Ultrathin Cross-Linked Networks at the Interface between Oil and

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Ultrathin Cross-Linked Networks at the Interface between Oil and Water: Structure, Properties, and Preparative Perspectives† Heinz Rehage,*,‡ Barbara Achenbach,‡ and F.-G. Klaerner§ Institute of Physical Chemistry and Institute of Organic Chemistry, University Essen, Universitaetsstr. 2-5, 45141 Essen, Germany Received January 15, 2002. In Final Form: March 29, 2002 The self-association process of surface-active monomers can be used to synthesize ultrathin cross-linked networks at fluid interfaces. We have systematically studied transient networks, which were stabilized and cross-linked by hydrogen bonds, Coulomb interactions, or van der Waals attraction forces. At the planar surface, we measured the rheological properties of these films. In addition to these experiments, we have also investigated typical structures of these networks by means of Brewster angle microscopy. The results of these investigations are that the surfactant molecules tend to build up two-dimensional aggregates, which are interconnected to form elongated network strands. Due to their simple synthesis, two-dimensional networks can serve as excellent model systems for advanced investigations of the stability and deformation properties of living cells. In addition to this, we analyzed the film-forming behavior of molecular tweezers, which are able to bind selectively electron deficient aromatic and aliphatic substrates.

Introduction Gel-like supermolecular structures are often observed in inorganic suspensions, biological systems, or macromolecular solutions;1 however, they can also occur in dilute solutions of surfactants or dyes.2-8 Surface-active molecules, under suitable conditions, tend to assemble reversibly into large aggregates, which might be connected to form supermolecular network structures. Besides the formation of three-dimensional gel phases, surface-active monomers can also form coherent structures at fluid interfaces.9-14 Two-dimensional network structures are sometimes formed by amphiphilic compounds due to their * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +49-(0)201-183-3987. Fax: +49(0)201-183-3951. † This article is part of the special issue of Langmuir devoted to the emerging field of self-assembled fibrillar networks. ‡ Institute of Physical Chemistry. § Institute of Organic Chemistry. (1) Ferry, J. D. Viscoelastic properties of polymers, 3rd ed.; John Wiley & Sons: New York, 1980. (2) Rehage, H.; Platz, G.; Struller, B.; Thunig, C. Tenside, Surfactants, Deterg. 1996, 33, 242. (3) Rehage, H. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prud’Homme, R. K., Eds.: Oxford University Press: New York, 1994; p 63. (4) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (5) Holz, T.; Fischer, P.; Rehage, H. J. Non-Newtonian Fluid Mech. 1999, 88, 133. (6) Hoffmann, H.; Rehage, H.; Schorr, W.; Thurn, H. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; p 425. (7) Hoffmann, H.; Platz, G.; Rehage, H.; Schorr, W.; Ulbricht, W. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 255. (8) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933. (9) Akkara, J. A.; Bruno, F.; Samuelson, L.; Ayyagari, M.; Tripathy, S.; Marx, K.; John, V. T.; Kaplan, D. L. Am. Chem. Soc. 1993, 34, 759. (10) Frank, C. W.; Naumann, C. A.; Knoll, W.; Brooks, C. F.; Fuller, G. G. Macromol. Symp. 2001, 166, 1. (11) Kim, J. U.; Lee, B. J.; Kwon, Y. S. Bull. Korean Chem. Soc. 1997, 18, 1056. (12) Kim, J. U.; Lee, B. J.; Kwon, Y. S. Thin Solid Films 1998, 327329, 486. (13) Marek, M.; Brynda, E.; Pientka, Z.; Schauer, J. Eur. Polym. J. 1997, 33, 1717. (14) Naumann, C. A.; Brooks, C. F.; Wiyatno, W.; Knoll, W.; Fuller, G. G.; Frank, C. W. Macromolecules 2001, 34, 3024.

