9726
J. Phys. Chem. B 2001, 105, 9726-9731
Structural Stabilities and Self-Assembly of Cucurbit[n]uril (n ) 4-7) and Decamethylcucurbit[n]uril (n ) 4-6): A Theoretical Study Kyung Seok Oh, Jungjoo Yoon, and Kwang S. Kim* National CreatiVe Research InitiatiVe Center for Superfunctional Materials, Department of Chemistry, DiVision of Molecular and Life Sciences, Pohang UniVersity of Science and Technology, San 31 Hyoja-dong, Pohang 790-784, Korea ReceiVed: May 21, 2001
Relative stabilities of cucurbit[n]uril homologues (CB[n]; n ) 4-7) and decamethylcucurbit[n]uril homologues (DCB[n]; n ) 4-6) have been inspected with ab initio and density functional theory (DFT) calculations. Results show that CB[6] and CB[7] are more stable among CB[n] homologues and DCB[5] among DCB[n] homologues, which confirms the previous experiments. This is also supported in consideration of the puckering angles of the respective dimeric building units. Our investigation on the template effect in the formation of CB and DCB macrocyclic structure suggests that H3O+ is important in tethering the building units being added to the macrocyclic structure in the course of the reaction. The presence of acid tends to favor the structures of smaller-size homologues. Thus, in consideration of the acid contents in the experiments, our calculations explain why CB[6] and DCB[5] are the major products. Moreover, it explains why considerable amount of CB[7] could be obtained in the recent report in which the amount of CB[7] was one-third of the amount of CB[6]. In a similar fashion, DCB[6] is predicted to exist as a coproduct with DCB[5].
Introduction Cucurbit[6]uril1 (CB[6], Figure 1a) has been one of the most celebrated cavitand,2 well noted for its remarkable features in host-guest complexation, molecular recognition, molecular switch phenomena, and catalysis.3 The structurally rigid internal cavity provides a binding site with high specificity. Its chemical and physical properties on forming complexes with cations and organic molecules4-6 and its applications in the construction of rotaxanes and catenanes7-9 have been reported. The rigid nonadecacyclic cage structure with two carbonyl-fringed portals and hollow interior makes it extremely attractive for molecular recognition and molecular switches.3 Complexation experiments by Mock et al. proved that CB[6] forms 1:1 complexes with a number of alkylammonium salts where the guest is situated in the cavity of the host.10 The binding mainly results from iondipole interactions and hydrogen bonding between the ammonium groups and the carbonyl oxygen atoms of the host. The hydrocarbon cation of the guest is situated in the cavity favored by the hydrophobic effect, which was proved by 1H NMR spectroscopy. Ammonium salts with bulkier hydrocarbon moieties are discriminated since they do not fit into the cavity of CB[6] due to their size. In addition, Mock et al. used the complexation of alkylammonium ions by CB[6] to catalyze 1,3dipolar cycloadditions of ammonium substituted alkynes and alkyl azides.11 This pericyclic reaction is accelerated by a factor of 5.5 × 104 under the catalytic influence of CB[6]. Despite its fascinating features, however, unclearness of the origin of its thermodynamic and structural stability has hindered the discovery of other CB homologues. The self-assembly of CB[6] is still a matter of controversy. Mock and co-workers had originally proposed that CB[6] was the product of an acidinduced, thermodynamically controlled rearrangement of an initially formed macromolecular condensation product of gly* Corresponding author. Email:
[email protected].
Figure 1. (a) Cucurbit[6]uril and (b) Decamethycucurbit[5]uril.
