Characterization of Solid Complexes between Aromatic Ketones and β

Paulo J. A. Ribeiro-Claro , António M. Amorim da Costa , Manuel L. Vueba , M. Eugénia Pina , Ana M. Amado. Journal of Raman Spectroscopy 2006 37 (4), ...
0 downloads 0 Views 86KB Size
Download PDF
10392

Langmuir 2000, 16, 10392-10397

Characterization of Solid Complexes between Aromatic Ketones and β-Cyclodextrin Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy J. C. Netto-Ferreira,*,†,‡ L. M. Ilharco,† A. R. Garcia,† and L. F. Vieira Ferreira† Centro de Quı´mica Fı´sica Molecular, Complexo I, Instituto Superior Te´ cnico, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal, and Departamento de Quı´mica, Universidade Federal Rural do Rio de Janeiro, Antiga Rio-Sa˜ o Paulo km 47, Serope´ dica, 23851-970, Rio de Janeiro, Brazil Received July 24, 2000. In Final Form: September 27, 2000

The diffuse reflectance infrared Fourier transform spectra of solid complexes 1-indanone/β-cyclodextrin (β-CD), 2-indanone/β-CD, 1′-acetonaphthone/β-CD, and 2′-acetonaphthone/β-CD exhibit a series of very important effects induced by complexation. For the complexes 1-indanone/β-CD, 2-indanone/β-CD, and 2′-acetonaphthone/β-CD, a dramatic decrease on the intensity of the band assigned to the water deformation mode was observed, showing that these probes are deeply included in the cyclodextrin cavity, displacing the crystallization water. Hydrogen bonding interactions involving the ketone carbonyl and the hydroxyl groups of β-CD are responsible for the stabilization of the complexes 1-indanone/β-CD and 2′-acetonaphthone/ β-CD. In what concerns the complexes 2-indanone/β-CD and 1′-acetonaphthone/β-CD, it is suggested that they are stabilized only by London interactions between the aromatic ring and the cyclodextrin cavity.

Introduction Cyclodextrins are naturally occurring olygosaccharides produced by enzymatic degradation of starch. They are composed of six (R-CD), seven (β-CD), eight (γ-CD), or more D-(+)-glucopyranose units joined by R-(1,4) linkages. They have a torus-shaped hydrophobic cavity, with variable cavity diameter (5.6 Å for R-CD, 6.8 Å for β-CD, and 8.0 Å for γ-CD), and are able to include a great variety of organic, inorganic, neutral, and ionic molecules.1-3 The driving force for the formation of such type of host/guest complex has been related to van der Waals interactions, release of cyclodextrin strain energy upon complexation, and hydrogen bonding between either the primary or secondary hydroxyl groups of the cyclodextrin and the guest. However, recent theoretical calculations of cyclodextrin complexes seem to indicate that van der Waals interactions are the main driving force in complexation.4-6 Due to their ability to form inclusion complexes, cyclodextrins have been used in several industrial processes, such as pharmaceutical, food, and chemical industries.7,8 The structural and thermodynamic parameters related to the formation of these inclusion complexes have been obtained by several spectroscopic9-19 and nonspectroscopic methods,20,21 mainly microcalorimetry,22 as well as by theoretical calculations.23-25 Among the spectroscopic methods, nuclear magnetic resonance is by far the most * To whom correspondence should be addressed. † Instituto Superior Te ´ cnico. ‡ Universidade Federal Rural do Rio de Janeiro. (1) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980, 19, 344. (2) Tabushi, I. Acc. Chem. Res. 1982, 15, 66. (3) Szeijtli, J. Cyclodextrins and Their Inclusion Complexes; Akade´miai Kiado´: Budapest, 1982. (4) Alvira, E.; Cativiela, C.; Garcia, J. I.; Mayoral, J. A. Tetrahedron Lett. 1995, 36, 2129. (5) Alvira, E.; Mayoral, J. A.; Garcia, J. I. Chem. Phys. Lett. 1995, 245, 335. (6) Alvira, E.; Mayoral, J. A.; Garcia, J. I. Chem. Phys. Lett. 1997, 271, 178. (7) Szejtli, J. Chem. Rev. 1998, 98, 1743. (8) Hedges, A. R. Chem. Rev. 1998, 98, 2035.

