J. Phys. Chem. 1992,96,7161-7164
7161
Y@Cs2,and 0.27 for La@Cs2. Considering the similar values seen in the fraction ratio of the major and minor isomers found in ESR and NMR studies, it might be possible to deduce that the main ESR signal is due to the metallofullerene with a C2 symmetry, just like as that the empty q2 is, and the sccond isomer is due to one of three minor isomers identitied for the empty Cg2 by 13CNMR. These considerations,in turn,strongly suggest that a metal is salectively encapsulated into the CS2cages with specific structures among four or more candidates. Furthermore,it is also expected that the 13Chyperfine structures, which were fvst distinguished in the present work, may provide very versatile and direct information on the molecular structure of M@Cg2. The analysis using semiempirical MO calculations is now in progress.
Note Added in proof. After submitting this report, we learned that Yannoni et al. have obtained results on Lacszsimilar to those reported here (ref 9). Acknowledgment. We thank Toyo Tanso Co. Ltd. for providing us La-containing carbon rods. We also thank Mr. Hitashi Kannari for the preparation of metal-containing carbon rods throughout this work. This work was partially supported by the grants from the Ministry of Education, Science and Culture of Japan. L
I
I
I
I
I
336.1 336.3 336.5 336.7 336.9 337.1 337.3 B /mT
Figwe 4. (a) ESR spectnun of Lacllzshown by an expanded scale. (b) Siulated ESR spectrum obtained by adding (c) and (d). (c) Simulated ESR spectrum for isomer I obtained by the same procedure in Figure 2c. (d) Simulated ESR spectrum due to isomer 11.
and La@CS2are summaflzcd in Table I, together with Sc@Cs2. As can be seen from the table, the general tendency of the c m in a g value and a hyperfine coupling constant from the isomer I to I1 is quite similar to each other, except the g value for Sc@C& Recent quantitative analysis of 13C NMR lines obtained for the empty Cg2strongly indicated that at least four isomers with C2,C, C, and another C2 (or C,) symmetries are formed with the fraction ratio of about 8:1 :1 :1, respectively.' On the other hand, from the prcptnt ESR spectra, we found two isomers (I and I1 in Table I) with the ratio of about 0.07 for Sc@CS2,0.15 for
References and Notes (1) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure, J.; Wang, L.; Alford, M.; Smalley, R. E. J. Phys. Chem. 1991, 95, 7564. (2) Alvarez, M. M.; Gillan, E. G.; Holczer, K.;Kaner, R. E.; Min, K.S.; Whetten, R. L. 1. Phys. Chem. 1991,95, 10561. (3) Johnson, R. D.; de Vries, M. S.;Salem, J.; Bethune, D. S.; Yannoni, S.Nature 1992, 355,239. (4) Weaver, J. H.; Chai, Y.;Kroll, G. H.; Jin, C.; Ohno, T. R.;Haufler, R. E.; Guo, T.; Alford, J. M.; Conceicao, J.; Chibante, L. P. F.; Jain, A.; Palmer, G.; Smalley, R. E. Chem. Phys. Lett. 1992, 190,460. ( 5 ) Shinohara, H.; Sato, H.; Ohkohchi, M.;Ando, Y.; Kodama, T.; Shida,
T.; Kat0 T.; Saito, Y. Nature 1992, 357, 52. (6) Yannoni, C. S.;Holnkis, M.; de Vries, M. S.;Bethune, D. S.;Salem, J. R.;Crowder, M. S.;Johnson, R.D. Science 1992,256, 1191. (7) Kikuchi, K.;Nakahara, N.;Wakabayashi, T.; Suzuki, S.;Shuomaru, H.; Miyake, Y.; Saito, K.;Ikcmoto, I.; Kaiiosho, M.; Achiba, Y .Nature 1992, 357, 142. (8) Kikuchi, K.;Nakahara, N.;Wakabayashi, T.; Honda, M.;Matsumiya, H.; Moriwaki, T.; Suzuki, S.;Shiromaru, H.; Saito, K.;Yamauchi, K.;Ikemoto, I.; Achiba, Y . Chem. Phys. Lett. 1992, 188, 177. (9) Yannoni, C. S.;Wendt, H. R.;de Vries, M. S.;Siemens, R. L.; Salem, J. R.;Lyerla, J.; Johnson, R.D.; Hoinkis, M.; Crowder, M. S.;Brown, C. A.; Bcthune, D. S.Synth. Met., in press.
