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Identification of AlFx(OR)y Species in Strongly Disordered Aluminum Isopropoxide Fluoride Solids: A Field-Dependent MAS NMR Study R. Ko¨nig,† G. Scholz,† A. Pawlik,‡ C. Ja¨ger,‡ B. van Rossum,§ and E. Kemnitz*,† Institut fu¨r Chemie, Humboldt-UniVersita¨t zu Berlin, Brook-Taylor-Straβe 2, 12489 Berlin, Germany, Bundesanstalt fu¨r Materialpru¨fung und -forschung, Richard-Willsta¨tter-Straβe 11, 12489 Berlin, Germany, and Leibniz-Institut fu¨r Molekulare Pharmakologie, Robert-Roessle-Straβe 10, 13125 Berlin, Germany ReceiVed: July 15, 2009
Highly disordered and X-ray amorphous aluminum isopropoxide fluorides can be seen as intermediates formed during the synthesis of high-surface AlF3. Their 27Al MAS NMR spectra were recorded at different magnetic fields of up to 21.1 T. The 27Al 3QMAS experimental data derived at B0 ) 14.1 T enabled the simulation of the 27Al MAS NMR spectrum. In addition to 6-fold coordinated species, the presence of four- and 5-fold coordinated AlFx(OiPr)CN-x (coordination number, CN, of 4 or 5) was unambiguously established for the first time for this system. A comparison of the chemical shifts observed for fluorine and aluminum isopropoxide fluorides with different F contents allows for a simple correlation for the appropriate species. As a main outcome, hints of corresponding F-species connected to 4-fold and 5-fold coordinated Al species were corroborated. Moreover, for the first time, AlFxO4-x and AlFxO5-x units present in the solids could be assigned based on a comprehensive graphical interpretation (27Al chemical shift trend analysis) that included known chemical shifts of solids containing pure AlO4, AlO5, AlF4, and AlF5 species. The correlations obtained are useful for the interpretation of 27Al MAS NMR spectra of related Al/F/O systems. 1. Introduction Just recently, we presented a study using multinuclear and multidimensional solid-state MAS NMR investigations to examine local structures and their changes in highly disordered solid aluminum isopropoxide fluorides as a function of the fluorination degree.1 The characterization of distinct local structural features of these solids leads to an enhanced knowledge of the properties and characteristics of the precursors of high-surface AlF3 (HS-AlF3) on a “molecular level”. These precursors can best be characterized as aluminum isopropoxide fluorides with the formula AlFx(OiPr)3-x · iPrOH (x ≈ 2.3 to 2.7), whereas HS-AlF3 represents a highly disordered, nanostructured metal fluoride. This new type of metal fluoride can be obtained via a fluorolytic sol-gel process starting from metal alkoxides and nonaqueous or aqueous HF.2,3 The potential of this fluorolytic sol-gel process as a soft chemistry approach is enormous: First, the sol-gel route offers a convenient way to tune the type, strength, and amount of Lewis/Brønsted acid/ base centers, for example, through the introduction of OH groups4 or to easily introduce other catalytically active species such as noble metals5 or redox-active metals.6 Additionally, these materials exhibit high surface areas (for HS-AlF3 about 200 m2/ g7), which is, for HS-AlF3, accompanied by an extremely high Lewis acidity, comparable to that of ACF or SbF5.7,8 Second, their small particle sizes and the sol-gel technique along with the possibility to use coating techniques make them advantageous for applications in optics or ceramic science.9-11 * Corresponding author. E-mail:
[email protected]. Fax: +49 (0)30 2093 7277. † Humboldt-Universita¨t zu Berlin. ‡ Bundesanstalt fu¨r Materialpru¨fung und -forschung. § Leibniz-Institut fu¨r Molekulare Pharmakologie.
However, due to the highly disordered and X-ray amorphous character of the solids, less or even no structural information can be obtained from routine methods, such as X-ray diffraction or IR spectroscopy. In contrast, solid-state NMR techniques have proven capable of directly probing the environment on a molecular level. Information on local structural features can be interpreted almost directly from the spectra. Nonetheless, for some aluminum isopropoxide fluorides with intermediate fluorination degrees (initial Al(OiPr)3/HF molar ratios of 1:1 to 1:2), this conclusion only holds true in part for the single-pulse 27Al MAS NMR spectra (even at high spinning speeds). The latter (spectra) are characterized by broad enveloping signals ranging from -40 to 60 ppm.1 This means the signals cover a range of chemical shifts known to be typical for 4-fold coordinated AlO4 and 6-fold coordinated AlF6, including all possible coordinations and F/O ligand combinations. In general, the signals of Al species are shifted to a higher field with an increasing coordination number (4-, 5-, 6-fold coordinated Al species)12,13 or, for a particular system, with an increasing number of surrounding F-atoms (AlO6, AlFO5, ..., AlF5O, AlF6).14,15 Therefore, a distribution of chemically different 4-, 5-, and 6-fold coordinated AlFx(OiPr)CN-x species (in the following, AlFx(OiPr)CN-x is used for both AlFx(HOiPr)CN-x and AlFx(OiPr)CN-x species) (CN: coordination number) probably exists in these aluminum isopropoxide fluorides. Due to the highly disordered character of the substances, each of these species has its own set of additional distributions of chemical shift and quadrupolar parameters, which produce additional spectral broadening. However, 27Al triple-quantum MAS NMR experiments recently presented (B0 ) 9.4 T) proved the distinct existence of the different species,1 but with a higher degree of uncertainty for some of them. Since some of the Al species have comparatively high quadrupolar frequencies or a low abundance in the solids or both, the intensities of the corre-
10.1021/jp9066795 CCC: $40.75 2009 American Chemical Society Published on Web 08/11/2009
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Figure 1. Sheared 27Al 3QMAS NMR spectra of AlFx(OiPr)3-x samples resulting from different initial molar ratios recorded at B0 ) 14.1 T (compound I: Al/F ) 1:1; compound III: Al/F ) 1:2) (I and III: νrot ) 27.5 kHz, TD1 (time domain in F1): 256, na (number of accumulation): 1200).