employed for solid-state samples,26-28 whereas absorption and fluorescence emission are more prevalent in solution.10,12-19 Fourier transform infrared spectroscopy in the diffuse reflectance mode (DRIFTS) has widely been employed to characterize solid-phase samples.29,30 These include several finely divided powders, such as silica,31 polystyrene,32 and cellulose.33,34 Recently, some of us have employed this technique to study the adsorption of aromatic ketones, namely, benzophenone and fluorenone, onto different types of cellulose, using the modifications produced in the ketone carbonyl stretching band upon adsorption.35,36 This is a very informative vibrational mode, given the fact that it is responsible for a strong absorption band whose wavenumber may range from ∼1650 to ∼1800 cm-1. Its position is strongly dependent on the carbonyl vicinity, namely, on the conjugated system in which the carbonyl partici(9) de Lucas, N. C.; Netto-Ferreira, J. C. J. Photochem. Photobiol., A: Chem. 1997, 103, 137. (10) Wang, X.-M.; Chen, H.-Y. Spectrochim. Acta 1995, 51A, 333. (11) Politi, M. J.; Tran, C. D.; Gao, G.-H. J. Phys. Chem. 1995, 99, 14137. (12) Milewski, M.; Maciejewski, A.; Augustyniak, W. Chem. Phys. Lett. 1997, 272, 225. (13) Brochsztain, S.; Rodrigues, M. A.; Politi, M. J. J. Photochem. Photobiol. A: Chem. 1997, 107, 195. (14) Barros, T. C.; Stefaniak, K.; Holzwarth, J. F.; Bohne, C. J. Phys. Chem. A 1998, 102, 5639. (15) Vieira Ferreira, L. F.; Lemos, M. J.; Wintgens, V.; Netto-Ferreira, J. C. Spectrochim. Acta, Part A 1999, 55, 1219. (16) Netto-Ferreira, J. C.; Casal, H. L.; Scaiano, J. C. J. Inclus. Phenom. 1985, 3, 395. (17) Kotake, Y.; Janzen, E. G. J. Am. Chem. Soc. 1988, 110, 3699. (18) Kotake, Y.; Janzen, E. G. J. Am. Chem. Soc. 1992, 114, 2872. (19) Kodaka, M. J. Phys. Chem. 1998, 102, 8101. (20) Rekharski, M. V.; Inoue, Y. Chem. Rev. 1998, 98, 1875. (21) Li, J.; Uzawa, J.; Doi, Y. Bull. Chem. Soc. Jpn. 1998, 71, 1953. (22) Rekharski, M. V.; Mayhew, M. P.; Goldberg, R. N.; Ross, P. D.; Yamashoji, Y.; Inoue, Y. J. Phys. Chem. 1997, 101, 87. (23) Lipkowitz, K. B. Chem. Rev. 1998, 98, 1829. (24) Madrid, J. M.; Pozuelo, J.; Mendicuti, F.; Mattice, W. L. J. Colloid Interface Sci. 1997, 193, 112. (25) Madrid, J. M.; Mendicuti, F.; Mattice, W. L. J. Phys. Chem. B 1998, 102, 2037. (26) Schneider, H.-J.; Hacket, F.; Ru¨diger, V.; Ikeda, H. Chem. Rev. 1998, 98, 1755.

10.1021/la001041b CCC: $19.00 © 2000 American Chemical Society Published on Web 11/21/2000

