J. Phys. Chem. 1984,88, 5959-5964 to the high dye concentrations employed and due to scattering from the edges. Optical improvements are in progress. The potential uses of fluorescent gel glasses in photoprocesses and optics are numerous. Examples are light guides for lasers and for luminescent solar concentrators and filters for linear and nonlinear optics. These and other uses as well as the material described in this paper are patent pending.
5959
Acknowledgment. We thank Zvi Grauer, N. Manor and H. Tzehoval for helpful discussions and technical assistance. This work was sponsored by the West-German Ministry of Science and Technology and assisted in part by the F. Haber Research Center for Molecular Dynamics, Jerusalem. Registry No. Rhodamine 6G, 989-38-8; silica, 7631-86-9.
Vibrational Spectroscopic Study of the Interlamellar Kaolinite-Dimethyl Sulfoxide Complex Clifford T. Johnston,*+Garrison Sposito,+ David F. Bocian,t and Robert R. Birge' Department of Soil and Environmental Sciences and Department of Chemistry, University of California, Riverside 92521 (Received: March 27, 1984; In Final Form: July 30, 1984)
The intercalation complex involving the layer silicate kaolinite and dimethyl sulfoxide (Me2SO) is studied through the combined application of Raman and infrared absorption spectroscopy. The spectral data indicated that the Me2S0 molecules were in a restricted, highly ordered interlamellar conformation, with the oxygen of the sulfonyl group hydrogen bonded to OH groups in the gibbsitesheet surface of kaolinite and the sulfur atom of the group keyed into the ditrigonal cavity on the opposing siloxane surface of the mineral. The methyl group in the intercalated molecule appeared to be influenced by both of the opposing mineral surfaces to the extent that vibrational degeneracy in the CH stretching modes was lifted.
Introduction The ability of interlamellar layer aluminosilicate surfaces to accommodate organic molecules and modify the structure and reactivity has prompted renewed interest in clay minerals as selective Of importance equal to the chemical conversions catalyzed by interlamellar surfaces are the structural perturbations that they induce in intercalated organic species. These perturbations provide a means of investigating the structure and reactivity of clay mineral surfaces themselves, similar to the use of adsorbed inorganic species as probes in electron spin resonance studies of interlamellar layer silicate surface^.^^^ Infrared (IR) absorption spectroscopy has been the only vibrational spectroscopic method used previously to study intercalation complexes formed between clay minerals and organic compounds. Although the IR technique is characterized by a high sensitivity to low concentrations of adsorbed species: it is limited severely by the opacity of the This restriction is especially evident in studies of adsorbed species on clay minerals and oxides, where strong IR absorption bands below 1300 cm-I, associated with framework vibrational modes, can obscure a considerable portion of the IR spectrum of an adsorbed ~ p e c i e s . ~ By contrast, the framework vibrational modes of clay minerals and oxides are only weakly Raman active.lOJ1 Thus, Raman spectroscopy may be useful for studying the vibrational spectra of clay mineral intercalation complexes. The objective of the present paper is to exploit this possibility by investigating both the vibrational Raman and IR spectra of the kaolinite-dimethyl sulfoxide (Me2SO) intercalation complex, with emphasis placed on the unique ability of the Raman technique to probe vibrational modes below 1300 cm-I. Kaolinite, a 1:l layer aluminosilicate," has been observed to intercalate a relatively large number of polar organic compounds including dimethyl ~ulfoxide,'~-~~ formamide, N-methylformamide, dimethylformamide, and a~etarnide,'"'~and pyridine N-oxide.20 The kaolinite-Me2S0 complex should provide an especially good Department of Soil and Environmental Sciences. *Department of Chemistry. Address correspondence to this author at the Center for Non-Linear Studies, MS B258, Los Alamos National Laboratory, Los Alamos, N M 87545.