sponding signals in the 3QMAS spectra presented earlier are low, complicating their identification. Yet another approach to enhance spectral resolution (for 27Al) is to use higher magnetic fields B0. This reduces quadrupolar broadening, and the observed chemical shifts move toward the true isotropic values. As was recently shown for aluminum hydroxy fluorides related to the samples discussed here, the resolution is clearly enhanced.14 Based on this, the aims of the present work are: (i) to unambiguously identify all the Al species involved using different methods including NMR experiments at ultra high fields up to 21.1 T and 3QMAS experiments at B0 ) 14.1 T and to derive their NMR parameters (δiso, νQη); (ii) to calculate accordingly the single-pulse 27Al MAS NMR spectra and to compare the obtained NMR parameters with data extracted from the application of the SORGE method and give a “rough” estimate for the contributing species as a further option for comparative purposes; (ii) to compare changes in the Al spectra with that in the F spectra resulting from different fluorination degrees; (iii) to derive with this method a possible assignment for the different AlFx(OiPr)CN-x species, which not only applies to 19F but also to 27Al species; (iv) to develop and find correlations in the chemical shifts (δ27Al) and the quadrupolar parameters (νQη) for 4- and 5-fold coordinated AlFx(OiPr)CN-x species, similar to that found for 6-fold coordinated AlFx(OH)6-x.14 2. Experimental Section Sample Preparation. The aluminum isopropoxide fluorides AlFx(OiPr)3-x · iPrOH were prepared as described in ref 1. Aluminum isopropoxide dissolved in dry isopropanol was reacted with nonaqueous isopropanolic HF under inert conditions. Depending on the molar ratio Al(OiPr)3/HF, a clear sol (Al/F ) 1:1 and Al/F ) 2: 3) or an opaque, slightly viscous gel (Al/F ) 1:2) formed. After removal of the solvent and drying in vacuum (70 °C), a transparent solid (Al/F ) 1:1 and Al/F )
2:3) or a white xerogel (Al/F ) 1:2) was obtained and further handled in a glovebox. To prepare MAS NMR experiments, the NMR rotors were filled directly in the glovebox, except when the sample was measured on the Bruker Avance 900. In this case, the samples were transferred to small vessels packed in bigger containers, both closed in the glovebox and opened directly before the preparation of the rotors. The molar ratios given in the following text refer to the initial Al(OiPr)3/HF molar ratio. Furthermore, the aluminum isopropoxide fluorides with initial Al/F molar ratios of 1:1, 2:3, and 1:2 will be considered as samples I, II, and III, respectively. Solid-State NMR. The MAS NMR spectra were recorded on the following spectrometers: a Bruker Avance 400 (Humboldt-Universita¨t zu Berlin, 9.4 T), a Bruker Avance 600 (Bundesanstalt fu¨r Materialpru¨fung and -forschung, Berlin, 14.1 T), a Bruker Avance 750 (Universita¨t Leipzig, 17.6 T), and a Bruker Avance 900 (Leibniz-Institut fu¨r Molekulare Pharmakologie, Berlin, 21.1 T) using a 2.5 mm MAS probe (Avance 400, 600, and 750) or a 3.2 mm MAS probe (Avance 900). The Larmor frequencies, relevant for the experiments discussed here, were ν19F ) 376.4 MHz (B0 ) 9.4 T) for fluorine nuclei and ν27Al ) 104.3 MHz (9.4 T), 156.4 MHz (14.1 T), 195.5 MHz (17.6 T), and 234.5 MHz (B0 ) 21.1 T) for aluminum nuclei, respectively. The 19F MAS NMR (I ) 1/2) spectra were recorded with a π/2 pulse duration of 2.0 µs, a spectrum width of 400 kHz, and a recycle delay of 3 s. Existing background signals (19F, 1H) could be suppressed either by application of a phase-cycled depth pulse sequence according to Cory and Ritchey16 (spectra recorded at 9.4 T) or by an echo pulse sequence. The 27Al MAS NMR (I ) 5/2) spectra were obtained with an excitation pulse duration of eπ/6 to ensure quantitative excitation. The recycle delay was chosen as 1 s. To record the spectra at 21.1 T, a Bruker probe head was used, which could be tuned very close to the 27Al frequency. Al triple quantum (3Q) MAS NMR spectra were achieved using a three-pulse z-filter sequence. For the creation of the 3Q-
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Figure 2. 27Al MAS NMR spectra of samples I and III obtained at different magnetic fields from 9.4 up to 21.1 T. Shown is the central region along with possible decompositions for the spectra recorded at B0 ) 14.1 and 21.1 T (black: experimental; colored: single contributions; and red: sum) based on the data obtained by 3QMAS (14.1 T: na ) 128-256, 17.6 T: na ) 256, 21.1 T: na ) 1024).
TABLE 1:
Al MAS NMR Parameters of Samples I and III as Obtained by 3QMAS at B0 ) 14.1 Ta
27
sample I, Al/F 1:1 δF1/ppm δF2/ppm PQ/MHz νQη/kHz δiso/ppm
1
2
3
4
5
6
7
8
81.3 18.4 12.7 1906 58.0
41.2 37.3 3.2 475 39.8
32.6 21.7 5.3 793 28.6
34 14 7.2 1075 26.6
25.6 16.6 4.8 721 22.3
3 0.5 2.5 380 2.1
2.3 -6 4.6 692 -0.8
-3 -9 3.9 589 -5.2
sample III, Al/F 1:2 δF1/ppm δF2/ppm PQ/MHz νQη/kHz δiso/ppm a
1
2
3
4
5
6
7
8
83.0 20.0 12.7 1908 59.7
59.1 48.1 5.3 797 55.0
41.6 37.1 3.4 510 39.9
33.4 20.8 5.7 853 28.7
25.5 16.3 4.9 729 22.1
4.5 0.9 3.0 456 3.2
6.6 -21.2 8.4 1267 -3.7
-0.5 -12.3 5.5 826 -4.9
Species of samples I and III listed in one column correspond to each other.
coherence, a pulse with a duration of 2 µs and for the conversion a pulse with a duration of 0.6 µs were applied. The selective z-filter pulse lasted for 7.7 µs. After a two-dimensional (2D) Fourier transformation, the spectra were sheared, resulting in pure absorption mode 2D contour plots.(see, e.g., ref 17). Generally, the characteristics of the observed signals were checked for longer recycle delays. Further experimental details (na: number of accumulations; TD1: time domain in F1 dimension/number of slices) are given with each spectrum. Some of the NMR spectra were simulated using the current version of dmfit2008.18 The isotropic chemical shifts δiso of 19F resonances are given with respect to the CFCl3 standard and 27 Al MAS NMR resonances with respect to AlCl3 in aqueous solution.
3. Results Aluminum isopropoxide fluorides with intermediate fluorination degrees may be considered key substances for clarifying local structures present in the precursors of HS-AlF3 and also the final substance HS-AlF3. Understanding their structural changes not only strengthens the knowledge on the type and the origin of the local structural features of the precursor/xerogel but also enables insight to be gained into the fluorolytic sol-gel process itself. Very recently, we presented a study concerning different solid-state NMR techniques (all experiments were carried out at B0 ) 9.4 T) and the assignement of local structures and their changes depending on a progressive fluorination degree.1 Some of the compounds discussed therein with intermediate initial Al/F molar ratios (ranging from 1:1 to 1:2) are also part of the study presented here.