Complexes from Ketones and Cyclodextrins

pates, on angular constraints when it is part of a ring system, and also on the molecular interactions in which the carbonyl is involved.37 Despite the potentiality of DRIFTS, as far as we know no studies have been carried out on the characterization of solid complexes of cyclodextrins using this technique. This technique can be of general use, provided that the absorption bands under observation are not superimposed by the strong absorption corresponding to the water deformation band at ∼1641 cm-1. The recently reported use of infrared spectroscopy in the study of solid complexes of sulfafurazole,38 cinnamyl alcohol,39 or phenytoin40 deals, in fact, with spectra obtained in transmission or reflection mode, and in none of the cases was the Kubelka-Munk treatment used.41 This treatment can be applied to an optically thick sample, in which any further increase on its depth does not change the measured reflectance. Thus, the reflectance spectrum may be transformed by the use of the remission function, i.e., the Kubelka-Munk equation for diffuse light,41 with the absorption spectrum of inclusion complexes being directly comparable to that obtained in transmission mode for transparent samples which is commonly expressed in absorbance units. Such treatment was successfully employed by us in the characterization of N-aryl-2,3-naphthalimides/β-cyclodextrin complexes in the solid state.42 In this work we are able to show that DRIFTS is indeed a very convenient tool for the structural characterization of solid complexes between aromatic ketones and β-cyclodextrin, since it provides extremely important information concerning these guest/host interactions. Among them, the most relevant are the existence of hydrogen bonds between the ketone carbonyl and β-CD hydroxyls, the presence of interactions between the ketone aromatic ring and the interior of the β-CD cavity, and the extent of water elimination from the inside of the cyclodextrin cavity. For this purpose, the structurally simple aromatic ketones 1-indanone (1), 2-indanone (2), 1′-acetonaphthone (3), and 2′-acetonaphthone (4) complexed with β-cyclodextrin were used.

Langmuir, Vol. 16, No. 26, 2000 10393

Experimental Section Materials. 1-Indanone (Aldrich) was recrystallized from n-hexane. 2-Indanone (Aldrich) was chromatographed on silica gel and recrystallized twice from n-hexane. 1′-Acetonaphthone and 2′-acetonaphthone were purchased from Aldrich and used as received. β-Cyclodextrin (Sigma) was recrystallized twice from water. Cyclohexane (Aldrich, spectroscopic grade) was used as received. Sample Preparation. The solid complexes ketone/β-cyclodextrin of molar ratio 1:10 were prepared by mixing a saturated solution of β-cyclodextrin in water (∼10-2 M) and a saturated solution of the ketone in diethyl ether. The resulting mixture was magnetically stirred for at least 72 h and then lyophilized until a completely dried sample was obtained. Removal of any noncomplexed ketone was achieved by thoroughly washing the dried solid complexes with diethyl ether. Finally, the complexes were dried under reduced pressure. The complexes ketone/β-cyclodextrin of molar ratio 1:1 were prepared following a literature procedure.1 Thus, a saturated solution of the corresponding ketone in diethyl ether was layered over a saturated aqueous solution of β-cyclodextrin for at least 96 h. The resulting precipitate was filtered out, washed with 20 mL of diethyl ether (10 times 2 mL aliquots) to remove noncomplexed ketone, and dried under reduced pressure. The solvent removal was performed overnight in an acrylic chamber with an electrically heated shelf (Heto, model FD 1.0110) with temperature control (25 ( 1 °C) and at a pressure of ca. 10-3 Torr. DRIFT Spectroscopic Analysis. The samples of pure β-CD and of ketones 1-4, as well as the ketone/β-CD complexes, were diluted in KBr (from Aldrich, FTIR grade), in a concentration of ∼4% w/w, and ground. The mixture obtained was placed in a sampling cup (11 mm diameter) and manually pressed. Infrared spectra were recorded with a Mattson Research Series 1 FTIR spectrometer in diffuse reflectance mode, with a Graseby/Specac Selector accessory. A wide-band mercury-cadmium-telluride (MCT) detector, cooled with liquid nitrogen, was used. The spectra were recorded at 4 cm-1 resolution, in the range 4000-500 cm-1. Diffuse reflectance spectra were obtained as a ratio of 1000 singlebeam scans of the sample to the same number of background scans for pure KBr. By use of the FIRST software, the diffuse reflectance spectra were transformed to Kubelka-Munk units. The IR transmission spectra of ketones 1-3 in cyclohexane solution were also recorded. They were obtained with 4 cm-1 resolution, as the result of 100 co-added sample scans, for a small amount of the ketone solution between two KBr plates, ratioed against the same number of background scans recorded for two clean KBr plates.