EWect of Confinement on the Resonant Intermolecular Vlbratlonai Coupling of the v2 Mode of Methyl Iodlde in Porous Silica Glasses
Y. T.Lee, S.L.Wallen, and J. Jonas* Department of Chemistry, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801 (Received: June 2, 1992; In Final Form: July 20, 1992) Using Raman scattering, the resonant transfer of vibrational energy was examined for methyl iodide confined in sol-gel prepared silica glasses with pore radii ranging from 12 to 70 A. The resonant intermolecularvibrational coupling (RIVC) of the v2 vibrational mode of methyl iodide reflects clearly the effect of geometric confinement. The RIVC induced line broadening and frequency shifts are in qualitative agreement with the Logan theory and the model of geometrically restricted energy relaxation as developed by Klafter et al.
hlroduch Although the dynamics of liquids in porous media have been studied by various techniques,'V2 attempts to unravel the effects of pure geometric confinement have proven to be experimentally difficult. Fkst of all, it is important to make sure that the observed signal arises only from the confined liquid and is not affected by the bulk liquid outside the pores. Another problem is related to the specific d a c e effects due to interactiom between the confined liquids and the confining surface; i.e., there is a significant dif-
ference between wetting and nonwetting liquids. The latter is especially important for studying polar wetting liquids since both the geometric confimement and surface interaction effects increase as pore size decreases. Optical t e c h n i q ~ e ssuch , ~ ~ as Raman scattering, avoid the fmt problem. Simple optical focusing within the transparent glass sample allows one to record signals directly from the confined liquid. Problems related to surface interactions can be partially Overcome by using porous glasses prepared by the sol-gel process
0022-3654/92/2096-7 161$03.00/0 Q 1992 American Chemical Society
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7162 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992
and by using nonwetting liquids. It is fortunate that the porous sol-pl glasses can be chemically &led to reduce significantly the effects of the surface interactions between the silanol groups and the confined liquids. Also, sol-gel glasses can be prepared with narrow pore size distributions and radii as small as 12 A. The chemical and physical stability of these glasses along with their traneparency in the visible region makes the porous sol-gel glasm an ideal hoat to study the effects of geometric confinement by Raman scattering. In the present work, the effect of geometric confinement on intermolecular resonant transfer of vibrational excitations was examined by Raman scattering of &ed methyl iodide in porous glasses with pore radii ranging from 12 to 70 A at room temperature. The resonant intermolecular vibrational coupling (RIVC) of the v2 vibrational mode of methyl iodide was investigated. In general, the observed line width broadening and peak shift are attributed to three mechanisms: RIVC, environmental frequency fluctuations (is., dephasing), and vibrational population relaxation? Isotopic dilution’ is a well-known method used to separate the contribution of RIVC from other relaxation mechanisms. In isotopically diluted solutions, the RIVC between neighboring molecules of the same species will be partially d e coupled while the effects due to other mechanisms remain essentially invariant. consequently, the difference W e e n the band parametem of the neat liquids and isotopically diluted liquids allow one to estimate the RIVC induced line broadening and peak shift as a function of thermodynamic state or, in the present study, the pore size. As shown in earlier studies,’-12 the RIVC mechanism contributes significantly to the line width and peak shift of the v2 vibrational mode of liquid methyl iodide. In this study, we examine the line broadening and peak shift of the u2 vibrational mode of methyl iodide confined to porous silica glassca prepared by the sol-gcl procea9.~13*’4 The experimental resultsof the present RIVC study are explained qualitatively by using the theoretical model pro@ by Logan” and the model of geometrically restricted relaxation developed by Klafter et a1.16
Expdmeatd Seetion Methyl iodide and deuterated methyl iodide (Aldrich Chemical Corp.) with purities of 99.5+% were used without further purification. The mole fractions of the isotopically diluted solutions were 0.1 (&O.Ol) in methyl iodide. Sol-gel glasses were prepared by mixing tetraethyl orthosilicate (TEOS)with ethanol and water according to the procedure described ~ l s e w h e r e . ~ J The ~ . ’ ~pore sizes, as determined by an AUTOSORB-1 BET instrument (Quantachrome), range from 12.2 to 76.3 A in radius. The majority of the glass samples prepared were of cylindrical shape with a diameter and height of approximately 1 and 2-3 cm, respectively. The porous glasses used in the experiment were placed inside the scattering cell, which was then connected to a high vacuum manifold Torr) and pumped overnight at a temperature of 200 OC before the introduction of the sample liquids. prior to filling the porte, the liquid samples wcre degassed by using at least four freezepumpthaw cycles. The degassing were performed and transferring of liquids into the porous glin the same vacuum manifold, eliminating any possible contamination. After the porous glass is immersed in the liquid for at least 12 h, the scattering cell is scaled and taken from the vacuum manifold for Raman spectroscopy. The spectroscopic system has bam previously described.” The power of the incident laser beam was 0.6 W,and the slits of the double monochromator were set at 100-200-200-100 pm or smaller. A small aperture was used to ensure that no signal from the bulk liquid outside the porous glass was recorded. Nonmodified lasses have approximately two surface silanol groups per 100 on their surfaces.’* In order to reduce the liquid-swface interactions arising from these moieties, the silanol groups on the surface were replaced with the less polar Si-0Si-(CH3), groups by reacting the porous glasses with toluene (Fisher Scientific) solutions of 1,1,1,3,3,3-hexamethyldisilazane (HMDS, Eastman Kodak Co.) as described by Sindorf and
x2
4000
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0
400
300 200 100 0 1230
1240
1250
1260
1270
Raman shift (cm-l) Figure 1. (A) Raman spectra of neat CHJ ( 0 )and isotopically diluted 0.1 mole fraction CH31 (0). Solid curves are the fitting results obtained as described in the text. (B Raman spectra of the sample liquid confined to porous glass with 12.2- pore radius and the same concentrations as above. The spectral intensities of the diluted solution have b a n multiplied by 3.
