J. phys. Chem. 1080, 84, 2819-2823
DTBPH+ cation in 81 wt % methanol-water, we see that TSRof the methine carbon has at least doubled upon protonation. From this we conclude that the reorientation of the tert-butyl group is much more hindered in the cation than the neutral species for 2,6-DTBP.
Conclusions From the T,and qc-H values for several tert-butylpyridines, the barriers to rotation for the methyl group have been estimated for the neutral and protonated species and found to be in the 2-4 kcal mol-’ range. A small increase is observed for 2,6-DTBP and 2-TBP upon protonation. The TsRvalues for the methine carbon have also been derived; an appreciable change is seen for 2,6-DTBP which is interpreted as being due to a larger barrier to rotation of the tert-butyl group in the 2,6-DTBPH+ cation than in the neutral species. Both of these results support our previous contention4that the internal rotations of the tert-butyl group in the 2,6-DTBPH+ cation are more hindered than in the unprotonated form. We have also shown that the chemicals shifts reported here for the neat and methanol phases are consistent with the hydrogen bonding of the solvent molecules to the pyridine nitrogen. This effect is substantially reduced in 2,6-DTBP because of steric hinderance. Larger changes for T1upon protonation are found for 2,6-DTBP and 2-TBP than for 4-TBP; this is interpreted as due to a large hydrophobic interaction caused by the proximity of the tert-butyl to the positive charge. References and Notes (1) Brown, H. C.; Kanner, B. J . Am. Chem. Soc. 1966, 88, 986. (2) McDaniel, D. H.;Orcan, M. J . Org. Chem. 1968, 33, 1922. (3) LeNoble, W. T.; Asano, T. J . Org. Chem. 1075, 40, 1179.
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(4) Hopkins, H. P., Jr.; AH, S. 2. J . Am. Chem. Soc. 1977, 99, 2069. (5) Aue, D. H.; Webb, H. M.; Bowers, M. T.; Uotta, C. L.; Alexender, C. J.; Hopkins, H. P., Jr. J . Am. Chem. Soc. 1076, 98, 854. (6) Pugmire, R. J.; Grant, D. M. J . Am. Chem. Soc. 1968, 90, 4232. (7) Levy, N. “Carbon-13 Nuclear Magnetic Resonance for Organlc Chemists”; Wiley-Intersclence: New York, 1972. (8) WeM, F. W.; wtthlh, T. “InteqxetaW~ of Carbcn-13 NMI spectra”; Heyden: London, 1976. (9) Noggle, J. H.; Schkmer, R. S. “The Nuclear Overhauser Effect. Chemical Appllcations”; Academic Press: New York, 1971. (10) Kuhlmann, K. F.; Grant, D. M. J. Chem. Phys. 1971, 55, 2998. (11) Alger, T. D.; Grant, D. M.;Harris, R. K. J . phys. Chem. 1972, 76, 281. (12) Lyerla, J. R., Jr.; Grant, D. M. J. phys. Chem. 1972, 76, 3213. (13) Collins, S. W.; Alger, T. D.; Grant, D. M.;Kuhlmann, K. F.; Srnlth, J. C. J. Phys. Chem. 1975, 79, 2031. (14) Ladner, K. H.; DaMq, D. K.; Grant, D. M. J. Phys. chem.1976, 80, 1783. (15) Axelson, D. E.; Holloway, C. E. Can. J. Chem. 1976, 54, 2820. (16) Blunt, J. W.; Stothers, J. B. J . Megn. Reson. 1977, 27, 515. (17) DoddreH, D.; Ghishko, V.; Allerhand, A. J . Chem. Phys. 1972, 56,
3683. (18) Woessner, D. E. J. Chem. Phys. 1962, 36, 1. (19) Woessner, D. E.; Snowden, B. S.; Meyer, 0. H. J. Chem. phys. 1969, 50, 719. (20) Woessner, D. W.; Snowden, 8. S. Adv. Mol. Relaxatkn Processes 1969, 3, 181. (21) Woessner, D. E. J. Chem. Phys. 1962, 37, 647. (22) Zens, A. P.; Ellis, P. D. J. Am. Chem. Soc. 1975, 97, 5685. (23) Tancredo, A.; Pizani, P. S.; Me-, C.; Farack, H. A.; Poole, C. P.,Jr.; Ellis, P. D.; Byrd, R. A. J. Megn. Reson., 1978, 32, 227. (24) Hopkins, H. P.; Jr.; All, S. 2. J . Phys. Chem. 1960, 84, 203. (25) Hopkh, H. P., Jr.; Alexander, C. J.; AH, S. 2. J. phys. Chem. 1978, 82, 1268. (26) Nakanlshl, H.; Yamamto, D.; Nakarnura, M.;ai,M. Tetrahedron Lett. 1973, 727. 127) Nakamura. N.: Oki. M.:Nakanishl. H. retrawm Lett. 1974.543. (28) Nakamura; Mi; Oki, Mi; Nakanishi, H.;Yamamto, 0. Bun. Chem. S0c. Jpn. 1974, 47, 2415. (29) During, J. R. J . Chem. phys. 1970, 52, 2046. (30) Alhger, N. L.; Htsch,J. A.; Mser,M. A.; Tymbrskl, I. J.; Vancatledge, F. A. J . Am. Chem. Soc. 1968, 90. 1199. (31) Bemheim, R. A.; Gutowsky, H. S.;Lav&son, 1. J. J . chem.Fttys. 1961, 34, 565.