tendency to build up ultrathin stable films at fluid interfaces. These phenomena might occur at the surface between air and water or in the contact layer between two immiscible solvents. Surfactants are often used as emulsifiers, dispersing agents, or foam-forming additives, because of their ability to stabilize interfaces. Many natural or biological surface-active compounds, such as proteins or phospholipids, exhibit similar properties. Phospholipids tend to form bilayer structures, which are basic components of the plasma membrane of living cells. Although the lipid molecules are known to form stable films, they are still in a liquid analogous state. In living organisms, however, this situation is often more complicated, because the bimolecular layers of phospholipid molecules are in close contact with two-dimensional or three-dimensional networks. It is easy to understand that these structures strongly enhance the stability of the fluidlike membranes. One well-known example for such structures is the red blood cell. A human erythrocyte consists of a lipid bilayer that is coupled at well-defined anchor points to a two-dimensional spectrin network, which confers elastic forces to the liquidlike phospholipid bilayer. As a consequence, a red blood cell is easily shearable, but strongly resistant to any increase of the local surface area. The stability of red blood cells depends very much on the rubber-elastic properties of the supporting spectrin network. The unique combination of elastic and viscous forces leads, hence, to interesting mechanical properties of the red blood cells. Since biological structures are rather complicated, we have synthesized well-defined model networks, which could give new insights into the basic features of these confined systems. Analogous to the spectrin network of red blood cells, we have used molecules that are partially fixed in a planar arrangement. In this publication, we shall discuss the basic rheological properties of these systems and their molecular structures, which could be visualized from Brewster angle microscopy (BAM). The present article gives a summary of our work in this area, and it was presented as plenary lecture at the SAFIN 2001 Conference in Autrans, France, November 24-28, 2001.

10.1021/la0255368 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/12/2002

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Materials and Experimental Techniques For measurements at the interface between dodecane and water, the surfactant was dissolved in desired amounts in the hydrophobic phase. Water was obtained from a pure water system (Seralpur PRO 90 CN). At the pure water surface, the surfactants were spread from a 10-3 M solution in chloroform (p.a. Merck). The synthesis of the amphiphilic dye molecules was realized using the method published by Czikkely et al.15 The surfactants Span 65 (trioctadecanyl ester of sorbic acid) and bis(2-ethylhexyl)sulfosuccinate sodium salt were purchased from Fluka. Molecular clips and tweezers were synthesized in the institute of Prof. Kla¨rner at the University of Essen, Germany.41 The shear rheological properties of the membranes were determined in a Rheometrics fluid spectrometer (RFS II), which was equipped with a modified shear system.16 The measuring cell consisted of a quartz dish (diameter, 83.6 mm) and a thin biconical titanium plate (angle, 2°; diameter, 60 mm), which could be placed exactly at the interface between oil and water. The dish was first filled with the aqueous phase. The titanium plate was then positioned at the water surface, and a solution of the surfactant was added. We measured the torque required to hold the plate stationary as the cylindrical dish was rotated with a sinusoidal angular frequency ω. In such experiments, the twodimensional storage modulus µ′(ω) and the loss modulus µ′′(ω) could be evaluated from the amplitude and phase angle of the stress and deformation signals. At the water surface, surface-pressure/area isotherms and BAM were performed with a BAM 2 apparatus constructed by the Nanofilm Technology Company in Go¨ttingen. The basic principles of BAM experiments are extensively described.17,18 If a p-polarized light beam is incident at the Brewster angle to the surface of water (53.1°), no light is reflected. A video camera, arranged in the direction of the reflected light beam, will now observe darkness. In the presence of surface-active compounds, however, the refractive index of the water surface is slightly changed. As a consequence, the video camera will now obtain light and the image of the network structure can be analyzed. One obtains, hence, a large optical contrast between the pure water surface (black regions) and those parts covered with surfactant molecules (white regions). The lateral resolution of the BAM is limited by the wavelengths of the incident laser beam (690 nm), but this technique allows investigation of monomolecular films with a thickness of only 1 nm.

