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Conclusions The preliminary observations described above serve to illustrate the sensitivity of TIRM technique for the study of the interactions between a particle and a surface. The manipulation of a particle close to a surface by the application of radiation pressure forces has been demonstrated to be a valuable extension to the TIRM technique. However, determination of the absolute separation of the particle from the surface still has to be established if a more complete analysis of the potentials obtained is to be performed. One way to establish the absolute separation of the particle from the surface is to subject the particle to a force that varies with distance
in a defined manner. Lubrication theory as developed by Brenner" indicates that the frictional force of a spherical particle moving orthogonal to a surface increases monotonically as the separation is decreased, The transient response of a particle subject to an instantaneous change in radiation pressure force is dependent on the measured gradient and this position-dependent frittional term, The results of such an analysis are to be the subject of a future publication. Registry No. Vitreous silica, 60676-86-0; polystyrene, 900353-6. _ _ -. (11) Brenner, H. Chem. Eng. Sei. 1961, 16, 242.
NMR Analysis of Chemisorbed Ligands for Isomeric Products: The p-SulfonylbenzylGroup on Silica Gel D. Slotfeldt-Ellingsen,f*tH. A. Resing,**+K. Unger,§ and J. F r y e l Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000, Fachbereich Chemie, Johannes Gutenberg Universitaet, 0-6500 Mainz, FRG, and Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received July 22, 1988. In Final Form: May 24, 1989 13C NMR spectra (cross-polarization magic angle spinning) have been recorded for the benzylsilyl derivative of silica gel and for its chlorosulfonated derivative. Resolution is sufficient to provide for the latter a four-line spectrum in the aromatic region with intensities and chemical shifts as expected for the sulfonyl group substituted para to the methylene group. The product of a surface reaction has thus been analyzed for its isomeric content.
The CPMAS (cross-polarization magic angle spinning) technique of carbon-13 NMR (nuclear magnetic resThe onance) has proven its utility in surface surface of silica gel can react to provide a surface-modified, chemically significant derivative;' the new surface functional group may itself be modified.' Such a preparative sequence may be used to provide specific reagents or even catalysts useful in the cleanup of air streams through contact of a gas stream with the porous modified adsorbent.6 In this note, it is shown by the CPMAS technique that when silica gel with attached benzyl groups
* Deceased. Address correspondence to: A. N. Garroway, Naval Research Laboratory, Chemistry Division, Code 6122, Washington, D.C. 20375-5000. Naval Research Laboratory. Current address: Center for Industrial Research, P.O. Box 124 Blindern, 0314 Oslo 3, Norway. Johannes Gutenberg Universitaet. Colorado State University. (1) Chang, J. J.; Pines, A.; Fripiat, J. J.; Resing, H. A. Surf. Sci.
*
1975, 47, 661.
(2) Resing, H. A.; Sloffeldt-Ellingsen, D.; Garroway, A. N.; Weber, D. C.; Pinnavaia, T. J.; Unger, K. Magnetic Resonance in Colloid and Interface Science; Fraissard, J., Resing, H. A., Eds.; Reidel: Dordrecht, 1980; p 239. (3) Shoemaker, R. K.; Apple, T. M. J.Phys. Chem. 1985,89,3185. (4) Bronnimann, C. E.; Maciel, G. E. J.Am. Chem. SOC. 1986, 108, 7154. (5) Unger, K.; Berg, K.; Nyamah, D.; Lothe, Th. Colloid Polym. Sci. 1974, 252, 317. (6) Carhart, H., personal communication. (7) Stewart, J. J. Chem. SOC.1922, 121, 2556.
is sulfonated,' sulfonation only occurs para to the methylene surface link. Sulfonation of toluene itself with chlorosulfonic acid yields a mixture of isomers;' thus the surface reaction might be expected to do the same. To show by other means that only the para isomer is formed would not be as straightforward. The preparation of the surface derivative has been described.' After reaction, the final concentration of the benzyl moiety was 4.1 pmol/m2; the BET area was 211 m2/g. Sulfonation with chlorosulfonic acid proceeds with little loss of organic ligand.' Spectra were recorded at NRL at 1.4 T and/or at 3.5 T at CSU. Experimental parameters are given in the figure captions. Spectra for the benzyl derivative with and without magic angle spinning are shown in Figure 1. A spectrum for the sulfonated benzyl derivative is given in Figure 2. Spectra similar to spectra c and d of Figure 1 have been analyzed earlier in terms of motion and orientation of the phenyl ring of the benzyl group.s For the purposes of this paper, they confirm the presence of both aromatic and aliphatic carbon in the benzyl derivative. Spectrum b of Figure 1, the high-resolution CPMAS spectrum, again shows these two contributions: aromatic (around 130 ppm) and aliphatic (at 21.1 ppm). Spectrum b of Figure 1could be simulated rather well by the 1:1:4:1 spectrum of shown as a, where the line at 138 ppm is assigned to the carbon atom attached to the methyl~~
~~~
~~~
(8) Slotfeldt-Ellingsen, D.; Resing, H. A. J. Phys. Chem. 1980, 84, 2204.
