Vibrational spectroscopy of a tetraureidocalix[4]arene based molecular capsule

Abstract

Structural models for self-assembled dimers composed of two urea calix[4]arenes which entrap benzene or cyclohexane are developed using Fourier transform infrared (FTIR) spectroscopy. Based on the host–guest ratio determined by ¹H NMR spectroscopy in solution, and confirmed for the solid state by a thermogravimetric analysis, it is possible to prove by a comparison of the FTIR data of host, guest, complex and model compounds, that the capsule is held together by a cyclic array of weak and strong hydrogen bonds between the urea units attached at the wide rim of the calixarenes. The dimerization of the two urea units leads to a loss of symmetry, and an averaged C₄ symmetrical arrangement is probable. Guest molecules, such as benzene or cyclohexane, are enclosed inside the container rotating fast on the IR timescale around a longitudinal axis of the guest. From the observed splitting of absorption bands upon dimerization and inclusion it follows that either two crystallographically independent types of capsules exist in the crystal lattice or that the guests are occupying two major orientations in the capsule. As indicated by a higher complexation induced shift for cyclohexane, this guest exhibits a tighter interaction with the host molecules compared to benzene.

---

Introduction

Molecular capsules based on calixarenes^1–3 or resorcinarenes can be formed by metal–ligand interactions^4,5 or by hydrogen bonds. Calix[4]arenes substituted by four urea functions at the wide rim represent an especially interesting example.^6–9 In apolar solvents they form dimeric capsules held together by a seam of hydrogen bonds between the interlocking urea moieties (Fig. 1). A guest molecule—often the solvent—is included in a cavity of about 200 ų and serves as a template for the capsule formation. Kinetically stable capsules in apolar solvents (e.g. benzene) were accessible using sterically demanding urea residues.⁸ The kinetic stability is in general drastically increased in cyclohexane¹⁰ in comparison to benzene, and in a special example it is observed also in DMSO.¹¹ Such (weak) molecular assemblies can be studied by single crystal structure determination in the solid state,^7,9 NMR spectroscopy in solution,^6,8,12,13 or mass spectrometry in the gas phase.¹⁴

We are interested in the development of alternative methods to determine structural models of calix[n]arene–guest complexes for those cases where a single crystal structure determination is not accessible.^15–17 Capsules based on tetra-urea calixarenes are interesting candidates for the use of a thorough Fourier transform infrared (FTIR) spectroscopic analysis. To gain a deeper insight into the solid state structure we have chosen for a first study dimeric capsules formed from calix[4]arene 1 with enclosed benzene or cyclohexane, respectively. Results gained by a FTIR spectroscopic analysis have already proved to give structural information for calixarene complexes in solution,^18–23 hydrogen bonded,^24–27 or surface-bound systems,^28,29 and structures in the solid state, which are comparable with results obtained by X-ray structure determination.^16,30

Results and discussion

Materials

The two complexes 1·C₆H₆·1 and 1·C₆H₁₂·1 were prepared as colourless powders by dissolving 1 in benzene or cyclohexane, respectively, evaporating the clear solution and drying the residue at 100 °C at <1 Torr. Monomeric 1 was obtained by recrystallisation of 1 from chloroform–methanol. From ¹H NMR spectra of 1·guest·1 measured in C₆D₁₂, where these capsules show a high kinetic stability,¹⁰ it could be concluded that the samples did not contain free guest (C₆H₆ or C₆H₁₂), and that one guest molecule (δ = 3.87 for benzene; δ = −1.44 for cyclohexane) is included per dimeric capsule.

Fig. 1 Tetraureido calix[4]arene 1 (R¹ = C₅H₁₁, R² = Me), Top right: schematic representation of the hydrogen bonds keeping the capsule together. Bottom: calculated structure for the dimer of tetraureido calix[4]arene 1 with included benzene (R¹ and R² are omitted for clarity).