Results Two-Dimensional Model Networks. Ultrathin crosslinked membranes can be formed at the surface of water or at the interface between oil and water.19-27 These structures can be divided into separate categories, depending on the nature and strength of bonding forces. Two extreme cases are often observed: transient networks and permanent structures. When the cross-linking process (15) Czikkely, V.; Dreizler, G.; Fo¨rsterling, H. D.; Kuhn, H.; Sondermann, J.; Tillmann, P.; Wiegand, J. Z. Naturforsch. 1996, 24A, 1830. (16) Pieper, G.; Rehage, H.; Barthe`s-Biesel, D. J. Colloid Interface Sci. 1998, 202, 293. (17) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (18) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (19) Rehage, H.; Veyssie, M. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 1166. (20) Burger, A.; Rehage, H. Prog. Colloid Polym. Sci. 1991, 84, 269. (21) Burger, A.; Rehage, H. Angew. Makromol. Chem. 1992, 202, 31. (22) Rehage, H.; Burger, A. Physica A 1993, 194, 424. (23) Husmann, M.; Achenbach, B.; Rehage, H. In The Wiley Polymer Networks Group Review; Stokke, B. T., Elgsaeter, A., Eds; John Wiley and Sons: New York, 1999; Vol. 2, p 444. (24) Husmann, M. Polyorganosiloxan-Filme zwischen fluiden Phasen: Rheologisches Verhalten von ultradu¨ nnen Membranen, Mikrokapseln und Kapselsuspensionen; Shaker Verlag: Aachen, 2001. (25) Rehage, H.; Achenbach, B.; Geest, M.; Siesler, H. W. Colloid Polym. Sci. 2001, 279, 597. (26) Burger, A.; Leonhard, H.; Rehage, H.; Wagner, R.; Schwoerer, M. Macromol. Chem. Phys. 1995, 196, 1. (27) Achenbach, B.; Husmann, M.; Kaplan, A.; Rehage, H. In Transportmechanisms across Fluid Interfaces; Kreysa, G., Eds.; WileyVCH: Weinheim, 2000; p 45.

Figure 1. Molecular structure of an amphiphilic dye molecule [3-stearyl-benzthiazol-(2)]-[3-stearyl-benzoxazol-(2)]monomethincyanin-iodide.

Figure 2. The two-dimensional storage modulus µ′(ω), the two-dimensional loss modulus µ′′(ω), and the magnitude of the complex viscosity |η*|(ω) as a function of the angular frequency ω for a monomolecular film of [3-stearyl-benzthiazol-(2)]-[3stearyl-benzoxazol-(2)]monomethincyanin-iodide dye molecules; surface concentration Γ ) 7.8 × 10-7 mol/m2, T ) 20 °C.

is a result of chemical reactions, then the structures are of the permanent type. Besides these chemically crosslinked membranes, self-assembling processes can also form coherent networks. In this case, the cross-linking reaction is induced by physical contacts. These interactions are caused by attractive forces, which might include complex formation, Coulomb forces, hydrogen bonds, or hydrophobic interactions. In the following chapters, we shall discuss basic structures and typical properties of physically cross-linked membranes, where coherent network structures were induced by aggregation processes. Amphiphilic Dye Molecules. Gel-like superstructures can be observed in adsorbed films if the molecules are strongly interacting. Typical examples are amphiphilic dye molecules, which tend to build up “Scheibe” or “Jelly” aggregates at fluid interfaces.28,29 A typical example of such a dye molecule is shown in Figure 1. The viscoelastic behavior of the ultrathin networks can be obtained from dynamic measurements. In this case, the shear strain is varied periodically with a sinusoidal alternation at an angular frequency ω. Relevant data of these experiments are summarized in Figure 2. The experimental curves exhibit typical properties of a generalized Maxwell material. In the regime of high frequencies, the two-dimensional storage modulus µ′(ω) attains a plateau value, and at these conditions the elastic response of the sample is dominant. It points out that µ′(ω) . µ′′(ω) for ω . 1. The plateau value describes the rubber-elastic properties of the sample. With decreasing angular frequency, the viscous properties become more important. In this regime, relaxation processes occur and the solution behaves as a liquid. The intersection point (28) Kuhn, H.; Mo¨bius, D. Angew. Chem. 1971, 17/18, 672. (29) O’Brien, D. F. Photogr. Sci. Eng. 1974, 18, 16.

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Figure 4. Two-dimensional foam structure of Span 65 during expansion; Γm ) 1.3 molecules/nm2.