0022-3654/84/2088-5959$01 S O / O
example for a comparativie Raman-IR study because of the high stability of the complex relative to other kaolinite intercalation comple~es,'~ the appearance of well-defined, discrete bands in the hydroxyl stretching region of the IR spectrum of the complex,14 and the high Raman activity of many of the vibrational modes of Me2SOe21 True intercalation of M e 2 S 0 into kaolinite, as opposed to multilayer adsorption on external surfaces, has been confirmed
(1) T. J. Pinnavaia, Science, 220, 365 (1983). (2) M. J. Tricker, D. T. B., Tennakiin, J. M. Thomas, and S.H. Graham, Nature (London), 253, 110 (1975). (3) J. M. Thomas in "Intercalation Chemistry", M. S. Whittingham and A. J. Jacobson, Eds., Academic Press, New York, 1982, Chapter 3. (4) M. B. McBride, Clays Clay Miner., 25, 6 (1976). (5) T. J. Pinnavaia in "Advanced Chemical Methods for Soil and Clay Minerals Research", J. W. Stucki and W. L. Banwart, Eds., D. Reidel, Dordrecht, 1980, p 391. (6) A. V. Kiselev and V. I. Lygin, "Infrared Spectra of Surface Compounds", Halsted Press, New York, 1975. (7) T. A. Egerton, A. H. Hardin, Y. Kozirovski, and N. Sheppard, J. Catal., 32, 343 (1974). (8) B. A. Morrow and A. H. Hardin. J . Phys. Chem., 83, 3135 (1979). (9) B. A. Morrow, J. Phys. Chem., 81, 2663 (1977). (10) H. Jeziorowski and H. Knozinger, J . Phys. Chem., 83, 1166 (1979). (1 1) C. T. Johnston, Ph.D. dissertation, University of California, Riverside, CA, 1983. (12) G. S.Garcia and M. S.Camazano, An. Edafol. Agrobiol., 24,495 (1965). (13) S.Olejnik, L. A. G. Aylmore, A. M. Posner, and J. P. Quirk, J . Phys. Chem., 72, 241 (1968). (14) 0. Anton and P. G. Rouxhet, Clays Clay Miner., 25, 259 (1977). (15) J. M. Adams and S. Waltl, Clays Clay M i n d . , 28, 130 (1980). (16) M. I. Cruz, A. Laycock, and J. L. White, Proc. Inr. Clay Con& 1, 775 (1969). (17) S.Olejnik, A. M. Posner, and J. P. Quirk, Clays Clay Miner., 19, 83 (1971). (18) S.Olejnik, A. M. Posner, and J. P. Quirk, J . Colloid Interface Sci., 37, 536 (1971). (19) R. L. Ledoux and J. L. White, J. Colloid Interface Sci., 21, 127 (1966). . (20) S.Olejnik, A. M. Posner, and J. P. Quirk, Spectrochim. Acta, Part A , 27, 2005 (1971). ( 2 1 ) M. T. Forel and F. Tranquille, Spectrochim. Acta, Part A , 26, 1023 (1970).
0 1984 American Chemical Society
5960
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984
Johnston et al.
in previous X-ray diffraction studies of the kaolinite-Me,SO ~ o m p l e x . ' ~The J ~ average amount of MezSO intercalated is approximately 1.4 molecules per unit cell of ka01inite.l~ The d(001) spacing of kaolinite after reaction with MezSO increases from 0.71 to 1.1 1 The observed increase of 0.40 nm in the intralamellar region is small, considering the molecular dimensions of Me2S0, and suggests that the intercalated MezSO molecules take on a highly ordered conformation within the interlamellar region. Olejnik et a1.I3 have interpreted this smaller-than-expected expansion in terms of a keying of the intercalated Me2S0 molecule into an interlamellar surface. ~
1
1
1
.
