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Five-Coordinate Silicon in Zeolites Probing SiO42F Sites in Nonasil and ZSM-5 with 29Si Solid-State NMR Spectroscopy.

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T. T. Dabrah, T. Kaneko, W. Massefski, Jr., E. B. Whipple. J Am. Chem.
SOC. 1997,119,1594.
10: yellow oil. R,=0.69 (ethyl acetateibenzene 911); F T I R (CH2C12cast): B=
T. T. Dabrah, H. J. Harwood, Jr., L. H. Huang, N. D. Jankovich, T. Kaneko,
2953, 2359. 2081, 1696, 1612, 1514, 1438, 1249, 1188, 1085, 836, 777 cm-I; 'H
J.-C. Li, S. Lindsey. P. M. Moshier, T. A. Subashi, M. Therrien, P. C. Watts, J.
NMR(600MHz,C,D,):b=7.23(m,2H,ArH),6.80(m,ZH,ArH),5.36(s,lH,
Anrrbiot. 1997.50. 1.
G C H H ) , 5.28 (s, I H. M H H ) , 5.24 (s, 1H, M H H ) , 5.23 (s, 1H, M H H ) ,
K. C. Nicolaou, M. W. Harter, L. Boulton, B. Jandeleit, Angew Chem. 1997,
4.54 (dd. J = 5.2.5.2 Hz. 1H,TBSO-CH), 4.34 (d, J = 11.5 Hz, I H , ArCHH). 4.31
109, 1243; Angew Chem. Int. Ed. Engl. 1997,36, 1194.
(d,J=11.5Hz.lH,ArCHH),4.06(d,J=12.5Hz,1H,C=CCHH),4.02(d,J= While this work was in progress a preliminary report appeared outlining a
12.5 Hz. 1 H, C%CHH), 3.36 (s, 3H, C(O)OCH,), 3.30 (s, 3H, ArOCH,), 2.23strategy for the preparation of the CP core using the divinylcyclopropane
2.38(m,2H,CH2). 1.74-1.84(m,2H,CH,),0.97(s,9H,(CH,),CSi),0.06(~,3H,
rearrangement: H. M. L. Davies, R. Calvo, G. Ahmed, Trtruhedron Lert.
CH$i), 0.04 (s. 3H. CH,Si); "C NMR (150.9MHz, C,D,): 6 = 159.7,148.7,143.2,
1997, 38, 1737.
130.8, 129.5. 114.1. 114.0, 112.9,72.5, 71.9, 71.8, 54.7, 51.3, 35.1, 26.0, 19.4. 18.4,
For recent insights on this reaction see: a) L. J. Brzezinski, S. A. Rafel, J. W.
-4.5, -5.1 (the carhonyl and diazo carbons do not appear in this spectrum);
Leahy, J. Am. Chem. SOC. 1997,119,4317; b) S. Rafel, J. W. Leahy, J. Org.
HR-MS: calcd. for CI,H3x0,N,Si [M+H']: 475.2628, found: 475.2644
Chem. 1997,62, 1521. and references therein.
15: clear oil: R, = 0.20 (hexanesldiethyl ether 6/4); FI-IR (CH2C12cast): 5= 2954,
N. Nakajima, K. Honta, R. Abe. 0. Yonemitsu, Tetrahedron Lert. 1988,62,
2856, 1688, 1613. 1513. 1464, 1362, 1302. 1250. 1174, 1093, 1038, 890, 836,
4139.
776cm I : ' H NMR (600MHz, C,D,): 6=7.25-7.22 (m. 2H, ArH). 6.86-6.83
D. F. Taber, K. You, Y. Song, J. Org. Chem. 1995.60. 1093.
(m. 2H, ArH), 5.38 (br. s. 1 H, W H H ) , 4.99 (br. s, 1 H, M H H ) , 4.41 (d, J =
For leading references to rhodium carbenoid chemistry, see: a) D. F. Taber,
11.5Hz,1H.ArCHH),4.38(d,J=11.5Hz,lH,ArCHH),4.14(d,J=5.0,Hz, Y. Song, J. Org. Chem. 1996,61,6706; b) H. M. L. Davies in Comprehensive
IH. TBSO-CH). 4.06 (d, J = 13.0 Hz, 1 H, M C H H ) , 3.78 (s, 3H, ArOCNJ,
Organic Synthesis, Vol. 4 (Ed.: B. M. Trost), Pergamon, New York, 1991,
3.70 (d. J = 13 Hz. I H. C=CCHH), 2.61 (ddd, J= 12.0, 12.0, 8.2 Hz. 1 H, CHH),
p. 1031.