couril and formaldehyde.1,3 They predicted that the hydronium ions at the carbonyl fringed portals provide the nuclei for assembly of the convex structure. However, it is so far the case that CBs can be synthesized using only sulfuric acid but not with other strong acids and, thus, has been suggested that the sulfate counterion might act as a template. Stoddart and coworkers synthesized decamethylcucurbit[5]uril (DCB[5], Figure 1b), which has raised the prospect of synthesizing a new family of CBs.15 Synthetic significance of the discovery of DCB[5] is that hydrochloric acid was used instead of sulfuric acid. Suppose that the synthetic mechanisms of CB[6] and DCB[5] are alike; then hydronium ion is the only possible cationic species available to tether the reactant molecules by their carbonyl oxygens. In addition, it is noteworthy that glycoluril itself acts as a chemical template in addition to being a reagent.12-14 In other words, the possibility for glycoluril to direct the formation of the product and participate in the macroscopic structure formation has been proposed, although in this case, the template is an integral part of the structure it helps to form. Recently, CB[5] (∼10%), CB[7] (∼20%), and CB[8] (∼10%) have been isolated and identified from the usual batch of product mixture containing CB[6], the major product (∼60%)0.16 It should be pointed out, however, that those CB[5], CB[7], and CB[8] were not intentionally synthesized but, rather, discovered
10.1021/jp011919n CCC: $20.00 © 2001 American Chemical Society Published on Web 09/07/2001
Cucurbit[n]uril and Decamethylcucurbit[n]uril within the batch suspected to be byproducts. Nevertheless, considering the relatively large amount, these homologues should not be ignored in the practical synthesis. On the contrary, the possibility for controlling the synthetic yield for each of these products should be recognized. To expand the scope of cucurbituril and supramolecular chemistry, we have investigated its energetic and geometric properties with the means of ab initio and density functional theory (DFT) calculations. Here, we report the molecular stabilities and self-assembly mechanism of CB[6] and DCB[5] and our prediction of the possible existence of other CB and DCB homologues. With ab initio and DFT calculations, energy profiles for various types of CB, DCB, and their H3O+ complexes are obtained to explain experimental preference for CB[6] and DCB[5] and to investigate how H3O+ acts as a template in the macrocyclic structure formation. The present work was actually done four years ago.17 However, due to the lack of any experimental evidence at the time, our theoretical results could not be reported. Now, we are able to belatedly shed light on this work, since the existence of three other homologues of CB[n] (n ) 5, 7, 8) in the batch of reaction products were isolated and characterized by another group (K. Kim and co-workers) only recently, 16 three years after our predictions. Thus, we report our studies on the formation of various homologues of CB[n] and DCB[n], in connection with our previous theoretical efforts in designing new functional molecular systems.18 Methods The energies and structures of CB[6], DCB[5], and their homologues, CB[n] (n ) 4, 5, 7) and DCB[n] (n ) 4, 6), were calculated with full geometry optimization using Hartree-Fock (HF) and density functional theory (DFT). HF and density functional calculations were performed using the suite of programs implemented in Gaussian 94.19 Becke’s threeparameter hybrid functional with the correlation functional of Lee, Yang, and Parr (B3LYP) was used for all DFT calculations. The general basis set 3-21G was used for geometry optimization with HF and DFT methods, and 6-31G* was used for singlepoint DFT with B3LYP/3-21G geometry. The geometries were optimized under the appropriate symmetry of each molecule, i.e., under Dnh symmetry. To directly compare the energetic stabilities between CB[n]s and DCB[n]s, we have devised the “unit” energy. It is entitled to the total energy of the structurally most unstrained structure of the repeating unit of CB[n] or DCB[n]. Specifically, it is defined as the difference of the total energies of monomeric glucouril (CB1, Figure 2a) and dimeric glucouril (CB2, Figure 2b), or of monomeric dimethylglucouril (DCB1, Figure 2c) and dimeric dimethylglucouril (DCB2, Figure 2d). These unit energies are therefore taken as the lowest possible energy that each moiety of a CB[n] or DCB[n] should have. By multiplying n to each unit energy, we can obtain the total energy value of an imaginary CB[n] or DCB[n] structure without the structural ring strain that are imposed on the real molecule.
