J. Phys. Chem. 1982, 86, 3061-3064
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High-Resolution Siiicon-29 Nuclear Magnetic Resonance Spectrum of Zeolite ZK-4: I t s Significance in Assessing Magic-Angie-Spinning Nuclear Magnetic Resonance as a Structural Tool for Aluminosilicates J. M. Thomas,' C. A. Fyfe,* S. Ramdas,+ J. Kilnowski,t and 0. C. Gobbit Department of Physicel Chemistry, Unlverslty of Cambridge, Cambridge, CB2 lEP, United Kindgom and Gueiph-Waterloo Centre for Orsduate Work in Chemishy, Universtty of Oueiph, college of Physical Sciences, Gueiph, Ontario N1G 2W1, Canads (Received: March 30, 1982)
The '%i MAS NMR spectrum of zeolite ZK-4, which has the same aluminosilicate framework as zeolite A but a Si/A1 ratio greater than unity, enables the nature of the Si,A1 local ordering to be determined. In line with the results of a recent Rietveld neutron powder diffraction analysis of T1-A, the MAS NMR spectra of ZK-4 point strongly in favor of the 4:O rather than the 3:l ordering scheme previously proposed. We suggest that the abnormal position of the Si(OAl)4MAS NMR signal in zeolites A and ZK-4 is due to the presence of a nearly linear T-0-T linkages in the aluminosilicate framework.
Introduction 29si magic-angle-spinning NMR (MAS NMR) and, more recently, ?-'A1 MAS NMR have contributed greatly to our understanding of the structure of aluminosilicates. Since these variants of the original MAS experiment' were introduced by Lippmaa and Engelhardt2J nearly 3 years ago, several major advances in the structural chemistry of zeolites have been registered through their agency. First, 29SiMAS NMR is capable of directly detecting the proportions of the five distinct structural groups4+ Si(OAl),(OSi), (n = 0, 1, ..., 4) which may be present in a zeolite of arbitrary Si/A1 ratio. Secondly, as MAS NMR is quantitatively reliable, it provides a convenient new method of determining Si/A1 ratios of the tetrahedral aluminosilicate framework7* (see below). Thirdly, it is capable of monitoring the structural reorganization of TO4 (T5 Si4+or AP+) units during the course of thermally induced solid-state processes such as ultrastabi1ization.lO Moreover, it can resolve" numerous crystallographically nonequivalent Si(OSi)4sites in ZSM-5/silicalitea And it throws new light on short-range Si& ordering in synthetic faujasites (zeolitesX and Y)covering a wide range of Si/Al r a t i o ~ . ~ * ~Furthermore, J?27AlMAS NMR easily distinguishes tetrahedrally from octahedrally coordinated AP+ ions in aluminosilicates-it has identified'O framework A1 in silicalite-and, combined with ?3i MAS NMR, it enables S i / A ratios to be quantitatively estimated in highly siliceous materials (such as silicalite/ZSM-5) even when these ratios exceed 1000. Another consequence of the application of 29SiMAS NMR to aluminosilicate structures has been the indication of the existence of two different kinds of Si,A1 ordering in certain zeolites. For example, evidence has been presented13 for the occurrence of two kinds of cancrinite (idealized formula Na&&S&ON.24H20),one with 4:O (i.e. Si(OAI),) and another with 3:l (i.e. Si(OAI),(OSi)) local ordering. But one of the most surprising results was the evidence12~~~J~ from %Si MAS NMR, that, in zeolite A, 3:l rather than the currently accepted 4:O Si,A1 ordering was found. 'Address correspondence to this author the University of Cambridge. University of Cambridge. University of Guelph.