This host–guest ratio was confirmed by thermogravimetric analysis.¹⁵ From the loss of weight during heating of the crystalline samples (Fig. 2) host–guest ratios of 2 ∶ 0.82 for cyclohexane and 2 ∶ 0.86 for benzene as guest molecule, respectively, can be deduced. This is in reasonable agreement with the data obtained by ¹H NMR spectroscopy in solution. Most guest molecules leave the crystal lattice when the samples are heated ca. 110–120 K above the boiling point of the free solvent (ΔT_bp). Similar ΔT_bp values can be found for p-tert-butylcalix[4]arene clathrates with THF (121 K), chloroform (121 K), or benzene (110 K) as guest molecules.¹⁵

At ca. 200 °C the interdigitating hydrogen bonds start to break and the entrapped guest molecules leave the interior of the capsule irrespective of their nature. Cooling the sample back to room temperature and re-heating up to 400 °C did not show any loss of weight, indicating that all guest molecules have left the capsule during the first heating cycle. The temperature interval ΔT in which the solvent is completely lost is somewhat broader for 1·C₆H₆·1 (83 K) compared to 1·C₆H₁₂·1 (36 K) and p-tert-butylcalix[4]arene·benzene (32 K). As expected, the loss of the entrapped molecule is endothermic (ΔH = 3.2 J g⁻¹ was determined by differential scanning calorimetry for 1·C₆H₁₂·1).

Fig. 2 Thermogravimetric analysis of calixarene-based capsules 1·C₆H₁₂·1 (a and b) and 1·C₆H₆·1 (c) (heating rate 10 K min⁻¹ under nitrogen, curve b) was obtained by heating 1·C₆H₁₂·1 up to 450 °C (a), cooling to room temperature and heating again to 470 °C.

General procedure for the vibrational analysis of calixarene-based molecular capsules

To gain a deeper insight into the structural properties of the urea-based capsules, FTIR spectra of monomer 1, the capsules 1·guest·1, and free guest were recorded both at room and at low temperature (80 K). The measurements at low temperature show better resolution of individual absorption bands because the molecular motion is decreased significantly at 80 K. After full assignment of all absorptions observed, structural information can be deduced by comparing the obtained spectra. Information about symmetry and interaction with the surrounding capsule of the included guests are accessible when the data of the free guest and signals observable for the guest in the dimers (1·guest·1) are compared (Tables 1 and 2). From the line width of significant absorptions conclusions for the dynamic behaviour of the guests can be drawn. In case the capsule itself comes into focus, a comparison of the signals observed for the calixarene monomer 1 and dimer 1·guest·1 instructs about type, strength and symmetry of intermolecular forces involved in the formation process of the capsule. Observed band splittings, changes of intensities and shifts of absorption are important experimental data for this purpose (Table 3).

Table 1 IR data (ν/cm⁻¹) and assignment of the N′-tolylureido capsule 1·C₆H₆·1 and benzene (sh = shoulder, w = weak)

T = 293 K	T = 80 K	Int.^a	C6H6	Assignment^33
a Interpretation of band splitting.b Superposition of guest and host bands.c Different orientations of the guest molecule in the crystal lattice/dimer.d Lifting of degeneracy.e Forbidden band becoming active.f Several components possible and/or combination tones with a degenerated component.
—	3075	^b	3090	ν2 + ν13 + ν18E1u
—	3054	^b	3070	ν13 + ν16A2u + E1u
3027 sh	3024	^b	3035	ν12E1u
—	1954	^b	1960	ν7 + ν19E1u
1477	1479	^b^,^c^,^d	1478	ν13E1u
	1471 sh
1387 sh	1386	^c^,^d^,^f	1385	ν2 + ν20E2u
	1378 sh
—	1285 sh		1308	ν9B2u
1244 sh	1243	^b^,^c^,^d^,^f	1245	ν11 + ν20E1u
1244	1224 sh
—	1171		1177	ν17E2g
1138 sh	1141 sh		1147	ν10B2u
—	1086	^e	1097 sh	ν4 + ν20E2g
1044	1043 sh	^b^,^c^e	1035	ν14E1u
	1031
1008 sh	1007 sh	^e	1008	ν6B1u or ν7B2g
850	857, 849 sh	^b^,^c^,^d^,^e	849	ν11E1g
—	793		793	ν17 − ν20A2u
673	679 sh, 671	^c^,^e	668	ν4A2u
—	592	^e	608 w	ν18E2g