E(total) ) n × E(building unit) + E(extra) The difference between the total energy of CB[n] and the n-fold unit energy, namely, E(extra), is directly related to the structural stability of CB[n] and DCB[n] and will be our criterion for the relative stability. This term includes the strain energy as well as electrostatic repulsion terms.20 To investigate template effects in the formation of a CB or DCB, two H3O+’s were
J. Phys. Chem. B, Vol. 105, No. 40, 2001 9727
Figure 2. Building units of cucurbit[n]uril (CB1 (a), CB2 (b)) and decamethylcucurbit[n]uril (DCB1 (c), DCB2 (d)).
positioned at each portal of each molecule. The same levels of calculations as for the uncomplexed CBs and DCBs were performed. Results and Discussion Stability of Cucurbit[n]uril and Decamethylcucurbit[n]uril. Table 1 lists the relative stabilization energies of CB[n] and DCB[n]. The results obtained by HF/3-21G and B3LYP/ 3-21G level optimization calculations show the energetic preferences for CB[6] and DCB[5]. Also, these two sets of calculations have similar relative energy values: CB[6] is lower than CB[4] by ∼20 kcal/mol and CB[5] by ∼5 kcal/mol, and DCB[5] is lower than DCB[5] by ∼10 kcal/mol. On the other hand, the relative energies of CB[7] and DCB[6] show that HF values are higher than DFT by ∼2-3 kcal/mol. The singlepoint calculations (B3LYP/6-31G*//B3LYP/3-21G) show slight deviation from the other two sets of calculations. CB[4], CB[5], and DCB[4] are higher than their counterparts by ∼2-6 kcal/mol, and most unexpectedly, the relative energy of CB[7] is lower than CB[6] by ∼2 kcal/mol. It should be pointed out, however, that from the cubic splines drawn on the relative stabilization energies of CB by different methods of calculations (Figure 3a), it can inferred that the missing optimization process in B3LYP/6-31G* single-point calculation has shifted the curve toward the CB[7] minimum. Such tendencies toward larger systems in the B3LYP calculations are also evident in DCB[n], and as a consequence, a shift of the cubic spline to the right is also clearly observed in Figure 3b. Analysis of the key structural features of CB[n], DCB[n] and their building units CB1, CB2, DCB1, and DCB2 has been made (Tables 2 and 3). The HF/3-21G and DFT/3-21G do not show much deviation from each other. The distance differences are within the order of magnitude of 0.01 Å, and the angles are within 0.1°, with some exceptions. The radius of the portal of CB[6] (∼3.5 Å) is in agreement with the experimental value. (Since the distances were measured with respect to the center of each atom, the calculated radius of the portal would be 3.5 Å minus the effective radius of carbonyl O. If van der Waals radius for O (∼0.7 Å) is considered, the diameter of the portal will be ∼5.6 Å, which is in agreement with the experimental value of ∼5.5 Å.1) The radii of the interior along the horizontal plane of symmetry are consistently greater than those of the portals by 1.5-1.7 Å, which confirms the barrel- or pumpkinlike shapes of all the optimized structures that we have obtained. The general trend of our calculations can be explained with the structural information. We have mentioned that the building units, CB1, CB2, DCB1, and DCB2, are the most unstrained
9728 J. Phys. Chem. B, Vol. 105, No. 40, 2001
Oh et al.
TABLE 1: Relative Stabilization Energies of CB[n] and DCB[n] (in kcal/mol)a,b cucurbit[n]uril HF/3-21G B3LYP/3-21G B3LYP/6-31G*c
decamethylcucurbit[n]uril
CB[4]
CB[5]
CB[6]
CB[7]
DCB[4]
DCB[5]
DCB[6]
21.1 19.5 25.5
4.10 5.20 7.41
0.00 0.00 0.00
2.10 0.05 -2.25
10.8 9.90 12.5
0.00 0.00 0.00
3.23 0.10 0.40
a All values are in kcal/mol b As described in the text, the stabilization energy is the E(extra) term, the difference between the total energy of a CB[n] or DCB[n] and n-fold the corresponding building unit energy. The CB unit energies (E(CB2) - E(CB1)) are -594.95490 au (HF/3-21G), -598.43997 au (B3LYP/3-21G), and -601.74825 au (B3LYP/6-31G*//B3LYP/3-21G). The DCB unit energies (E(DCB2) - E(DCB1)) are -672.59564 au (HF/3-21G), -676.64845 au (B3LYP/3-21G), and -680.37326 au (B3LYP/6-31G*//B3LYP/3-21G). The E(extra)’s were converted to kcal/mol units. c Single-point calculations with B3LYP/3-21G optimized structures (B3LYP/6-31G*//B3LYP/3-21G).