*
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Although three other experimental techniques (X-ray, electron, and neutron diffraction) all yield14JSbsuggestive information which, to a greater or lesser degree, is incompatible with the accepted 4:O (Fm3c) model for the structure, it is the single dominating resonance at -89.7 f 0.5 which is the most compelling item of evidence supportive of the 3:l ordering scheme. It has been argued,'& incorrectly, that 3 1 ordering gives a unit cell of 12.1 A and a space group Pm3, and that the 3:l ordering scheme is not compatible with the 24.0 A repeat found in X-ray diffraction studies. Taken in conjunction with powder (neutron) diffraction data, which revealed14 small but unmistakable, and hitherto undetected, rhombohedral distortion in dehydrated Na-A zeolite, the MAS NMR data led to the formulation of an alternative structure model based on 3 1 ordering and described fully elsewhere14 with the space group being R3. (1)E. R. Andrew, A. Bradbury, and R. G. Eades, Nature (London), 182,1659 (1958). (2)G. Engelhardt, D.Kunath, M. M U , A. Samoson, M. Tarmak, and E. Lippmaa, Workshop on Adsorption of Hydrocarbons in Zeolites, Berlin-Adlershof, Nov, 1979. (3)(a) E. Lippmaa, M. Miigi, A. Samoson, G. Engelhardt, and A.-R. Grimmer, J.Am. Chem. Soc., 102,4889-93 (1980);(b) E. Lippmaa, M. Miigi, A. Samoson, M. Tarmak, and G. Engelhardt, ibid., 103, 4992-6 (1981). (4)S. Ramdas, J. M. Thomas, J. Klinowski, C. A. Fyfe, and J. S. Hartman, Nature (London),292,228-30 (1981). (5)G. Engelhardt, U. Lohse, E. Lippmaa, M. Tarmak, and M. Miigi, 2.Anorg. Allg. Chem., 482,49-64 (1981). (6)M. T. Melchior, D. E. W. Vaughan, and A. J. Jacobson, J. Am. Chem. Soc., in press. (7)E. Lippmaa, G.Engelhardt, et al., Zeolites, in press. (8)J. Klinowski, S. Ramdas, J. M. Thomas, C. A. Fyfe, and J. S. Hartman, J. Chem. Soc., Faraday Trans. 2, in press. (9)J. M. Thomas, S. Ram&, G. R. Millward, J. Klinowski, M. Audier, J. Gonzalez-Calbet, and C. A. Fyfe, J.Solid State Chem., in press. (10)J. Klinowski, J. M. Thomas, C. A. Fyfe, and G. C. Gobbi, Nature (London),296,533-6 (1982). (11)C. A. Fyfe, G. C. Gobbi, J. Klinowski, J. M. Thomas, and S. Ramdas, Nature (London),296,530-3 (1982). (12)G. Engelhardt, E. Lippmaa, and M. Magi, J.Chem. SOC.,Chem. Commun., 712-3 (1981). (13)J. Klinowski, J. M. Thomas, C. A. Fyfe, and J. S. Hartman, J. Phys. Chem., 85, 2590-4 (1981). (14)L. A. Bursill, E. A. Lodge, 3. M. Thomas, and A. K. Cheetham, J. Phys. Chem., 85,2409-21 (1981). (15)(a) J. V. Smith and J. J. Pluth, Nature (London),291,265(1981); (b) L. A. Bursill, E. A. Lodge, and 3. M. Thomas (reply to J. V. Smith and J. J. Pluth), Nature (London),291,265-6 (1981).
0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86, No. 16, 1982
Very recently, however, further evidence has come to light, which prompts a reexamination of the structure of zeolite A. This new information consists of the following facts: (a) The rhombohedral distortion, present in dehydrated Na-A, is vanishingly small16 in dehydrated TI-A and dehydrated Ag-A. (b) The extent of rhomobohederal distortion, now shown to be detectable in refined X-ray work, is dependent upon the degree of hydration of the Na-A; there are indication~l~ that the crystal symmetry of the dehydrated material may also be a function of temperature. (c) Our recent neutron powder diffraction data for TI-A refines smoothly16 in the cubic space group Fm3c and yields convincing bimodal T-O separation distances ( S i 4 = 1.599 A, A1-0 = 1.728 A). Unlike any other zeolite, Linde A contains the so-called double four-membered rings in the structure, and each tetrahedral Al atom is part of one such double ring. It has been suggested that the intrinsic strain associated with this unique structural feature causes MAS NMR resonances to be shifted with respect to corresponding resonances in other zeolites. This led us to design an experiment which would test afresh the correctness of the assignment of the single resonance in Linde A to a 3:l local ordering. The strategy adopted consists in using zeolite ZK-4 which has been shown18to posseas the same aluminosilicate (TO,) framework as zeolite A, but its &/A1 ratio, unlike that of zeolite A where it is close to unity, is in the vicinity of 1.5. It must, therefore, exhibit seuerul %Si MAS NMR peaks, just as zeolites X and Y do. One of the peaks in the spectrum of ZK-4 should therefore correspond with the peak in the spectrum of Linde A. We have shown89g that, for a given aluminosilicate structure, 29Sichemical shifts of Si(OAl), and Si(OAl),(OSi) peaks are only marginally dependent on composition. Thus, assignment of the signal in the spectrum of Linde A is in principle possible by matching it with one of the peaks in the spectrum of such highly siliceous zeolite even if the ordering pattern of the latter is different. Moreover, by determining the Si/Al ratio independently (using analytical electron microscopy) one may further test whether the resonance at -89.7 is attributable to 3:l or to 4:O by comparing the (Si/l&MR ratio (calculated as shown below) with that obtained by electron microscopy using X-ray emission (@i/A1)xRd.