~
~
9
~
~
Experimental Section The kaolinite investigated was a KGa-1 sample (Washington County, Georgia) obtained from the Clay Minerals Society Clay Repository. A description of the properties of this clay is given by van Olphen and FripiatSz2A stock suspension of Na-saturated kaolinite containing the fraction having an esd less than 2.0 pm was prepared in a background electrolyte of 10 mol m-3 NaCl. The clay concentration in the suspension was 20% (w/w) and the pH value of its supernatant solution was 5.5. A complete description of the clay preparation is given by Johnston." Commercially available, spectral-grade Me2SQ, distilled in glass under vacuum, was used with no additional purification. A 100-mL aliquot of the stock clay suspension (containing 20.0 g of the KGa-1 clay) was washed with three 100-mL aliquots of a MezSO solution containing 9% H 2 0 (v/v) using centrifugation. Olejnik et al.13 measured the rate of intercalation for a series of MezSO solutions containing different water contents and observed that the maximum rate of intercalation occurred at a water content of 9% (v/v). The clay concentration of the kaolinite-Me2S0 suspension was 20 g of clay suspended in 100 mL of the Me2SO/H20 solution. The resulting clay suspension was placed in a 125-mL glass Erlenmeyer flask and mixed further on a shaker for 48 h. Raman spectra were obtained on a computer-controlled Spex Industries Ramalog 6 spectrometer equipped with a thermoelectrically cooled Hamamatsu Model R955 photomultiplier tube and a photon-counting detection system. The 488.0-nm line of an argon ion laser (Spectra Physics Model 164-05) was used at an incident power output of 100 mW. The spectral slit width ranged from 2 to 5 cm-' for all of the spectra reported in this paper. Samples were prepared for Raman analysis as follows. Aliquots (35 mL) of the kaolinite-Me2S0 suspension which had been allowed to react while mixing on a shaker for 72 h were placed in 50-mL Pyrex beakers. The clay suspension in the beaker was left open to the atmosphere and stored at 25 OC. The Raman spectra were obtained by placing the beaker containing the kaolinite-Me,SO complex directly in the Raman illuminator. The Pyrex beaker did not present spectral interference in the 5040OO-cm-' region. The illumination geometry involved collection of the scattered radiation of an angle of 90° relative to the incident light source. All Raman scans were obtained by using a OS-cm-' step size and a counting time of 5 s at each step, with the exception of the 100-lOOO-cm-l scans, which were obtained by using a 1.O-cm-' step size with a 5-s counting time. The Raman spectrometer was calibrated with a low-pressure neon lamp. IR spectra were recorded with a Perkin-Elmer Model 283B filter-grating double-beam instrument interfaced to a HewlettPackard Model 85 computer through a Hewlett-Packard Model 3937 data acquisition system. Samples were prepared by evaporating clay suspensions deposited on thin AgCl plates in a vacuum dessicator under 0.01 torr. Oriented films of kaolinite were prepared for X-ray diffraction (XRD) analysis by evaporating the solvent from the clay suspension on a glass slide in a vacuum desiccator under a pressure of 0.01 torr. XRD patterns were recorded on a Philips diffrac(22) H. van Olphen and J. J. Fripiat, "Data Handbook for Clay Materials and Other Non-Metallic Minerals", Pergamon Press, New York, 1979. (23) A. Weiss, H. 0. Becker, H. Orth, G. Mai, and M. Wimmer, Proc. In?. Clay Conf, 1975, 411 (1976).
59 DAYS WET
31 DAYS
-
NON I NTEF
3725
3600
WAVE NUMBER Figure 1. Raman spectra of kaolinite in suspension (a) and kaoliniteMezSO complexes (b-f) in the hydroxyl stretching region.
3620 3661
1
A f
60 DAYS
40 HOURS
e
30 HOURS
d
24 HOURS
C
12
b
HOURS
3800
3400
WAVE NUMBER
Figure 2. IR spectra of kaolinite (a) and kaolinite-MezSO complexes (b-f) in the hydroxyl stretching region.
tometer using Ni-filtered Cu K a radiation at 30 kV and 15 mA. The scanning rate was 1 deg-28 min-I.
Results and Discussion OH Stretching Region. The Raman and IR spectra of kaolinite reacted with M e 2 S 0 for varying periods of time are shown in Figures 1 and 2, respectively. The decrease in the intensity of the 3695-cm-' band and the appearance of three new bands at 3661, 3538, and 3502 cm-I in the spectrum (Figures If and 2f) agree well with previous IR studies of the kaolinite-MezSO ~ o m p l e x . ~The ~ , ' ~3695- and 3661-cm-l bands appear in both the
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 5961
Spectroscopic Study of Kaolinite-Me2SO Complex
b a
INTERCALATED
NON- I NTERCALATED
XYL
r
?==-
d =.714nrn
i L
\ ’
b
\
L
b
INNER -SURFACE HYDROXYL
Figure 3. Projection of four half-unit-cells of kaolinite (a) and a kaolinite-Me2S0 complex (b) onto the (100) plane.
