5969
J . Phys. Chem. 1989, 93, 5969-5972
-
emergence of this liquid-crystalline microphase LC’,, constitutes the transient phase separation L2 L2 LC. The birefringence of LC’,, is smaller than that of its progenitors. This can be seen from the central oscilloscope trace of Figure 4b where the rate of increase of turbidity is fortuitously balanced by the rate of decrease of birefringence, yielding a steady-state signal. 5. After termination of the electric field, the system returns to equilibrium, but via a route different from the forward path in Figure 6) of just described. All but two (represented by the forward steps are irreversible (+), connecting states that are accessible only under electric field. For the species in those states we have used primed symbols. The reequilibration at E = 0 is characterized by three rate processes (Figure 4b-d). A largeamplitude, fast ( T - ~= 74 p ) decay is followed by a slower ( T - ] = 506 ~ s ) small-amplitude , relaxation of the birefringence. , amplitude of the T-, process is smaller than Although T~ = T - ~ the that of T~ because the population of A’, has been depleted in the preceding cluster-growth steps. The T - ] relaxation corresponds A,, and to the collapse of the micellar induced dipoles, Le., A’, the 7-2 to the disintegration of the anisotropic order of the liquid-crystalline microphase LC’,,. Subsequently, the turbidity vanishes nonexponentially in about 1.5-2 s. This last, slow process seems to involve subdivision of the macrodroplets by successive constriction steps which requires a massive rearrangement of the
+
-
-
surfactant molecules as they reconstruct the original equilibrium species and populations.
Conclusion Electric field greater than 1.2 kV cm-’ induces L2 L2 + LC phase separation in homogeneous microemulsions of AerosolOT/isooctane/H20 with compositions sufficiently close to the boundary between the L2 and L2 + LC phases, even at temperatures several degrees below the upper critical phase separation temperature T,. The evolution of birefringence and delayed turbidity exhibited by the system are manifestations of distinct steps associated with the formation of oriented anisotropic structures, linear cluster growth, and fusion, leading to the transient LC phase. The slow reequilibration of the LC phase, which occurs upon termination of the field, represents the different return path in the overall thermodynamic cycle.
-.
Acknowledgment. Helpful suggestions by Dr. A. C. Hall, discussions with Professor V. Degiorgio and Dr. R. Zana, and technical assistance by D. Miller are gratefully acknowledged. This material is based in part upon work supported by the Texas Advanced Research Program under Grant No. 1766, the National Science Foundation (Grant No. CHE-8706345), and the R. A. Welch Foundation.
Trigonal Level Splittings of Cr3+ Doped in NaMg[ Al(ox),]*8H20 Single Crystals T. Schonherr, J. Spanier, and H.-H. Schmidtke* Institut f u r Theoretische Chemie, Universitat Diisseldorf, 0 - 4 0 0 0 Diisseldorf, FRG (Received: March 30, 1989)
The optical absorption spectrum of C r ( o ~doped ) ~ ~in NaMg[Al(ox3)].8H20is reported. Since the chromium occupies equivalent positions the band pattern arising from spin-forbidden transitions is less complicated compared to the corresponding compound of nine-crystalline water. Anisotropic electric dipole selection rules allow for assignments of the Kramers doublets resulting from 2E,(Oh) which is split by only 2-3 cm-I. The experimental energy level scheme, which is in some respect unusual compared to other tris-chelated compounds, can be rationalized by angular overlap model calculations on the basis of D3 symmetry, demonstrating the importance of the angular geometry on quartet and doublet band splittings.
Introduction Previous interpretations of electronic spectra of trisoxalatochromate(II1) in neat crystalline surroundings or doped materials were limited by the low-symmetry environment to which the chromium chromophore is Band assignments were further complicated by the presence of nonequivalent sites in various crystals which concerns also the narrow doublet lines arising from spin-forbidden transitions between t2g3levels. In a luminescence study, Coleman4 measured the amount of splitting of the lowest 2E, state (in Oh notation) varying by a factor of almost 100 for different host materials which has been attributed to outer-sphere interactions being generally not explicitly considered in conventional ligand field theory. On the other hand, Hoggard5 considered on the basis of an angular overlap model (AOM) calculation, including anisotropic metal-ligand T-bonding, the possibility of very large ZE,splittings. Forster et a1.1,6noted (1) Condrate, R. A.; Forster, L. S. J. Mol. Spectrosc. 1967, 24, 490. ( 2 ) Kawasaki, Y.; Forster, L. S. J. Chem. Phys. 1969, 50, 1010. ( 3 ) Mortensen, 0. S. J . Chem. Phys. 1967, 47, 4215. (4) Coleman, W. F. J . Lumin. 1975, 10, 163. (5) Hoggard, P. E. Coord. Chem. Rev. 1986, 7 0 , 85. ( 6 ) Coleman, W. F.; Forster, L. S. J . Lumin. 1971, 4, 429.