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TABLE 2: Comparison of δiso and νQη Obtained by Different Methods for Specific Species (Samples I and III)a,b sample I, Al/F ) 1:1 species
1
2
3
4
δ27Al (14.1 T)/ppm δ27Al (17.6 T)/ppm δ27Al (21.1 T)/ppm SORGE δiso/ppm νQη/kHz R2 3QMAS (9.4 T)d δiso/ppm νQη/kHz 3QMAS (14.1 T) δiso/ppm νQη/kHz possible assignmente
34.7 38.3 43.9
25.3 29.2 30.8
19.3 24.9 26.7
49.9 1202 0.916
35.4 959 0.995
58.0 1906 AlO4
AlFxO4-x x ) 1, 2
5
6
7
8
14.2 19.4 22.5
1.6 1.7 2.1
-4.9 -2.6 -1.3
-13.0 -11.1 -9.9
33.1 1117 0.986
29.0 1168 0.999
2.3 265 0.723
1.6 770 0.999
-7.5 713 0.999
41.5 450
31.2 919
28.5 758
21.8 762
2.3 273
0.0 524
-5.5 473
39.8 475 AlF4
28.6 26.6 793 1075 AlFxO5-x x)1-4
22.3 721 AlF5
2.1 380 AlO6
-0.8 692
-5.2 589 AlFxO6-x x)3-5
sample III, Al/F ) 1:2 species δ27Al (14.1 T)/ppm δ27Al (17.6 T)/ppm δ27Al (21.1 T)/ppm SORGE δiso/ppm νQη/kHz R2 3QMAS (9.4 T)d δiso/ppm νQη/kHz 3QMAS (14.1 T) δiso/ppm νQη/kHz
1
2
3
4
5
6
8f
7
9
45.8 50.3 52.8
34.3 36.5 37.7
24.2 28.4 28.8
13.6 18.2 19.9
1.9 3.3 2.7
-12 -10.3 -9.2
58.3 1072 0.999
40.4 750 0.999
33.2 895 0.922
25.2 1027 0.993
n.d. n.d. n.d.
-7.0 677 0.999
42.1 483
32.7 758
23.8 533
39.9 510
28.7 853
22.1 729
59.7 1908
55.0 797
species #8
comparison of CT/ST δ27Al (CT)/ppm δ27Al(ST)/ppm δiso/ppm νQη/kHz
3.2 456
c
-2.5 1151
-3.3 562
-3.7 1267
-4.9 826
-6.7 416
7
8f
9
21.1 T -9.2 -8.3 -8.4 400
17.6 T -10.3 -8.1 -8.3 529
14.1 T Czjzek -9.6 670
a The denotation of the species follows Table 1. b CT, Central transition; ST, Center of gravity of the inner satellite transitions; n.d., not determinable. c Value taken directly from the spectrum. d See ref 1. e O denotes (OiPr/HOiPr). f Average of species 7, 8, and 9.
Analysis of the 27Al Spectra Obtained at Different Magnetic Fields and Comparison with Quadrupolar Parameters Deduced from 3QMAS Data. The 27Al MAS NMR singlepulse spectra1 underline the assumption that some of the visible noise peaks are attributable to additional species. That a clear interpretation is difficult is obvious when the 27Al single-pulse MAS NMR spectra obtained at a magnetic field B0 ) 9.4 T are used as a starting point in the examination of these systems. In the case of compound I, remaining Al(OiPr)6 units are indicated by the sharp signal at δ27Al ) 2 ppm; significant amounts of octahedrally coordinated Al sites can be concluded for both compounds (I and III). They must occur in a mixed oxygen/fluorine environment, as the maximum of the observed signal (-6 to -12 ppm with asymmetric upfield decay) is clearly high-field shifted from values typical for pure AlO6/Al(OH)6 (δiso(AlO(H)6) ≈ 10...0.16 ppm).19,20 A more detailed description of the structural features of the aluminum isopropoxide fluorides is given elsewhere.1,21,22 The presence of 4- and 5-fold coordinated Al species is conceivable, but the resolution is not adequate to distinguish them more clearly (Figure 2, 9.4 T).
To confirm the results, two methods were used, both of which in principle should allow for the extraction of the amount of contributing species and their NMR parameters. On one hand, higher magnetic fields were applied; on the other hand, 27Al 3QMAS NMR spectra were rerecorded. This time 3QMAS NMR was carried out at a magnetic field of B0 ) 14.1 T, to achieve an enhanced spectral resolution. The corresponding 27Al 3QMAS spectra of samples I and III are shown in Figure 1 and exhibit several contributions for Al species in 4-, 5-, and 6-fold coordination. Table 1 lists the identified species, and Table 2 compares the species and their quadrupolar parameters obtained so far, including the data presented earlier1 and values obtained using a rough approximation method described later on. The isotropic chemical shifts and the quadrupolar frequencies (the use of the quadrupolar frequency νQ means always the quadrupolar product νQη, since the effect of the asymmetry parameter η is, for a first approximation, negligible) were calculated using the corresponding values for (F1,F2) taken from the sheared spectrum and eqs 1 and 2.
δiso ) (17δF1 + 10δF2)/27
(1)
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PQ ) √85/900 · ν0 · √δF1 - δF2
(2)
νQη ) PQ · 3/((2I - 1) · 2I)
(3)
and
Applying higher magnetic fields, the enhancement of the spectral resolution is evident (Figure 2, samples I and III, B0 ) 14.1 to 21.1 T). The following effects are considerable upon a first glance: (i) As already proven by the 3QMAS experiments at B0 ) 9.4 T, the aluminum isopropoxide fluorides consist of several unambiguously distinguishable AlFx((H)OiPr)CN-x species (CN: 4, 5, 6). (ii) As the influence of quadrupolar broadening is reduced more and more with higher field, the signals become significantly narrower. (iii) The observable maxima of the signals shift to lower fields. The latter finding can easily be understood when eqs 4 and 5 are considered.