Results and Discussion Indanones. The DRIFT spectra of 1-indanone (1) crystals and of the solid 1/β-CD complex (1:1) are shown in Figure 1A, in the region 1500-1800 cm-1. These spectra were normalized to 1 in the carbonyl stretching mode maximum absorption. The transmission IR spectrum of a 1-indanone solution in cyclohexane (Figure 1C) enabled us to characterize the carbonyl stretching mode of this

(27) Ripmeester, J. A.; Ratcliffe, C. I. In Comprehensive Supramolecular Chemistry; Davies, J. E. D., Ripmeester, J. A., Eds.; Pergamon/ Elsevier: Oxford, 1996; Vol. 9. (28) Udachin, K. A.; Ripmeester, J. A. J. Am. Chem. Soc. 1998, 120, 1080. (29) Griffiths, P. R.; Fuller, M. P. In Advances in Infrared and Raman Spectroscopy; Hester, R. E., Ed.; Heyden: London, 1982. (30) Christy; A. A.; Kvalheim, O. M.; Velapoldi, R. A. Vib. Spectrosc. 1995, 9, 19. (31) Boroumand, F.; van den Bergh, H.; Moser, J. E. Anal. Chem. 1994, 66, 2260. (32) Christy, A. A.; Liang, Y. Z.; Hui, C.; Kvalheim, O. M.; Velapoldi, R. A. Vib. Spectrosc. 1993, 5, 233. (33) Hulleman, S. H. D.; van Hazendonk, J. M.; van Dam, J. E. G. Carbohydr. Res. 1994, 261, 163.

(34) Kondo, T.; Sawatari, C. Polymer 1996, 37, 393. (35) Ilharco, L. M.; Garcia, A. R.; Lopes da Silva, J.; Vieira Ferreira, L. F. Langmuir 1997, 13, 4126. (36) Ilharco, L. M.; Garcia, A. R.; Lopes da Silva, J.; Lemos, M. J.; Vieira Ferreira, L. F. Langmuir 1997, 13, 3787. (37) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.; Chapman and Hall: London, 1980; Vol. 2. (38) Szafran, B.; Pawlaczyk, J. J. Inclusion Phenom. Mol. Recognit. Chem. 1995, 23, 277. (39) Yu, S.-Z.; Li, J.-H.; Wang, J.-Y.; Tian, S.-J. J. Thermal Anal. 1997, 49, 1517. (40) von Plessig, C.; Jaramillo, V. H.; Knopp, C. Bol. Soc. Chil. Quim. 1997, 42, 157. (41) Kubelka, P.; Munk, F. Z. Tech. Phys. 1931, 12, 593. (42) Netto-Ferreira, J. C.; Wintgens, V.; Garcia, A. R.; Ilharco, L. M.; Lemos, M. J. Vieira Ferreira, L. F. J. Photochem. Photobiol., A: Chem. 2000, 132, 209.

10394

Langmuir, Vol. 16, No. 26, 2000

Netto-Ferreira et al.

Figure 1. DRIFT spectra of (A) 1-indanone crystals (- ‚ ‚ -) and 1-indanone/β-CD complex (molar ratio 1:1) (s), normalized in the carbonyl stretching band, and (B) pure β-CD (- - -) and 1-indanone/β-CD complexes of molar ratio 1:10 (‚ ‚ ‚) and 1:1 (s), normalized to the more intense band of β-cyclodextrin (at 1032 cm-1, not shown). (C) FTIR transmission spectrum of 1-indanone solution in cyclohexane (saturated).

ketone in the absence of the strong dipole interactions usually present in the solid state. The wavenumber of the ν(CdO) band for 1 in cyclohexane, ca. 1726 cm-1, is typical of a carbonyl group in an aliphatic five-member ring, with the bond order reduced by conjugation with the aromatic moiety.37 In crystalline phase, due to a variety of strong interactions and solidstate effects, the carbonyl-stretching band is broader, with a clear shoulder, and red shifted (to 1709 cm-1). In the inclusion complex with β-CD, the carbonyl band shows a slight reduction in the full width at half-height, when compared to the ketone crystals (although keeping the shoulder) and, more important, shifts about 5 cm-1 to higher wavenumbers. This can be explained by assuming that the laterally oriented CdO group in 1-indanone is able to form highly directional hydrogen bonds, probably with the secondary hydroxyl groups of the β-CD. Two effects on the carbonyl group are then expected, when compared to the pure ketone crystals: loss of resonance with the aromatic ring, with the consequent shift of the carbonyl band to higher wavenumbers (usually about 3040 cm-1), and reduction of the CdO bond force constant, with the consequent shift back to lower wavenumbers (about 15-25 cm-1). The net effect is a slight shift to higher wavenumbers, as observed in Figure 1A. In conjugated ketones, the coupling of ν(CdC) and ν(CdO) modes may be responsible for a band close to the carbonyl, at lower wavenumbers.37 This band is absent whenever molecular deformations or interactions reduce the possibility of resonance. In the case of 1-indanone crystals, such intramolecular coupling results in the band observed at 1663 cm-1. However, this coupling mode is absent in the DRIFT spectrum of the 1/β-CD complex. This is a good indication that new interactions between the carbonyl group and/or the aromatic ring of the ketone are being formed with the interior of the β-CD cavity, which are responsible for decoupling the ν(CdC) and ν(CdO) modes.35 This is a complementary confirmation that an inclusion complex is formed. The ν(CdC) skeletal ring breathing modes of 1-indanone, observed at 1590 and 1611 cm-1 in solution, are enhanced in the crystals, and an additional band appears at 1601 cm-1. The presence of this new band may be