Figure 3. The two-dimensional storage and loss modulus as a function of the angular frequency for a Span 65 film formed at the interface between chlorcyclohexane and water (c ) 0.5 mmol/L, T ) 20 °C).

where µ′(ω) ) µ′′(ω) is characterized by an average value of the relaxation time. This constant can be calculated using the simple relation

λ)

1 ωint

(1)

This relaxation time describes the dynamic features of ultrathin networks. In monolayers of surfactant or dye molecules, stress decay can only occur by rupture processes of junction points. We can, hence, conclude that the ultrathin networks exhibit striking dynamic properties. These structures are fluctuating; they are continuously built up and get destroyed by the formation and breaking process of cross-linking points. In such systems, the relaxation time describes an average lifetime or breaking time of the junction points. At experimental conditions where the frequency is short in comparison to the reciprocal lifetime, a junction point cannot open during one oscillatory cycle. In this regime, the adsorbed layer behaves as a permanent cross-linked network. A completely different structure can be observed for small angular frequencies. In this regime, there are numerous breaking and reformation processes within the time scale of observation. As a consequence, an applied shear stress will completely relax and a fluidlike behavior results. The intermediate frequency range is characterized by an ambivalent behavior where both processes occur simultaneously. This regime is characterized by striking viscoelastic properties. Networks Stabilized by Hydrogen Bonds. The surfactant Span 65, trioctadecanyl ester of sorbic acid, tends to form very stable emulsions. These special features lead to the frequent use of this surface-active compound for technical applications. Typical examples of these technologies include the formation of foams, microcapsules, or concentrated emulsions. Enhanced emulsion stabilities are often correlated with large values of the surface viscosity or elasticity.30 It is interesting to note that ultrathin films formed by adsorption of Span 65 molecules exhibit such viscoelastic features. Relevant data are summarized in Figure 3. Span 65 molecules are only soluble in organic solvents such as dodecane or chlorcyclohexane, and at the interface (30) Edwards, D. A.; Brenner, H.; Wasan, D. T. Interfacial Transport Processes and Rheology; Butterworth-Heinemann: Boston, 1991.

toward water a monomolecular film is instantaneously formed, due to the surface activity of this detergent.25 The rheological data of such films exhibit typical properties of a generalized Maxwell material, and we have observed the same behavior as already discussed with the dye molecules. In monolayers of surfactant molecules, stress decay can occur by rupture processes of junction points. We can, hence, conclude that the Span 65 networks exhibit striking dynamic properties. In comparison to the experiments at the waterdodecane interface,25 the plateau value here is much lower. This indicates that the density of junction points at the interface of water-chlorcyclohexane is lower than at the interface of water-dodecane. The experimental data of the concentration dependence of the surface shear modulus suggest that hydrogen bonds may be responsible for this special behavior. This seems, however, not to be the only source of cross-linking processes because the surfactant Span 60, a similar compound, does not show the formation of these supermolecular structures.25 The number of paraffin chains gives the main difference between these surface-active molecules: Span 65 has an average value of three, whereas Span 60 only has one of these hydrophobic chains. Since long paraffin chains are also interacting by van der Waals attractions, there might also be a combination of hydrogen bonds and hydrophobic forces, which finally leads to the observed dynamic cross-linking process.31 In addition to the rheological measurements, we investigated Span 65 by means of Brewster angle microscopy.25 During compression, the isotherm shows a typical pattern of a condensed film. An interesting phenomenon can be observed, when the film relaxed: starting from a surface concentration Γm < 3 molecules per nm2 the film tears off. The formed structure was similar to that of twodimensional foams. A typical picture of this special structure is summarized in Figure 4. Membranes Stabilized by Coulomb Interactions. If cationic or anionic surfactants are combined with multivalent counterions, then networks with rubber-elastic features are often formed at fluid interfaces.27,32 In this case, ionic surfactant molecules are cross-linked by oppositely charged counterions. After synthesizing the membranes, we studied the rheological properties at the oil/water and the air/water interfaces. The data of film formation at different interfaces were compared, and we did not observe significant differences. The films formed at the air/water interface were analyzed with the aid of an interfacial shear rheometer and subsequently with the Brewster angle microscopy images. Typical properties of (31) Olinga, A.; Winzen, R.; Rehage, H.; Siesler, H. W. J. Near Infrared Spectrosc. 2001, 9, 19. (32) Rehage, H.; Achenbach, B.; Kaplan, A. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1683.