2.10 (s. 3H, O=CCH,), 1.78 (dd. J = 12.5, 7.5 Hz, CHH), 1.61-1.55 (m.3H,
The stereochemistry of compound 11 was assigned through the use of
CHH. includes d ( O = I.%), J=5.0 Hz. cyclopropyl-H), 1.44-1.40 (m. 2H,
COSY, NOESY, ROESY, and HMQC NMR experiments on the correCHH), 0.90 (d, J - 5 . 0 Hz, 1H, cyclopropyl-H), 0.83 (s. 9H, (CH,),CSi), -0.01
sponding primary alcohol obtained by reduction (DIBAL 2.2 equiv).
(s. 3H. CH,Si). -0.05 (s, 3H, CH,Si): ',C NMR (150.9MHz, C,D,): 6=206.9,
D. L. Hughes, Org. React. 1992,42,335.
159.1. 140.5. 130.7. 129.2, 118.4. 113.7,76.9,72.8. 72.0.55.3, 50.0,44.5. 31.4, 28.5.
S. V. Ley, J. Norman, W. P. Griffith, S. P. Marsden, Synthesis 1994, 639.
[M+Na-1:
26.8, 25.9. 20.0. lX.1, -4.6, -4.8; HR-MS: calcd. for CZsH3X04Si
R. E. Ireland, M. D. Varney. J. Org. Chem. 1986,51,635.
453.2437. found: 453.2446
P. A. Wender. M. P Filosa, J. Org. Chem. 1976,41,3490; for a review on this
4: clear oil: R, =0.26 (diethyl ethedhexanes 812); FTIR (CH2CI, cast): B= 3418
type of reaction see: E. Piers, Comprehensive Organic Syntkesrs, Vol. 5 (Ed.:
(OH), 2924. 2820. 2360, 1696, 1612, 1513, 1453, 1248, 1174, lO34,820cm-'; 'H
B. M. Trost), Pergamon, New York, 1991, p. 971.
NMR (600 MHz. C,D,,): 6-7.28-7.24 (m.2H, ArH), 6.79-6.75 (m, 2H, ArH),
J. A. Marshall, D. G. Clearly, J. Org. Chem. 1986,51, 858.
4.37(d,J=11.3Hz.IH.C=CCHff),4.31 (d,J=l1.3Hz,lH,ArCHff),4.26(d,
a) Y. Ueno, S. Aoki, M. Okawara, J. Am. Chem. SOC.1979, 101. 5414; b)
J=11.3Hz. 1H.ArCHH).4.10(d,J=11.3Hz,~CCHH),4.04(ddd,J=10.6,6.0,
G. E. Keck, J. H. Byers, K. A. M. Walker, J. Org Chem. 1985, 50, 5444; c)
6.3 Hz, 1 H. HO-CH), 3.26 (s, 3H, ArOCH,). 2.98 (d, J=6.0 Hz, l H , CHOH),
D. E. Ward, Y. Gai, B. F. Kaller. rbid. 1995.60.7830.
2.53(ddd..1=16.0. 11.1.6.4H2, lH,CHH),2.42(ddd,J=lS.5,2.9,2.9Hz,lH,
These operations were performed on an SGI Indigo-2 workstation using the
CHH). 2.41 -2.35 (m. 2H. CHH), 2.13 (ddd, J = 13.7, 11.1, 2.2 Hz, 1 H, CHH),
program Insight II (Biosym Technologies, Inc., San Diego. CA)
Table 1. Selected physical properties of compounds 4,10,15, and 22.