explained by a strong resonance of the CdC with the carbonyl mode, favored by the crystalline structure. In the solid complex, the spectrum becomes more similar to that in solution. This suggests that molecules of 1 are isolated from each other and only one molecule is entrapped in each cavity of β-CD. The fact that these skeletal breathing modes are still present after complexation indicates that interactions of the 1-indanone phenyl group with the interior of the β-CD cavity are not strong. Possibly, the position of the molecule inside the cavity is determined by hydrogen bond formation between the carbonyl group and β-CD hydroxyl groups and, therefore, the molecule is not as free to penetrate inside the cavity. In Figure 1B, the same region of the DRIFT spectra is shown for pure β-CD and the 1-indanone/β-CD complexes of molar ratio 1:10 and 1:1, with the spectra being normalized to the more intense band of β-CD at 1032 cm-1 (not shown). These spectra allow the characterization of the solid-phase complexes from the β-CD point of view, confirming the inclusion of the ketone in the β-CD cavity. This complexation involves expelling and replacing water molecules from the interior of the cyclodextrin cavity. In fact, the intense water deformation band present in the pure β-CD spectrum at ∼1641 cm-1 is reduced in the 1:10 complex and even more in the 1:1 complex. The spectra obtained in an equivalent study of 2-indanone (2) and its β-CD complexes are shown in Figure 2. On comparison of the spectra of the two ketones, i.e., 1- and 2-indanone, in a nonpolar solvent, the large blue shift of the ν(CdO) band of the later to 1761 cm-1 (Figure 2C) reflects the fact that the carbonyl group is also in a five-member ring, despite the fact that in this case the carbonyl is not conjugated with the phenyl group. Similarly to 1-indanone, the carbonyl band shifts to a lower wavenumber in the crystalline phase (1751 cm-1), due to solid-state effects. The shape of the νCdO band in the solid-phase complex 2-indanone/β-CD (2/β-CD) (Figure 2A) resembles the one in solution. This is an indication that in the complex the carbonyl group is not involved in the same kind of interactions as in the crystalline phase, although the wavenumber of maximum absorption is the same in the two cases. These results may be understood

Complexes from Ketones and Cyclodextrins

Langmuir, Vol. 16, No. 26, 2000 10395

Figure 2. DRIFT spectra of (A) 2-indanone crystals (- ‚ ‚ -) and 2-indanone/β-CD complex (molar ratio 1:1) (s), normalized in the carbonyl stretching band, and (B) pure β-CD (- - -) and 2-indanone/β-CD complexes of molar ratio 1:10 (‚ ‚ ‚) and 1:1 (s), normalized to the more intense band of β-cyclodextrin (at 1032 cm-1, not shown). (C) FTIR transmission spectrum of 2-indanone solution in cyclohexane (saturated).