δ27Al ) δiso + δQIS
(4)
I(I + 1) - 3 - 9m(m - 1) · 106 30ν02
(5)
2 δQIS ) -νQη ·
Since δQIS (quadrupolar-induced shift) for a given transition m of the nucleus with spin I (27Al: I ) 5/2) also depends on the Larmor frequency ν0, the observed chemical shift δ27Al moves toward the isotropic chemical shift δiso for high magnetic fields B0. Along with the extraction of parameters from 3QMAS, two other possibilities can be derived for the calculation of quadrupolar parameters: (a) eqs 1 and 2 and following the SORGE method (second order graphical extrapolation), introduced by Massiot;23 (b) the same equations and the consideration of the two transitions m, whereas the shift of the center of gravity of both of the inner satellites is found by extrapolation of the spinning side bands (e.g., from n ) 2) to n ) 0 since they (center of gravity and spinning sideband n ) 0) completely overlap.24,25 The prerequisite for the latter two methods is a reliable decomposition of the 27Al MAS NMR spectra. In this case, the spinning side bands have a very small intensity, and the species are not resolved. This means that method b can only be used to approximate the data of the most intense species in the 6-fold coordinated region (sample III). Since we know that in this sample more than one octahedrally coordinated AlFx(OiPr)6-x species exists1,21 (Table 1, Figure 1), the values obtained must be mean values (Table 2). Generally, due to strong superimpositions, decomposition is extremely difficult without knowledge of the true line shapes. To compare the results from 3QMAS spectra, for which the true line shapes were derived, the deconvolution of the singlepulse spectra was first rough approximated with mixed Gaussian/ Lorentzian functions for the contributing species with consideration of the following: (i) that the true line forms possibly vary and (ii) that the error introduced with this approach is larger for the spectra recorded at fields lower than 21.1 T. True line forms may have shapes broadened by second-order quadrupolar interactions or a Czjzek-type line form with an asymmetric
decay in the upfield part of the signal as often found for amorphous compounds.26 In this case, the spectra obtained at B0 ) 21.1 T are the best starting point for such an approach. The results applying the SORGE method using the approximated models are given in Table 2. (The results of the approximated deconvolutions are given in the Supporting Information.) Consequently, the values derived for δiso and νQ using these methods should be seen as rough estimations; nonetheless, the approximations give strong indications of the real amount of contributing species. Comparing the quadrupolar parameters obtained by several methods (3QMAS, SORGE), for each species, generally the determined isotropic chemical shifts fit very well, whereas the values for the quadrupolar frequencies are only slightly higher on comparison of the data derived by the SORGE method with the data from 3QMAS. Since all the Al species in the respective highly disordered solids have a more or less disordered environment, each quadrupolar frequency is also distributed. The excitation efficiency at any given excitation frequency of 3QMAS experiments is higher for species with lower quadrupolar frequencies.27 Thus, it is possible that species and proportions of distinct species with lower quadrupolar frequencies are overestimated in these experiments. The values calculated from the 3QMAS data should be therefore smaller. Nevertheless, a higher frequency was also found with the other methods for most of the species with high quadrupolar frequencies (in the range of 800-1000 kHz, as determined by 3QMAS). It is to be noted that the SORGE method in this case should give approximate values. The results of the experiments agree upon comparison of the data obtained by the 3QMAS experiments at B0 ) 9.4 and 14.1 T. This applies also to the species which were identified with a higher uncertainty in the 9.4 T spectra.1 All of the species have been identified. Due to the high quadrupolar frequency of species 1 (see Table 1), the most downfield shifted species for I and III were not detected with the 3QMAS technique at 9.4 T. Based on the number of different species, their line shapes, and NMR-parameters (δiso, νQη) derived from 3QMAS, a calculation of the strongly superimposed 27Al MAS NMR spectra of samples I and III should be possible. Figure 2 shows along with the 27Al single-pulse MAS NMR spectra obtained at B0 ) 14.1 T and B0 ) 21.1 T possible deconvolutions under consideration of models for the different lines using the NMR parameters δiso and νQη. The calculation of the experimental spectra works very well with the parameters worked out, and the results of the calculations are given in Table 3. Nevertheless, simplifications have to be made. The signals for species in the chemical shift range characteristic for 6-fold coordinated AlFx(OiPr)6-x (0 to -20 ppm) are strongly superimposed in all of the spectra (Figures 1 and 2). A clear differentiation remains difficult. For example, the 21.1 T spectra and the 3QMAS spectrum of sample I clearly indicate the existence of at least two different AlFx(OiPr)6-x species in an amorphous environment. Due to the strong superimposition, a simulation is possible using only one contribution, representing an averaged species. Additionally, a full reproduction of the spectrum is only feasible when a contribution of other species is assumed: this has the same parameter set as that found for species 7 in sample III, exhibiting a line shape broadened by second-order quadrupolar interactions, which is typical for species in a more ordered environment. The strong chemical
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TABLE 3: NMR Parameters Obtained by Deconvolution of the 27Al MAS NMR Spectra (B0 ) 14.1 T and B0 ) 21.1 T) of Samples I and III sample I, Al/F ) 1:1 line #
line modela
corresponds to species as identified from 3Q
#1 #2 #3 #4 #5 #6 #7 #8
Q mas 1/2 Gaus/Lor CzSimple CzSimple CzSimple Gaus/Lor CzSimple Q mas 1/2
1
#1 #2 #3 #4 #5 #6 #7 #8
Q mas 1/2 CzSimple CzSimple CzSimple CzSimple Gaus/Lor CzSimple Q mas 1/2
3 4 5 6 7+8 7 sample III
line modela
corresponds to species as identified from 3Q
3 4 5 6 7+8 7 sample III 1
δiso/ppm
dCS/ppm
B0 ) 14.1 T 57.9 32.7b 29.6 8.2 26.6 9.0 22.3 7.8 1.6b -1.3 8.8 -3.7 57.1 45.9 33.5 29.0 22.2 2.7b 0.4 0.4
B0 ) 21.1 T 3.0 3.2 3.0 3.0 1.7 -
νQ/kHz
LB or fwhm/kHz
intensity/%
νQ 1859
η 0.14 793 900 721 692 η 0.13 νQ 1265
2.4 1.2 1.6 2.0 1.3 0.3 1.2 1.4
39.9 1.9 12.3 6.8 6.5 4.5 23.1 4.9
η 0.14 νQ 1851 659 947 782 700 602 νQ 1164 η 0.1
2.6 1.1 2.3 1.6 0.8 0.6 1.4 0.9
50.1 1.0 5.3 6.6 2.6 10.1 7.8 15.8
νQ/kHz
LB or fwhm/kHz
intensity/%
νQ 1856
η 0.14 787 730 νQ 1242 η 0.10 716
2.4 0.7 1.6 1.4 0.6 1.0 1.3
20.8 0.8 11.5 6.8 1.6 18.9 39.6
νQ 1857
1.6 1.4 1.1 2.2 1.0 1.2 1.2 1.8
13.1 1.1 1.4 18.0 2.5 3.8 27.0 33.0
sample III, Al:F ) 1:2 line #
δiso/ppm
dCS/ppm
#1 #2 #3 #4 #5 #6 #7
Q mas 1/2 Gaus/Lor CzSimple CzSimple Gaus/Lor Q mas 1/2 CzSimple
1 3 4 5 6 7 8
B0 ) 14.1 T 59.6 37.1b 30.2 8.2 22.1 7.8 1.7b -3.9 -5.9 8.6
#1 #2 #3 #4 #5 #6 #7 #8
Q mas 1/2 Gaus/Lor Gaus/Lor CzSimple CzSimple Gaus/Lor Q mas 1/2 CzSimple
1 2 3 4 5 6 7 8
60.7 45.7b 39.5b 32.3 21.4 2.8b 0.8 -6.5
B0 ) 21.1 T 7.6 3.8 3.0
η 0.14 935 627 νQ 1318 η 0.10 842
a Line models used in dmfit:18 Qmas1/2: (second-order broadened) central transition of a quadrupolar nucleus under MAS conditions with infinite νrot; CzSimple: Czjzek distribution for quadrupolar nucleus in amorphous environments with dCS (distribution of chemical shifts) and distributed values for quadrupolar frequency (d ) 5, η fixed to 0.61). b Given value is δ27Al, line with less intensity, calculated with mixed Gaussian/Lorentzian model. LB, Gaussian Line Broadening for lines calculated with Czjzek or Qmas1/2 model; FWHM, Full width at half maximum for Gaussian/Lorentzian lines.
