symmetry axis which is highly (with o-xylene) and moderately (with p xylene) ordered along the channel axis d. Below 3 10 K , we find that ring reorientation slows down in p-xylene and becomes nonuniform in o-xylene. Rapid internal methyl rotation prevails at all temperatures with both molecules. Throughout the temperature range investigated cyclohexane executes simultaneously rapid ring inversion and reorientation about the molecular symmetry axis, whereas dioxane experiences the latter process only. Dramatic temperature-induced spectral modifications are interpreted in terms of order parameters describing the preferential orientation of the molecular symmetry axes along the channel axis d or in terms of temperature-dependent tilt angles between C2 and d. We find that tetrahydrofuran (THF) assumes exclusively a C2 half-chair conf~rmation,~ orients in an upright position within the channels, and spins rapidly about the tunnel axis. The spectral consequences of varying the temperature from 370 to 150 K are interpreted as changes in the molecular geometry. The acetone methyls reorient rapidly at all temperatures. We find that the axes of internal methyl rotation do not orient uniquely within the channels; rather, they wobble rapidly about a mean orientation, and we detect two types of methyl groups differing in the extent of these fluctuations. Experimental details are summarized in section 11. The theoretical background required to follow our interpretation of the N M R spectra is given in section 111. Results are presented and discussed in section IV, and our conclusions appear in section V. 11. Experimental Section
p - and o-xylene-dlo,cy~lohexane-d,~, dioxane-d8, tetrahydrofuran-d,, and acetone-d6 (at least 99% D) were purchased from Merck Sharpe and Dohme (Canada). ( 5 ) Freed, J. H. In "Spin Labeling: Theory and Applications"; Berliner, L. J.; Ed.; Academic Press: New York, 1976; Vol. I, Chapter 3. (6) (a) Mehring, M. NMR Basic Princ. Prog. 1976,11,30. (b) Haeberlen, U. "High Resolution NMR in Solids"; Academic Press: New York, 1976. (c) Griffin, R. G. "Methods Enzymol. 1981, 72, 108. (d) Skarjune, R.; Oldfield, E. Biochemistry 1979, 18, 5902. (7) (a) Freed, J. H.; Bruno, G. V.; Polnaszek, C. F. J . Phys. Chem. 1971, 75, 3386. (b) Campbell, R. F.; Meirovitch, E.; Freed, J. H. J . Phys. Chem. 1979, 83, 525. (c) Meirovitch, E.; Freed, J. H. Chem. Phys. Lett. 1979, 64, 311. (d) Baram, A.; Luz, 2.; Alexander, S.J . Chem. Phys. 1973,58,4558. ( e ) Alexander, S.;Baram, A,; Luz. 2. Ibid. 1974,61, 992. (f) Alexander, S.; Baram, A.; Luz, 2. Mol. Phys. 1974, 27, 441. (g) Alexander, S.;Luz, 2.; Naor, Y.; Poupko, R. Ibid. 1977, 33, 1119. (h) Baram, A,; Luz, 2.; Alexander, S. J . Chem. Phys. 1976,64, 4321. (i) Spiess, H. W. Chem. Phys. 1974, 6, 217. (j)Spiess, H. W.; Grosescu, R.; Haeberlen, U. Ibid. 1974, 6,226. (k) Pschorn, 0.;Spiess, H. W. J . Mugn. Reson. 1978, 39, 217. (8) (a) Davis, J. H.; Jeffrey, K. P.; Bloom, M.; Valic, M. I.; Higgs, T. P. Chem. Phys. Lett. 1976, 42, 390. (b) Bloom, M.; Davis, J. H.; Valic, M. I. Can. J . Phys. 1980, 58, 1510. (c) Huang, T. H.; Skarjune, R. P.; Wittebort, R. J.; Griffin, R. G.; Oldfield, E. J . Am. Chem. Sot. 1980, 102, 7377. (d) Wittebort, R. J.; Schmidt, C. F.; Griffin, R. G. Biochemistry 1981, 20, 4223. (e) Jacobs, R. E. Oldfield, E. Prog. Nucl. Magn. Reson. Spectrosc. 1981, 14, 113. (f) Wittebort, R. J.; Blume, A.; Huang, T. H.; DasGupta, S.K.; Griffin, R. G. Biochemistry 1982, 21, 3437. (9) Rice, D. M.; Wittebort, R. J.; Griffin, R. G.; Meirovitch, E.; Stimson, E. R.; Meinwald, Y. C.; Freed, J. H. J . Am. Chem. SOC.1981, 103, 7707. (h) Meirovitch, E.; Krant, T.; Vega, S. J. Phys. Chem. 1983, 87, 1390. (9) Mak, T. C. W.; McMullan, R. K. J . Chem. Phys. 1965, 42, 2732.
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 4309 n
n
Figure 1. (a) Axial rigid-limit powder spectrum obtained with a quad= 0.21 rupole constant Qo = 165 kHz and a natural line width T2*-' kHz; 6, = (3/4)Q0 and zero denotes the position of the Larmor frequency. (b) Partially averaged powder spectrum obtained by allowing the C-D bond associated with trace a to spin rapidly about a diffusion axis orthogonal to the C-D bond. (Le., 1(1/2)(3 cos2 a - 1)1 = 0.5). ( c ) Asymmetric rigid-limit powder spectrum obtained with Qo= 165 kHz, 9 = 0.205, T2*-I(x) = T2*-I b) = 2.4 kHz, and T 2 * - l ( z )= 4.7 kHz (x, y , and z denote the principal axes of the quadrupole tensor).