0022-3654/89/2093-5969$01.50/0
-
that transitions in the intercombination band region (4A2s 2Eg) of C r ( o ~ ) are ~ ~ considerably dependent on the amount of crystalline water present in the N a M g [ A l ( o ~ ) ~ ] - x H ( x~=0 8 or 9) host materials. Similar observations have been reported from the emission spectra of C ~ ( O X )doped ~ > in K , [ A I ( o x ) ~ ] . ~ Hwhere ~O partial dehydration on the crystalline surface was claimed to be responsible for these band ~plittings.~ The quartet absorption spectra, on the other hand, could be satisfactorily explained by the presence of a trigonal ligand field acting on the C r 0 6 octahedron.’ From EPR measurements, Bernheim and Reichenbecher8 concluded that the Cr3+ion when doped in N a M g [ A l ( o ~ ) ~ ] . 8 H ~ 0 crystals is located on an axis of trigonal symmetry occupying equivalent positions. Therefore, this system ought to be very appropriate for investigating the electronic structure of C r ~ x , ~ in a specific environment, in particular for determining the spin-orbit split levels in the sharp-line spectrum which arise from transitions within the :,t configuration. In the present Letter we report results on single-crystal emission and polarized absorption spectra measured from .the title com(7) Piper, T. S.; Carlin, R. L. J . Chem. Phys. 1961, 35, 1809. (8) Bernheim, R. A,; Reichenbecher, E. F. J . Chem. Phys. 1969,51, 996.
0 1989 American Chemical Society
5910
The Journal of Physical Chemistrv, Vol. 93, No. 16, 1989
Letters
-
Figure 2. (a) High-resolution spectrum of the 4A2, 2E, origins in different polarizations obtained at T = 5 K. (b) Level sequence and electric dipole selection rules for zero phonon transitions. Polarizations are u (-) and ?r (---). Figure 1. Single-crystal absorption spectrum of 1% Cr" doped in NaMg[Al(o~)~l.IH~0 obtained at T = 90 K. The light is linearly polarized with the electric vector parallel, r (---), or perpendicular, u and a (-), the trigonal molecular axis. The insets show band patterns of intercombination transitions in more detail which were measured from a 5% chromium-doped crystal. pound. For rationalizing the experimental level scheme, crystal field and AOM calculations are performed which closely depend on the geometry of the coordination ~ p h e r e . ~
Experimental Section The complex compounds N a M g [ M ( o ~ ) ~ ] . 9 H ~with 0 , M = Cr and AI, were synthesized by using standard methods3 Crystals containing eight molecules of water per formula unit were obtained by annealing in the temperature range 96-98 0C.8 The loss of one H 2 0 molecule was checked by gravimetric and DTA methods. Endothermic DTA peaks measured at 98 and 126 OC are due to a stepwise loss of water molecules as has been described also for K3[Cr(o~)3].3H20.10 Mixed crystals were grown by slow evaporation of aqueous solutions which contained varying amounts of the two compounds. Under the polarization microscope the crystals proved to be uniaxial. Polarized absorption bands were measured on a Cary 17 spectrophotometer equipped with a microscopical device as described earlier." High-resolution spectra in absorption and emission were recorded with a McPherson 0.5-m double monochromator and a red-sensitive GaAs photomultiplier. Temperature down to 5 K were achieved by using commercial flow-cryostat technique. In order to prevent water exchange with the atmosphere, the samples were coated with silicon grease, which beforehand was checked for transparency in the considered spectral region.