d
Maciel.19 Before surface modification, the porous glasses were heated to 200 OC under vacuum Torr) Overnight to remove adsorbed molecules from the surface. After modication, exccss solution reactants were evaporated at a temperature of 110 OC under vacuum Torr). The modified glass was then transferred to the scattering cell in an inert nitrogen environment and heated to 110 OC under vacuum (-W5 Torr) for at least 6 h before the degassed liquid samples were loaded. All porous glassca were modified at the same time and in the same reaction flask to assure consistency in the modification procedure. The glass peaks found below 1200 cm-I do not interfere with the v2 vibrational mode which is located at approximately 1241 cm-’. Since the VH signal was too weak for accurate data analysis, the VV spectra were assumed to be approximately equal to the isotropic spectra? As the concentration of the isotopically diluted solution of methyl iodide is 0.1 (iO.01) mole fraction, the line broadening due to RIVC, AFRIVC(R)for pore radius R, can be calculated according to ArRIVC(R) = (10/9)[rO(R)
- rd(R)1
(1)
where ro(R) and rd(R) are the full width at half-maximum intensity (fwhm) for the pure methyl iodide and the isotopically diluted methyl iodide, respectively, in the porous glass of pore radius R. Similarly, the line shift due to RIVC, AvRIvc(R)for pore radius R, can be calculated according to
-
AVRIVC(R) (10/9)bO(R) vd(R)I (2) where vo(R) and ud(R) arc the peak positions of the v2 vibrational mode for the pure methyl iodide and the isotopically diluted methyl iodide, respectively, in the porous glass of pore radius R. Results and DLscussioa Typical polarized Raman spectra of methyl iodide are shown in Figure 1. The top two curves (A) are the raw d a t ~of~neat and isotopically diluted methyl iodide (0.1 mole fraction) solutions in the bulk phase while the bottom two curves (B) reprawnt the raw data for neat and isotopically diluted methyl iodide solutians in the 12.2-A modified porous glass. The raw spectral data for the neat (0)and isotopically diluted methyl iodide (0)arc fit by a single Lorentzian bandshape (solid curves) with a flat background using standard spectroscopic peak analysis software
The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 7163
Letters
R tA)
R (A) 1244.0
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2L
12.2
69.2 52.1 26.7
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1
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5
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0.00 0.02 0.04
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0.06
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Figure 2. Peak positions for the u2 mode of CHJ ( 0 )and isotopically diluted CH,I (0)plotted as a function of the reciprocal of pore radius, 1/R (A-I), for the nonmodified glasses. The mole fraction of CHJ in the CDJ isotopic dilution experiment is 0.1.
r
I
0.00 0.02
1
I
0.04 0.06
I
0.08 0.10
1/R (A-1) Figure 4. (A) Line widths for the v2 mode of CH31( 0 )and isotopically diluted CHJ (0)as a function of the reciprocal of pore radius, 1/R (A-l), for the nonmodified glasses. (B) Line widths for the u2 mode of CHJ ( 0 )and isotopically diluted CHJ (0)as a function of the recip rwal of pore radius, 1/R (A-l), for the modified glasses. The mole fraction of CHJ in the CD31isotopic dilution experiment is 0.1.
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1241
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521
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1 I
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1/R (A-1) Figure 3. (A) Peak positions for the u2 mode of CHJI ( 0 )and isotopically diluted CHJ (0)plotted as a function of the reciprocal of pore radius, 1/R @-I), for the modified glasses. The mole fraction of CHJ in the CDJ isotopic dilution experiment is 0.1. (B) The RIVC induced line shift plotted as a function of the reciprocal of pore radius, 1/R (A-I), for the modified glasses.