similarity of samples I and III, two intermediates in the synthesis of the precursor of HS-AlF3, supports this assumption. Furthermore, a full reproduction of the experimental spectra was sometimes only possible when small contributions with integral intensities less or equal to 1% were taken into consideration. Interestingly, nearly all Al species with coordination numbers lower than 6 (4 and 5) can be identified for both samples, whereas in the case of the superimposed 6-fold coordinated AlFx(OiPr)6-x species, a general high-field shift, meaning a higher degree of fluorination x in AlFx(OiPr)6-x, can be observed (Table 3). Both samples I and III clearly exhibit the existence of AlF4 and remaining Al(OiPr)3. The latter was indicated by the signals for the symmetric Al(OiPr)6 unit and the Al(OiPr)4 units with similar parameters (δiso, νQ, η) as stated by Abraham.28 Strong arguments for the fluorolytic sol-gel pathway were recently
demonstrated.1 The existence of further realistic intermediate structures AlFx(OiPr)5-x (x ) 1 - 5) was shown for the first time. 27 Al Spectra in Comparison with 19F in Dependence on the “Fluorination Degree”. The determination of the intensities of the single Al species as given in the Supporting Information from ultrahigh-field MAS NMR or as calculated based on 3QMAS, considering for the most intense species their quadrupolar parameters, isotropic chemical shifts, and line forms, generally allows for changes in their amounts with a progressive fluorination degree to be monitored. As recently shown, direct 19 F-27Al-HETCOR experiments exhibit only very broad crosspeaks; the different sites superimpose; and an unambiguous F-Al species assignment cannot be proven.1 An indirect method is to track the development of the intensities of certain F species with increasing fluorination degrees and to compare the development in the corresponding
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Figure 3. 19F MAS NMR spectra of samples I, II, and III obtained at 9.4 T. Shown is the central region along with possible decompositions for each (blue: experimental; colored: single contributions; and red: sum). The decomposition of the spectra of samples II and III was performed with the parameters obtained for I as the starting point for further optimizations. The results of the decompositions are listed in Table 4 (na for each spectrum 192).
F and Al plots. Therefore, the 19F MAS NMR spectra obtained at B0 ) 9.4 T were reinvestigated (samples I, II, and III, Al/F ) 1:1, 2:3, and 1:2)1 and calculated here. The corresponding 19 F MAS NMR spectra along with possible decompositions are displayed in Figure 3. The results and calculated NMR parameters are summarized in Table 4. Starting with the decomposition of the 19F MAS NMR spectrum of sample I, the spectra of the following samples were calculated taking into account for the first approximation the same number of signals with fixed values for the chemical shifts (optimizable parameters: fwhm and amplitude). On that basis and in further calculations, the chemical shift and the other parameters were optimized, assuming that similar species exist in the solids and contribute to the spectra but to different extents.
Assignment of the Specific AlFx(OiPr)CN-x Species. Tables 2 (27Al) and 4 (19F) give along with the corresponding data a possible assignment for each AlFx(OiPr)CN-x species. However, to the best of our knowledge, a publication does not exist that treats the clear and unambiguous assignment of 4- and 5-fold coordinated Al species in a mixed F/O environment. This is mainly due to a lack of crystalline reference compounds. Nonetheless, in a few papers, AlFxO4-x species have been formulated and characterized with NMR spectroscopy, especially the work of Lacassagne.13 Furthermore, tetrahedrally coordinated Al-F species were identified by solid-state MAS NMR techniques during dealumination processes of zeolites.29,30 Therefore, the Al species as given in Table 2 were assigned based initially on a general classification into 4-, 5-, and 6-fold coordinated Al species following the trends mentioned in the introduction and as presented in a comprehensive way by Lacassagne.13 The 27Al MAS NMR spectra of the precursors of HS-AlF3 show that the majority of the Al atoms are octahedrally coordinated in a mixed F/O-environment.1,21,22 The initial molar ratio of Al(OiPr)3 to HF for the preparation of the “standard” AlF2.3(OiPr)0.7 · z iPrOH precursor is 1:3. The intensities for different coordinated Al species found in the chemical-related aluminum isopropoxide fluorides with lower F contents were worked out here. Following the development of the sums of intensities of each Al species group (Al species with CN 4, 5, and 6) with an increasing degree of fluorination, insight into the changes of the appropriate local structures during the fluorolytic sol-gel process can be gained. Surprisingly, the plot of the sum of intensities of each species group versus the fluorination degree gives three almost linear correlations (Figure 4). Both the intensities obtained by the approximation of the 21.1 T spectra and the intensities obtained by calculation of the 14.1 T spectra were used for the correlations. The differences between the two approaches are negligible. The amount of 4- and 5-fold coordinated Al species decreases, and the amount of 6-fold Al species increases with the degree of fluorination. A similar plot for the F species (as given in Table 4) is obtained by plotting the intensities of each species against the initial molar F/Al ratio as shown in Figure 5. The data for the F species of the precursor of HS-AlF3 (Al/F ) 1:3) were presented in ref 31. The comparison of the general trends of the Al and F species observed in both plots should allow for an indirect correlation (Table 5).