2.08-2.07(m.lH.CHH),2.00(ddd,J=15.2,10.7,2.0Hz,1H,CHH),1.94-1.87
(m, 1 H. CHH). 1.58-1.51 (m, l H , CHH), 1.30 (dd, J=12.7, 2.6Hz. CHH),
1.17 - 1.10 (m,1 H, CHH); "C NMR (150.9 MHz, CnDa):6 = 209.1, 159.7, 145.4,
130.8. 129.6. 127.3. 114.1, 76.4, 71.8, 70.0, 54.7, 51.2, 39.6, 39.2, 33.9, 28.2, 27.0;
HR-MS: calcd. for CIYHIIO,[M+Na']: 339.1572, found: 339.1565
22: beige oil: R, = 0.20 (diethyl ether); FT-IR (CHfZI, cast): B= 2925,2855. 1778,
1699, 1461. 139Y, I184, 1020 cm I: 'H NMR (600 MHz, C,D,): b=4.66 (m, 1 H,
G C H ) , 3.73 (d. .J=Y.OHz, l H , OXOC HH) , 3.38 (d, J=9.0Hz, I H ,
O=COCHH).2.04- I.Y5 (m, 3H, CHH), 1.93 (dd,J= 18.2, 1.3 Hz, CC(O)CHH),
1.86 (dd. J=IX.2, 1 . 3 H ~ CC(O)CHH),
.
1.77 (ddd, J=12.0, 6.0, 2.5Hz. I H ,
CHH). 1.72-1 64 (m, l H , CH H) , 1.62-1.58(m, l H, C HH) , 1.45-1.26(m,3H,
CHH), 1-16 (d, J = 13.6 H L , 1 H, CHH), 0.95 (ddd, J = 13.4, 3.9, 1.2 Hz, 1 H,
CHH); "C NMR (150.9 MHz, C,D,): d=212.5, 174.3, 144.9, 122.8, 75.4, 47.7,
46.4.42.7, 3X 6.37.4, 30.2.28.6,23.6,21.0; HR-MS: calcd. for CI3H,,O, [M+H*]:
221.1178. found: 221.1169
with the opposite stereochemistry to that of 3) and the
bridgehead olefin, two of the most challenging structural
elements in 1 and 2. The structure of this cyclization product
was assigned by spectroscopic means (see also Table 1).Thus,
'H NMR spectroscopy (1D-GOESY) showed NOE of the H2
signal upon irradiation of H1, whereas molecular dynamics
and minimization calculations (CV Force
revealed
interatomic distances of 2.5 and 4.3 A for H1 -H2 and H1H3, respectively, supporting structure 22 rather than 3.
The described chemistry demonstrates an efficient entry
into the unusual carbon skeleton found in the CP core by
combining three powerful synthetic operations (intramolecular cyclopropanation, divinylcyclopropane rearrangement,
and radical cyclization). Although modifications are required,
this strategy should facilitate the chemical synthesis of both
natural products and relatives thereof for chemical biology
investigations.
Received: July 8,1997 [Z10653IE]
German version: Angew. Chem. 1997,109,2922-2925
Keywords: cyclizations
ucts rearrangements
-
- cyclopropanations - natural prod-
Angew Chern Inr Ed Eng/ 199%36, No. 24
Five-Coordinate Silicon in Zeolites: Probing
Si0,F- Sites in Nonasil and ZSM-5 with
29SiSolid-state NMR Spectroscopy**
Hubert Koller,* Axel Wolker, Hellmut Eckert,
Christian Panz, and Peter Behrens
The primary building blocks in a wide variety of tectosilicates are corner-sharing SO,,* tetrahedra."] Silicon typically
has a coordination number of four in inorganic silicates, and
only a few structures are known with other coordination
numbers. Six-coordinate silicon has been found in a variety of
minerals and silicon phosphates.[*] Recently, van de Goor
et al. found a five-coordinate silicon site (SiO,,F-) by
single-crystal X-ray structure analysis of the clathrasil nonasil,
which was hydrothermally crystallized from a silica gel in the
presence of cobaltocenium cations and fluoride ani0ns.1~1This
nonasil will be designated as [Cp2Co]-F-[Si-NON].I4]
Fluoride ions can be used as a substitute for OH- in the
syntheses of zeolites and clathrasils. Both F- and OH- are
[*] Dr. H. Koller, A. Wolker, Prof. H. Eckert
lnstitut fur Physikalische Chemie der Universitat
Schlossplatz 4/7, D-48149 Munster (Germany)
Fax: Int. code + (251)83-29159
e-mail: hkoller@uni-muenster.de
C. Panz, Prof. P. Behrens
Institut fur Anorganische Chemie der Universitat Munchen (Germany)
[**I We thank the Fonds der Chemischen Industrie, the German Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie, and the
Deutsche Forschungsgemeinschaft (KO 1817/1-1)for financial support.