Primary Reactions in Irradiated Deuterio a-Amino Acids Studied by Pulse Radiolysis and ESR Spectroscopy P.-0. Samskog,’ 0. Nllsson, A. Lund, Ihe Studsvik Sclence Research Laboratory, S-617 82 Nyk@ing, Sweden
and T. Glllbro Oepartment of phvslcal Chemkhy, Urn& Unhersity, S-90187 UmeB,Sweden (Received: October 1, 1979; In Final Form: Merch 26, 1980)
Transient optical absorptions have been observed in a pulse radiolysis study of glycine-do,L-alanine-da,and (a-amino-d2)isobutyricacid-d. By comparing the kinetics of the transient optical absorptions with the decay of the line intensities in the ESR spectra, we have been able to assign the optical transients to the primary oxidation and reduction products of the amino acids. The radical anions have their absorption maxima at 480, 415, and 365 nm, respectively, and the activation energies for deamination are 5.5, 10.4, and 11.7 kcal mol-’. The radical cation of glycine-d3absorbs at 365 nm. The radical cations of the other two amino acids absorb in UV, and all decarboxylate with an activation energy of about -11 kcal mol-’.
Introduction
The primary oxidation and reduction products formed in irradiated single crystals of a-amino acids have been studied by ESR and ENDOR spectroscopy. They have
been a matter of concern for a long time.’ Muto et a1.2 recently studied the oxidation products formed in glycine and a-aminoisobutyric acid at 4.2 and 77 K. They found that the oxidation products formed in
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Samskog et al.
TABLE I
species I (cation) e G, EA cm-’ M-’ kc al (100eV)” mol-’ 7300 11.1 11.0 11.7 9
amino acid
a
glycined L-alanine-d, (a-amino-d,) isobutyric acid-d Data from ref 7.
Amax,
nm 365
uv
UV
the deuterated acids are mainly +D2NCRR’C02-where R = R’ = H for glycine and R = R’ = CH3 for a-aminoisobutyric acid, The oxidized form of alanine where R = H and R’ = CH3has probably the same structure. The hole is primarily trapped in the nonbonding orbital of the carboxyl oxygen. This is followed by dedeuteration of the amino group, and the unpaired electron is finally found in a u orbital delocalized over the whole molecule.2 These radical cations have all nearly the same g tensor which exhibits a characteristic negative g shifta2v3On warming they may decompose by decarboxylation. The reduced forms of these amino acids have also been extensively ~tudied.~fj In the protonated form they have the general structure +H3NCRR’cOOH-with the unpaired electron localized primarily on the carboxyl carbon atom. On warming they decompose by deamination. The anionic decay of L-alanine and a-aminoisobutyric acid has been investigated before by using ESR at low temperatures.6 We have previously studied the decay kinetics and the optical absorption properties of the primary radical ions formed in irradiated glycine-d3.’ This could be done by combining the techniques of pulse radiolysis and ESR spectroscopy. The same method has now been used to study the primary radical ions formed in irradiated Lalanine-d3 and (a-amino-d2)isobutyricacid-d. The motivation of the present work was to study the influence of the R and R’ groups on the rate constants and activation energies for the decomposition of the primary species.