Results and Discussion Optical Spectra. The low-temperature polarized absorption spectra of doped single-crystals of NaMg[AI(ox),]43H20 containing about 1% or 5% C r ( o ~ ) anions ~ ~ - are illustrated in Figure 1. Although the type of space group for the modification of the host crystal has been controversially all structural investigations agreed that the A1 ions are located on a trigonal symmetry axis. From EPR measuremend the site symmetry was determined to be D3 which should not be significantly changed when AI is substituted by Cr. Since the absorption band pattern also does not alter very much between room and liquid helium temperature, a structural phase transition will not be expected. The trigonal contribution to the crystal field acting on Cr3+ is apparent from the highly anisotropic absorpti2n observed for the quartet-quartet transitions. Alignment of the E vector parallel (A) or perpendicular ( u ) to the optical axis of the crystal, the direction of which is determined by the 3-fold axis of the anion,8 (9) Atanasov, M. A,; Schonherr, T.; Schmidtke, H.-H. Theor. Chim. Acta 1987, 7 1 , 59. (10) Nagase, K. Chem. Lett. 1972, 5 8 7 . ( I I ) Teutsch, U.; Schmidtke, H . - H . J . Chem. Phys. 1986, 84, 6034. (12) Frossard, L. Schweiz. Mineral. Petrog. Mitt. 1956, 36, 1.
was obtained with the aid of a polarization microscope. The a-spectrum was measured with light propagating along this axis showing no azimuthal dependences for the polarization. Since the polarization relations are found to be a = u # n,the intensity primarily arises from an electric dipole transition mechanism. Then, according to D3selection rules, the a-polarized quartet bands must be attributed to the electronic transitions 4A2 4E(4T2K) and 4E(4T,g)in agreement with corresponding assignments given in the literature for different host material^.^^' In n-polarization only one trigonal split component, Le., 4A2 4A1(4TzK) is dipole allowed, whereas the forbidden 4A2 4A2(4T,s)transition obtains some minor intensity by a vibronic coupling mechanism. The low-energy part of the a-absorption spectrum obtained from a sample of higher Cr3+concentration is depicted in the inset A of Figure 1 showing a well-resolvedvibronic structure (the intensity profile of the n-spectrum differs only for a few weaker sidebands). The number of sidebands is, however, significantly smaller compared to the corresponding band region of the nonahydrate compound3 which can be attributed to the presence of nonequivalent chromium sites in the latter species. The ZE,(Oh) zero-phonon origin can be readily identified with the most intense band of lowest energy in the absorption spectrum. This assignment is supported by the emission spectrum which exhibits a corresponding line at similar wavelength. At low temperature, recorded at high spectral resolution, the zero-phonon line of ZE,(Oh) in the u- and Aspectrum is split by about 2-3 cm-' (cf. Figure 2a) which is due to the combined effects of trigonal molecular symmetry and spin-orbit coupling. In the a-spectrum this splitting could not be resolved. The D3 low-symmetry component of the ligand field is, however, large enough, destroying the inversion center (vide infra), so that most of the intensity of the E, zero-phonon lines can be assumed to be due to electric dipole transitions. This is supported by the relatively large intensity of these peaks compared to the vibronic side bands (cf. inset A of Figure 1). Since from the EPR investigation8 the zero field splitting parameter D is calculated to be negative;I3 Le., the level sequence arising from the 4A2, ground state is E312 < Ell* (in the D3* double group notation), the sequence for the resulting 'E, split levels (E312, El/z) then can be determined from the a- and n-spectrum. When electric dipole selection rules are applied, the slightly larger band separation for the a-polarized transitions supplies a level ordering for these Kramers doublets of E3/2 < with an assignment as given in Figure 2b. The observed band pattern cannot be explained by two similar nonequivalent sites which might have been formed by a structural phase transition. The level energies then are determined from the a-polarized absorption peaks arising from a *E, splitting of about 3 cm-I (h0.08 due to the limited spectral resolution) in good agreement with the observed u-polarized transitions. The measured splitting is in the same order of magnitude as the ground-state splitting which, on the other hand, is much smaller in other comparable low-symmetry chromium( 111) complexes. It is noticed that the 'E, splitting is drastically reduced in the present compound also compared to the ninefold-hydrated
-
- -
(13) Lahiry, S.; Kakkar, R. Chem. Phys. Lett. 1982, 88, 499.