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim. 1997
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~~
called "mineralizing agents" since they solubilize the silica
components in hydrothermal syntheses. The synthesis with
fluoride results in zeolite structures containing only few defect
sites;[5,61however, the structural rationale for this phenomenon is not known. In contrast, high-silica zeolites (that is, those
that have a high silicon content) made in the presence of OHhave many defect sites which form SiO- ... HOSi hydrogen
bonds.l61
Since the first clathrasils and zeolites were crystallized by
the fluoride route,{7]the range of structures that are synthesized in the presence of F- (instead of OH-) keeps increasing.[*] Aside from the aforementioned case of n ~ n a s i l , [the
~]
structural role of fluoride was only clarified for the clathrasil
octadecasil, in which F- is located in double four ring
In the case of the high-silica zeolite ZSM-5, which is made
with tetrapropylammonium cations and fluoride ions (designated as [TPAI-F-[Si-MFI]), the location of the fluoride ions
was investigated by X-ray diffraction experiments on powders[l01and twinned crystals.[l]lHowever, the results of the two
studies provide different fluoride positions, and neither
implies five-coordinate silicon. Apparently, the disorder of
the tetrapropylammonium cations, the lack of good single
crystals, and (as will be shown below) a motion of the fluoride
ions caused some difficulties for the use of diffraction
techniques. Here we show with 29Si magic-angle spinning
(MAS) NMR experiments that five-coordinate silicon in the
form of SiO,,,F- exists in [Cp2Co]-F-[Si-NON] and [TPAI-F[Si-MFI]. 29Si NMR spectroscopy is a suitable method for
distinguishing between four-, five- and six-coordinate silicon
sites by considering their chemical shifts.[*]
Specifically, the cross-polarization (CP) technique, which
relies on heteronuclear dipole - dipole interactions, allows
one to probe the spatial proximity among spins. 29SiNMR
signals for silicon sites near fluorine can be selectively
enhanced by polarization transfer from I9Fin 29Si(I9F)CPMAS
NMR experiments. Therefore, Si04/2F- sites will give a
relatively strong signal due to the short Si-F distance.
Figure l a shows the 29SiNMR spectrum of [Cp2Co]-F-[SiNON] . There is partial overlapping of the resonances for ten
distinct crystallographic silicon sited3] between 6 = - 104 and
- 116. Particularly interesting is a weak additional signal at
6 = - 145, which we assign to the five-coordinate silicon in the
structure of [Cp2Co]-F-[Si-NON] . A similar chemical shift by
d = - 150 for five-coordinate silicon was observed by solidstate 29SiNMR spectroscopy in a quenched potassium silicate
glass by Stebbins.[121Figure l b shows that the line at 6 = - 145
in the 29Si(19F]CPMAS NMR spectrum is dramatically
enhanced, hence confirming the assignment to a Si0,/2Fspecies.
The 29Si{1H)CPMAS NMR spectrum of [TPAI-F-[Si-MFI]
at 298 K is shown in Figure 2a. In the range of chemical shifts
for four-coordinate silicon (6 = - 107 to - 117) there are
narrow signals for distinct crystallographic Si04i2tetrahedra.
-70
-100
-150
-180
-8
Figure 2. 29Si('H)CPMAS NMR spectra of the ZSMd sample [TPA]-F-(SiMFI],Bo=705T,contact time 10ms:a) T=298K,d=-107.4,-108.9,-111.6,
-112.3. -114.5, -116.2, -116.8, -125.0; b) T=140K, 6=-103.0, -105.5,
-107.5, -108.1, -109.3, -110.3, -111.4, -113.7, -114.8, -1154, -117.3,
- 144.1, - 147.0.
However, not all of the expected signals are resolved due to
overlapping of some lines. A significantly broader line is
observed at 6 N- 125. Although several groups obtained
similar 29SiNMR spectra of [TPAI-F-[Si-MFI], this broad
signal at d = - 125 has not been commented on before. The
intensity of this signal is greatly enhanced in the 29Si(19F]
CPMAS NMR spectrum of [TPAI-F-[Si-MFI] . Therefore, we
conclude that this signal must be due to silicon sites that are in
close proximity to fluorine. The chemical shift at d = - 125
falls between that expected for a Si04,2F-site (6 = 145) and
the range for four-coordinate silicon (6 = - 107 to - 117). In
the following it will be shown that the line at 6 = - 125
corresponds to an averaged signal due to dynamic exchange
between four- and five-coordinate silicon.