Experimental Section The deuterated crystals of the amino acids were obtained by several recrystallizations from heavy water. In the pulse radiolysis experimenta the crystals were placed in a cryostat and irradiated from opposite sides by single 3-ns pulses of 800-keV electrons from the double-beam Febetron 708. Light from a pulsed 450-Wxenon lamp was used for analyzing the transient absorption and passed the samples at right angles to the electron beams. The optical path length was 4-5 mm, and the thickness of the crystals in the direction of the electron beams was 2 mm. The overall rise time of the detection system is -5 ns. Since the dose rate was very high, calorimetry was used for the dose and relative dose measurements. The average dose was 108 krd/pulse (for experimental details see ref 8). The crystals used for the ESR measurements were irradiated with a dose rate of 210 krd h-’ for 4 h in a @‘Co gamma cell at 77 K. The ESR spectra were then recorded with a Varian E9 spectrometer as first derivatives. In all experiments the samples were held at the desired temperature by means of a flow of thermostated nitrogen gas.
Results and Discussion Pulse Radiolysis. On pulse radiolysis of glycine-dJ and ~-alanine-d~, two transient absorptions were observed for each sample. At short wavelengths (l rno1-I
850 985
5.5 10.9 11.7
t,,,, PS
at 2 2 ° C 1.0 0.7 18.5
WAVELENGTH lnm)
Figwe 1. Absorption spectra of radical cations (specles I) in kradlated cramino acMs: (X) glyclnad,6 and (0)L-alanlnad,. The dose was 108 krd and the optical path length 0.37 cm.
lengths and extrapolating the optical density to the end of the electron pulse, we obtained the two optical absorption spectra shown in Figures 1and 2. The absorption maxima are at 365 and 480 nm, respectively.’ At room temperature the two species in ~-alanine-d, have very different half-lives (Table I). The kinetics of the short-lived species could therefore be studied at room temperature where the long-lived species gives a constant background in the optical absorption. It decays by firstorder kinetics, and its absorption spectrum with maximum at 415 nm is shown in Figure 2. At 100 “C this species is very short-lived and does not disturb the kinetic analysis of the long-lived species which also decays by first-order kinetics. Its absorption maximum is below 350 nm (Figure 1).
In (d-amino-d2)isobutyric acid-d only one first-order decay was observed. The species in question has an absorption peak at 365 nm. The increasing absorption a t shorter wavelengths might, however, indicate the presence of a second species with absorption maximum in UV by analogy with the other two amino acids. These two absorbing species have almost the same half-lives and activation energies, since the kinetics is the same in the whole wavelength interval between 300 and 450 nm at all temperatures. All acids therefore seem to form two species on irradiation, one (I) absorbing at short wavelengths (365
The Journal of phvsical Chembtry, Vol, 84, No. 21, 1980 2821
Primary Reactions in Irradiated Deuterio a-Amino Acids 0.10
I
g*20012
I
, 20G
~
V
2 v, 2 W
0.05 -I
4
I ! In 0
400
300
500
600
WAVELENGTH (nm)
Figure 2. Absorption spectra of radlcai ankns (species 11) in hadiated aamino acids: (X) glyc1ne-d3,' (0)~-alanine-d,, and (0) (adminod2)isobutyrica&d. (The absorption bebw 300 nm is due to the catbn.) The dose was 108 krd and the optical path length 0.37 cm. I '
I
I
1
u i i i l + 25
30
35
bo
C5
l 4 ( K 4 L 10-9
BOln-
Figure 3. Arrhenius plots for the decay of the transient species in irradiated L-alanine-d, (radical cation (0) and radical anion (X)) and in (a-amino.d,)isobutyric a c M d (0).
Figure 4. ESR spectra of an irradiated crystal of ~-alanlne-d~:a 4 irradiated at 77 K, heated to the temperatures indicated, and measured at 77 K. Spectrum d is obtained from a protonated s a w .All spectra are measured along the [OlO] axis at a microwave power of 5 MW.
nm or less, Figure 1) and one (11) absorbing at longer wavelengths (365 to 480 nm, Figure 2). The activation energies for the species were obtained from the Arrhenius plots in ref 7 and Figure 3. In Table I the optical properties of the species are collected. ESR Spectroscopy. In order to identify the species, we had to find their signals in the ESR spectra. We therefore looked for components of the ESR spectra of the deuterated acids which had the same kinetics as the optical transients. Glycine-d3. Using this technique we identified the 365-nm absorption of irradiated glycine-ds as the radical cation +D2NCH2C02-and the 480-nm absorption as the radical anion +D3NCH2COOD-.' L-Alanine-& In Figure 4 are shown the ESR spectra at different temperatures of a single crystal of ~ d a n i n e - d , irradiated at 77 K and measured along the [OlO] axis. The sample was heated to the temperatures indicated in the figure, and the ESR spectra were then recorded at 77 K.