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 5971
Letters
species (20 cm-1)29396whereas the ground-state splitting is very similar (ca. I .5 cm-I).',* Bands due to the less intense 4Azg7ZT,, lines at higher energy are usually covered by the rich vibrational fine structure of transitions into ZE, and are difficult to detect. Likewise, emission spectra cannot yield information about higher electronic states since they rapidly decay after excitation by radiationless transitions. Band assignments are further complicated by the fact that transition probabilities of corresponding vibrational sidebands can be. very different in the absorption and emission process. The band pattern (cf. the inset A of Figure 1) can be, however, almost completely interpreted in terms of vibrational fine structure upon the 'E, zero-phonon transitions. For the most intense lines frequency intervals of 120, 140, 360, 41 1, 460, 640, and 970 cm-' are calculated corresponding to vibrational modes of C r ( o ~ ) ~ , in the excited state. They all can be identified with frequencies of the ground state as measured in the infrared14 or luminescence spectra, which are expected to be almost equal since the electric transitions occur between levels belonging to the same configuration t22. Only one less intense band, which has no corresponding peak in the emission or IR spectrum, would be a candidate for a spin-orbit component of 'TI (indicated by an arrow in the inset A of Figure 1) being 565 cm- k higher in energy than the 'E, lines. This interval would be too large to be explained by the second component of zE,, split by the low-symmetry field. Some sharp peaks of low intensity are also found in the region between the quartet bands (inset B of Figure 1) which correspond to transitions into the highest levels of tz: electron configuration. In trigonal symmetry, three spin-orbit states (Kramers doublets) can result from 'Tzs from which those arising from trigonal 2E(2T2g)are appreciably increased in energy due to interaction with the lower 'E('E,, 'TI ) levels. On the other hand is the second 'Tzs component, Le., bAl(2T2,),only influenced by the combined effect of spin-orbit coupling and reduction of symmetry. Therefore, we may assign the main peak at 20682 cm-l to the 'A, electronic origin and the others to vibronic sidebands. This assignment is in accordance with the higher intensity measured in a-polarization which is expected from electric-dipole selection rules. Crystal Field Calculations. A further objective of this study is to compare the experimental level scheme with that predicted from crystal field and AOM calculations. In the framework of conventional ligand field theory the parameters K and K'are introduced for trigonal symmetry in addition to the cubic ligand field parameter Dq.I5 Considering only diagonal elements of the perturbation matrix in the strong field approach, the quartet band splittings are calculated in terms of the low-symmetry parameters as
4E - 4Az = ( 3 / 2 ) K 4E-4A
I
- ( 3 / 2 ) K - 2Il2K'
for 4T2, for 4T,,
(la) (Ib)
From the splitting of the first quartet band K is fitted to be about 300 cm-l. The observed level sequence of the 4T,, components requires K ' t o be much larger and to have a negative sign (K'-1500 cm-I). The derived parameter ratio of K'/K = -5 is not in agreement with an early assumption of comparable magnitudes for K and K'.I6 The relation does not significantly change when these parameters are obtained from complete ligand field calcu= 120 wave functions are considered lations where all possible (complete CI in d3) and spin-orbit coupling is included. As has been demonstrated for trigonal d3 complexes of rhenium" and chromium,Is a relation between ligand field parameters can be used for an estimate of the trigonal distortion of the CrO,
(io)
(14) Gouteron, J . J . fnorg. Nucl. Chem. 1975, 38, 5 5 . ( 1 5 ) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier Science: Amsterdam, 1984. (16) Sugano, S.; Peter, M. Phys. Reu. 1961, 122, 381. (17) Schoenen, N.; Schmidtke, H.-H. Mol. Phys. 1986, 57, 983. (18) Urushiyama, A,; SchBnherr, T.; Schmidtke, H.-H. Ber. Bunsenges. Phys. Chem. 1986, 90, 1195.