(Peakft by Jandel Scientific). Since there is a weak side band occurring at the lower wavenumber region of the v2 mode, the fitting was performed by using only the data on the right-hand side of the peak and the data located approximately 3 cm-’to the left of the peak maximum. All reported line widths and peak maxima are values averaged from at least four different spectra. In the plots of the neat and isotopically diluted methyl iodide data, the uncertainties are determined as the standard deviations for the replicate runs at each pore size. The uncertainties in the plots of the RIVC induced line shift and bandwidth are the square roots of the sums of the squared standard deviations. In Figure 2, the peak frequency of the v2 mode is plotted as a function of 1/R for both neat liquids and the diluted solutions confined to the nonmodified glasses. The s l o p of both lines are the same indicating that, within the experimental error, AvRIVC does not depend on pore size. However, in the case of the modified glasses a plot of the v2 peak frequency as a function of 1/Rfor both neat liquids and the diluted solutions shows a dramatic difference in their slopes (Figure 3A). This indicates that the RIVC peak shift, AvRIVc, increases as pore size decreases (Figure 3B). The experimental line widths of neat and isotopically diluted
1 1.6 0.00
I
0.02
I
0.04
I
0.06
I
0.08
0.10
1/R ( a 1 )
Figure 5. RIVC induced line broadening for the u2 mode of CHJ as a function of the reciprocal of pore radius, 1/R (A-I), for the nonmodified (A;full line) and the modified ( 0 ;dashed line) glasses. Data points are calculated from Figure 4 according to eq 1.
methyl iodide are plotted as a function of the reciprocal of pore radius in Figure 4, A and B, for the nonmodified and modified glasses, respectively. The ArRIVC’s,calculated according to eq 1, are shown in Figure 5 where the open squares (dashed line) represent the data of the modified glasses and open triangles (solid line) represent the data of the nonmodified glasses. It is of interest Ar, decreases as pore size decreases to note that, unlike bmC, for both nonmodified and modified glasses. Moreover, the absolute values of ArmC in modified and nonmodified glasses are the same within the experimental error. The trends observed in AVR’VC and AFRIVccan be qualitatively explained by using the theoretical predictions from the models proposed by Logan15 and by Klafter et a1.I6 In fact, this study of RIVC in porous glasses was partly motivated by the results of these two theoretical studies. Logan’s modells considers only the RIVC mechanism in bulk liquids, whereas the model developed by Klafter et a1.16 treats energy transfer and relaxation in codined geometric spaces. In Logan’s model, the RIVC contribution to the line shift of bulk liquids, AuRIVc, is represented by the following expression (3)
7164
J. Phys. Chem. 1992,96, 7164-7166
where &/dQ is the transition dipole, 6 is the permittivity of free space, m is the reduced mass of the harmonic oscillator, uo is the vibrational frequency of the free molecule, and u is the hard-sphere diameter. The liquid structure parameter i(p,T) approaches zero for random molecular orientations and increases as the liquid structure becomes more orientationally ordered. Vibrational frequencies are predominantly determined by static molecular structure rather than by the dynamical processes occurring through intermolecular and intramolecular interactions. Since the liquid molecules confined to spaces as small as several molecular diameters are less freely randomized compared to molecules in a bulk liquid, the local orientational order of the restricted molecules should be enhanced which, in turn, will result in a larger AuRIVC according to eq 3. Obviously, Logan's model does not consider any surface interactions, and hence it is not too surprising to find that b R w C ( R ) of the nonmodified glasses does not follow the theoretically predicted trend. However, for modified glasses, where the surface interactions have been reduced significantly, AYRIVC(R) does increase as pore size decreases (Figure 3). The possible confined geometry-induced changes in AuRIVcfor the nonmodified glasses, although not observable within the experimental error, are by no means as large as the effects for the modified glasses as shown in Figure 3. This is indicative of the local molecular structure for confined liquids in nonmodified glasses being significantly affected by the surface interactions and, therefore, may counteract the confined geometry-induced changes, with the net result of a smaller change in A Y R ~ V C ( R ) compared to that observed in the modified glasses. The theoretical work of Klafter et a1.16 concerning energy relaxation under the effect of geometrical confinement was a strong motivating force in initiating this investigation. One of their major conclusions in studying the direct resonant energy transfer of restricted molecules is that the geometry (such as the cylindrical pores) slows down the relaxation rate of the direct energy transfer. A slower relaxation rate implies a narrower line width for a particular relaxation mechanism, and so in our experimental work it is expected that ArRnc will also decrease as the pore size is decreased. These theoretical predictions were qualitatively confmed by our experimental results (Figures 4 and 5). While peak shifts are determined by the static molecular structure, the line widths are much more dependent on the dynamic processes involving intermolecular and intramolecular interactions. Resonant intermolecularvibrational coupling occurs through energy transfer between molecules of the same species. Therefore, any surface perturbation, regardless of magnitude, will change the energy of the vibration for molecules associated with the surface resulting in nonresonance with the original energy levels. The degree to which the surface perturbation affects the energy level is unim-
portant as any difference will result in a reduced ability to relax through interaction with surface associated molecules. One would expect that any surface perturbation will affect the RIVC relaxation rate in a similar manner. In agreement with our expectations, it appears that the surface interactions of the nonmodified glasses do not affect significantly the RIVC relaxation rate as shown in Figure 5.