---

Table 2 IR data (ν/cm⁻¹) and assignment of the N′-tolylureido capsule 1·C₆H₁₂·1 and cyclohexane³¹ (sh = shoulder, w = weak)

T = 293 K	T = 80 K	Int.^a	C6H12	Assignment
a Interpretation of band splitting.b Superposition of guest and host bands.c Different orientations of the guest molecule in the crystal lattice/dimer.d Lifting of degeneracy.e Forbidden band becoming active.f Several components possible and/or combination tones with a degenerated component.
3131	3130		3155	ν3 + ν19 + ν32Eu^b
				2ν19 + ν32Eu
3071	3075		3094	ν5 + ν19 + ν31Eu^b
2925	2919		2952	ν12A2u^b
—			2898	ν25Eu/ν1A1g
—	2881		2898	ν25Eu/ν1A1g^b
—	2846		2847	ν26Eu/ν2A1g^b
2661	2662		2660	ν21 + ν28Eu
2476	2476		2477	ν14 + ν22Eu/2ν21A1g + Eg
—	2365		2366	ν3 + ν30Eu/ν19 + ν30Eu
—	2128		2136	ν21 + ν31Eu
1446	1444	^c^,^d	1450	ν14A2u/ν27Eu^b
1429	1429
1386	1389	^c^,^d^,^f	1350	ν4 + ν32Eu^b
	1374
1243	1243	^c^,^d	1257	ν29Eu
1225	1226
1047	1050		1039	ν5 + ν32Eu^b
1012	1015	^c^,^e	1014	ν23 + ν32A2u + Eu^b
933	936	^c^,^e	941	ν15A2g
	928
903	906	^c^,^d	904	ν30Eu
	898
866	867	^c^,^d	862	ν31Eu
845	846
—	802		821	ν5A1g^b
781	785		806	ν23Eg
524	522		524	ν16A2uν25Eu/ν1A1g^b

---

Table 3 Selected IR data (ν/cm⁻¹) and assignment of the urea monomer 1, urea dimer 1·C₆H₆·1, and urea dimer 1·C₆H₁₂·1 [I_rel: estimated relative intensities of the members of a cluster (%-transmission)³⁰]

1		1·C6H6·1		1·C6H12·1
T = 293 K	Irel	T = 80 K	Irel	T = 80 K	Irel	Assignment^‡
3599		3599		3590		ν(NH)
3479	30	3482	30	3482	30
3339	100	3408	70	3408	70
3309	100	3354	100	3350	100
3221	70	3290	100	3283	80
3138		3203	70	3258	70
		3134		3226	65
				3196	60
				3134
3076		3081		3081		ν2; ν20a.(CH)
3030	100	3025	100	3025	100	ν20a; ν20b (CH)
3002	20	2998	20	2998	20	ν7b, ν20b; ν20a (CH)
2952		2952		2952		νas (CH3)
2926		2927		2927		νs (CH3)νas (CH2)
2869	100	2869	100	2869	100	νs (CH3)
2857	95	2859	95	2857	95
2760	50	2763	50	2763	50	ν13 + ν17
2730	100	2733	100	2731	100
1889		1889		1893		ν7 + ν19
1705 sh		1705	40	1705 sh	24	ν (CO)
		1682	65
1658		1666	100	1665	100
		1648	70
1602		1616	80	sh	80	β (NH), ν8a; ν8a
		1604	100	1608	100	β (NH)
		1584	80	sh	80	β (NH)
1553	100	1554	100	1558	100	β (NH)
1516	80	1512	80	1514	80	ν19a

---

Because it is not possible to rule out any hydrogen bonding between the urea units in the calixarene monomer 1 itself, a reference system based on model compounds 2–5 (Fig. 3) can be used to obtain complexation induced shifts (CIS) and estimate the strength of such hydrogen bonds in the monomer as well as in the dimer 1·guest·1 (Table 4). The chosen model substances 2–5 reflect important parts of the calix[4]arene 1 concerning bond strength and substitution pattern. In the latter case, the weight of the substituents must reflect the real situation rather than their actual chemical structure. Further details about the general procedure used for the vibrational analysis have been published earlier.³⁰

Table 4 IR data (ν/cm⁻¹) and assignment of the empty urea derivative 1 and its dimer 1·C₆H₆·1 for the calculation of interactions