Raman and the I R spectra, but the 3538- and 3502-cm-’ bands were only detected in the I R spectra. A careful examination of the intermediate-time IR spectra (Figure 2b-f) suggests that the 3661-cm-’ band in the spectrum of the kaolinite-Me2S0 complex a t 60 days (Figure 2 0 is related to the 3668-cm-’ band in the spectrum of nonintercalated kaolinite (Figure 2a). The intensity of the 3652-cm-’ band in both the Raman and IR spectra appears to decrease as the extent of intercalation increases. After 60 days, this band appears as a weak shoulder on the 3661-cm-’ band. The kaolinite-Me2S0 spectrum labeled “wet” (Figure 1) is that of a colloidal suspension in the Me2SO/H20 solvent, whereas the spectrum labeled “dry” (Figure If) is that of a sample presented as a dry powder. The close similarity between the two spectra indicates that the Me2SO/H20 solvent produces no significant Raman interference in the hydroxyl stretching region, despite the fact that the solvent contains a significant amount of water. The 3620-cm-’ band is the only hydroxyl stretching band which is not affected significantly by the presence of MezSO molecules in the interlamellar region. The 3620-cm-’ band in the kaolinite IR spectrum (Figure 2a) has been assigned to the inner hydroxyl g r o ~ p ? ~ The , ~ ~position * ~ ~ and orientation of this hydroxyl group are illustrated qualitatively in the projection onto the (100) plane of kaolinite shown in Figure 3a.25.27 The inner hydroxyl group (24) R. L. Ledoux and J. L. White Science, 145, 47 (1964). (25) S. Motherwell, “A Program for Plotting Molecular and Crystal Structures”, University Chemical Laboratory, Cambridge, England, 1978. (26) B. R. Zvyagin, ’Electron Diffraction Analysis of Clay Mineral Structure”, Plenum Press, New York, 1967. (27) R. F. Giese, Clays Clay Miner., 21, 145 (1973). (28) P. G. Rouxhet, N. Samudacheata, H. Jacobs, and 0. Anton, Clay Miner., 12, 171 (1977). (29) R. F. Giese, Bull. Mineral., 105, 417 (1982). Recent neutron dif-
fraction studies of the hydrogen positions in kaolinite by J. M. adams, Clays Clay Miner., 31,352 (1983) and by P. R. Suitch and R. A. Young, Clays Clay Miner., 31, 357 (1983) suggest strongly that the third inner-surface hydroxyl group is oriented more nearly perpendicular to the ab plane than parallel to it. (30) V. C. Farmer in “The Infrared Spectra of Minerals”, V. C. Farmer Ed., Mineralogical Society, London, 1974, p 331. (31) K. Wada, Clay Miner., 7, 51 (1967). (32) W. Wiewiora, T. Wieckowski, and A. Sokolowska, Arch. Mineral, 35, 5 (1979).
is highly resistant to isotopic exchange with deuterium and to dehydroxylation at elevated temperatures because of its recessed location within the kaolinite structure. This fact also accounts for the lack of change in the 3620-cm-’ band in the Raman and IR spectra of the kaolinite-Me2S0 complex upon intercalation. The 3695-, 3668- and 3652-cm-’ bands in the IR spectrum of kaolinite (Figure 2a) have been assigned to hydroxyl groups present on the interlamellar surface and are termed the innersurface hydroxyl groups. On the basis of isotopic dilution and IR polarization studies of thin films of kaolinite in the hydroxyl stretching region, Rouxhet et a1.28assigned the 3965- and 3668cm-’ bands to the stretching vibrations of two coupled inner-surface hydroxyl groups lying almost perpendicular to the (001) plane. The 3652-cm-’ band has been assigned to the stretching motion of an inner-surface hydroxyl group concluded on the basis of pleochroic behavior to lie almost parallel with the clay surface.28 However, some doubt has been expressed recently as to the near-parallel orientation of this O H group.29 The two bands at 3538 and 3502 cm-’ in the IR spectrum of the kaolinite-Me2S0 complex indicate clearly that the intercalated MezSO molecules hydrogen bond to the inner-surface hydroxyl groups of kaolinite. The wavenumber differences between the 3695- and 3661-cm-’ bands and that between the 3538- and 3502-cm-’ bands in Figure 2f are similar (34-36 cm-I). Because of this similarity, the 3538- and 3502-cm-’ bands may correspond to the stretching frequency of coupled inner-surface hydroxyl groups perpendicular to the (00 1) plane hydrogen bonded to Me2S0 molecules in the interlamellar region. The observed red shift of the inner-surface hydroxyl stretching frequency of approximately 160 cm-’ indicates that an intermediate hydrogen bond forms upon intercalation. This proposed interaction between the intercalated Me2S0 molecules and the perpendicular innersurface hydroxyl groups is illustrated by the dashed lines in Figure 3b. Both the IR and Raman spectra in the hydroxyl stretching region provide evdidence that the inner-surface hydroxyl groups of kaolinite are hydrogen bonded to intercalated MezSO molecule~.’~3’~ The interpretation of these spectra is restricted, however, to discussions of changes that occur in the vibrational modes of kaolinite and only general bonding mechanisms between inter-
5962
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984
Johnston et al.