-
Oh
I
LA29
D3
D3d
I
a)
b)
1
0
I
I
I
I
I
1
2
3
L
5
c
chromophore under the restriction of fixed metal-ligand distances and small deviations from cubic symmetry: ( K iK')/Dq = 10/2'12(19c,b- O ) ~ / 1 8 0 O
(2)
in which O is the angle between a trigonal axis of the octahedron and one of the metal-ligand bond vectors. Using the parameter values given above, O is increased by about At9 = O - OCUb = 6' leading to significantly compressed CrO, octahedra of D3d symmetry which would explain the strong trigonal contribution to the ligand field potential. Structural investigations on other [Cr(ox)J3+ salts supplied somewhat smaller values of AO( 10-30);19320 however, the larger trigonal compression of the Cr-doped AI complex with respect to the pure chromium compound could be due to the misfit of the ionic radii of Cr3+ (64 pm) and AI3+ (57 Pm). The unusually large zero-field 4A2, ground-state splitting of 1.55 cm-l as derived from EPR measurementss can be explained by a complete (all configuration) ligand field calculation using a spinvorbit coupling parameter { in the expected range of 180-250 cm-'. On the other hand, the small energy separation between the spin-orbit levels arising from the lowest 'E, excited state could not be fitted satisfactorily by the ligand field as given above by using a unique parameter set which is varied within the wide limitations of a trigonal field restricted by the measured size of quartet band splittings. The model so far does, however, not consider chelate effects and/or additional geometric distortions of the chromophore. For instance, should a mutual twisting of the two oppositely coordinated oxygen triangles around the trigonal axis (by an angle p) be expected as is required also for other Cr(ox),,- compounds.20 This additional distortion leading to D, symmetry was shown to have significant influence on the doublet splittings of K , [ C r ( ~ x ) , l . ~Since, first of all, angular distortions are to be considered, the more appropriate model would be the AOM which uses local bonding parameters independent of angular variations of the coordination sphere. Eventual bond length variations are then considered by fitting the AOM parameters to the experiment. Very recently it has been demonstrated ( e g , for chromium tris(a~etylacetonate)~~'~ and a mixed osmium(1V) oxalate complex2') that the AOM can appropriately account for (19) Niekerk, J . N . van; Schoening, F. R. L. Acta Crystallogr. 1952, 5 , 499. (20) Taylor, D.Aust. J . Chem. 1978, 31, 1455.
5972
J . Phys. Chem. 1989, 93, 5972-5974
relations between the electronic spectra and the structure of the compounds. Since in the case of N a M g [ C r ( o ~ ) ~ ] - 8 H the~ 0required structural data are still missing, we started our calculations using atomic positions which are available for the ammonium and the potassium salt^.^^^^^ Figure 3a shows from some model calculations, which were performed by using the perturbation matrices (including CI) given by H ~ g g a r d the , ~ splitting of the 2E, state being more than 10 times larger than the ground-state splitting for a comptessed C r 0 6 = 54.74O) when restricting to D3dsymchromophore (3 > dCub metry. Deviations from the octahedral value of the twist angle, Le., cp = 60°, are connected to the chelate bite angle a and the trigonal angle 3 by the relation (cf. Figure 1 of ref 9) cos ( a / 2 ) = sin d cos (9/2) (3)
If we take the d value of 60.74O which was derived from the quartet band splittings, and the octahedral angle cp = 60°, a bite angle of a = 82' is obtained from eq 3 which is in good agreement with the oxalate bite in other first-row transition complexes as well as with the corresponding valence angle of the free anion. Therefore, we can assume that an additional geometric distortion lowering the D3dsymmetry should not significantly influence the bite angle that is predominantly determined by the electronic structure of the oxalate ligand. Figure 3b illustrates the calculated energy dependence of ,A2, and ZE,split levels with respect to A 9 (=60° - (p), when a is fixed at 83'. With growing Acp the splitting of 2E, is drastically reduced whereas the ground-state splitting decreases only moderately. This behavior that is mainly due to strong contributions of a-bonding is different from the band splittings calculated for twisted [Cr(en),13' for which metal-ligand