TABLE 4: NMR Parameters Obtained by Deconvolution of the 19F MAS NMR Spectra (9.4 T) of Samples I, II, and III sample
I
II
III
fwhm/ fwhm/ fwhm/ species # δiso/ppm kHzb intensity/% δiso/ppm kHz intensity/% δiso/ppm kHz intensity/% 1 2 3 4 5 6 7 8 9 10 11a a
-130.4 -136.8 -148.5 -156.6 -162.0 -162.9 -171.5 -172.2 -181.8 -189.0 -198.9
4.8 2.6 5.1 2.8 0.6 3.0 0.8 5.0 2.4 3.7 2.9
1 3 24 18 3 23 2 21 3 3
-130.4 -136.8 -146.3 -155.0 -162.1 -162.6 -171.4 -171.2 -180.0 -187.6
4.8 2.6 4.4 3.4 0.7 3.3 0.8 4.5 5.3 16.4
2 2 13 24 3 34 0.3 15 7 1
-130.4 -137.0 -147.1 -154.2 -162.6 -162.0 -169.5 -172.7 -183.3
4.8 2.3 5.4 3.2 0.6 3.6 1.4 4.6 5.2
Overlapping with spinning sideband of species 1. b fwhm: full width at half maximum.
1 1 11 18 1 40 0.5 19 8
possible assignment AlFxOCN-x x ) 1 (?) bridging F AlFxOCN-x x ) 2 (?) bridging F AlFxOCN-x x ) 3 (?) bridging F AlFxO6-x x)4 bridging F ordered units/Al3O8F x)5 bridging F AlFxO6-x ordered units/terminal sites ? AlFxO6-x x)4 terminal F AlFxOCN-x x ) ? terminal F “AlF4/AlF5/AlF6” terminal F
Identification of AlFx(OR)y Species
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Figure 4. Development of the approximated contributions to the 27Al MAS NMR spectra recorded at B0 ) 21.1 T (a) and obtained contributions considering for calculation δiso, quadrupolar frequencies, and line shapes of the main species (B0 ) 14.1 T, (b)) in dependence on the initial molar ratio HF/Al(OiPr)3 for 4-, 5-, and 6-fold coordinated species groups. Green: AlFx(OiPr)6-x; blue: AlFx(OiPr)5-x; and red: AlFx(OiPr)4-x. If the values derived for (a) were equal to those of (b), no discrimination is given within the figure.
TABLE 5: Possible Correlations of Corresponding Species Deduced from Figures 4 and 5 F-species i
AlFx(O Pr)CN-x
Al
“bridging”
terminal
δiso/ppm
δiso/ppm
δiso/ppm
-188a CN: 4, x ) 4 38 -155a CN: 5, x ) 2 - 5 29, 27, 22 -130, -137, -149 (-172), -182, -189 CN: 6, x ) 3 - 5 -1 to -10 -156, -162, -172 a
See ref 1.
Therefore, the broader 19F signal at δiso ≈ -162 ppm is the only one, whose intensity clearly increases with F content. The same applies to the intensity of the signal for the octahedrally coordinated AlFx(OiPr)6-x species. Thus, it is clear that these two species correspond. Furthermore, it cannot be assumed that the F species with chemical shifts at δiso ≈ -156 and -172
ppm correspond to 4- or 5-fold coordinated Al species. As deduced, the observed chemical shifts (19F: δiso ) -154 and -162 ppm,27Al: δiso ) -7 to -10 ppm) can be assigned to AlF4(OiPr)2 and AlF5(OiPr) octahedra present in the xerogel matrix.14,31 On the basis of this, the F species at δiso ≈ -130, -137, and -149 ppm more likely can be assigned to the 5-fold coordinated AlFx(OiPr)5-x species than to 6-fold coordinated Al species (Table 4, species 1 to 3) or to the 4-fold coordinated species. In the case of the latter, the main contributing species is Al(OiPr)4 (Table 3). The 19F signals in the high-field part of the spectra (δiso ) -182, -189 ppm) indicate terminal F sites in several species, since known chemical shifts for AlF4- to AlF63- species were determined in the range of about -190 to -200 ppm13,32 and are predicted by several models.33,34 However, a clear assignment remains difficult, since species superimpose at similar positions. For example, the isotropic chemical shift for µ2F in AlF2O4 units is often found at δiso ≈ -136 to -143 ppm (see ref 31 and references cited therein). Additionally, as explained in ref 1 and considered in the models used here, species can be observed that can be attributed to more ordered local environments. The relatively sharp peak at 19F chemical shift of -163 ppm could possibly be assigned to a species similar to Al3(OiPr)8F · D (D ) DMSO or pyridine) as presented in ref 35 (see Table 4, species 4) since we observed an 19F chemical shift of δiso ) -160 ppm for the pure crystalline compound Al3(OiPr)8F · D (D ) pyridine).36 However, due to the several superimposition effects already mentioned, contributions for ordered local environments attributable in the 27Al MAS NMR spectra can not be clearly differentiated. Chemical Shift Trend Analysis for 4 and 5-Fold AlFx(OiPr)CN-x Species. The different Al species can be more specifically assigned when a gradual (nearly linear) change of isotropic chemical shift for each species group (CN 4 or 5) with a stepwise introduction of F in the AlFxOCN-x polyhedra is assumed. One might think that a gradual change in the chemical shifts observed could also be produced by a gradual distortion of the corresponding polyhedra, as presented by Weller for
Figure 5. (A) Development of the contributions to the 19F MAS NMR spectra recorded at B0 ) 9.4 T in dependence on the initial molar ratio HF/Al(OiPr)3. The graphs for the species at δiso ) -163 and -148 ppm just follow the general characteristics of the intensities of the respective species and are guide lines for the eyes. (B) Comparison for species with low amounts: chemical shifts lower than -180 ppm representing terminal F sites (black) and higher than -137 ppm representing polyhedra with less F content x (gray). The results of the decompositions are given in Table 4.