Materials Zeolite ZK-4, synthesised from the mixture of sodium hydroxide and tetramethylammonium hydroxide (TMAOH) as bases, is isostructural with zeolite A, but its chemical composition is significantly different. A typical unit cell formula of ZK-4 is Na7(TMA)2[(A102)?(Si02)15].27H20,as compared with Na12[(A102)12(S~02)12]-27H20for Linde A. Its (cubic) unit cell parameter is smaller by 1.33%,on account of the higher &/A1 ratio. The tetramethylammonium cation is too large to pass through the zeolitic channels and cannot be exchanged for another cation. The sodium form of ZK-4 is prepared by calcining at 550 “C in an air purge and subsequent treatment with sodium hydroxide solution. This materials may be regarded as a “high-silica”variant of zeolite A. Two separate preparations of TMA-ZK-4 were kindly provided
TABLE I: Chemical Shifts (in ppm from Tetramethylsilane) of the Single MAS NMR Signal Observed in Various Cationic Forma of Zeolite Linde A cationic form Na-A Ba-A Ag-A Ag-A with enclatherated AgNO, Tl-A Li-A
-88.9 -90.5 -87.5 -88.2 -88.8
-85.1
* 0.5 * 0.5 f f
0.5 0.5
? f
0.5 0.5
by Dr. L. V. C. Rees of Imperial College, London. Spectra of both calcined and uncalcined preparations of zeolite ZK-4 have been recorded. We have previously examined a wide variety of cation-exchangedzeolite A samples. The relevant information pertaining to them is given in Table I.
Experimental Section ?3i MAS NMR spectra were recorded at 79.80 MHz on a Bruker WH-400 multinuclear spectrometer equipped with a home-made single-frequency probe. Rotors were spun at a rate of 3.0 kHz. Intervals of 5 s were allowed between pulse sequences and 3000 free induction decays were accumulated per sample. A tenfold variation in the repeat time of the experiment produced no perceptible change in the relative intensities of peaks in the spectrum which indicates that they are quantitatively reliable. Spectra were computer simulated by using Gaussian peak profiles, and the relative intensities were calculated. Chemical shifts were measured from tetramethylsilane with high-field shifts being negative. The Si/A1 ratio of the zeolites (designated (Si/Al)-) was determined by analytical electron microscopy; X-ray emission peaks of Si and A1 (Ka) were compared with those of a variety of known standards.
Results For the two samples of ZK-4 we studied, the (SiAl), were 1.77 (sample 1)and 1.56 (sample 2), similar to that of the sample described by Kerr and Kokotailo.ls” Relative peak intensities (normalized to 100) of uncalcined samples are given in Table I1 (see Figure 1). Spectra of calcined samples were essentially very similar, although the chemical shifts of all the peaks were approximately 1 ppm to higher field than in uncalcined preparations. 27AlMAS NMR spectra were measured at 104.22 MHz with a home-made probe. For all the samples a single absorption was observed at 58.5 f 0.5 ppm from [Al(H20)6]3’,a value typical for zeolite A. It has been shown elsewhere5g8that the Si/Al ratio may be determined from %Si MAS NMR spectral data. Assume that the first-order neighborhood of every A1 atom is Al(OSi), (i.e., that the so-called Loewenstein rule applies). Then each Si-0-Al linkage of a Si(OAl), structural unit is equivalent to A114 atoms. It follows that the Si/Al ratio in the tetrahedrally bonded framework is 4
2 Isi(oM), n=O
(Si/Al)NMR = (16) A. K.Cheetham, M. Eddy, D. A. Jefferson, and J. M. Thomas, submitted for publication. (17) T. Rayment, to be published. (la) (a) G. T.Kerr and G . T. Kokotailo, J. Am. Chem. Soc., 83,4675 (1961); (b) G.T. Kerr, Znorg. Chem., 5, 1537-9 (1966); (c) G.T. Kerr, U S . Patent 3 314 752 (1967).