Figure 3. The relative stabilization energies of (a) cucurbit[n]urils and (b) decamethylcucurbit[n]urils plotted and fitted to cubic splines.
structures. They do not form a ring, but they have the structural features of the unitary moiety of CB[n] and DCB[n]. Therefore, inspections have been made on the structural changes that have been made by the ring formations. In Tables 2 and 3, the key structural features are lined up for simple and direct comparison. For CB[n], CB[6] shares the closest similarity with CB2 on the distance between the two adjacent carbonyl O’s at the portal (d(O-Oh)). The bond angles of CB2 are closely matched with both CB[6] and CB[7] and the vertical distance of the carbonyl O’s (d(O-Ov)) with CB[7]. DCB[n]s show similar features as well, in which d(O-Oh) of DCB2 is closely matched with that of DCB[5], angles with both DCB[5] and DCB[6], and d(OOv) with DCB[6]. This structural analysis gives explanation why the macrocyclic rings smaller than the naturally occurring CB[6] and DCB[5] are unstable by ∼5 and ∼10 kcal/mol, respectively. The structural strain is thus the largest factor in the energetic barrier in the formation of these smaller supramolecular systems. The possibility for glycouril or dimethylglycouril to direct the formation of the products and participate in the macroscopic geometry can be qualitatively ascertained because of this important role of ring strain involved. Mulliken population analysis gives the effective atomic charge densities on CB and DCB. In Table 4, we have listed the atomic
charges of the carbonyl O, carbonyl C, glycouril N, glycouril C, and methyl C of DCB. The values are similar, on the order of magnitude of 0.001 au for CB[5-7] and DCB[4-6]. On the other hand, those of the building units show considerable differences. For example, the charge at carbonyl O is reduced from -0.649 in CB1 to -0.624 in CB[6], and the charges at the carbonyl C and the nitrogens are increased from 1.118 and -0.919 in CB1 to 1.265 and -0.934 in CB[6], respectively. Also, the charge at the carbon of the bridging methylene group is slightly reduced from 0.133 in CB2 to 0.127 at CB[6]. This charge shift can be ascribed to the ring formation where the bridging and connection of the building units induce delocalization of the electron density held at the carbonyl double bonds. Indeed, DFT calculations show that the bond lengths of carbonyl C-O in CB’s increase by ∼0.2 Å from 1.21 Å in CB1 and CB2 to 1.23 Å in CB[5-7]. Likewise, similar observation is made in DCB’s, where carbonyl C-O bond increase by ∼0.1 Å from 1.22 Å in DCB1 and DCB2 to 1.23 Å in DCB[4-6]. Because of this overall similarities in structures and charge distributions, the total B3LYP/3-21G energies for CB[6] and CB[7] and for DCB[5] and DCB[6] differ only slightly by 0.05 and 0.10 kcal/mol, respectively. The B3LYP/6-31G*//B3LYP/ 3-21G calculations tend to give a slightly more stabilized energy for CB[7], possibly because of not carrying out geometry optimizations on them. The favorable energy for CB[7], comparable to CB[6] (second to CB[6] (∼60%) and twice the amount of CB[5] and CB[8] (∼10% each)), explains why the product yield of ∼20% from the batch of reaction product could be isolated. Consideration of Hydronium Ion as the Template. Meanwhile, consideration of H3O+ as the template in the formation of the supramolecular structure of CB[n] and DCB[n] has been made on the energetic basis. The relative binding energies of CB[n] and DCB[n] on H3O+ complexation are shown in Table 5, and the pictorial view of the structures are shown in Figure 4. For DCB[n]-H3O+ complexes, the binding energy of DCB[5] to H3O+ is lower than that of DCB[4] and DCB[6] complexes by more than 10 kcal/mol. Thus, in the formation of DCB[n], it seems very likely that H3O+ ions play the role of tethering the building units to form the macromolecular ring structure. Structurally, hydrogen bonding in DCB[5]-H3O+ complex is optimal. In the DCB[4]-H3O+ complex, H3O+ bears only one hydrogen bond with the carbonyl O’s because of the small size of the ring, and in DCB[6]-H3O+, the hydrogen bond length is longer that that of the DCB[5]-H3O+ complex by an average value of ∼0.17 Å. Also, the DCB[4] complex displays H3O+ that is too far off from the rim of DCB[4] molecule. In addition, DCB[5]-H3O+ complex has the most similar structure to the DCB1, which is well displayed by the distance between hydrogen bonded carbonyl O’s, implicating less distortion of
Cucurbit[n]uril and Decamethylcucurbit[n]uril
J. Phys. Chem. B, Vol. 105, No. 40, 2001 9729
TABLE 2: Structures of CB[n] and the Building Units (CB1, CB2) Predicted by HF/3-21G and DFT(B3LYP)/3-21G Calculationsa CB[5]
CB[6]
CB[7]
CB1
CB2
D5h
D6h
D7h
(Figure 2a)
(Figure 2b)
symm
HF
DFT
HF
DFT
HF
DFT
interior exterior d(O-Oh) d(O-Ov) ∠_ ∠COCtb ∠CCtC ∠NCNb
4.509 2.777 3.264 6.149 108.0 174.8 124.0 115.3
4.399 2.780 3.268 6.230 108.0 175.3 123.9 115.6
5.195 3.508 3.508 6.114 120.0 175.0 120.9 116.0
5.160 3.548 3.548 6.225 120.0 175.0 123.6 116.0
5.893 4.255 3.693 6.098 128.6 175.1 122.3 116.4
5.919 4.310 3.740 6.218 128.6 174.9 123.3 116.3
HF
DFT
5.826
5.90
178.5 116.4
178.2 116.5
HF
DFT
3.483 5.973 124.9 178.3 119.4 116.5
3.593 6.097 127.5 177.8 120.4 116.7
a Calculations were performed with 3-21G basis set. Distances are in Å; angles in deg. All measurements are done with respect to the center of each atom. b Ct is the center of two horizontal carbon at glycouril.
TABLE 3: Structures of DCB[n] and the Building Units (DCB1, DCB2) Predicted by HF/3-21G and DFT(B3LYP)/3-21G Calculationsa DCB[4]
DCB[5]
DCB[6]
DCB1
DCB2
D4h
D5h
D6h
(Figure 2c)
(Figure 2d)
symm
HF
DFT
HF
HF
DFT
HF
interior exterior d(O-Oh) d(O-Ov) ∠_ ∠COCt ∠CCtC ∠NCNb
3.633 2.129 3.011 6.237 90.0 172.6 126.0 114.6
3.666 2.085 2.949 6.288 90.0 174.0 125.1 115.0
4.406 2.773 3.260 6.110 108.0 174.3 121.9 116.0
4.444 2.782 3.271 6.200 108.0 174.8 122.1 116.1
5.175 3.494 3.494 6.050 120.0 174.9 120.1 116.9
5.217 3.532 3.532 6.165 120.0 175.0 120.9 116.8
a
HF
5.788 178.7 114.7
DFT
5.871 178.5 114.9
HF
DFT
3.285 5.945 115.6 177.9 117.8 116.8
3.383 6.058 118.2 177.7 118.6 117.2
See Table 2 for detailed descriptions of the notations.
TABLE 4: Mulliken Charges of CB[n], DCB[n] and Their Building Units Predicted by HF/3-21Ga cucurbit[n]uril
decamethylcucurbit[n]uril
CB[5]
CB[6]
CB[7]
CB1
CB2
DCB[4]
DCB[5]
DCB[6]
DCB1
DCB2
O Ccb N
-0.617 1.263 -0.937
-0.624 1.265 -0.934
-0.628 1.264 -0.931
-0.649 1.188 -0.919
-0.616 1.254 -0.934
-0.625 1.266 -0.936
-0.632 1.267 -0.933
-0.655 1.191 -0.918
Cbc Chd
0.123 0.333
0.127 0.333
0.129 0.332
0.291
-0.638 1.226 -0.925 -0.930 0.133 0.299 0.325
0.106 0.474
0.107 0.481
0.110 0.484
0.438
-0.541
-0.559
-0.578
-0.538
-0.643 1.229 -0.924 -0.930 0.115 0.452 0.468 -0.539 -0.568
Cme a
All charges are in atomic units (au). b Carbonyl carbon at the glycouril unit. c Carbon at the bridging methylene groups. d Carbon at the horizontal of the glycouril unit. e Carbon at the methyl group of dimethylglycouril.