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Figure 6. Possible correlation of 27Al isotropic chemical shifts in dependence on the composition x in AlFx(OiPr)CN-x (red: CN ) 4, blue: CN ) 5, assuming linearity). Additionally, some reference points are incorporated (given values for isotropic chemical shifts): ∆1, Lacassagne et al.: AlFxO4-x in NaF/AlF3/Al2O3 melts;13 ∆2, Stößer et al.: AlO4 in γ-Al2O3;20 ∆3, Abraham et al.: Al(OiPr)4 in [Al(OiPr)3]4 and AlO4 in Al2O3 and Al(OEt)5 in [Al(OEt)3]n and AlO5 in Al2O3;28 ∆4, Groβ et al.: AlF4 and AlF5 in NMe4AlF4 and (NMe4)2AlF5;38 and ∆5, Robert et al.: AlF4/AlF5/ AlF6 model for NaF/AlF3 melts.12
4-fold coordinated AlO species in certain crystalline aluminate sodalites;37 however, the chemical situation is different here: The substances discussed are highly disordered and X-ray amorphoussaway from regular ordered structures as found in the sodalites stated above. More importantly, a structural distortion of the corresponding units is often obtained by different environments around the polyhedra, for instance induced by different cations in the case of the sodalites. However, other metal ions can be excluded for the aluminum isopropoxide fluorides. Therefore, a reasonable model is the assumption of a change in the amount of coordinating F atoms in the surrounding of aluminum in AlFx(OiPr)CN-x species (CN ) 4, 5). We found a similar correlation for the 27Al chemical shifts of octahedrally coordinated AlFx(OH)6-x species in crystalline aluminum hydroxy fluorides AlFx(OH)3-x and showed that the chemical shifts of AlFx(OiPr)6-x species can easily be explained via correlation.14 Since more comparable literature concerning solid-state NMR on defined (crystalline) compounds with octahedrally coordinated AlFxO6-x units exists, this trend is thoroughly supported. Solids with 4- and 5-fold coordinated AlFxOy species do not appear often in the literature, except for some 4-fold coordinated AlFxO4-x species existing in NaF/AlF3/Al2O3 melts13 and the pure species: AlO4, AlO5, AlF4, and AlF5. The first are known for some alumina20,28 or aluminum alkoxides,28 whereas the latter were used by Robert to explain the development of 27Al chemical shifts observed in cryolite-based melts depending on the AlF3 content: δ27Al(AlF52-) ) 21 ppm and δ27Al(AlF4-) ) 38 ppm.12 In the course of our investigations, we determined chemical shifts of δ27Al(AlF52-) ) 23 ppm and δ27Al(AlF4-) ) 49 ppm for different ammonium fluoroaluminates.38 These data are given in Figure 6, which offers a comprehensive graphical interpretation for assignment of the species in the aluminum isopropoxide fluorides. Data points for the graphs are the isotropic values obtained from 3QMAS spectra (Figure 1 and Table 1). Surely we can identify the limits at δiso ) 59 ppm for the 4-fold coordinated Al(OiPr)4 unit of the tetrameric Al(OiPr)3 with a quadrupolar frequency similar to that determined by Abraham28 and an AlF4 species at δiso≈ 40 ppm, which is also present in isopropoxide fluorides with lower fluorination degrees and is identified by 19Ff27Al CP MAS NMR experiments.1 The species in sample III, with a chemical shift of δiso ) 55 ppm,
would match the correlation under assumption of an AlF(OiPr)3 unit (x in AlFx(OiPr)4-x ) 1). The values reported by Lacassagne13 are slightly downfield shifted with regard to this correlation, similar to the value for the ionic AlF4- in solid NMe4AlF4, which agrees more with the known values for AlF4- in DMSO or CD3CN solution (49 ppm).32 However, the values were determined for the ionic species, whereas the units identified and involved in intermediates during the fluorolytic sol-gel process are presumably strongly cross-linked. Similar considerations hold for the interpretation of the correlation graph for 5-fold coordinated AlFx(OiPr)5-x species. However, even less is known, and the presented correlation (Figure 6, lower graph) incorporates well the known values for AlF5 and AlO5 species. In summary, comprehensive correlation graphs have been worked out for the first time that may be used as an interpretation basis for further investigations on these and related systems. This also applies to 4-fold coordinated AlFx(OiPr)4-x and 5-fold coordinated AlFx(OiPr)5-x species. Similar to the correlation graph for octahedrally coordinated AlFxO6-x, the graphs presented in Figure 6 may be applied to related Al/F/O systems. 5. Conclusions The use of ultra high-field MAS NMR up to 21.1 T enabled the distinguishing of both 4- and 5-fold coordinated AlFx(OiPr)CN-x species as well as 6-fold coordinated AlFx(OiPr)6-x species as intermediate structures in aluminum isopropoxide fluorides. The existence of certain four- and 5-fold coordinated AlFx(OiPr)CN-x species has been unambiguously shown for the first time using ultra high-field MAS NMR at magnetic fields B0 up to 21.1 T. On the basis of 3QMAS, reliable parameters, and the knowledge of the shape and the amount of the contributing species, a calculation of strongly superimposed 27Al MAS NMR spectra is possible. The quadrupolar parameters deduced by the SORGE method, using approximated lines, are in good agreement with those found by application of 3QMAS techniques. A comparison of the values obtained by the different methods for the strongly superimposed Al species shows that the given chemical shifts vary about ( 3 ppm.
Identification of AlFx(OR)y Species 27
Al and 19F signals can be correlated with a comparison of the development of single species (19F) and species groups (27Al) with increasing fluorination degrees. However, the assumption is confirmed for the first time that species with 19F chemical shifts at -130 to -149 ppm correspond to 4- and 5-fold coordinated aluminum isopropoxide fluoride species or both for this system. Additionally, a variety of possibly terminal fluorine sites are evident for the highly disordered and amorphous aluminum isopropoxide fluorides in the upfield part of the spectra. A plausible interpretation for the occurrence of different species in the aluminum isopropoxide fluorides identified in this study is the assumption of certain AlFx(OiPr)CN-x species that vary in the amount of surrounding F atoms. This issue is supported by the studies stated earlier for related octahedrally coordinated AlFx(OH)6-x species in crystalline aluminum hydroxy fluorides14 and explains the general development of the observed chemical shifts. Furthermore, known chemical shifts for pure oxygen or fluorine coordinated species are fully reproduced by the correlations found and presented in this study. Even more generally, applying the knowledge of the correlation of chemical shifts to 6-fold coordinated AlFxO6-x species, the graphs presented here for 4- and 5-fold coordinated AlFxOCN-x species should be suitable and transferable to related Al/F/O systems, as already indicated by the values for the species assigned by Lacassagne.13 Acknowledgment. The authors thank the Deutsche Forschungsgemeinschaft (Project Ke489/29-1 and Ja552/24-1) for financial support. Prof. Dr. J. Haase (Universita¨t Leipzig, Fakulta¨t fu¨r Physik und Geowissenschaften, Postfach 100920, D-04009 Leipzig, Avance 750 spectrometer) and Prof. Dr. H. Oschkinat (Leibniz-Institut fu¨r Molekulare Pharmakologie, Robert-Roessle-Straβe 10, D-13125 Berlin, Avance 900 spectrometer) are kindly acknowledged for providing measurement time. Dr. D. Heidemann and Dr. H.A. Prescott are acknowledged for fruitful discussions. Supporting Information Available: NMR parameters obtained by approximation of the 27Al MAS NMR spectra to obtain data for the SORGE method, starting from the deconvolution of the 21.1 T spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ko¨nig, R.; Scholz, G.; Kemnitz, E. J. Phys. Chem. C 2009, 113, 6426–6438. (2) Kemnitz, E.; Groβ, U.; Ru¨diger, S.; Shekar, C. S. Angew. Chem., Int. Ed. 2003, 42, 4251–4254. (3) Ru¨diger, S.; Kemnitz, E. Dalton T. 2008, 9, 1117–1127. (4) Wuttke, S.; Coman, S. M.; Scholz, G.; Kirmse, H.; Vimont, A.; Daturi, M.; Schroeder, S. L. M.; Kemnitz, E. Chem.-Eur. J. 2008, 14, 11488–11499.