Conclusion The influence of geometric confinement on the molecular dynamia of liquid methyl iodide was studied by Raman scattering. The RIVC of the u2 vibrational mode in methyl iodide turns out to be an excellent probe to explore the effects of geometric confinement as the results for the resonant intermolecular vibrational coupling mechanism of this mode provide qualitative experimental evidence of changes due to geometric confinement. The experimental results, which show that the smaller the pore size the smaller the line widths and the larger the line shifts induced by RIVC, are qualitatively explained by using the theoretical models of and Klafter et d.I6This finding suggests that the liquid methyl iodide confined to very small ports has a higher reorientational order and slower resonant energy transfer relaxation rates. Acknowledgment. This work was partially supported by the National Science Foundation by Grant NSF CHE 90-17649.
Reference and Notes (1) Drake, J. M., Klafter, J., Kopelman, R., Eds. Dynamics in Small Confining Systems; Proceedings of Symposium M; 1990 Fall Meeting of the Materials Research Society; and references therein. (2) Liu, G.; Mackowiak, M.; Li, Y.; Jonas, J. Chem. Phys. 1990,149,165. (3) Warnock, J.; Awschalom, W. W.; Shafer. M. W. Phys. Reo.B 1986, 34, 475. (4) Nikiel, L.; Hopkins, B.; Zerda, T. W. J. Phys. Chem. 1990,94,7458. ( 5 ) Watson, J.; Zcrda, T. W. Appl. Specfrosc. 1991, 45, 1360. (6) Rothchdd, W. G. Dynamics of Molecular Liquids; John W h y & Sons: New York, 1989. (7) mge, G.; Lindner, D. Ber. Bunsen-Ges.Phys. Chem. 1990,94,408 and references therein. (8) Dbge, G.; Amdt, R.; Khuen, A. Chem. Phys. 1977, 21, 53. (9) Trisdale, N.; Schwartz, M. Chem. Phys. Lett. 1979, 68, 461. (10) Baglin, F. G.; Wilkes, L. M. J . Phys. Chem. 1982, 86, 3793. (11) Baglin, F. G.; Versmold, H. Chem. Phys. Lcrr. 1983, 96, 656. (12) Wilde, R. E.; Zyung, T. Mol. Phys. 1985, 55, 809. (13) Shafer, M. W.; Awschalom, D. D.; Wamock, J.; Ruben, G. J . Appl. Phys. 1987, 61, 5438. (14) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Asley, C. S.J . NonCrysr. Solid. 1982, 48, 47. (15) Logan, D. E. Mol. Phys. 1986, 58,97. (16) Klafter, J.; Blumen, A.; Drake, J. M. In Molecular Dynamics in Restricted Geometries; Klafter, J., Drake, J. M., Us.; John Wiley & Sons: New York, 1989; Chapter 1. 117) Lee. Y. T.: Wallen. S. L.: Jonas. J. J . Phvs. Chem. 1992.96,4282. . . (18) Unpublished data obtained by klid NMR in our group. (19) Sindorf, D. W.; Maciel, G. E. J . Phys. Chem. 1982, 86, 5208.
Gas-Phase Reactions Involving Hot '*O('P) Atoms and Formaldehyde Richard A. Femeri* and Alfred P.Wolf Department of Chemistry, Brookhaven National Laboratory. Upton, New York 11973 (Received: June 9, 1992: In Final Form: July 15, 1992) We report an investigation of the low-pressure gas-phase reactions involving hot I*O('P) atoms with translational energies in excess of 1 eV and formaldehyde. Using mass spectrometry, I8Oincorporation in C'60180 and HC'Q products was obscr~ed. These results strongly suggested that carbonyl bond addition was an important reaction at these energies.