1	1·C6H6·1	BS^a	CIS^b
(T = 293 K)	(T = 80 K)	|Δ|/cm^−1	|Δ|/cm^−1	Assignment
a BS: band splitting.b CIS = complexation induced shift; Δ = ν(1·C6H6·1) − ν(model substance).c 3221, 3309, 3339 are split into two components, e.g. 3221 into 3203 and 3290.
3599	3599	19–99^c	9–69	ν (NH)
3479	3482
3339	3408
3309	3354
3221	3290
3138	3203
	3134
1658	1705	58	8–65	ν (CO)
	1682
	1666
	1648
1602	1616	62	11–21 2	β (NH)
	1604			β (NH)
	1584		2	β (NH)
1553	1554			β (NH)
1418	1431	29	8–21	ν (C–N)
1402	1418
	1402
640	638	—	2	γ (NH)
	sh
619	619	—	5
	sh
601	601	37	4–33	γ (CO)
593	594
585	586
578	576		2
	564
553	553	—

---

According to the NMR data in solution,⁸ the dimers posses S₈-symmetry while the X-ray structure of a comparable urea capsule⁹ reveals C₄ symmetry. In case of deviation from this symmetry, C₂ or C₁ must be considered as the corresponding point groups. According to the vibrational spectroscopic analysis of cyclohexane,³¹ this guest belongs to the point group D_3d or has lower symmetry when the structure deviates from the chair conformation; benzene belongs to the point group D_6h. Hence it follows for included cyclohexane or benzene, respectively, that all vibrations are IR- and Raman active and all degeneracies are lifted.

Fig. 3 Model compounds 2–5 used for the vibrational analysis of the empty cavitand and molecular capsules.

Vibrational analysis of 1·C₆H₆·1

The FTIR spectra (KBr disks) obtained for 1·C₆H₆·1 are shown in Fig. 4 and the results of the analysis of these data are summarised in Tables 1 and 3.

Apart from the lifting of degeneracy, the splitting of several absorptions (e.g. 1245 → 1244, 1224 cm⁻¹, cf.Table 1) of included benzene is clearly observed.

Because benzene molecules are found only inside the capsules, there are two possible explanations for this observation. Either there are two independent orientations of the capsule in the crystal lattice or included benzene has two major orientations inside the cavity. Because the line width of the included benzene is similar to that of pure benzene and interactions between encapsulated benzene molecules are unlikely due to size-limitations, the band width is presumably a result of fast rotation of the guest molecules around their longitudinal axes at the two main orientations.

Fig. 4 IR spectra of benzene and 1·C₆H₆·1 (bottom) (T = 293 K, KBr disks).

Most frequencies of benzene are shifted downwards (e.g. 3090 → 3075). Large effects up to 16 cm⁻¹ for the C–H and 23 cm⁻¹ for the C=C valence vibrations are observed, while the average shift for all absorptions is about 11 cm⁻¹. Few frequencies are shifted to higher values with shifts up to 9 cm⁻¹. This can be explained as complexation-induced shift (CIS) by CH–π interactions³² of the benzene molecules in the inner cavity. For the 1 ∶ 1 complex of p-tert-butylcalix[4]arene with benzene a similar averaged CIS is observable (12 cm⁻¹).¹⁶

However, in this case a maximum shift of 26 cm⁻¹for the C–H and 30 cm⁻¹ for the C=C valence vibration, respectively, is observable indicating a somewhat stronger contact of benzene in the cavity of p-tert-butylcalix[4]arene. This difference can be rationalized by additional CH–π interactions of the tert-butyl groups of the p-tert-butylcalix[4]arene skeleton towards the included guest which increase the binding strength. Such an additional binding force is not possible for the urea–calixarene 1, because the urea moieties are involved in hydrogen bonds which hold the capsule together.

Comparison of IR data of the monomer and the dimers (Table 3) clearly reveals some significant shifts of the frequencies. As expected, the shift of the amide bands in the monomer is up to 69 cm⁻¹ and in the dimer up to 99 cm⁻¹, compared to model compounds.