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Figure 7. Brewster angle microscope image of AOT and aluminum(III)-sulfate (AOT, c ) 5 × 10-6 mol/L; aluminum(III)-sulfate, c ) 10-3 mol/L, pH ) 4; T ) 20 °C). Figure 5. The two-dimensional storage modulus µ′ and loss modulus µ′′ of AOT (bis(2-ethylhexyl)sulfosuccinate sodium salt) as a function of the time t (AOT, c ) 5 × 10-6 mol/L; counterions: aluminum(III)-sulfate, c ) 10-3 mol/L, ω ) 10 rad/s, deformation γ ) 0.1%, pH ) 4; T ) 20 °C).

Figure 8. Scheme of the investigated tweezers (n ) 0, Rd O-Ac).

Figure 6. The two-dimensional storage modulus µ′, loss modulus µ′′, and the magnitude of the complex viscosity |η*| as a function of the angular frequency ω (AOT, c ) 5 × 10-6 mol/L; aluminum(III)-sulfate, c ) 10-3 mol/L, deformation γ ) 0.1%, pH ) 4; T ) 20 °C).

these ultrathin membranes built by the anionic surfactant AOT and aluminum(III)-sulfate are summarized in Figure 5. These data provide information on the cross-linking reactions of such a film at the pure water surface. This experiment illustrates the fact that the cross-linking reaction is very slow. The moduli reach plateau values after 5 h of reaction time. At this stage, complete crosslinking is accomplished. Judging from the lower moduli, it appears that the AOT/ AlIII films are less cross-linked than their cationic pendants.25 The flow properties of these films can be analyzed by measuring both moduli as a function of the angular frequency ω at constant deformation (frequency sweep experiment). Figure 6 gives relevant information on this type of test. The almost constant values of both moduli indicate that the formation is independent of the frequency. The elastic properties, described by the storage modulus µ′, are much more pronounced than the energy dissipated as heat, which is represented by the loss modulus µ′′. The complex viscosity |η*| decreases with increasing angular frequency. A solid-analogue film is formed with few flow properties. If a critical stress is exceeded, the network will be destroyed. For further investigations of the structure, the networks were visualized by means of Brewster angle microscopy. In analogy to the flow cell of the rheometer, a static measuring cell was constructed. The film was observed in the gap between the plate and the glass dish.

According to the rheological measurements, a concentration series of AOT was studied. In the regimes of lower concentrations, a typical network structure of film was examined. Figure 7 shows an image of this network (AOT, c ) 5 × 10-6 mol/L). This image was observed after 2 h. We can detect both separate fragments and cross-linked areas. Ultrathin networks, stabilized by Coulomb forces, can be formed from a large number of anionic or cationic surfactants. It is interesting to note that all structures we have investigated so far exhibit similar properties. The basic network structure consists of disklike aggregates having typical diameters of several micrometers in the vicinity of the sol-gel transition. The aggregates are in close contact, and they are forming infinite large clusters of fractal geometry (see Figure 7). These networks have many pores, but with increasing surfactant concentration, the uncovered areas become much smaller and a homogeneous film structure is formed. As these membranes are easy to synthesize, they may be used in technical applications for the stabilization of emulsion droplets and foams. Molecular Clips and Tweezers. Molecular clips and tweezers tend to bind selectively electron deficient aromatic and aliphatic substrates. Syntheses and supermolecular properties are extensively described in several papers of Prof. Kla¨rner.40-43 The film-forming behavior of one of these molecular tweezers was investigated by pressure-area isotherms and Brewster angle microscopy at the air-water interface. A schematic drawing of those tweezers is shown in Figure 8. To prepare the spreading solution, the tweezers were dissolved in chloroform (p.a. Merck) at a concentration of 10-3 M. Figure 9 shows the surface pressure-area isotherm for a monolayer of the tweezers at 20 °C. To specify remarkable changes of the film properties, arrows indicate some interesting points. The corresponding Brewster angle microscopy images are shown in Figure 10. The isotherm shows a plateau regime at 7 × 10-3 N/m. Such a constant region is typical for a first-order phase transition from a low-density fluidlike to a condensed phase.