[TPAI-F-[Si-MFI] has a phase transition at 175 K that is
associated with a change in the space-group symmetry. [I3]
Figure2b shows the 29Si(1H]CPMAS NMR spectrum of
[TPAI-F-[Si-MFI] below this phase-transition temperature. In
this spectrum the broad line at 6 = - 125 is not observed, and
instead two sharp lines for at least two distinct Si0,,2F- sites
are found at 6=-144.1 and -147.0. The strong dipolar
interaction between the 29Siand 19Fnuclei, which would result
in broader lines, is effectively averaged out by the MAS
technique. Therefore, 19F decoupling was not necessary to
obtain the narrow lines in Figure 2b. At room temperature the
fluoride ion exchanges between different SiOdi2tetrahedra of
the MFI structure, which leads to the broad feature at 6 =
- 125. This motion is frozen out at 140 K, producing the sharp
lines for the SiO,,& sites. These observations clearly show
that five-coordinate silicon exists in [TPA]-F-[Si-MFII that is,
~
ii
n
Figure 1. Nonasil sample [Cp,Co]-F-[Si-NON]: a) "SL('H) CPMAS NMR
spectrum, B0=7.05 T, contact time 10 ms; b) "Si(19F) CPMAS NMR spectrum,
B, = 4.7 T, contact time 5 ms.
2824
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
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7
Angew. Chem. Znf. Ed. Engl. 1997.36,No. 24
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high-silica Z S M - 5 made with fluoride as the mineralizing
agent.
When crystals of [TPAI-F-[Si-MFI] are calcined, then
tetrapropylammonium cations and fluoride anions are removed from the structure to leave behind a bare microporous
Si02 framework of ZSM-5 without five-coordinate silicon.
This calcined zeolite was studied in detail by other^.[^^'^] Work
is now in progress to identify in more detail the fluoride
binding sites, to characterize the motional process of the
fluoride ions, and to measure the relevant ‘9FA9Si distances.
Received: July 10,1997 [210668IE]
German version: Angew. Chem. 1997,109,2939-2940
Keywords: coordination modes
silicon zeolites
-
-
NMR spectroscopy
-
[l] F. Liebau, Structural Chemistry of Silicates-Structure, Bonding, and
Classification, Springer, Berlin, 1985.
[2] a) G. Engelhardt, H. Koller, NMR Basic Principles and Progress 1994,31,1;
b) G. Engelhardt, D. Michel, High-Resolution Solid-state NMR of Silicafes
and Zeolites, Wiley, Chichester, 1987.
[3] G. van de Goor, C. C. Freyhardt, P. Behrens, Z. Anorg. Allg. Chem. 1995,
621, 311.
[4] “NON” is the three-letter code recommended by the International Zeolite
Association for this framework structure: W. M. Meier, D. H. Olson, C.
Baerlocher, Atlas of Zeolite Structure Types, Elsevier, New York, 1996.
[5] J. M. ChCzeau, L. Delmotte, J. L. Guth, M. Soulard, Zeolites 1989, 9, 78.
I61 H. Koller, R. F. Lobo, S. L. Burkett, M. E. Davis, J. Phys. Chem. 1995, 99,
12588.
171 a) E. M. Flanigen, R. L. Patton, US-A 4073865, 1978; b) J. L. Guth, H.
Kessler, J. M. Higel, J. M. Lamblin, J. Patarin, A. Seive, J. M. ChCzeau, R.
Wey. ACS Symp. Ser. 1989,398, 176.
[ S ] a) J. L. Guth, H. Kessler, €? Caullet, J. Hazm, A. Merrouche, J. Patarin, Proc.
9th Inr. Zeolite Conf (Eds.: R. von Ballmoos, J. B. Higgins, M. M. J. Treacy),
Butterworth-Heinemann, Montreal, 1993, p. 215; b) M. A. Camblor, C.
Corell, A. Corma, M. J. Diaz-Cabaiias, S. Nicolopoulos, J. M. GonzalezCalbet. M. Vallet-Regi, Chem. Mater 1996, 8, 2415; c) M. A. Camblor, A.