673
1 1 1400
!
l l 1200
l
l
' l 1000
I
~
~
800
1
l
I
600
l
I
j
637
l
KGa-1
4
WAVE NUMBER
Figure 4. IR spectra of kaolinite (a) and a kaolinite-Me2S0 complex (b) in the low-wavenumber region.
a
500
1000
WAVE NUMBER
Figure 5. Raman spectra of kaolinite (a), a kaolinite-Me2S0 complex (b), and (c) liquid Me2S0 in the low-wavenumber region.
alated Me2S0 molecules and the interlamellar surface of kaolinite can be inferred, since the spectra provide no information regarding the vibrational modes of intercalated Me2S0. Sulfonyl Stretching Region. The comparative IR spectra of nonintercalated and intercalated kaolinite in the 1400- to 400-cm-' region, shown in Figure 4, demonstrate the inability of IR spectroscopy to study the low-frequency vibrational modes of the intercalated species because of interference produced by the intense absorption bands associated with the framework modes of kaolinite. Although a significant amount of MezSO has been intercalated (23% (w/w) according to Adams and Waltl'4, the IR spectrum of the intercalation complex is not significantly different from the IR spectrum of nonintercalated kaolinite. The only differences apparent between the two spectra are some very weak bands (1320, 1110, and 960 cm-') and a slight reduction in the intensity of the 940-cm-I band in the spectrum of the kaolinite Me2S0 complex (Figure 4b). The 1320-, 11 lo-, and 960-cm-' bands correspond to vibrational modes of Me2S02' and the 940-cm-' band has been assigned to the A1-O-H bending motion of the inner-surface hydroxyl group.31 The reduced intensity of the 940-cm-' band in the spectrum of the kaolinite-Me2S0 complex complements the observed decrease in intensity of the inner-surface hydroxyl stretching bands and supports the hypothesis that intercalated MezSO molecules hydrogen bond to the kaolinite surface through the inner-surface hydroxyl groups. The Raman spectra in the 1000-100-cm~' region of intercalated and nonintercalated kaolinite samples are shown in Figure 5. The position, bandwidth, and peak intensity of the large band at 141 cm-' did not change significantly upon intercalation and, consequently, the low-wavenumber Raman spectra of intercalated and nonintercalated kaolinite could be normalized relative to the peak intensity of this band. The Raman spectrum of the kaoliniteM e 2 S 0 complex is very similar to that of liquid M e 2 S 0 (Figure 5c). With the exception of the strong 141-cm-' band, the lowwavenumber Raman spectrum of kaolinite (Figure 5a) can scarcely be detected in the spectrum of the intercalation complex. Raman spectra in the 900-1 150-cm-' region for kaoliniteMe2S0 complexes corresponding to different intercalation ratios are shown in Figure 6. The spectrum of the Me2SO/H20 solution (Figure 6a) consists of a strong band at 1044-cm-', with a shoulder occurring at 1015 crn-l, and a less intense band at 955 cm-'.There is general agreement among theoretical studies of the vibrational spectrum of M e 2 S 0 that the 1044-cm-I band results from the sulfonyl stretching m o t i ~ n . ~As ~ .the ~ ~extent of intercalation increases, the frequency of the 1044-cm-' band decreases and a new band appears at 1024 cm-' (Figure 6e), a red shift of 20 cm-'.
Similar findings have been reported in Raman investigations of M e 2 S 0 soluitons with different water content^,^^,^^ where a red shift of the sulfonyl stretching frequency was observed as the water content increased. This lowering of the sulfonyl stretching frequency was attributed by Bertoluzza et to a transition from a system characterized by dipoldipole interactions, such as occur in pure, liquid Me2S0, to one characterized by an increased number of hydrogen-bonding interactions between water molecules and the sulfonyl group of M e 2 S 0 molecules. The sulfonyl stretching frequencies of transition-metal sulfoxide complexes have been studied extensively with vibrational spectroscopic method^.^',^^
(33) W. D. Honocks and F.A. Cotton,Spectrochim. Acta, 17,134 (1961). (34) F. Tranquille, P. Labarbe, M. Foussier, and M. T. Forel, J . Mol. Struct., 8, 273 (1971).