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decreases up to 0.5 nm2/molecule (images 10.3 and 10.4), the surface concentration of the clothespins increases. Image 10.4 indicates that these particles can also form two-dimensional networks. To characterize this phenomenon, rheological measurements were carried out. In addition to this, it should be possible to illuminate the tweezers’ orientation at the surface by further experiments. This work is still in progress. Figure 10.5 shows the film being totally compressed. The higher reflectivity of the film at the left side indicates the formation of multilayers.

Figure 9. Surface pressure-area isotherm of a monolayer of molecular tweezers at T ) 20 °C.

Figure 10. Brewster angle microscopy images of the five marked points.

The tweezers form a stable monolayer at the air-water interface, as indicated by the surface pressure/area isotherm (Figure 9). The observed reflectivity of the film is generally lower than that of multilayers. By extrapolation of the isotherm from higher pressures to zero pressure (35-45 mN/m), the size of the tweezers could be estimated assuming a dense packing of these amphiphilic compounds. In this way, the required tweezers space is approximately 0.5 nm2. For small surface pressures and large areas (see Figure 10.1), the tweezers tend to aggregate and form small needles with typical dimensions of approximately 35 µm. Most of them are oriented in the direction of film flow. Image 10.2 shows the results of the phase transition of the aggregates. Now the tweezers look like “clothespins” with an average particle length of 50 µm. If the area

Conclusions In this study, we discussed typical properties of twodimensional model networks. These structures were formed from tensio-active molecules at the pure water surface or at the interface between two immiscible fluids. In all films or membranes investigated so far, the adsorption process of single molecules induced the formation of well-defined aggregates. Hydrogen bonds, Coulomb interactions, van der Waals attraction forces, or steric interactions could enhance these association processes. In the vicinity of the two-dimensional sol-gel transition, the adsorbed aggregates had typical sizes of several micrometers, so that they could easily be observed by means of Brewster angle microscopy. This is in fairly good agreement with percolation theories, which predict the existence of large clusters or aggregates near phase transitions.33-39 With increasing concentration, the surfactant aggregates were cross-linked, and they finally formed coherent, two-dimensional network strands. The rheological properties of these stabilized films were analyzed at the plane interface using two-dimensional rheometers. It turns out that the elastic moduli were time dependent. Fluctuations and stress relaxation processes, induced by the limited average lifetime of junction points, could explain this striking phenomenon. Ultrathin networks exhibit interesting rheological properties, such as yield values and rubberlike and transient flow behavior. They can, hence, be used to stabilize emulsions, foams, or microcapsules, and this might be interesting for a large number of new technical applications. Acknowledgment. Financial support of this work by grants from the “Deutsche Forschungsgemeinschaft” SFB 1690 and the “Fonds der Chemischen Industrie” is gratefully acknowledged. Thanks are due to M. Madani and R. Ko¨nig for technical assistance. LA0255368 (33) Daboul, D.; Aharony, A.; Stauffer, D. J. Phys. A: Math. Gen. 2000, 33, 1113. (34) Heo, M.; Cheon, M.; Chang, I.; Stauffer, D. Int. J. Mod. Phys. C 1999, 10, 1059. (35) Stauffer, D.; Chang, I. J. Stat. Phys. 1999, 95, 503. (36) Cheon, M.; Heo, M.; Chang, I.; Stauffer, D. Phys. Rev. E 1999, 59, R4733. (37) Stauffer, D. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1672. (38) Jan, N.; Stauffer, D. Int. J. Mod. Phys. C 1998, 9, 341. (39) Stauffer, D. Physica A 1997, 242, 1. (40) Kamieth, M.; Burkert, U.; Corbin, P. S.; Dell, S. J.; Zimmermann, S. C.; Kla¨rner, F.-G. Eur. J. Org. Chem. 1999, 2741. (41) Kla¨rner, F.-G.; Benkhoff, J.; Boese, R.; Burkert, U.; Kamieth, M.; Naatz, U. Angew. Chem., Int. Ed. 1996, 35, 1130. (42) Kla¨rner, F.-G.; Burkert, U.; Kamieth, M.; Boese, R.; BenetBuchholz, J. Chem.sEur. J. 1999, 5, 1700. (43) Kla¨rner, F.-G.; Panitzky, J.; Preda, D.; Scott, L. T. J. Mol. Model. 2000, 6, 318.