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Langmuir, Vol. 5 , No. 6, 1989 1325
NMR Analysis of Chemisorbed Ligands
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150
100
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Chemical Shift bpm) Figure 2. 13C CPMAS NMR spectrum at 3.5 T of the sul-
fonated benzyl derivative of silica gel. The parameters associated with the spectrum are as follows: cross-polarization time, 500 ms; number of scans, 80 OOO; broadening, -30 Hz (ca. -0.7 ppm); spinning rate, 3880 Hz. Peaks are at 22.2, 126.9, 129.7, 139.5, and 141.9 ppm. The small peak at 39 ppm and others in ' ' V the 220-250-ppm range (not shown) are spinning sidebands.
I
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Chemical Shift (PPm) Figure 1. 13Ccross-polarization NMR spectra at 1.4 T of silica gel reacted with benzyltrichlorosilane. The parameters asso-
ciated with each spectrum are cross-polarization time (ms)/ repetition period (s)/number of scans (thousands)/broadening (ppm). Spectra d (0.5/0.5/12/2) and c (0.5/0.6/118/2) were obtained without MAS, d at about 77 K and c at room temperature; for these, the sweep width is 500 ppm. Spectrum b is a CPMAS spectrum (1/0.3/45/1) at a spinning rate of ca. 2 kHz. Spectrum a is a simulation of spectrum b obtained by using the chemical shifts (relative intensities) 20.1 (l),125 (l),129.3 (4), and 138 (1)ppm. For a and b, the sweep width is 250 ppm, as shown.
ene group (by analogy with many benzyl compounds) and where, therefore, the line at 125 ppm is assigned to carbon para to the methylene. The sulfonated derivative exhibits a clear, four-line 2:2:1:1 aromatic spectrum at 3.5 T (Figure 2) and indicates quite strongly, by the following arguments, that para substitution predominates. Note that resolution of the individual lines in Figure 2 was achieved by means of resolution enhancement (negative line broadening). Earlier results' at 1.4 T for the sulfonate derivative had also pointed to the para substitution, though the spectral resolution was far less than the resolution at 3.5 T. A set of theoretical spectra for the ortho, meta, and para isomers was constructed by using the shifts assigned for the various carbon sites of benzenesulfonicacidlo (with respect to benzene) under the assumption that the con(9) Resing, H. A.; Slotfeldt-Ellingsen, D.; Pinnavaia, T. J.; Garroway,A. N. 175th National Meeting of the American Chemical Society,
September 1978, Abstract P133.
tributions of the sulfonyl group are additive to those found for the benzyl ligand above. For the para isomer, let carbon 1 be that attached to the sulfonyl group; then the calculated (found) line positions are (1)139.8 (139.5), (2) 127.1 (126.9), (3) 130.6 (129.7), and (4) 141.9 (141.9) ppm. Thus the number of lines, their positions, and their relative intensities all indicate that the contributions of the para isomer dominate the spectrum. The six-line spectra for the other isomers tend to fill in the center of the aromatic region to a greater degree. For the ortho isomer, the predicted positions are 126.4,127.2,130.7,133.3, 135.9, and 144.2 ppm. For the meta isomer, they are 122.9, 127.2, 130.7, 133.3, 139.4, and 144.8 ppm. The agreement of the theoretical line positions for the para isomer suggests that the predicted line positions should also be valid for the meta isomer, the isomer more likely to be formed in addition to the para. That there is little spectral intensity around 122.9 ppm suggests that the metasulfonated benzyl derivative is effectively absent. From the point of view of NMR spectroscopy, we have shown that experimental resolution is sufficient to distinguish such isomers; from the point of view of surface modification, we have shown that the surface steric constraints are such that the para isomer dominates.
Acknowledgment. The 3.5-T13C NMR spectrum was obtained at the Colorado State University Regional NMR Center, funded by National Science Foundation Grant No. CHE-8616437. The cooperation of the reviewers is explicitly acknowledged (A.N.G.). (10) The Sadtler Standard Spectra; Carbon-13NMR; Sadtler: Philadelphia, 1976; p 902C.