if, upon formation of the inclusion complex, only one molecule of 2-indanone is entrapped in each cavity of β-CD, with the carbonyl group involved only in shortrange dipole interactions with β-CD. This leads us to suggest that, similarly to 1-indanone complexes, in the 1:1 complex 2/β-CD each cyclodextrin cavity accommodates only one ketone molecule. Furthermore, the fact that the carbonyl group is separated from the ring by one carbon atom, which could be seen as a minor structural alteration, is indeed a determinant of its orientation in the cavity: it must be orthogonally oriented to the larger axis of the CD cavity, without the possibility of forming highly directional hydrogen bonds with the hydroxyl groups of β-CD. The ν(CdC) skeletal ring breathing modes of 2-indanone in solution are strong, at 1616 and 1638 cm-1.43 The band shapes are different in the crystals, slightly red shifted and with lower relative intensities (when compared to the carbonyl band). In the solid complex they cannot be clearly identified. This may be due to constraints imposed by the molecular packing, which become more important when the molecule is confined to the β-CD cavity. From these results, one can suggest that in the complex 2-indanone/β-CD, contrary to the previous case, the probe deeply penetrates the cyclodextrin cavity. Thus, inhibition of the ν(CdC) skeletal ring breathing modes is a consequence of strong interactions between the interior of the cyclodextrin and the phenyl ring. In what concerns the replacement of water molecules contained in the β-CD cavity by 2-indanone, the DRIFT spectrum of the complex (Figure 2B) reinforces the conclusion that this ketone is more deeply included in the cyclodextrin cavity than 1-indanone due to the fact that more volume for each CD cavity is occupied by the guest molecule. On the other hand, since the intense water deformation sharply decreases with the increasing in the molar ratio for the complex, one can conclude that, as more β-CD cavities are occupied by ketone molecules, more crystallization water is removed from their interior. Acetonaphthones. The characteristic wavenumber of the carbonyl band of 1′-acetonaphthone (3) in solution (43) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Investigation of Organic Compounds, 4th ed.; Wiley: New York, 1981.

(1684 cm-1) is much lower than that for indanones, reflecting the fact that the ν(CdO) mode is not affected by ring constrictions. Rotation of the carbonyl relative to the naphthalene group is not hindered in solution and therefore the coupling of the ν(CdC) and ν(CdO) modes is very weak, originating a hardly detected band at ∼1620 cm-1 (see Figure 3C). The transmission IR spectrum of the pure liquid 1′-acetonaphthone is similar to that in solution, with the carbonyl band red shifted about 8 cm-1 (Figure 3A). The carbonyl band in the solid complex 3/β-CD is shifted within the spectral resolution but apparently is much broader than that observed for the pure liquid. This may easily be explained by inspection of Figure 3B. In fact, taking into account that both the water deformation and the carbonyl stretching modes have close frequencies in this ketone, the carbonyl band in the solid complex is modified and apparently broadened by the deformation band of the remaining water. This indicates that the ketone has displaced only a small part of the water molecules from the β-CD cavity. Thus, the DRIFT spectra allow us to conclude that the carbonyl group is not involved in strong interactions with the host cavity. On the other hand, the relative intensities of the ring breathing modes are slightly reduced when compared to the liquid ketone. This is a clear indication that the naphthalene moiety is not completely included in the β-CD cavity, otherwise these modes would be hindered. Unfortunately, the observation of the very informative ν(CdO)/ν(CdC) coupling mode becomes quite difficult in this case, due to the abovementioned superposition of water and CdO bands. Very different results were obtained for the inclusion of 2′-acetonaphthone (4) in the β-CD cavity (Figure 4). The coupling between CdC and CdO stretching modes is revealed by the very intense band at 1628 cm-1, observed in the DRIFT spectrum of the crystal phase (Figure 4A). The intensity of this band in the solid 2′-acetonaphthone/ β-CD (4/β-CD) complex is reduced to about a half, while the ring breathing modes at 1595 and 1578 cm-1 are even more reduced. These two observations point to a strong interaction between the naphthalene part of 4 and β-CD. Additionally, the carbonyl band shifts about 8 cm-1 to higher wavenumbers, which may be interpreted by the

10396

Langmuir, Vol. 16, No. 26, 2000

Netto-Ferreira et al.

Figure 3. (A) Transmission FTIR spectrum of pure liquid 1′-acetonaphthone (- ‚ ‚ -) and DRIFT spectrum of 1′-acetonaphthone/ β-CD complex (molar ratio 1:1) (s), normalized in the carbonyl stretching band. (B) Pure β-cyclodextrin (- - -) 1′-acetonaphthone/ β-CD complex of molar ratio 1:1 (s), normalized to the more intense band of β-CD (at 1032 cm-1, not shown). (C) FTIR transmission spectrum of 1′-acetonaphthone solution in cyclohexane (saturated).