chemical shift
n=O
(1)
o*25nISi(OAl)n
where Isi(oAl) represents the relative intensity of the Si(OAl), signaf. Equation 1 is valid if, and only if, no Al-
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TABLE 11: Chemical Shifts in ppm from MeJi (Upper Values) and Relative Signal Intensities in Percent of Total Signal (Lower Values) for the Two Uncalcined Samplesa of Zeolite ZK-4 sample
(Si/Al)XRE
1
1.77
-87.9 -92.7 -98.3 -104.8 -109 16.9 34.2 26.9 16.3 5.7 2 1.56 -83.6 -88.4 -93.3 -99.3 -105.2 -1 10 2.1 26.4 36.9 18.5 11.6 4.5 a In both cases a very weak signal (marked “m”in Figure 1) was also measured at -75.9 ppm, corresponding to a metasilicate impurity.
-70
-90
-110
ppm from T M S
FlQve 1. %i MAS NMR spectra of two uncalcined samples of zeoHte ZK-4 at 79.80 MHz. Chemical shifts are given in ppm from tetramethylsilane. Numbers above NMR signals are “ n ” in SYOAl),(OSI), for the most likely assignment. Signals marked with “m” are caused by small amounts of Slot- (metasilicate) impurity. (a) (SiIAlh, = 1.77 (sample 1); (b) (Si/Al)XRE = 1.56 (sample 2).
0-Allinkages are present. It is thus a convenient tool for checking whether the Loewenstein rule applies in a particular case. We have shown it to be so in a wide range of synthetic faujasites in which (Si/A1)NMRagrees very closely with the value obtained by XRE. Discussion Table I1 reveals that in sample 1 there is no peak at -83.6 ppm (where the &(ON), signal normally occurs), and that in sample 2 there is only a very small signal in this region, corresponding to 2.1% of the total signal intensity. This we have assigned to an impurity, such as a small amount of zeolite X which is known to cocrystallize with highly siliceous A synthesized from nitrogenous bases. For both samples there is a significant signal at -88.4 and -87.9 ppm, respectively. In order to ascertain to what extent the position of this peak is affected by the TMA+ cation, we have measured the spectra of the calcined samples 1 and 2. In both cases the chemical shift of the first significant peak is virtually identical with that observed in Na form of Linde A (-88.9 ppm), and only approximately 1 ppm different from that of the uncalcined samples. There are two possibilities for the assignment of the signals in the spectrum to different Si(OAl)n(OSi)4-n structural units. First, the five peaks can be attributed, from left to right (as indicated in Figure 1) to Si(OAl),, Si(OAl),(OSi), Si(OAl)2(OSi)2,Si(OAl)(OSi),, and Si(OSi), building blocks, respectively. Now, assuming the validity of the Loewenstein rulemin the framework of zeolite ZK-4, (19) V. Gramlich and W. M. Meier, Z.Kristallogr., 133, 134 (1971).