Hexachlorocyclotriphosphazene (I) (Aldrich) was purified by sublimation, and two recrystallizations from hexane were performed, giving material melting at 113-1 14 "C. Tris( 1,2-dioxyphenyl)cyclotriphosphazenewas prepared according to Allcock and Walshl& in 35% yield (mp 245 "C) (after recrystallization from chlorobenzene) and was sublimed (200 "C/0.1 mmHg) before use. The adducts were obtained by spontaneous addition, as described by Allcock and Siegel.lob The polycrystalline powders were packed tightly in 5-mm-0.d. N M R tubes 3 cm in length. The 2H N M R experiments were performed on a Brucker CXP-300 spectrometer. The deuterium Larmor frequency was 46.07 MHz, and a horizontal solenoid coil high-power probe was used. All 'H N M R spectra were obtained with the quadrupole echo (gokx, re,90y,t)experiment with a delay time of 20-30 ps and a 90' pulse length of 3.3 ps and by Fourrier transformation of the quadrature detected echo signal after t = 2r,, following the first pulse. Instrumental artifacts due to finite pulse length were found to be negligible. Spectra were obtained with 1000-5000 accumulations with a repetition time of 1-2 s , and alteration of the phase of the first 90" pulse was performed to reduce distortions in the echo signals due to dead-time effects after the second pulse. The temperature at the sample was controlled with a flow of N2 gas and stabilized with a temperature control unit with a precision of roughly f l "C. 111. Theoretical Background The main anisotropic term in the spin Hamiltonian of a deuterium nucleus in a solid is the quadrupole interaction."J2 The NMR spectrum of a carbon-bonded deuterium atom with the C-D bond oriented at a unique angle I9 with respect to the external field Ho is a doublet with a splitting g:"J2
g = ( 3 e 2 q Q / 2 h ) ( 1 / 2 ) ( 3cos2 I9
- 1)
in frequency units. e2qQ/his the quadrupole constant Qo of the axial tensor Qo. For a polycrystalline morphology the spectrum will be given by properly weighted superposed doublets, resulting in a typical pattern such as that illustrated in Figure la, obtained with Qo = 165 kHz and a natural line width T2*-' = 0.21 kHz. The spectrum is symmetric about the Larmor frequency vo, and the dominant features of this line shape are two strong peaks disposed symmetrically about vo and separated by 6o = ( 3 / 4 ) & and two extreme shoulders separated by 2a0, the former corresponding to I9 = 90" orientations (we shall refer to these as the (10) (a) Allcock, H. R.; Walsh, E. J. Inorg. Chem. 1971, 10, 1643. (b) Allcock, H. R.; Siegel, L. A. J . Am. Chem. SOC.1964,86, 5140. (1 1) See, for example: Hsi, S.; Zimmermann, H.; Luz, 2. J . Chem. Phys. 1978, 69, 4126 and references cited therein. (12) Mehring, M. NMR: Basic Princ. Prog. 1976, 11, 30.
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“perpendicular peaks”) and the latter to 0 = 0’ orientations (to be called the “parallel peaks”). The spectrum we just described is a rigid-limit axial powder spectrum. With onset of motion, the molecules will span the various orientations in space in a fashion determined by the nature of the dynamic process, and the line shape will alter accordingly. In principle, the type of motion and the symmetry and magnitude of local orienting potentials as well as geometric features such as tilt angles between diffusion and magnetic axes can be derived from the spectral analysis.’ A simple case, relevant to the forthcoming discussion, is rapid reorientation about a diffusion axis tilted at an arbitrary angle CY rellative to the C-D bond. Should this motion be uniformly diffusive or should the molecule execute discrete jumps between equivalent sites of symmetry equal to or higher than C30,partial averaging of the original tensor Qo to an effective axially symmetric tensor Q with its principle axis z’along the diffusion axis will occur? The principal value of Q can be calculated straightforwardly from those of Qoand the orientation of the diffusion axis in the local frame of the latter and vice versa; given Q, one can derive goetric parameters. With a polycrystalline powder one would therefore expect a pattern analogous to a true rigid-limit powder spectrum “scaled down” by the factor (1/2)(3 cosz CY - 1) (Le., Q = Qo(l/2)(3 cosz a l)), as illustrated in Figure l b for a = 90’. Since the quadrupole tensor is traceless, isotropic motions will lead to complete collapse of the quadrupole structure to give, in the motional narrowing regime, a single line centered at vo. Note, however, that a similar spectrum will result with a = 54.70°, the so-called “magic angle”, for which 3 cosz a - 1 = 0. We shall refer repeatedly in the following presentation to partially averaged axial powder spectra obtained as a result of fast reorientation about internal diffusion axes. In many cases, Q will be determined directly from the experimental spectrum by measuring the distance 6 between the perpendicular peaks. Should the molecule investigated contain several inequivalent C-D bonds, in the sense of identical quadrupole constants Qobut different orientations a,relative to the internal diffusion axis, the zHN M R spectrum will consist of a superposition of ai powder spectra. The potential of obtaining conformational characteristics is straightforward and will be illustrated below. When motional averaging is complete, it is not possible to differentiate between planar Brownian diffusion and jumps between equivalent sites of symmetry equal to or higher than C3”. However, for slower motions, of the order of the anistropy of the quadrupole interaction, a time regime wherein spectral features particular to given orientations in space have not yet been averaged out completely, the line shape is model d e ~ e n d e n t .These ~ aspects has been borne out by the experiment in several cases, that of the CPZ-benzene-d6I3 being most relevant to the present discussion. Another extension of the simple considerations outlined previously relates to nonuniform reorientation about a unique diffusion axis.14 For low-symmetry motions such as 180° jumps about a tilted axis, the formerly axial powder pattern of the type illustrated in Figure l b will transform into an asymmetric powder pattern, such as shown in Figure IC, as a consequence of some orientations in the plane perpendicular to the diffusion axis becoming preferred over others. Finally, should the diffusion axis itself experience additional motions, the spectrum will be further averaged. We consider a simple extension of a spatially fixed diffusion axis, namely wobbling rapidly within a cone centered at a given orientation in space. It is easy to see that the axial powder pattern obtained in the absence of wobbling will be further averaged to another axial powder pattern of reduced width, and an order parameter S, similar to the parameter used to estimate ordering in anisotropic fluids,I5 can be defined as the ratio of the perpendicular peak separation in the presence and in the absence of wobbling. (13) Meirovitch, E.; Belsky, I.; Vega, S. J . Phys. Chem. 1984, 88, 1522. (14) Wemmer, D. E. Ph.D. Dissertation, University of California, Berkeley, 1975. (15) See, for example: Void, R. R.; Vold, R. L.; Szeverenyi, N . M. J . phys. Chem. 1981, 85, 1934 and references cited therein.