Due to dimerization there is a splitting of some bands of the calixarene 1 into two components, e.g. the methyl group vibrations of the N-tolyl-ureido group and the vibrations of the pentyloxy group (e.g. 2926 → 2927 and 2869 cm⁻¹). The ratio of the split components is 1 ∶ 1. Because the splitting is mainly observed for molecular regions on the outside of the capsule, influences of the guest molecule are not likely. Therefore, the splitting is probably due to a C₄ symmetrical arrangement in which both calixarenes are not exactly twisted by 45° as required for a S₈ symmetrical arrangement. This is in accordance with the crystal structure of urea calixarene 1 (R¹ = CH₂CO₂Et) in which a twist of 43° was observed.⁹ This observation is diagnostic irrespective of whether a dimer or monomer is present.

For the calixarene monomer 1 two types of NH groups are present, i.e. a calixaryl-NH and a tolyl-NH. Because one expects symmetrical [ν_s (NH)] and anti-symmetrical [ν_as (NH)] vibrations for both types of amide band, involving these NH-groups in identical hydrogen bonds would lead to a signal set consisting of four components. However, each of these bands is split again into two, partly overlapping, components: one is shifted downwards (up to 19 cm⁻¹, 3309 → 3290 cm⁻¹), one upwards (up to 99 cm⁻¹, 3309 → 3408 cm⁻¹). This must be caused by the arrangement of the dimer and gives evidence for a weak and a strong hydrogen bond.

The included benzene has no further influence on the conformation and geometry of the dimer.

In summary, the observed complexation induced shifts and shifts derived from the comparison with model compounds 2–5 give evidence for two types of hydrogen bonds between the urea monomers as well as CH–π interactions between included benzene and the aromatic rings of the host molecule.

Vibrational analysis of 1·C₆H₁₂·1

The line width of the included cyclohexane is similar to that of benzene. Therefore, a similar dynamic behaviour (rotation) can be assumed.

Most frequencies of cyclohexane are shifted up to 33 cm⁻¹ down, with an average shift of about 14 cm⁻¹; few frequencies are shifted to higher values by up to 39 cm⁻¹ with an average shift of 13 cm⁻¹. This can be explained as complexation induced shift, which is clearly stronger compared to the inclusion of benzene. This may be due to the ca. 20% higher molecular volume of cyclohexane compared to benzene which results in a tighter fit of this guest inside the capsule. The different spatial fit of the two different guests inside the cavity is also reflected in solution. However, in the latter case a higher CIS could be detected for benzene (−3.38ppm) compared to cyclohexane (−2.88 ppm) by ¹H NMR spectroscopy.¹⁰ This is not a contradiction because the complexation-induced shift observed by NMR spectroscopy is mainly based on the fact that a guest molecule is located in the anisotropic cone of the phenyl rings, whereas CIS obtained by FTIR spectroscopy depends on the anisotropic fields induced by all surrounding functional groups.

Comparison of the monomer and the dimer shows, that there are some significant shifts in the frequencies. The shift of the amide bands is up to 99 cm⁻¹, the other shifts are up to 16 cm⁻¹.

All amide bands of the dimer are split into four components. These components are again split caused by the arrangement of the dimer (cf.1·C₆H₆·1). Because the C=O stretching mode of both capsules (1666 for C₆H₆ and 1665 cm⁻¹ for C₆H₁₂) is very similar but significantly different from the monomer (1658 cm⁻¹), one can assume that the hydrogen bonding towards this oxygen atom is of comparable strength in both dimeric systems. The included cyclohexane has no further influence on the conformation and geometry of the dimer. The splitting of the signals for both benzene and cyclohexane is therefore due to two different arrangements of the guest molecules in the cavity of the capsule or by two types of crystallographically independent capsules. On the basis of the experimental data available up to now, it is not possible to distinguish between the two explanations.

Again, due to the dimerization some bands are split into two components, and the complexation induced shifts obtained as before give evidence for two types of hydrogen bond between the calixarene monomers and CH–π interactions of the included cyclohexane to the aromatic rings of the calixarenes.

Some low lying vibrations which cannot be assigned to any of the modes of the subunits have to be characterised as specific for the calixarene capsule in toto. They are denoted as “calixarene ring mode”.