a-effects are generally neglected (cf. Figure 4 of ref 5). Although much larger 2E, splittings (about 400 cm-l) have been predicted from the AOM for C r ( o ~ ) ~ ,in- view of a dominant role of aani~otropy,~ the experimental findings determine this level splitting to be rather small in the present system which can be rationalized by the same model using a reasonable trigonal geometric distortion of the tris-chelated molecule as the basis for the calculation. The 10 times larger 2E, splitting in the compound with nine crystalline H 2 0 (AE = 30 cm-l) can be explained by a further reduction of symmetry moving the two opposite planes of coordinated oxygens away from parallel position. This tipping of the oxalate ligands leading to Cz symmetry is actually suggested by X-ray structure results obtained for the ammonium and the potassium ~ a l t s . ~ ~ , ~ ~ For this symmetry reduction the calculation predicts larger splittings for both the ground state and 2E, in accordance with EPR and other optical experiment^.^^^ A satisfactory fit to the experimental findings is obtained if the geometric angles are chosen p = 55', 29 = 59O, and a = 83' (&lo in all cases). The twisting of the oxygen triangles around the trigonal axis does not have a large influence on the quartet level scheme: the calculated splitting is small compared to the broadness of the measured absorption bands which could not be resolved. The AOM calculation, however, can explain the unusual splittings of the lower octahedral states resulting from the t 2 2 electron configuration which give rise to intercombination bands. For a further test of the given description of the C r ( o ~ ) molecule, ~~an identification of higher electronic states would be necessary which seems to be prevented by the intense vibronic sideband structure arising from 'Eg in the long-wavelength part and by the 2T2,transitions. low spectral resolution in the region of ,A2,
(21) Schonherr, T. Photochemistry and Photophysics of Coordination Compounds; Yersin, H., Vogler, A., Eds.; Springer: Berlin, 1987; p 31.
Acknowledgment. We are grateful to the Fonds der Chemischen Industrie, Frankfurt/Main, for financial support.
-
Ordering in the Framework of a Magnesium Aluminophosphate Molecular Sieve Patrick J. Barrie and Jacek Klinowski* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1 E W, England (Received: April 5, 1989)
MgAPO-20 is the magnesium aluminophosphate equivalent of the zeolite sodalite with magnesium, aluminum, and phosphorus in the tetrahedral framework sites. We have measured *'AIand 3'P magic-angle-spinning(MAS) NMR spectra of this material and demonstrate that (i) all spectral signals can be assigned; (ii) the framework composition can be calculated from the spectra; (iii) the magnesium in the framework is strictly ordered; (iv) the precise nature of this ordering can be determined.
Introduction Since 1982 several new families of porous solids have been synthesized. The A1P04 molecular sieves, with structures built from alternating AIO, and PO4 tetrahedra, were the first to be discovered.' Some of them have the framework topologies of known zeolites, but many have novel structures. AIPO, sieves are synthesized under mild hydrothermal conditions from gels containing sources of aluminum, phosphorus, and at least one organic structure-directing template. Incorporation of a silicon source into an aluminophosphate gel results in the formation of silicoaluminophosphates, denoted SAPO, and the incorporation of a metal, Me (such as Mg, Mn, Fe, Co, or Zn), into AIPO, and SAPO gives the MeAPO and MeAPSO sieves, respectively.2
Some of these have high Bransted acidities and thus a considerable potential as catalysts as well as ion exchangers and sieves. The sodalite cage (a truncated octahedron) is the simplest space-filling polyhedron apart from the cube and divides space with minimum partitional area.3 Many natural and synthetic materials with the sodalite structure and a variety of framework and enclathrated species have been described. One of these, ultramarine, was recently shown by MAS NMR4 to be disordered and thus to violate the Loewenstein rule5 ("the aluminum avoidance principle") which is universally obeyed in hydrothermally prepared aluminosilicates. By contrast, the mineral lazurite, the natural equivalent of ultramarine, is strictly ordered. The sodalite structure, in which all tetrahedral and all cationic sites are
( I ) Wilson, S. T.; Lok, B. M.; Messina, C. A,; Cannan, T. R.; Flanigen, E. M . J . A m . Chem. Soc. 1982, 104, 1146. (2) Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S. T. Pure Appl. Chem. 1986, 58, 1351.
(3) Thomson, Sir William. London, Edinburgh, Dublin Philos. Mag. J . Sci. 1887, 24 (5th series), 503. (4) Klinowski, J.; Carr, S. W.; Tarling, S. E.; Barnes, P. Nature 1987, 330, 56. (5) Loewenstein, W. A m . Mineral. 1954, 39, 92.
0022-365418912093-5972$01.50/0 0 1989 American Chemical Society