M
IO kHz Figure 2. Experimental *HNMR spectra obtained at the temperatures denoted on figure from a polycrystalline sample of CPZ-p-xylene-dlo. The insert shows quadrupole echo line shapes calculated for discrete jumps about a Cp, symmetry axis with 7, = 30 ws, a tilt angle of 67.5’ between the principal axis of an axial quadrupole tensor and the jump axis, and the following jump rates: (a) T ~ =- 0, ~ (b) 1.2 X lo4, (c) 3.0 X lo4, (d) 1.0 X lo5, (e) 1.7 X lo5,( f ) 6.7 X los s-l.
IV. Results and Discussion p - and o-Xylene-dlo. We show in Figure 2 zH N M R spectra of p-xylene-dlo as a function of temperature. Most line shapes in this figure are quite unusual. However, close examination reveals that the 308 K spectrum is dominated by what appears to be a common axial powder pattern, with the innermost pair of (perpendicular) peaks separated by 6 kHz and the next pair of (parallel) peaks by 26 kHz. We also observe a pair of rather broad peaks at low and high frequencies which appear to belong to a second spectral component. We denote the latter as component I1 and the former axial powder spectrum as I, and with the molecular structure of xylene in mind we rationalize as follows. Perdeuterated xylene contains six aliphatic and four aromatic deuterium atoms. Typical rigid-limit values Qoof the quadrupole constants associated with aliphatic and aromatic deuterons are 165 and 190 kHz, respectively. Let us assume that the outermost peaks pertaining to component I1 represents the parallel peaks of an axial powder pattern; we measure their separation in frequency units to find that it is about 1/3 of that expected for a true rigid-limit spectrum, i.e. roughly 53.5 kHz. The splitting between the intense inner peaks, pertaining to component I, is about of the rigid-limit value, i.e. approximately 21 kHz. Finally, lowering the temperature is apparently affecting component I alone, and the gradual and continuous changes in the line shape are suggestive of slow-motional effects. All the features mentioned above are interpreted with the following model. First, we assume that internal methyl rotation is rapid at all temperatures. Then, for a typical value Qo = 165 kHz for aliphatic deuterons and a tetrahedral angle of 109O, we predict an axial powder spectrum associated with a partially averaged quadrupole constant of Q, = 165(1/z)l(3 cos3 109’ 1)1 = 56.3 kHz, in accord with our former assumption that the outermost peaks represent the perpendicular components of an axial powder pattern with a quadrupole constant of 53.5 kHz. We therefore associate component I1 with the methyl deuterons and component I with the ring deuterons. This assignment was confirmed by a ZHN M R spectrum obtained with a p-xylened4-cyclophosphazene adduct. As component I is narrower than
Cyclophosphazene Inclusion Compounds
,
J ,
-150
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 4311
QX105
0 KHr
-150
IS0
0 KHr
150
--J I
Ill
\
...
IOkHz
Figure 4. Same as in Figure 2 for CPZ-o-xylene-d,,. The dashed line superimposed on the 175 K experimental spectrum is a powder spectrum calculated with q = 0.932 and a natural line width of roughly 5% of eo.