Conclusion

By the use of FTIR spectroscopy, the complexes 1·C₆H₆·1 and 1·C₆H₁₂·1 could be characterised. The capsules are held together by two types of hydrogen-bond bridges from two types of different urea NH groups, one showing strong and the other less favourable interactions as indicated by the splitting of the amide bands. The structures of the monomers and corresponding subunits in the dimer are nearly identical; the included guest molecules have little influence on the overall geometry of the surrounding non-covalently assembled container molecule. The mobility of benzene and cyclohexane, respectively, is similar taking the half-width of the corresponding absorption into consideration. Compared with 1·C₆H₆·1, the IR data of the capsule 1·C₆H₁₂·1 exhibit larger shifts, which suggests stronger interactions. Because the overall geometry of the capsule is nearly independent from the guest, which is rotating fast on the IR timescale, the splitting observed for the included solvent molecule is likely to be due to two main orientations of the guests inside the capsule. However, on the basis of the current experimental data, it remains unclear whether the guest molecules are located at two different sites in the capsule or two different types of capsules are existing in the crystal.

Three tables containing fully assinged IR data of tetraureido calix [4] arene 1, their clathrates with cyclohexane and benzene and e the corresponding model substances 2–5 as well as ¹H NMR spectra of the capsule 1·C₆H₁₂·1 in solution.

Experimental

The preparation of the calixarene 1 and the capsules have been described earlier.^8,10,34 Infrared spectra were recorded on a FTIR spectrophotometer IFS 113v (Bruker) using KBr windows and a DGTS detector, with a resolution of approximately 0.5 cm⁻¹. Raman spectra were recorded with the Raman-Laser spectrophotometer Dilor XY (multi- and single-channel detector, resolution 1 cm⁻¹) using an Ar-Laser (Coherent, exciting line 514.53 nm). The temperature of the samples was 293 K and 80 K, respectively. Host–guest ratios of the samples used for the IR analysis were determined by NMR spectroscopy¹⁰ and thermogravimetric analysis as described earlier.¹⁵

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (Scha 685/2-2, 2–4 and Bo523/14-1) and the Fonds der Chemischen Industrie. Generous support from Professor Dr G. Maas is gratefully acknowledged.