Corma, L. A. Villaescusa, Chem. Commun. 1997,749; d) R. E. Moms, S. J.
Weigel, N. J. Henson, L. M. Bull, M. T. Janicke, B. F. Chmelka, A. K.
Cheetham, J. Am. Chem. SOC. 1994, 116, 11849; e) J. E. Lewis, C. C.
Freyhardt. M. E. Davis, J. Phys. Chem. 1996, ZOO, 5039; f) A. Kuperman, S .
Nadimi, S. Oliver, G. A. Ozin, J. M. GarcCs, M. M. Olken, Nature 1993,365,
239.
[9] P. Caullet. J. L. Guth, J. H a m , J. M. Lamblin, H. Gies, Eur. J. Solid State
lnorg. Chem. 1991,28, 345.
[lo] B. F. Mentzen, M. Sacerdote-Peronnet, J. L. Guth, H. Kessler, C.R. Acad.
Sci. Paris Ser II 1991, 313, 177.
1111 G. D. Price, J. J. Pluth. J. V. Smith, J. M. Bennet, R. L. Patton, J. Am. Chem.
SOC.1982,104,5971.
[12] J. F. Stebbins, Nature 1991,351, 638.
[13] J. M. ChCzeau. L. Delmotte, T. Hasebe, N. B. Chanh, Zeolites 1991ZZ, 729.
1141 C. A. Fyfe. Y. Feng, H. Grondey. G. T. Kokotailo, H. Gies, Chem. Rev. 1991,
91. 1525.
Highly Precise Shape Mimicry by a Difluorotoluene Deoxynucleoside, a ReplicationCompetent Substitute for Thymidine**
Kevin M. Guckian and Eric T. Kool”
Until recently, the large majority of studies of the origins of
DNA replication fidelity have concluded that complementarity of hydrogen bonding is the chief source of energetic
selectivity between the four nucleotides at the transition state
for initial insertion.11.21As part of this field of study, a wide
number of modified nucleoside analogues have been incorporated into DNA in an effort to study the origins of
mutagenesis and mechanisms for the fidelity of DNA
replication.[3] Virtually all of these differed from natural
nucleosides both in their hydrogen bonding arrangement and
in their size and shape. While such approaches have led to
valuable insights, with such analogues it is extremely difficult
to distinguish between steric and hydrogen bonding effects as
sources of observed differences in replication p r ~ p e r t i e s . ( ~ ~ ~ l
To address this problem we proposed the structures of four
nucleoside analogues designed to mimic as closely as possible
the structure of natural nucleosides, but lacking standard
polar hydrogen bonding functionality. For example, compound 1, a difluorotoluene deoxynucleoside, was designed to
Hd
Hd
1
2
mimic the shape of the natural deoxynucleoside thymidine
(2). Its difluorotoluene “base” is isoelectronic with thymine,
and through the replacement of the carbonyl groups with C-F
and the polar N-H group with C-H the two compounds are
isosteric as
Studies of the properties of nucleoside 1 have indicated that
the compound is quite nonpolar and that the difluorotoluene
moiety shows no measurable hydrogen bonding tendency in
aqueous and organic solutions. For example, titration of 1 with
adenine derivatives in chloroform does not lead to measurable complex formation. Further, replacement of thymine
with difluorotoluene in the center of a DNA duplex 12 base
pairs in length causes strong destabilization, and the difluorotoluene shows no preferential pairing with adenine
over the other three bases; in contrast, thymine shows a
3 -4 kcal mol-I preferen~e.[~,~]
Based on the hydrogen bonding
complementarity model of DNA replication fidelity, it was
expected that during replication this analogue would serve as
a very poor enzyme substrate and would show very little
[*] Prof. E. T. Kool, K. M. Guckian
Department of Chemistry, University of Rochester
Rochester, NY 14627 (USA)
Fax: Int. code + (716)473-6889
e-mail : etk@etk.chem.rochester.edu
I**] This research was supported by the National Institutes of Health
(GM52956). We thank Prof. T. R. Krugh for helpful discussions, Prof.
W. D. Jones and Dr. R. Lachicotte for assistance with the crystallographic
data, and Dr. J. Perlstein for assistance with the graphics.
Angew. Chem. Int. Ed. Engl. 199736, No. 24
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