-150
150 KHz Figure 3. (a) Theoretical *HNMR line shapes for a quadrupole tensor with Qo= 178 kHz; the intrinsic line width T2*-I = 2.4 kHz; calculations were done as described in ref 7c; L = 22 for the slower rates, and L = 8 for the faster rates used; 8 = Oo, R,, the rate perpendicular to the unique diffusion axis, as denoted; RIlcan assume any value since for an
0
axially symmetric quadruple tensor, motion about the principal axis does not affect the spectrum. The spectrum at the top is the rigid-limit pattern, obtained with R, 5 X lo2s-l. (b) 8 = 34' 44', R, = 1.0 X lo3 sd, and RII as denoted. (c) 8 = 5 5 O , R, = 1.0 X lo's-', and R,, as
denoted. 11, the molecule is necessarily experiencing a second dynamic process which affects the ring deuterons alone. A straightforward thought is considering reorientation about the molecular C2axis. If we assume that the symmetry of this process to is C,,or higher and its rate rapid, the powder spectrum of the ring deuterons will be reduced from, say, 190 kHz to QII= 190(1/2)(3 cos2 a - 1) kHz which, provided the aromatic ring is a perfect hexagon, Le., a = 60°, is predicted to be equal to 23 kHz. With the principal axis of the rapidly spinning methyl deuterons parallel to the C-CD3 bond, the angle a is zero for these nuclei and ring reorientation about C2 will not affect the methyl spectrum. The experimental value of the quadrupole constant associated with component I being 21 kHz, we conclude that all predictions of the model outlined above are borne out by the 360 K spectrum. We now inquire whether the evolution of the spectrum between 308 and 150 K can also be interpreted with this model. The motionally narrowed component I spectrum observed at 360 K is model insensitive, provided reorientation about the C2axis occurs by means of a process of symmetry C,, or higher. Slow-motional spectra are, however, model sensitive, and intricate details of the motion about C2need be specified for a quantitative analysis. In general, reorientation about a unique axis can proceed either diffusively or by means of discrete jumps compatible with the symmetry of the mobile molecule or with that of its immediate cyrstal environment. As questions related to the nature of such motions are repetitively addressed in the following, we show in Figure 3 illustrative examples of diffusive reorientation about axes tilted respectively at 0, 34.44, and 5 5 O relative to the principal axis of an axial quadrupole tensor for various motional rates. Calculations were performed using a general method to analyze slow-motional N M R spectra for Z = 1 nuclei7c based on the
previous work of Freed et al.7a In particular, the formalism employed is treating "very anisotropic" reorientation, i.e. relative!y more rapid rotation about a particular axis in a rigid molecule or a particular axis (or axes) of internal rotation in a nonrigid molecule which is, in general, tilted relative to the principal magnetic axes. The internal rotations are treated by Brownian diffusion with R,, and R, denoting the motional rates parallel and perpendicular to the diffusion axis, and the spin Hamiltonian consists of an anisotropic quadrupole interaction. It can be easily seen that for all values of a the line shapes evolve continuously and smoothly, as expected for an even-averaging mechanism. On the contrary, conspicuous sharp discontinuous features appear in slow-motion spectra generated by discrete jumps, examples of which we will be discussing below. In analogy with a recent 2H N M R study of the solid benzene-d6-cyclophosphazene inclusion compound,i3 we assume that C2is parallel to the channel axis d and the guest molecule is hopping between three equivalent minima in the free energy potential surface set up by the host lattice, compatible with the 32-site symmetry of the latter. For a perfect hexagon geometry of the p-xylene ring, these are threefold jumps about a diffusion axis tilted at a = 60° relative to the C-D bonds. We have calculated the deuterium N M R line shape with this model for decreasing motional rates and could not reproduce the typical features of the Figure 2 spectra (Le., the broad centrally located peak observed at 190 and 183 K and the sharp peaks appearing at lower and higher frequencies in the 175 and 150 K spectra). On the other hand, we found that the agreement between theory and experiment is quite remarkable with a = 67.5 f l o , as shown by comparing the experiment spectra in Figure 2 with the calculated line shapes in the Figure 2 insert. These results are interpretable in terms of a distortion of the benzene ring from a perfect hexagon. Indeed, there is recent experimental evidence obtained with N M R for deformed benzene rings in the solid state,'6a although there is no quantitative estimate as to the extent of deformation. Also, a former 2H N M R study of the benzene-d6-cyclophosphazene inclusion compound13 indicates that the hopping process of the aromatic rings about the channel axis is of a more complex nature than the postulated discrete jumps between three symmetrically disposed minima. It is possible that the combined effect of geometric deformations and complex dynamics manifests spectrally in the form of discrete jumps about a diffusion axis tilted, on the average, at 67.5 f l o relative to the aromatic C-D bonds, and we suggest this as a (16) (a) Suwelack, D.; Rothwell, W. D.; Waugh, J. S. J . Chem. Pbys.
1980, 73, 2559. (b) Schwartz, L. J.; Meirovitch, E.; Ripmeester, J. A.; Freed, J. H.J . Phys. Chem. 1983,87, 4453.
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4312