References

1. D. C. Gutsche, Calixarenes, in Monographs in Supramolecular Chemistry, ed. F. J. Stoddart, vol. 1, Royal Society of Chemistry, Cambridge, 1989.
2. D. C. Gutsche, Calixarenes Revisited, in Monographs in Supramolecular Chemistry, ed. F. J. Stoddart, Royal Society of Chemistry, Cambridge, 1998.
3. Calixarenes 2001, ed. Z. Asfari, V. Böhmer, J. Harrowfield and J. Vicens, Kluwer Academic Publishers, Dordrecht, 2001.
4. Z. Zhong, A. Ikeda, M. Ayabe, S. Shinkai, S. Sakamoto and K. Yamaguchi, J. Org. Chem., 2001, 66, 1002.
5. N. Cuminetti, M. H. K. Ebbing, P. Prados, J. de Mendoza and E. Dalcanale, Tetrahedron Lett., 2001, 42, 527.
6. Y. L. Cho, D. M. Rudkevich and J. Rebek, Jr., J. Am. Chem. Soc., 2000, 122, 9868.
7. A. Shivanyuk, V. Böhmer and E. F. Paulus, Angew. Chem., 1999, 111, 3091; A. Shivanyuk, V. Böhmer and E. F. Paulus, Angew. Chem., Int. Ed., 1999, 38, 2906.
8. M. O. Vysotsky, I. Thondorf and V. Böhmer, Angew. Chem., 2000, 112, 1309; M. O. Vysotsky, I. Thondorf and V. Böhmer, Angew. Chem., Int. Ed., 2000, 39, 1264.
9. O. Mogck, E. F. Paulus, V. Böhmer, I. Thondorf and W. Vogt, Chem. Commun., 1996, 2533.
10. M. O. Vysotsky and V. Böhmer, Org. Lett., 2000, 2, 3571.
11. M. O. Vysotsky, I. Thondorf and V. Böhmer, Chem. Commun., 2001, 1890.
12. O. Mogck, M. Pons, V. Böhmer and W. Vogt, J. Am. Chem. Soc., 1997, 119, 5706.
13. N. Chopra, R. G. Chapman, Y. F. Chuang, J. C. Sherman, E. E. Burnell and J. M. Polson, J. Chem. Soc., Faraday Trans., 1995, 91, 4127.
14. C. A. Schalley, R. K. Castellano, M. S. Brody, D. M. Rudkevich, G. Siuzdak and J. Rebek, Jr., J. Am. Chem. Soc., 1999, 121, 4568.
15. J. Schatz, F. Schildbach, A. Lentz and S. Rastätter, J. Chem. Soc., Perkin Trans. 2, 1998, 75.
16. J. Schatz, F. Schildbach, A. Lentz, S. Rastätter, J. Schilling, J. Dormann, A. Ruoff and T. Debaerdemaeker, Z. Naturforsch., Teil B, 2000, 55, 213.
17. J. Schatz, A. C. Backes and H.-U. Siehl, J. Chem. Soc., Perkin Trans. 2, 2000, 609.
18. B. T. G. Lutz, G. Astarloa, J. H. van der Maas, R. G. Janssen, W. Verboom and D. N. Reinhoudt, Vib. Spectrosc., 1995, 10, 29.
19. J. W. M. Nissink, H. Boerrigter, W. Verboom, D. N. Reinhoudt and J. H. van der Maas, J. Chem. Soc., Perkin Trans. 2, 1998, 1671.
20. J. W. M. Nissink, H. Boerrigter, W. Verboom, D. N. Reinhoudt and J. H. van der Maas, J. Chem. Soc., Perkin Trans. 2, 1998, 2541.
21. J. W. M. Nissink, H. Boerrigter, W. Verboom, D. N. Reinhoudt and J. H. van der Maas, J. Chem. Soc., Perkin Trans. 2, 1998, 2623.
22. B. Paci, G. Amoretti, G. Arduini, G. Ruani, S. Shinkai and T. Suzuki, Phys. Rev. B: Condens. Matter, 1997, 55, 5566.
23. R. G. Janssen, W. Verboom, B. T. G. Lutz, J. H. van der Maas, M. Maczka and J. P. M. van Duynhoven, J. Chem. Soc., Perkin Trans. 2, 1996, 1869.
24. J. A. Kanters, A. Schouten, E. Steinwender, J. H. van der Maas, L. C. Groenen and D. N. Reinhoudt, J. Mol. Struct., 1992, 269, 49.
25. B. Brzezinski, H. Urjasz and G. Zundel, J. Phys. Chem., 1996, 100, 9021.
26. L. Frkanec, A. Visnjevac, B. Kojic-Prodic and M. Zinic, Chem. Eur. J., 2000, 6, 442.
27. D. M. Rudkevich, Chem. Eur. J., 2000, 6, 2679.
28. W. C. Moreira, P. J. Dutton and R. Aroca, Langmuir, 1994, 10, 4148.
29. J. W. M. Nissink and J. H. van der Maas, Appl. Spectrosc., 1999, 53, 528.
30. J. Dormann, A. Ruoff, J. Schatz, O. Middel, W. Verboom and D. N. Reinhoudt, J. Phys. Org. Chem., 2001, 14, 707.
31. H. Takahashi and T. Shimanouchi, J. Mol. Spectrosc., 1964, 13, 43.
32. M. Nishio, M. Hirota and Y. Umezawa, The CH/π Interaction: Evidence, Nature and Consequences, Wiley-VCH, New York, Chichester, Weinheim, Brisbane, Singapore, Toronto, 1998, ch. 9, pp. 141–144.
33. G. Varsányi, Assignments for vibrational spectra of seven hundred benzene derivatives, London, 1974, vol. 1.
34. O. Mogck, V. Böhmer and W. Vogt, Tetrahedron, 1996, 52, 8489.

---

Footnote

† Electronic supplementary information (ESI) available: three tables containing fully assigned IR data of tetraureido calix[4]arene 1, its complex with cyclohexane and benzene, and the corresponding model substances 2–5 as well as ¹H NMR spectra of the capsules 1·C₆H₆·1 and 1·C₆H₁₂·1 in solution. See http://www.rsc.org/suppdata/p2/b1/b108055p/

---