in the case of naphthalene has been previously related to the equilibrium constant of complex formation. Equilibrium constants for 2′-substituted naphthalenes are similar to those observed for naphthalene,44,45 whereas naphthalenes substituted at 1′ position are much lower. These results are supported by recent theoretical calculations, from which it was shown that naphthalenes substituted at the 2′ position are included axially into the cyclodextrin cavity.24 The lower value for the equilibrium constant of 1′-substituted naphthalenes is probably due to steric interactions between the substituent and the β-cyclodextrin, which lead to different inclusion modes for these complexes.46 Conclusions Figure 4. DRIFT spectra of (A) 2′-acetonaphthone crystals (- ‚ ‚ -) and 2′-acetonaphthone/β-CD complex (molar ratio 1:1) (s), normalized in the carbonyl stretching band, and (B) pure β-CD (- - -) and 2′-acetonaphthone/β-CD complex of molar ratio 1:1 (s), normalized to the more intense band of β-CD (at 1032 cm-1, not shown).

formation of hydrogen bonds with the hydroxyl groups of β-CD, in a similar way to 1-indanone. As complementary information, the comparison of the DRIFT spectrum of the solid-phase complex (1:1) with that of pure β-CD (Figure 4B) shows that the water content is much reduced upon complex formation. From the whole set of information obtained by using DRIFT spectroscopy, one can propose a structure for the 2′-acetonaphthone/β-CD complex in which the aromatic part of the ketone must be deeply immersed in the β-CD cavity. Strong dispersion interactions between the aromatic ring and the host, as well as hydrogen bond formation between the carbonyl and probably the secondary hydroxyl groups of β-CD, would be responsible for complex stabilization. These results are fully in accord with those previously reported for naphthalene substituted in position 1′ or 2′.44,45 The penetration of the guest molecule into the host cavity (44) Hamai, S.; Mononobe, N. J. Photochem. Photobiol., A: Chem 1995, 91, 217. (45) Hamai, S. Bull. Chem. Soc. Jpn. 1996, 69, 2469.

The diffuse reflectance infrared Fourier transform spectroscopy has allowed us to characterize solid inclusion complexes of the type simple aromatic ketone/β-CD. There are two main differences between the complexes 1-indanone/β-CD and 2-indanone/β-CD. For the former, hydrogen bonds between the carbonyl group and the β-CD hydroxyls are responsible for complex stabilization, whereas for the latter the stabilization is reached through interactions between the ketone aromatic ring and the interior of the cyclodextrin cavity. Thus, it was possible to predict that 1-indanone (1) will be less deeply included into the CD cavity than 2-indanone (2). Due to the larger kinetic diameter for 1′-acetonaphthone (3) when compared to 2′-acetonaphthone (4), it is possible to postulate different inclusion modes for 3 in the β-CD cavity. In the two possible structures for this complex, in which the probe is either axially or equatorially oriented, the probe does not fully penetrate the CD cavity after inclusion. On the other hand, the absence of water molecules inside the CD cavity after inclusion of 4, as well as a clear evidence for hydrogen bond formation and London interactions between this probe and the host, enabled us to suggest that, in the complex, 4 is profoundly located inside the cyclodextrin cavity. In conclusion, it was possible to distinguish, through DRIFT spectra, cases where complex formation is due (46) Harata, K.; Uedaira, H. Bull. Chem. Soc. Jpn. 1975, 48, 375.

Complexes from Ketones and Cyclodextrins

only to dispersion interactions, from those where specific interactions such as hydrogen bonding between the ketone carbonyl and β-CD hydroxyl groups are present. Of the four examples studied, 2-indanone/β-CD and 1′-acetonaphthone/β-CD complexes do not show evidence for such hydrogen bond interactions. Thus, by employing the DRIFTS technique it was clearly possible to determine in which cases hydrogen bonds where formed as well as the degree of hydration of the CD cavity. Further work is

Langmuir, Vol. 16, No. 26, 2000 10397

currently in progress, aiming to show the generality of this behavior with compounds containing other functional groups. Acknowledgment. This work was financed by Project PRAXIS/212/Quim22/94. J.C.N.F. thanks Fundac¸ a˜o para a Cieˆncia e a Tecnologia (FCT) for a Visiting Professor fellowship. A.R.G. wishes to express her gratitude for a FCT/PRAXIS XXI Ph.D grant. LA001041B