tentative interpretation of our experimental observations. 2H N M R spectra of o-xylene obtained as a function of temperature are shown in Figure 4. The 380 K spectrum is clearly dominated by an axial powder pattern associated with a quadrupole constant of 27 kHz, to be referred to below as component I; at low and high frequencies we observe very broad and diffuse peaks, disposed symmetrically with respect to the Larmor frequency, which apparently belong to a second component. At about 308 K the central portion of the spectrum changes precipitously; down to approximately 240 K we apparently encounter a two-phase region; at roughly 240 K the lower phase line shape stabilizes and remains invariant down to 175 K. Because of the very poor signal to noise ratio of the component I1 peaks, we feel we cannot qualify their temperature dependence, and therefore the subsequent discussion will focus on the component I spectrum. To interpret the o-xylene spectra, we adopt the model used to rationalize the 'H N M R line shapes of the p-xylene (and the benzeneI3) adducts. Namely, we assume that methyl groups spin rapidly about their symmetry axis; the aromatic rings orient on the average with their C, symmetry axis along the channel axis d, and the entire molecule rotates about C,. Should this latter motion be rapid, the ten deuterons of o-xylene will generate three axially symmetric powder spectra which, for a perfect hexagon, are associated with angles a = 30' for the six methyl deuterons and a = 30 and 90' for two pairs of ring deuterons. Consequently, the partially averaged quadrupole constants will be QII = 56.3(1 2)(3 cos2 30° - 1) kHz = 35 kHz for the methyl nuclei and Qr = 190(1/2)(3 cosz 30' - 1) kHz = 100 kHz and Q: = 190(1/2)(3 cos2 90' - 1) kHz = 125 kHz for the two pairs of aromatic nuclei. Experimentally, we observe at 380 K an axial powder spectrum associated with a quadrupole constant of roughly 27 kHz and broad peaks separately by approximately 80 f 10 kHz. We can interpret the 380 K spectrum with the model outlined above, associating the axial powder pattern (component I) with the methyl deuterons and the low- and high-field peaks (component 11) with the ring deuterons, provided we also allow for rapid fluctuations of C2 about the channel axis d which would further reduce 35 kHz to 27 kHz and 100 kHz (125 kHz) to roughly 70 kHz (90 kHz). Using component I, we estimate the order parameter at approximately 0.77. As already mentioned, we observe at roughly 308 K quite a dramatic change in the spectrum of the methyl deuterons: the parallel shoulders of the axial powder spectrum apparently preserve their position in field and the perpendicular peaks conspicuously split into two peaks. In other words, the axial powder pattern observed at high temperatures alters to become an asymmetric powder spectrum, with 7 at 175' estimated to be 0.932 (with 7 = (Vxx- V,)/V,, and IVJ > V Il, > lVxxl),with vi, ( i = x, y, z ) denoting the principal components of the quadrupole tensor in Cartesian coordinates, as shown by the calculated dashed line superimposed on the experimental 175 K spectrum. The asymmetric powder spectrum described above reflects a change in the dynamics experienced by the methyl deuterons.'4J6J7 Moreover, as the overall spectral width is preserved, it can only arise from a change in the nature of the rapid molecular reorientation about the C, symmetry axis, whereas internal methyl rotation stays fast. Although it is not possible to specify unambiguously the nature of these mechanistic changes, which, in most general terms, indicate that in the lower phase the symmetry of this motion becomes lower than C,,, one can envisage a simple process which would generate the N M R line shapes observed at the lower temperatures, namely 180'jumps about an axis tilted relative to the C-CD3 bonds. Obviously, the symmetry of this motion is compatible with the molecular symmetry if we take the jumps axis to lie parallel to the C,axis. We have calculated such spectra for various a values to find that the 175 K spectrum associated with 7 = 0.932 can be obtained
Meirovitch and Belsky
I
(17) Barnes, R. G. In "Advances in Nuclear Quadrupole Resonance"; Smith, J. A. S., Ed.; Heyden: London, 1972, Vol. 1, Chapter 26.
Figure 5. Same as in Figure 2 for CPZ-cyclohexane-d12.
with a = 54 f 1'. For a perfect hexagon geometry of the aromatic ring a = 60°;thus, as before, the N M R results are suggestive of geometric distortions and/or complex jump-type motions. Apparently, the symmetry characteristics of the mobile molecules on one hand and those of the host lattice on the other compete in determining the nature of the dynamics experienced, with the latter dominating in the upper phase and the former in the lower phase. Cyclohexane-d12 and Dioxane-d8. We pursued the study of guest structure and dynamics in CPZ adducts by examining several aliphatic guest molecules differing in structure and flexibility. 2H N M R spectra from a polycrystalline powder of CPZcyclohexaned,, are shown in Figure 5. First, note that all spectra are single-component line shapes identifiable with the common axial powder pattern, with structureless singlets centered at the Larmor frequency, or with a composite line shape given by a superposition of the two. Then, it is quite obvious from the considerable reduction in the overall width of the spectra relative to the rigid-limit pattern (see Figure 5 scale) that motional averaging necessarily takes place. Finally, the typical quadrupole structure, characteristic of most spectra in Figure 5, indicates that the motions experienced by the guest molecules at the appropriate temperatures must be anisotropic processes. In fact, these general characteristics impose rather severe restrictions on any structural and dynamic model one may suggest to interpret the Figure 5 line shapes, as spectral equivalence in the presence of anisotropic reorientation implies not only similarity of the rigid-limit magnetic parameters but also identity of geometric features such as the orientation of the various C-D bonds relative to the preferred axes of diffusion (see Theoretical Background section). In the particular case of cyclohexane, simultaneous ring inversion, which makes axial and equatorial deuterons equivalent, and rapid reorientation about the molecular C3,symmetry axis of the average moleclar geometry, which makes all deuterons equivalent, are inferred by this uniformity in the spectral response, and we could not conceive of any other physically plausible alternative.