we can use eq 1 to calculate (Si/Al)”. This yields 1.66 for sample 1 and 1.51 for sample 2, both close within the experimental error to the compositions determined by XRE. Alternatively, we may assume that no Si(OAl)4units are present in either sample and that the first four peaks in each spectrum correspond to Si(OAl),(OSi), Si(OAU2(0Si)2,Si(0Al)(OSi),, and Si(OSi),, respectively. In this case the signals at -109 ppm in sample 1 and at -110 ppm in sample 2 (amounting to ca. 5% of total intensity) have to be attributed to amorphous silica, not otherwise easily detectable, present in the sample. Equation 1 gives for (Si/Al)” the values of 2.74 in sample 1 and 2.22 in sample 2, both very different from the real zeolite composition. It is clear that the first assignment of the NMR signals (as indicated in Figure 1) is correct. As a consequence, the single peak observed in the spectrum of Linde A must be assigned to Si(OAl), and not to Si(OAl),(OSi). The values of 2eSichemical shifts for the Si(OAl), peaks in zeolites X and Y (synthetic faujasites*) range between -83.7 and -84.1 ppm, whereas in ZK-4 and Linde A Si(0Al)4resonates at ca. -88 ppm. How can this difference be explained? A unique feature of the structure of zeolites A and ZK-4 is the presence of the already mentioned strained double four-membered rings, with T-0-T angles21p22of 129, 152, 152, and 177’. We believe that the presence of a linear or nearly linear T-0-T linkage modifies= the bonding (i.e., T-O attraction and T-T repulsion) characteristic for a the central Si atom in a Si(OAl),(OSi), unit, which in turn affects the value of the chemical shift at which this Si atom resonates. In silicalite,l’ where linear T-0-T linkages are present, the various crystallographically nonequivalent Si(OSi), structural units resonate in the range of -109 to -116.3 ppm, whereas in zeolites X and Y in which T-0-T angles are close to the mean value of ca. 145’ observed in the majority of silicates and alumin o s i l i c a t e ~the , ~ ~Si(OSi)4signal appears between -101.9 and -106.0 ppm. It seems therefore that there is at least (20) It can be argued that the spectra can be interpreted in terms of notional ”rearrangement”of the Si0,’- and NO4&tetrahedra, as discussed in connection with synthetic faujasites.8 Imagine that Al atoms are being replaced by Si in a non-loewensteinian Si(OAl),(OSi) structure of zeolite A, so that the framework compositions corresponding to those of samples 1 and 2 are reached. As a result of such a rearrangement some A1-O-Al linkages remain. If a fraction p of all Al atoms belong to Al(OSi),(OAl) groupings and a fraction (1 - p ) belong to A1(OSiI4groupings, it can be shown that
This expression is more general than eq 1 and reduces to it for p = 0. When the Si/Al ratio is determined by chemical methods, the value of p can be calculated, yielding 0.49 for sample 1 and 0.61 for sample 2. As p must decrease as the Si/Al ratio increases, the trend observed is in the wrong direction. Another drawback of this interpretation, which we feel must be rejected, it that it cannot account for the NMR signals observed at -109 ppm in sample 1 and -110 ppm in sample 2. (21) J. J. Pluth and J. V. Smith, J . Phys. Chem., 83,741-9 (1979). (22) J. J. Pluth and J. V. Smith, to be published. (23) E. P. Meagher, J. A. Tossell, and G. V. Gibbs, Phys. Chem. Miner., 4, 11-21 (i979). (24) W.H.Baur, Acta Crystallogr., Sect. B,36, 2198-2202 (1980).
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a qualitative correlation between the value of the chemical shift and the T-O-T angles in the zeolitic frameworks. The details of this interesting effect, as well as the role of the cation (see Table I), await fuller investigation.
Conclusions The weight of evidence favors the view, based originally on X-ray studies,lBthat 4:0, i.e., Loewensteinian, rather than 3:l Si,Al ordering prevails in the ZK-4 structure. It is now apparent that, in the structural framework of zeolite
A, the single Zssi MAS NMR resonance at -88.7 ppm (from Me4&) is attributable to a Si(OAl), peak. For faujasitic and numerous other aluminosilicates a resonance close to this value is attributable to Si(OAl),(OSi). The implications are that in strained structures, including possibly amorphous aluminosilicates, the range of resonance previously published may have to be extended by a few parts per million. The range of the chemical shift for the Si(OAl), grouping may thus be 2.5-3.0 ppm wider than is thought at present.
Vibronlc Charge Oscillations in the Infrared Spectra of 1,-Doped ,&Carotene and (CH), J. Paul Devlln Deparfment of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 (Received: April 29, 1982)
It is known that trans-@-caroteneresembles polyacetylene structurally and in the response of its infrared, Raman, and visible spectra to doping with I2and other electron acceptors. Further, like I,-doped (CH),, I2 complexes with &carotene are electron conductors. New observations on a series of codeposita of Iz and @-caroteneindicate that the infrared spectra previously analyzed have been for fully ionized 8-carotene which is a relatively poor conductor. Contrary to previous implications, infrared spectra for the more highly conductive forms of Izcomplexed @-caroteneare found to be characterized by intense absorption bands that are coincident with the strong resonant Raman bands of neutral @-carotene.This confirms that vibronic charge oscillations are the primary source of the intense Irdoped &carotene infrared bands and adds to the credibility of a similar assignment for conducting (CHI,.