(35) H.Kelm, J. Kqsowski, and E. Steger, J . Mol. Srrucr., 28, 1 (1975). (36) A. Bertoluzza, S. Bonora, M. A. Battaglia, P. Monti, J. Raman Spectrosc., 8, 231 (1979).
1024
59 DAYS
>-
k v)
z
31 DAYS
z Q
E
a a
24 DAYS
10 DAYS
DMSO
I 1 50
900
WAVE NUMBER Figure 6. Raman spectra of liquid MezSO (a) and kaolinite-Me,SO complexes (b-e) in the sulfonyl stretching region.
Spectroscopic Study of Kaolinite-MezSO Complex
The Journal of Physical Chemistry, Vole88, No. 24, 1984 5963
.
n2g'g 5z
.
.
,
,
.
,
59 DAYS
59 DAYS
31 DAYS
I-
z
.
673
W
z
.
31
DAYS
U
5
24
LT
DAYS 24
DAYS
IO DAYS
IO DAYS 10% WATER I N DMSO
DMSO
31 00
28 50
WAVE NUMBER Figure 7. Raman spectra of liquid MezSO (a) and kaolinite-MezSO complexes ( b e ) in the CH stretching of region.
Red (bathochromic) shifts of the sulfonyl stretching frequency of Me2S0 have been attributed by Davies3' to oxygen coordination of M e 2 S 0 molecules to metal centers, whereas blue (hypsochromic) shifts have been attributed to coordination of M e 2 S 0 to metal centers via sulfur. The red shift of the sulfonyl stretching frequency in the Raman spectrum of the kaolinite-Me2S0 complex as the extent of intercalation increases thus can be interpreted as evidence in support of the hypothesis that the M q S O molecules hydrogen bond to the inner-surface hydroxyl groups through the sulfonyl group, with the sulfur atom keyed into the opposing ditrigonal cavity of the siloxane surface (Figure 3b). The observation of this red shift in the IR spectrum of the kaoliniteMezSO complex is not possible because the intense Si-0 stretching bands of kaolinite completely obfuscate the I R vibrational spectrum of MezSO in the 1150-950-cm-' region. CHStretching Region. Raman spectra in the CH stretching region for kaolinite-MezSO complexes corresponding to different intercalation ratios are shown in Figure 7. The most prominent difference between the spectrum of the kaolinite-MezSO complex at 59 days (Figure 7e) and the spectrum of liquid MezSO (Figure 7a) is the appearance of three bands at 2936,3006, and 3 116 an-'. The 2936-cm-' band is relatively sharp and distinct, whereas the 3006- and 31 16-cm-' bands appear as shoulders on the 3001-crn-' band. The frequency of 2913 cm-', which has been assigned to ~ . ~ shifted ~ by 6 the symmetric CH stretching f r e q ~ e n c y ,is~ blue cm-' to 2919 cm-' and the band at 2998 cm-', corresponding to the antisymmetric CH stretching motion, is blue shifted by 3 cm-'. The appearance of three bands at 2936,3006, and 31 16 cm-' suggests that there are at least two distinct types of C H bonds present in the intercalated kaolinite-Me2S0 complex. The C H bonds which are oriented toward the inner-surface hydroxyl groups of the gibbsite sheet would be expected to experience greater electrostatic repulsion than the CH bonds which are oriented toward the siloxane surface. This effect would be enhanced if
-
(37) J A. Davies, A h . Znorg. Chem. Radiochem., 24, 115 (1981). (38) B. A. Sexton, N. R. Avery, and T. W. Turney, Surf.Sci., 124, 162 (1983).