Cyclophosphazene Inclusion Compounds
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984 4313
Let us now calculate the magnitude of the quadrupole constants predicted with this model. Ring inversion can be visualized as a twofold jump process about an axis bisecting the D-C-D angle whereby the axial and equatorial deuterons are being interchanged. In the limit where the rate of this motion is of the order of lo7 s-l or higher (see, for example, ref 8g), an asymmetric tensor Q obtains, with the following values for the quadrupole constants along its principal axes x , y, and z : ,6 = -D, ,6 = D(3 cosz a - l), and ,a, = D(2 - 3 cos2 a),where D 3eqQo/4h and a is the tilt angle between the jump axis and the two C-D bonds. For typical values of Qo= 165 kHz and 2a = 109’, one obtains, ,a = 123.8 kHz, Syy = -1.44 kHz, and 6,, = 125.2 kHz. This tensor (Q) is further averaged to Q by rapid reorientation about the C,, axis. The orientation of the C,, axis in the x,y,z frame is defined by the polar angle 0 = 33.8’ and the azimuthal angles q5 = -90°, and Q was shown previously7c to be axially symmetric. Then 6 , = 3/4Q’ is given by the expression7c
6, = (1/3)(1/2)(3
COS’
0 - 1)6,
+
/J”
300
+
- (1/2)(6,, 6,,y) (3/4)(6,, - fiyyyY) sinZ0 cos 24
(the factor I / , comes from the fact that 6i, = 2oIi and 6,, = in terms of the uII)s given ( 2 / 3 ) z , with the expression for in ref 7c). By inserting the proper figures, we obtain 61 = 43 kHz, subject to possible variations in the values of Qo and 2a. Yet, none of the spectra in Figure 5 bear out these expectations, as the largest 6 value, measured at 190 K, is of the order of 15 kHz, rather than 43 kHz. Moreover, the dramatic temperature-induced spectral alterations clearly indicate that dynamic processes, other than the two motions discussed above, must necessarily operate. There are two simple models which would explain both the absolute and the relative magnitude of the partially averaged quadrupole tensors associated with the Figure 5 line shapes. One was invoked in a previous study of the thiourea-cyclohexane-dI2 adductsh in assuming that the cyclohexane rings are tilted relative to the channel axis d about which they reorient rapidly, with concomitant rapid reorientation about the molecular symmetry axis C,,, implied by ring inversion. The overall width of the resulting axial powder patterns are then “scaled” by the factor (1/2)(3 cos2 { - l ) , with {denoting the angle between the C,, axis and d. the 190 K spectrum is typical of a phase we call “phase I” with { = 41.2O which persists up to 255 K. At 223 K an additional component, which appears to be a relatively narrow structureless line centered at the Larmor frequency, becomes visible. We associate the latter with a new phase (11) and realize that, within this phase {increases gradually from 56.3’ at 255 K to 43.4’ at 360 K (alternatively, {is equal to 68.2’ a t 190 K, 56.3’ at 255 K, and 66O at 360 K, an ambiguity inherent in the deuterium spectrum). The second model consists in assuming rapid ring inversion of (on the average) horizontal cyclohexane rings and rapid fluctuations, with temperature-dependent amplitudes, of the molecular symmetry axis about the channel axis d. The width of the powder pattern obtained as a result of ring inversion will be, therefore, further reduced, according to the amplitudes of these rapid vibrations. We materialize these in terms of an order parameter S , as discussed previously, which is, in fact, equal to the previous geometric factor of (1/2)(3 cosz { - 1). We obtain S = 0.35,0.04, and 0.29 at 190, 255, and 360 K, respectively, assuming “positive” ordering, i.e. preferential orientation of the C,, axis parallel to d . (Note that negative ordering, associated with absolute values of the order parameter S twice as large as that of positive ordering, is ruled out, as at some temperatures S would be smaller than
-0.5). The line shapes observed at 190 and 155 K are indicative of slow-motional effects. We cannot, for technical reasons, explore the entire slow-motional regime which would, very likely, allow us to differentiate between the two models suggested above and gain deeper insight into the microscope characteristics of the dynamics. This is, at present, prevented by the very limited
I *,% ZOKi-z
t_
Figure 6. Same as in Figure 2 for CPZ-dioxane-d8
temperature range over which slow-motional spectra are being observed. The diOXane-ds spectra shown in Figure 6 are visually similar to the cyclohexane-dlz spectra shown in Figure 5. However, we measure Q = 50.8 kHz at 160 K, which is larger than the maximum value of 43 kHz predicted with cyclohexane for rapid ring inversion. This is not surprising, as inversion of the dioxane ring may not occur under similar circumstances and, moreover, since dioxane does not possess a C,, symmetry axis. On the other hand, the axis being within the plane of the four carbon atoms parallel to the CD2-CDzbonds is a Dzhmolecular symmetry axis, and rapid spinning of the molecule about it would render all eight deuterons equivalent and result in an axial powder pattern of the order of 50 kHz, taking the tetrahedral angle to be 109.4’. To explain the temperature dependence, we again invoke either rapid reorientation about Dzhand d combined with molecular tilt within the channels or fast spinning about DZh,with the latter fluctuating rapidly about d . Assuming S = 1 at 190’, we take the tetrahedral angle to be 109.4’ and obtain S = 0.02 and 0.1 a t 240 and 370 K, respectively. Alternatively, with the model wherein the dioxane ring, similar to the cyclohexane ring, is inclined relative to the channel axis d with a temperature-dependent tilt angle {, we obtain { = 0, 53.9, and 50.8’ at 190, 240, and 70 K, respectively. Note that with dioxane two phases coexist over much of the temperature range investigated, and there appear to be three distinct phases, with the 160 K spectrum typical of phase I, the 240 K spectrum characteristic of phase 11, and the major component of the 370 K line shape associated with phase 111. We wish to highlight the unique observation that, within quite a large temperature range, molecules such as cyclohexane-d,, trapped within thiourea inclusion channels,sh cyclohexane-dI2, dioxane-& THF-d,, and acetone-d6 (see below) trapped within the CPZ tunnels give narrow-line 2H NMR spectra. This indicates (a) isotropic (or nearly so) molecular reorientation, (b) incidental molecular tilt at the magic angle combined with particular assumptions regarding structure and dynamics, or (c) prevalence
4314
Meirovitch and Belsky