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15585 (5) Patil, P. T.; Dimitrov, A.; Radnik, J.; Kemnitz, E. J. Mater. Chem. 2008, 18, 1632–1635. (6) Scheurell, K.; Scholz, G.; Pawlik, A.; Kemnitz, E. Solid State Sci. 2008, 10, 873–883. (7) Krahl, T.; Vimont, A.; Eltanany, G.; Daturi, M.; Kemnitz, E. J. Phys. Chem. C 2007, 111, 18317–18325. (8) Krahl, T.; Kemnitz, E. J. Fluorine Chem. 2006, 127, 663–678. (9) Kru¨ger, H.; Kemnitz, E.; Hertwig, A.; Beck, U. Thin Solid Films 2008, 516, 4175–4177. (10) Stosiek, C.; Scholz, G.; Eltanany, G.; Bertram, R.; Kemnitz, E. Chem. Mater. 2008, 20, 5687–5697. (11) Ahrens, M.; Schuschke, K.; Redmer, S.; Kemnitz, E. Solid State Sci. 2007, 9, 833–837. (12) Robert, E.; Lacassagne, V.; Bessada, C.; Massiot, D.; Gilbert, B.; Coutures, J. P. Inorg. Chem. 1999, 38, 214–217. (13) Lacassagne, V.; Bessada, C.; Florian, P.; Bouvet, S.; Ollivier, B.; Coutures, J. P.; Massiot, D. J. Phys. Chem. B 2002, 106, 1862–1868. (14) Ko¨nig, R.; Scholz, G.; Pawlik, A.; Ja¨ger, C.; van Rossum, B.; Oschkinat, H.; Kemnitz, E. J. Phys. Chem. C 2008, 112, 15708–15720. (15) Dambournet, D.; Demourgues, A.; Martineau, C.; Durand, E.; Majimel, J.; Vimont, A.; Leclerc, H.; Lavalley, J.-C.; Daturi, M.; Legein, C.; Buzare, J. Y.; Fayon, F.; Tressaud, A. J. Mater. Chem. 2008, 2483– 2492. (16) Cory, D. G.; Ritchey, W. M. J. Magn. Reson. 1988, 80, 128–132. (17) Amoureux, J. P.; Fernandez, C. Solid State Nucl. Magn. Reson. 1998, 10, 211–223. (18) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z. H.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70–76. (19) Xue, X.; Kanzaki, M. J. Phys. Chem. B 2007, 111, 13156–13166. (20) Sto¨βer, R.; Scholz, G.; Buzare´, J. Y.; Silly, G.; Nofz, M.; Schultze, D. J. Am. Ceram. Soc. 2005, 88, 2913–2922. (21) Pawlik, A.; Ko¨nig, R.; Scholz, G.; Kemnitz, E.; Ja¨ger, C. J. Phys. Chem. C 2009, submitted. (22) Ko¨nig, R.; Scholz, G.; Kemnitz, E. Solid State Nucl. Magn. Reson. 2007, 32, 78–88. (23) Massiot, D.; Mu¨ller, D.; Hu¨bert, T.; Schneider, M.; Kentgens, A. P. M.; Cote´, B.; Coutures, J. P.; Gessner, W. Solid State Nucl. Magn. Reson. 1995, 5, 175–180. (24) Neuville, D. R.; Cormier, L.; Massiot, D. Geochim. Cosmochim. Acta 2004, 68, 5071–5079. (25) Florian, P.; Sadiki, N.; Massiot, D.; Coutures, J. P. J. Phys. Chem. B 2007, 111, 9747–9757. (26) de Lacaillerie, J. B. D.; Fretigny, C.; Massiot, D. J. Magn. Reson. 2008, 192, 244–251. (27) Medek, A.; Harwood, J. S.; Frydman, L. J. Am. Chem. Soc. 1995, 117, 12779–12787. (28) Abraham, A.; Prins, R.; van Bokhoven, J. A.; van Eck, E. R. H.; Kentgens, A. P. M. J. Phys. Chem. B 2006, 110, 6553–6560. (29) Kao, H. M.; Liao, Y. C. J. Phys. Chem. C 2007, 111, 4495–4498. (30) Grey, C. P.; Corbin, D. R. J. Phys. Chem. 1995, 99, 16821–16823. (31) Ko¨nig, R.; Scholz, G.; Bertram, R.; Kemnitz, E. J. Fluorine Chem. 2008, 129, 598–606. (32) Herron, N.; Thorn, D. L.; Harlow, R. L.; Davidson, F. J. Am. Chem. Soc. 1993, 115, 3028–3029. (33) Liu, Y.; Tossell, J. J. Phys. Chem. B 2003, 107, 11280–11289. (34) Bureau, B.; Silly, G.; Buzare´, J. Y.; Emery, J. Chem. Phys. 1999, 249, 89–104. (35) Ru¨diger, S.; Groβ, U.; Feist, M.; Prescott, H. A.; Shekar, C. S.; Troyanov, S. I.; Kemnitz, E. J. Mater. Chem. 2005, 15, 588–597. (36) Koch, J. Diploma thesis, Humboldt-Universita¨t zu Berlin, Berlin, 2008. (37) Weller, M. T.; Brenchley, M. E.; Apperley, D. C.; Davies, N. A. Solid State Nucl. Magn. Reson. 1994, 3, 103–106. (38) Groβ, U.; Mu¨ller, D.; Kemnitz, E. Angew. Chem., Int. Ed. 2003, 42, 2626–2629.
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