Radical Anion. The two-line spectrum at 77 K is obtained at all orientations of the crystal and is known to come from the interaction of the unpaired electron with the 0 proton of the radical anion +D3NCH(CH3)COOD-.Qdsd On warming to 153 K the radical anion decays and a new radical is formed as shown by the spectrum in Figure 4b. The spectrum consists of a doublet of quartets as seen by the stick plot in Figure 4c. The doublet splitting is anisotropic, and the quartet splitting is almost isotropic with a coupling of 23-25 G. The same spectrum was obtained in a protonated sample at 153 K after the anion had decayed. This means that the decay product does not have any couplings due to exchangeable protons on the amino group. This is an indication that the anion decays by deamination, and we therefore assign the 153-K spectrum to the .CH(CH3)COODradical. The deamination of the anion has been proposed before.Qd16 The decay of the 415-nm optical absorption should have a half-life of 30 s at 153 K. This is obtained from our
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The Journal of physical Chemistry, Vol. 84, No. 21, 1980
optical data (Table I) by extrapolation to low temperatures. Since a half-life of 30 s was observed for the radical anion in the ESR spectrum, we assign the 415-nm absorption to the radical anion. The decay we observe by pulse radiolysis is therefore +D3NCH(CH3)COOD- CH(CH3)COOD ND3
-
+
The activation energy of the reaction is 10.9 kcal mol-' (Table I). Radical Cation. The additional lines in the 77-K spectrum (Figure 4a) were seen at certain orientations. These lines have been observed and the spectrum was assigned to a cationic species by Friday and Miyagawa.lo The structure of this species is probably +D2NCH(CH3)COO-by analogy with the structure of the radical cation of glycine-d3 and (a-amino-d2)isobutyic acid-dq2 After the crystal is warmed to 153 K, the cation spectrum might be hidden under the spectrum of the anion product CH(CH3)COOD. On further annealing to 193 K the anion product still remains, but new lines appear at that temperature on the wings of the spectnun (Figure 4c). These lines are much broader than the other lines, and their positions depend upon the crystal orientation. In Figure 4c the outer lines have their maximum separation and the main spectrum has its minimum width. In the protonated crystal of L-alanine, the outer lines are split into a doublet with a hyperfine coupling of ca. 9 G at the same crystal orientation (Figure 4d). The anion product spectrum is unaffected. We tentatively assign the new lines to the H2NCH(CH&radical in the protonated crystal. This suggestion is based on (a) the effect of protonation, (b) the anisotropy of the spectrum caused by the a proton, and (c) the broad line width which might originate from an unresolved coupling to the 14Nnucleus. A stick plot diagram for the H2NCH(CH3)radical is indicated in Figure 4d. The change in the ESR spectrum at 193 K is accompanied by a fading of the yellow color of the crystal. This fading and the formation of the D2NCH(CH3)radical is attributed to decarboxylation of the radical cation +D2NCH(CH3)C02- D2NCH(CH3)+ C02 The decay is supported by the fact that COPis formed with G(C02) values of 0.2-l.0.11-'3 The kinetics of the decarboxylation reaction is in agreement with the kinetics of the decay of the UV-absorbing species, the half-life being 30 s at 193 K (Table I). We therefore assign the UV absorption of irradiated ~-alanine-d,to the cation radical. The anion and cation products CH(CH3)COOD and D2NCH(CH3)seem to have different coupling constants. For the particular orientation in Figure 4 the hyperfine couplings of CH(CH3)COOD are a, = 16 G and U C H ~= 23 G. The hyperfine couplings of DZNCH(CH3) are a, = 33 G and aCH3= 25 G. This suggests that the two radicals are differently oriented with respect to the crystallographic axes. This is expected since they are formed by fragmentation of the molecule at opposite ends. a-Aminoisobutyric Acid. In Figure 5 are shown the ESR spectra at low temperatures of a single crystal of (aamino-d2)isobutyric acid-d irradiated at 77 K and measured along the [Ool] axis. The sample was heated to the temperatures indicated in the figure, and the ESR measurements were then performed at 103 K. The broad single line in Figure 5a has been assigned to the anionic species or its deuterated form, +D3NC(CH3)2COOD-.4f~g*6 On warming the signal is replaced by a seven-line spectrum with a binomial distribution of intensities (Figure 5b) arising from the radical .C-
-
Flgue 5. ESR spectra of an irradiated crystal of a-amlnokbutyrlc acid irradiated at 77 K, heated to the temperatves Indicated, and measured along the [ O O l ] axis at 77 K.