DMSO 000
600
WAVE NUMBER Figure 8. Raman spectra of liquid M e 2 S 0 (a), liquid Me,SO containing 10% water (b), and kaolinite-Me2S0 complexes (c-f) in the CS stretching region.
all three inner-surface OH groups (Figure 3b) were in fact oriented perpendicularly to the ab plane, as proposed very re~ently.~'On the other hand, one would anticipate that the negative charge on the oxygen atoms in the siloxane surface3' would result in a much less repulsive interaction between C H bonds directed toward this surface. These proposed interactions between the C H bonds of the methyl groups and the interlamellar surfaces of kaolinite are illustrated in Figure 3b. The sharp, distinct line shape of the 2936-cm-' band in Figure 7 suggests that the methyl groups of intercalated M e 2 S 0 are in a highly ordered conformation over the vibrational time scale. Additional spectroscopic studies of the kaolinite-MezSO complex with proton N M R spectroscopy might provide greater insight into the conformation and mobility of MezSO in the interlamellar region over longer time scales. CS Stretching Region. Raman spectra in the 600-800.cm-' region for kaolinite samples intercalated with MezSO are shown in Figure 8. For comparison, the Raman spectrum of liquid MezSO is shown in Figure 8a. The 696- and 666-cm-' bands in the spectrum of liquid MezSO have been assigned to antisymmetric and symmetric CS stretching motions, r e s p e ~ t i v e l y . ~ ~The *~~ spectrum of the kaolinite-MezSO complex after 59 days of intercalation (Figure 8 0 is significantly different from the spectrum of liquid M e 2 S 0 (Figure 8a), in that two distinct bands appear at 720 and 688 cm-' in the spectrum of the intercalation complex. The appearance of these bands corresponds with the appearance of the 3 116- and 2936-cm-I bands in the high-wavenumber Raman spectrum of the kaolinite-MezSO complex as the extent of intercalation increases. There is a 15-cm-' difference between the 720- and 705-cm-' bands and between the 688- and 673-cm-' bands. This result suggests that the 720-cm-' and corresponds to the perturbed, antisymmetric C S stretching motion and, similarly, that the 688-cm-' band corresponds to a perturbed, symmetric C S stretching mode. Olejnik et a1.I8 studied the IR spectrum of an interlamellar kaolinite-acetamide complex and (39) V. S.Urosov, Geochem. Znt., 7, 143 (1970). (40)G.Gieseler and G. Hanschmann, J. Mol. Srrucr., 8, 293 (1971).
J . Phys. Chem. 1984, 88, 5964-5971
5964
observed that the wavenumber of the amide I band decreased as the extent of intercalation increased. This behavior was attributed to hydrogen bonding between the carbonyl group of intercalated acetamide and the inner-surface hydroxyl groups of kaolinite. Corresponding to the wavenumber decreases of the amide I band, the wavenumbers of C N stretching bands were observed to increase, also indicative of hydrogen bonding through the carbonyl g r o ~ p . ~ ’These findings tend to support the proposed hydro-
gen-bonding mechanism between the sulfonyl group of Me,SO and the inner-surface hydroxyl groups of kaolinite. The observed increase in wavenumber of the C S stretching bands, similar to an increase in the wavenumber of C N stretching bands, is evidence that the intercalated MezSO molecules coordinate to the innersurface hydroxyl groups through the oxygen atom of the sulfonyl group.
(41) J. Bellamy, “The Infrared Spectra of Complex Molecules”, Methuen, London, 1974.
Acknowledgment. This research was supported in part by NSF Grant CME-79-16336.
Triplet-State Resonance Raman Spectrum of all-trans-Diphenylbutadiene R. Wilbrandt, R i m National Laboratory, 4000 Roskilde, Denmark
W. E. L. Grossman: P. M. Killough, J. E. Bennett, and R. E. Hester* Department of Chemistry, University of York, York YO1 5DD, England (Received: March 28, 1984)
The preresonanceRaman spectrum of all-trans-diphenylbutadiene(DPB) in its ground state and the resonance Raman spectrum (RRS) of DPB in its short-lived electronically excited triplet state are reported. Transient spectra were obtained by a pumpprobe technique using two pulsed lasers. The preresonance spectrum of the ground state is not significantly changed from that of the nonresonance spectrum. In the resonance spectrum of the triplet state the double-bond stretching mode of the butadiene part is shifted by 43 cm-I downward to 1582 cm-’, whereas the single-bond stretching mode is essentially unchanged. A transient band at 1556 cm-’ is tentatively assigned to the 8a ring mode shifted downward by 39 cm-’. These results are discussed in detail.