The Journal of Physical Chemistry, Vol. 88, No. 19, 1984
TABLE I: Quadrupole Constants Q , Natural Line Width T2-' *, and Corresponding 01 Values temu. K T,*-', kHz 0,kHz deg 3 2 1
370 300 220 370 300 220 370 300 220
1.7 2.5 2.5 1.7 1.7
5.9 1.7 1.7 1.7
8.7 1.7 5.7 23.3 26.0 30.0 31.3 36.0 46.1
52.8 53.0 53.4 49.6 49.0 48.1 47.8 46.8 44.5
56.6 56.4 56.0 59.8 60.4 61.3 61.6 62.6 64.9
"Calculated with Qo= 178 ~ H z . ' ~
Figure 7. Same as in Figure 2 for CPZ-THF-$. The three groups of C-D bonds, classified according to their tilt a],CY*, and 0 1 ~from the channel axis d, are labeled 1, 2, and 3, respectively. The right insert is a room-temperature spectrum obtained by accumulating approximately 25 000 scans, with arrows denoting the three pairs of parallel peaks in the powder spectra of 1, 2, and 3. The left insert is a structure of the C, half-chair conformer and its intrachannel orientation. of a cubic phase combined with rapid translational diffusion and thereby effective averaging of the various sites in the polycrystalline powder. The latter alternative is ruled out by X-ray crystallography experiment^^^ and by measurements of the translational diffusion constants;'S2 the second alternative involves peculiar fortuitousness. The first hypothesis entails the unusual phenomenon of increasing order with increasing temperature within a given phase (see Figure 6). We offer both models a and b as plausible alternatives. THF-d8 and Acetone-d6. 2H NMR spectra from a polycrystalline powder of CPZ-THF-d8 are shown in Figure 7. Although rich in features, the spectrum appears to consist of three superposed axial powder patterns, with an additional contribution from a centrally peaked line. Indeed, at all temperatures we observe three pairs of symmetrically disposed peaks, corresponding to the perpendicular components of a given powder spectrum, and three pairs of weaker peaks, with a twice larger splitting, corresponding to the parallel components. The right insert in Figure 7 is a room-temperature spectrum obtained following extensive signal averaging. The parallel shoulders, which in most spectra in Figure 7 are quite weak, are prominent in the insert. All powder spectra in Figure 7 are considerably narrower than the true rigid-limit spectrum. Again, a simple and straightforward interpretation of the powder pattern associated with a static C-D bond being averaged into an axial powder pattern of reduced overall breadth is to assume that the particular C-D bond under consideration is spinning rapidly about a diffusion axis tilted at an angle a. The three spectral components would then correspond to different tilt angles a , and the temperature dependence of the various splittings are suggestive of a being temperature dependent. On the basis of these considerations, we suggest the following model for the conformation of THF, its orientation within the channel, and the nature of the anisotropic dynamic process it experiences. Let us consider spinning about the channel axis d of a C, envelope conformer shown in Figure 7, with d being within the plane defined by the four carbon atoms. This would classify the eight deuterons into three groups denoted respectively as 1, 2, and 3 (see Figure 7, insert), according to the magnitude of the angle a the corresponding C-D bonds form with d. If one assumes that reorientation about d is fast and uniform, the three different values of a associated respectively with the deuterium atoms labeled 1, 2, and 3 on the schematic structure of THF in Figure 7 will generate three distinct axially symmetric
powder patterns with splittings 61, 62, and 83 given as 6, = Qo(1/2)(3 cosz ai- 1) and relative intensities of 1:1:2, in accord with the number of deuterium atoms in each group. The partially averaged quadrupole constants Q, = (4/3)6, are, therefore, given uniquely by the magnitude of a,.Hence, spectral simulation consisted in superposing three axial powder patterns with Q, and the natural line width T2*-I (which are the two parameters determining such a spectrum) allowed to be free variables and relative intensities as noted above. The dashed lines in Figure 7 are calculated line shapes obtained with the best-fit values of a j (i.e., Q,) and T2*-' from Table I. As the sign of the quadrupole constant is not known, a particular axial powder pattern can be reproduced with two different values of a. We find that the D-C-D angles of the group 3 deuterons increase (decrease) from 105.6 (113.2') to 106.8' (112.0') from 370 to 220 K whereas the D-C-D angles adjacent to the oxygen decrease (increase) from 107.6 (111.2') to 105.8') (113') over the same range of temperatures. (The latter statement is based on the requirement that the magnitude of the various D-C-D bond angles be close to the typical tetrahedral value of 109', implying that the series of smaller (larger) a1values be associated with the series of larger (smaller) az values). This ambiguity can be rather than an I = 1 nucleus, resolved, in principle, with an I = and it is our intent to study the I3C spectrum of T H F to this end. A comment on the uniqueness of our model is in order. Classification of the eight T H F deuterons into three groups of respectivelyfour, two, and two equivalent atoms (equivalence being defined by the relative orientation of the C-D bond and the channel axis d) imposes very restrictive requirements on the conformation of the molecule, its intrachannel orientation, and the nature of its dynamics. Expect for the C2envelope conformation, theoretical calculations also invoke a C, half-chair conformation for THFS9 However, we could not conceive of any plausible anisotropic motion of this conformer compatible with the experiment. There is a systematic discrepancy in the relative intensities of the three spectral components of a given spectrum in Figure 7, in terms of a clear theoretical deficiency in the contribution of components 1 and 2 relative to that of 3. We have checked whether differences in the spin relaxation times Tl associated with the three individual components may (through selective saturation in the signal-averaging process, whereby subsequent experiments are being performed with the magnetization allowed to recover in-between consecutive experiments) account for this observation and found that varying the delay time between runs from 1 to 10 s had no effect on the NMR line shape. We believe that these discrepancies are due to the contribution of the centrally peaked spectral component, visible in all spectra shown in Figure 7. We refer the reader to our previous discussion of the dioxane-d8 and cyclohexane-d,2 adducts and to the corresponding figures (5 and 6), where we have pointed out that very broad two-phase regions are apparently common phenomena in crystalline inclusion compounds. A similar phenomenon was Hence, detected in a recent study of thiourea-cy~lohexane-d~~.~~ we may regard all spectra in Figure 7 as reflecting an extremely broad two-phase region. Since the precise shape of the centrally peaked line is not known, we have not carried out extensive simulations aimed at an exact reproduction of the experimental traces