(CHJ2COOD.'g The coupling is isotropic and the coupling constant is 22 G. The kinetics of the decay of the radical anion agrees well with the decay of the optical absorption at 365 nm (Figure 2), its half-life being 30 s at 173 K. By analogy with the two other amino acids investigated, we assign the long-wavelength component (-365 nm) to the radical anion. I t decomposes by deamination +D3NC(CHJ&OODC(CH3)2COOD + ND3
-
the activation energy being 11.7 kcal mol-' (Table I). The satellite lines in Figure 5a represent a second primary radical. The ESR spectrum of this radical is masked by the main features in Figure 5a. The spectrum was, however, studied by Muto et aL2and found to represent the primary oxidation product which has the unpaired electron delocalized over the molecule. It is supposed to decay by decarboxylation by analogy with the case of glycine and milanine. The kinetics of the decay in the UV region was found to be the same as at 365 nm. The conclusion that the two species have the same decay kinetics must be taken with some reservation since the two decays could not be resolved. The corresponding cation product ND2C(CH3), could not be detected after warming the crystal to several temperatures between 103 and 223 K. However, it seem reasonable also in this case to assign the UV absorption (i.e., the absorption below 320 nm in Figure 2) to the radical cation. Conclusions By comparing the kinetics of the optical transients in some irradiated a-amino acids with the kinetics of the decay of certain hyperfine line intensities in the corresponding ESR spectra, we have been able to assign some optical properties and kinetic data to the primary oxidation and reduction products of these amino acids. The identification of the optical transients with certain ESR lines was based on the calculation of rate constants at low
J. Phys. Chem. 1980, 84, 2823-2827
temperature from constants measured at high temperatures using an Arrhenius equation with a temperatureindependent preexponential factor. This is justified by the fact that for all amino acids the temperature dependence of the rate constant can be described by an Arrhenius equation of this type over a wide temperature range (see Figure 3). The results show that ,A, of the radical anions are displaced to shorter wavelengths when the hydrogen atoms on the a-carbon atom of glycine is replaced by methyl groups. At the same time the activation energy for deamination increases. On the other hand the activation energy for decarboxylation of the radical cation is almost independent of these structural changes. Kinetic studies of the anionic decay by means of ESR spectroscopy have been performed before.6 Our results for the half-lives and the activation energies differ considerably from the ESR data. In principle, however, kinetic measurements by pulse radiolysis should be more accurate than kinetic measurements by ESR spectroscopy. The reason is that by pulse radiolysis it is possible to measure reaction rates over a wide temperature range without going to measuring times longer than a few microseconds. I t is thus easier to keep the experimental conditions constant during the pulse radiolysis measurements as compared to kinetic measurements by means of ESR spectroscopy
2823
which generally require measuring times of minutes or hours. References and Notes (1) H. C. Box, "Radatkn Effects, ESR and ENWR Amlysb", AcadenIc Press, New York, 1977, Chapters 6 and 7. (2) H. Muto, M. Iwasaki, and Y. Takahasl, J . Chem. Phys., 66, 1943 (1977). (3) A Mlnegishi and Y. SMnozaki, 11th ESR Symposium, Konazawa, Japan, Oct 8, 1977, p 23. (4) (a) M. A. Collhs and D. H. W e n , Mol. phys., 10, 317 (1966); (b) J. shrdair, J. Chem. phys., 55, 245 (1971); (c) P.B. A y w n ~ &and K. Mack, J. Chem. Soc.,Faraday Trans. 1 , 68, 1139 (1972); (d) J. Slnclalr and M. W. Hanna, J . Phys. Chem., 71, 84 (1967); (e) J. Shrclak and M. W. Hanna, J . Chem. Phys., 50, 2125 (1969); (f) H. C. Box and H. G. Freund, W.,44, 2345 (1966); (9) M. FupmOto, W. A. Won, and D. R. Smith, ibM., 48, 3345 (1966). (5) M. Iwasakl and H. Muto, J. Chem. Phys., 61, 5315 (1975). (6) H. shields, P. J. Hamrick,&., C. smith,andY. Ham,J. chem.phys., 58. 3420 (1973). (7) P.0. Samskog, T. Giilbro, and G. Nilsson, Chem. phys. Lett., 64, 162 (1979). (8) P. 0. Samskog, G. Nllsson, and A. Lmd, J. Chem. phys.. 68, 4986 (1976). (9) I. Miyagawa, N. Tamua, and J. W. cook,J. Chem. Phys., 51, 3520 (1969). (10) E. A. Frklay and I. Miyagawa, J . Chem. Phys., 55, 3569 (1971). (11) W. C. Gottschall, Jr., and B. M. T O M , A h . Chem. Ser., No. 81, 374 (1968). (12) W. C. oottshal.Jr., and B. M. Tobert J. phys. chem.,72,922 (1966). (13) A. Mlnegishl, Y. Shinozakl, and 0. Meshttsuka, Bull. Chem. Soc. Jpn., 40, 1271 (1969).