A controversy surrounds the interpretation of the infrared vibrational spectrum of doped conducting polyacetylene, (CH),. The spectrum is generally agreed to be dominated by three intense bands ( 1400,1290, and 900 cm-’) that emerge during the doping process and which apparently have intensities more than an order of magnitude greater than the infrared absorption bands for the pristine polymer.’* Recently Harada et al.? having noted that the truns-@-carotenemolecule structurally resembles trans-(CH), and that, like (CH),, the 0-carotene electrical conductivity is strongly enhanced by I, complexation? proceeded to show that trans-@-carotene is, indeed, a promising model for the study of doped conjugated conducting polymers. In particular, it was established that the visible and Raman spectra of 0-carotene respond to I2doping in a manner similar to the response of the spectra of (CH), and that, as for (CH),, the vibrational infrared spectrum of 12-doped 0-carotene is dominated by three intense bands that emerge during the doping process (1464, 1122, and 972 cm-’1. Rabolt et al. had previously noted the unusual response of the (CH), infrared spectrum to acceptor doping and, impressed with the intensity of the dominant new bands as well as an apparently very large shift in the C=C stretching frequency reminiscent of that for TCNE for one-electron reduction (- 150 cm-’), argued that the infrared bands of the conducting polymer are produced by
electron charge oscillations resulting from the vibronic coupling of the totally symmetric (CH), modes with charge-transfer excitations along the polymer backbone.’ Such charge oscillations are well-known to be a common source of the dominant infrared bands in charge-transfer systems including the C = C mode of TCNE. The infrared spectra for the alkali metal salts of both TCNE4 and TCN05 are dominated by absorption originating from the totally symmetric vibrational modes. In particular for KTCNQ, vibronic-coupling-based oscillations give rise to vibrational dipoles parallel to the stacks of TCNQ anions but produced by symmetric mode atomic displacements that are perpendicular to the anion stack The mechanism for the vibronic coupling that is basic to the charge oscillations which produce the intense infrared “vibrational” absorption bands has long been understood for charge-transfer systems.6 Since I,-doped (CH), is quite well established as a charge-transfer (CT) system, with positively charged chain segments balanced by 13-and/or Is- counterions, Rabolt’s suggested parallelism to the simple CT salts of TCNE and TCNQ seems appropriate. However, Harada et al. used new data, in particular that for @-carotenedoped with Iz, to discount the comparison with CT salts. The primary evidence was the observation that the intense @-caroteneIR bands produced by I, doping are signficantly shifted (-50 cm-’ average) relative to the most intense Raman bands. This
(1)(a) J. F. Rabolt, T. C. Clarke, and G. B. Street, J. Chem. Phys., 71, 4614 (1979); (b) C. H. Fincher, Jr., M. Ozaki, A. J. Heeger, and A. G. MacDiarmid, Phys. Reu. B, 19,4140 (1979). (2)I. Harada, Y.Furukawa, M. Tasumi, H. Shirakawa, and S. Ikeda, J. Chem. Phys., 73, 4746 (1980). (3)(a) H. Shirakawa and S. Ikeda, Polym. Prepr. Jpn. 28,3, 465 (1979). (b) C. M. Huggins and 0. H. LeBlanc, Nature (London),186,552 (1960).
(4) J. J. Hinkel and J. P. Devlin, J. Chem. Phys., 58, 4750 (1973). (5)(a) G. R. Anderson and J. P. Devlin, J. Phys. Chem., 79, 1100 (1975);(b) R.Bozio and C. Pecile, J. Chem. Phys., 67,3864 (1977); (c) R.Bozio, I. Zanon, A. Girlando, and C. Pecile, J. Chem. SOC.,Faraday Trans. 2,74,235 (1978). (6)(a) E.E.Ferguson and F. A. Matsen, J. Am. Chem. SOC.,82,3268 (1980);(b) H. B. Friedrich and W. B. Person, J. Chem. Phys., 44,2161 (1966).
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