Cyclophosphazene Inclusion Compounds
Figure 8. Same as in Figure 2 for CPZ-acetone-d6. The arrrows denote the parallel and perpendicular peaks in the powder spectra associated with the two groups of acetone molecules, as explained in the text. The orientation of acetone within the channels is arbitrary (see text). The insert shows *H NMR spectra from a polycrystalline powder of acetophenone-d,-apocholic acid inclusion compound at room temperature and at 150 K.I8
in Figure 7. Qualitatively, however, a spectral component with sharp features at zero frequency and broad wings would contribute to a higher extent at lower and higher frequencies and less to the central portion of the spectrum, in accord with our experimental observations. In principle, the partially averaged quadrupole tensors associated with the experimental spectra may reflect both rapid spinning about the molecular symmetry axis and less than perfect ordering (which we assumed above in calculating the a values) about the channel axis d . The very reasonable values we obtained for the various D-C-D bonds, which are all, as expected, in the vicinity of the 109O tetrahedral angle, indicate that ordering is close to being perfect, justifying our taking S = 1. Yet, the precise values of the various bond angles are subject to the uncertainty in the value of S , although for very high ordering, spectral sensitivity to variations in S is quite low.l5 2H N M R spectra from a polycrystalline powder of CPZacetone-d6 are shown in Figure 8. The 165 K spectrum is readily identified with the common axial powder pattern with Q = 42.3 kHz measured directly from the spectrum with the splitting between the low- and high-field shoulders, corresponding to the parallel orientations, twice the splitting of the perpendicular peaks. As mentioned previously, we expect internal methyl rotation to reduce the quadrupole constant to roughly 56 kHz. Hence, additional motional averaging must operate in these compounds. Further reduction in the overall width of the spectrum is observed upon heating. Concomitantly, the typical features of the axial powder pattern, conspicuous and sharp at 165 K, become somewhat diffuse at the higher temperatures. Thus, the position of the outer shoulders becomes blurred and the relatively sharp decrease to the base line is replaced by what appears to be a stepwise drop in intensity. Likewise, the relative intensity of the central part of the spectrum increases and a pair of weak yet noticeable peaks become visible (see inner pair of arrows in Figure 8). To interpret the temperature-dependent reduction in the overall spectral breadth relative to 56 kHz, we suggest a rapid wobbling motion of the C-CD3 diffusion axis about a mean orientation
The Journal of Physical Chemistry, Vol. 88, No. 19, I984
4315
within the channels, with the wobbling amplitude decreasing and, hence, the value of Q increasing upon cooling. We also offer a tentative interpretation of the line-shape alterations described above. It is suggested that the 308 K spectrum is a composite line shape consisting of two axially symmetric powder spectra, the four pairs of parallel and perpendicular peaks denoted by arrows in Figure 8 corresponding to the extreme orientations. From the perpendicular peaks we estimate QI= 36 kHz and Qn = 28 kHz, with I and I1 denoting the two components. These may be related to the two methyl groups of a given acetone molecule which, due to inequivalence in their spatial disposition within the inclusion channels, experience more or less motional freedom. On the other hand, we may encounter two different acetone sites within the channels which differ in the extent to which the C-D, bonds are spatially restricted by their immediate environment. In any event, the wobbling amplitude is apparently reduced upon cooling, with the differences in local mobility between the two types of methyl groups becoming negligible at 165 K. As spectral resolution is quite low, these latter thoughts should be considered with some reservation. It is, however, of interest to note that spectra very similar to, yet better resolved than, the 308 K spectrum in Figure 8 were obtained recently with acetophenone-d,-deoxycholic acid adducts,'* where prevalence of two acetone sites within the channels at ambient temperature was confirmed by X-ray crystallography. In support of our two-site suggestion we present these results as an insert in Figure 8. The presence of a second site at room temmperature is conspicuous and so is the additional motional averaging due to the wobbling mode. Upon cooling to 150 K, both components converge to the axial powder spectrum expected for a rapidly reorienting methyl group about a unique C-CD3 axis. Note that contrary to the 2H N M R results obtained with all the other guests, the acetone-d6 results do not infer reorientation about, or ordering with respect to, the channel axis. Hence, the intrachannel orientation of this guest cannot be specified.
V. Conclusion Dynamic solid-state 2HNMR line shapes from guest molecules trapped within CPZ inclusion channels were analyzed in terms of molecular conformation, dynamics, and intrachannel orientation. Extension to studying other nuclei, lower temperatures, and employing single crystal crystals are suggested by these results. Information on intrachannel orientation, molecular geometry, the nature and rate of internal and overall dynamic models, and temperature-induced changes in these parameters was obtained. Acknowledgment. This study was made possible in part by funds granted by the Charles H. Revson Foundation (to E.M.) and the Minerva Foundation of Munich, West Germany. Registry No. p-Xylene, 106-42-3; o-xylene, 95-47-6; cyclohexane, 110-82-7;tetrahydrofuran, 109-99-9; acetone, 67-64-1; dioxane, 1239 1-1; hexachlorocyclotriphosphazene, 940-71-6; tris( 1,2-dioxyphenyl)cyclotriphosphazene, 91 190-13-5;p-xylene-dlo,4105 1-88-1;o-xylene-dlo, 56004-61-6;cyclohexane-d12,1735-17-7;tetrahydrofuran-d8,1693-74-9; acetone-& 666-52-4;dioxane-d,, 17647-74-4. (18) Meirovitch, E. J . Phys. Chem., submitted for publication.