Introduction The shorter polyenes have attracted renewed attention in recent years. The reason for this can be found in two different facts. On the one hand, they serve as model compounds for the chromophores in larger biological systems, such as retinal in the visual pigment rhodopsin and carotenoids in carotenoprotein complexes which are widely distributed in nature and are of importance in photosynthetic systems. On the other hand, both calculations and experiments with the shorter polyenes have led to change in the ordering of electronic states of the singlet manifold of some of these molecules. A one-photon forbidden lA, state has been proposed below the ‘B, state, which previously was believed to be the lowest excited electronic singlet state. Consequently, most of the recent theoretical work has involved extensive semiempirical and ab initio calculations designed to improve the prediction of the energies of excited electronic and vibrational singlet states through the inclusion of extended configurational interaction in the calculations. On the experimental side, one- and two-photon absorption and fluorescence studies, both stationary and time resolved, have been used to obtain information on the vibronic structure of these molecules. The species studied in this way, at low temperatures in solid matrices and glasses, in liquid solution, and in supersonic jets in the gas phase, were mainly the unsubstituted all-trans-butadiene, -hexatriene, and -0ctatetraene and their phenyl-substituted analogues all-trans-diphenylbutadiene (DPB), -diphenylhexatriene (DPH), and -diphenyloctatetraene (DPO). With the exception of butadiene and hexatriene these molecules (i) fluoresce and (ii) exhibit strong two-photon absorption cross sections to the ‘A, state, which has established the existence of an ‘A, state below the ‘B, state for octatetraene at low temperatures,’ for DPB in EPA mixed solvent (diethyl ether, isopentane, ethanol; 5 5 2 ) at 77 K2 in solution at room temper+Onleave from the Department of Chemistry, Hunter College, New York, 10021.
0022-3654/84/2088-5964!§01 S O / O
ature,’ and in the gas phase: for DPH in solution at room temperatures and at 77 K,6 and for DPO in solution at room temperature,$ at 77 K,8 and at liquid-He temperature.’ Conflicting results were, however, recently obtained for DPB in solution by investigations of its excited singlet-state absorption ~ p e c t r u m . ~In spite of considerable effort to detect a low-lying ‘A, state in butadiene and hexatriene, no such state has been found in these molecules. For a recent review see the work of Hudson, Kohler, and Schulten.lo The triplet states of the shorter polyenes have received less attention. Theoretical calculations have, with few exceptions, not so much aimed a t the determination of exact vibronic energies but rather attempted to map the potential energy surfaces of the lowest triplet states as a function of angle of twisting of one or more double bonds of the ground state. Varying degrees of sophistication have been used in both ab initio and semiempirical methods.11-16 The problem of twisting of double bonds in the (1) Kohler, B. E.; Snow, J. B. J . Chem. Phys. 1983, 79, 2134. (2) Bennett, J. A,; Birge, R. R. J . Chem. Phys. 1980, 73, 4234. (3) Swofford, R. L.; McClain, W. M. J. Chem. Phys. 1973, 59, 5740. (4) Heimbrook, L. A.; Kohler, B. E.; Spiglanin, T. A. Proc. Nut/. Acad. Sci. U.S.A. 1983, 80,4580. ( 5 ) Holtom, G. R.; McClain, W. M. Chem. Phys. Lett. 1976, 44, 436. (6) Fang, H. L.-B.; Thrash, R. J.; Leroi, G. R. Chem. Phys. Lett. 1978, 57, 59. (7) Hudson, B. S.; Kohler, B. E. Chem. Phys. Lett. 1972, 24, 299. (8) Fang, L.-B.; Thrash, R. J.; Leroi, G. J. Chem. Phys. 1977, 67, 3389. (9) Goldbeck, R. A,; Twarowski, A. J.; Russell, E. L.; Rice, J. K.; Birge, R. R.; Switkes, E.; Kliger, D. S. J . Chem. Phys. 1982, 77, 3319. (10) Hudson, B. S.;Kohler, B. E.; Schulten, K. In “Excited States”; Lim, E. C., Ed.; Academic Press: New York, 1982; Vol. 6, p 1. (11) Baird, N . C.; West, R. M. J. Am. Chem. SOC.1971, 93, 4427. (12) Bonacic-Koutecky, V.; Shingo-Ishimaru J. Am. Chem. SOC.1977,99, 8134. (13) Gregory, A. R.; Williams, D. F. J . Phys. Chem. 1979, 83, 2652. (14) Ohmine, I.; Morokuma, K. J . Chem. Phys. 1980, 73, 1907.
0 1984 American Chemical Society