Crystal Structure of Hydrated Partially Zinc( 11)-Exchanged Zeolite A, Zn5Na2-A Yang Kim and Karl SeH' Depafimnt of chemlshy, UnlverSny of Hewall, k d u h ~Hewali , 96822 (Received: June 4, 1979; In Flnal Form: May 27, 1080)
The crystal structure of fully hydrated, partially Zn(I1)-exchangedzeolite A, stoichiometry Zn5Na2-A per unit cell, has been determined from three-dimensionalX-ray diffraction data gathered by counter methods. The structure was solved and refined in the cubic space group Pm3m: a = 12.196(2) A at 24(1) OC. Zinc(I1) ions are located at three distinct crystallographic sites. At the center of the sodalite unit, one zinc(I1) ion per unit cell is coordinated at 2.11(3) A by a distorted octahedron of water molecules. Three Zn(I1) ions per unit cell lie on threefold axes just inside the large cavity where they are 2.25(1) A from three framework oxide ions. Each of these Zn(I1) ions is further associated at 2.19(6) A with a hydroxide ion (probablynot a water molecule) recessed further into the large cavity to complete a distorted tetrahedral coordination sphere. The f i h Zn(I1) ion lies deep within the large cavity and is associated with a distorted octahedron of water molecules at -2.2 A. The two Na' ions per unit cell are associated with &ring oxide ions and with two water molecules each. Altogether, 24 water or hydroxide oxygens were located per unit cell. Full-matrix least-squares refinement converged to a conventional R index of 0.078 using the 237 reflections for which Io > 3a(Io).
Introduction The properties of zeolites are sensitive to the kinds of cations, their numbers, and their positions within the lattice. A knowledge of these positions can provide a structural basis for understanding these properties. In particular, the structures of solvated ion-exchangedzeolites are important in understanding ion-exchange selectivities and limits,as well as hydrolysis and instability phenomena Thus far, a number of first-row transition-metal-exchanged zeolite A systems of stoichiometry M,Na12-23Si12Al120e. yH20 per unit cell,' where M = Mn(II),2 Fe(II),3 CO(II),~ Ni(II),3 or Zn(II), and 2.7 Ix 5 5.0 have been studied by single-crystal X-ray diffraction techniques. The structures of these hydrated zeolites reveal a variety of structural features, depending upon the cation ex0022-3854/80/20862823$01 .OO/O
changed. In hydrated Mq5Nai-A,' the Mn(I1) ions are five-coordinate trigonal bipyramidal and interact strongly with the rigid anionic aluminosilicate frameworks5 In contrast, the Co(II) ions in the crystal structure of hydrated Co4Na4-A adopt positions so distant from the framework that conventional coordination to framework oxide ions is not possible. One Co(I1) ion at the center of the sodalite unit is octahedrally coordinated by water molecules; the remaining three, located in the large cavity6 of the zeolite, have promoted an extensive stoichiometric hydrolysis of the aluminosilicate framework. In Ni3Nh-A and Fe2.7Nh.6-A,the principle transition metal ion positions, like those of most of the Co(I1) ions in hydrated C O ~ N ~ ~ are - A along , ~ threefold axes deep in the large cavity. A hexaquo Fe(I1) ion is found at the center of the 0 1980 American Chemical Society