TABLE 5: Relative Binding Energies of CB[n] and DCB[n] to H3O+ Ion (in kcal/mol)a cucurbit[n]uril HF/3-21G B3LYP/3-21G B3LYP/6-31G*b
decamethylcucurbit[n]uril
CB[4]
CB[5]
CB[6]
CB[7]
DCB[4]
DCB[5]
DCB[6]
1.51 -4.89 -5.93
-15.1 -18.9 -16.0
0.00 0.00 0.00
10.9 11.6 5.08
17.2 14.1 11.1
0.00 0.00 0.00
15.1 18.8 14.7
a The values were obtained by subtracting the total energy of the complex to the total energy of the uncomplexed CB or DCB (Table 1). The binding energy of CB[6]-H3O+ and DCB[5]-H3O+ were chosen as zero. b Single-point calculations with B3LYP/3-21G optimized structures.
the monomeric building unit on being combined to the macromolecular structure. However, for CB[n]-H3O+ complexes, the CB[5] complex turned out to have the largest binding energy, in all levels of calculations. The CB[5]-H3O+ binding strength is ∼17 kcal/ mol stronger than that of CB[6]-H3O+ and ∼26 kcal/mol stronger that that of the CB[7]-H3O+ complex. This result does not seem to be in connection with the relative energy of CB[n] that we obtained. Explanations for such result are, first, that the H3O+’s are positioned at the center of the ring in CB[5]
and CB[6] whereas they are driven aside in CB[7]. Consequently, CB[5] and CB[6] complexes have three hydrogen bonds, but CB[7] has only two. Also, the hydrogen bonding in CB[5] has shorter bond lengths (∼1.70 Å), which indicates stronger binding than that in CB[6] (1.87 Å). Second, comparison between the structures of each CB[n] with and without H3O+ shows that CB[6] undergoes large deformation, while the geometry of CB[5]-H3O+ complex is similar to that of the CB1, which indicates structural stability with least deformation of the building unit.
9730 J. Phys. Chem. B, Vol. 105, No. 40, 2001
Oh et al.
Figure 4. Top views of the structures of cucurbit[n]uril-H3O+ (n ) 5, 6, 7) and decamethycucurbit[n]uril-H3O+ (n ) 4, 5, 6) complexes.
conditions.16 Although this is only a crude and qualitative description of the actual process, we can see that the minimum of the energy curve is shifted toward CB[6] from CB[5]. Also, the flattening of the dotted curve explains why the isolation of CB[5], CB[6], CB[7], and CB[8] in the experiments was possible.16 For more accurate description, the supplementary role of the sulfate ion (not clear), entropy, and solvent effects could be included, although it is very unlikely that these terms are the determining factors. This reasoning is also applicable to DCB formation which both the relative stabilization energy and binding energy profiles does not show any ambiguity.
Figure 5. Plot of the relative stabilization energies (in kcal/mol) of CB[n] in CB[n]-H3O+ complexes (a) and DCB[n] in DCB[n]-H3O+ complexes (b). The bold lines represent the acid free energies of CB and DCB and thin lines the binding energy of CB and DCB to H3O+. The dotted lines represent the relative stabilities when about ∼20% (in consideration of mole percentage of the acid used in the experimental condition16) of the binding effect of H3O+ is taken into account.
To rationalize the templating role of H3O+ in the formation of CB, we must take into account both the relative stabilization energy of CB[n] and the H3O+ binding energy. In other words, we believe that H3O+ is initially used to tether the building units to combine into the macromolecular structure in the beginning of the reaction process. Subsequently, toward the end of the reaction, the stability of the CB itself takes over the reaction kinetics. This can be visualized in Figure 5a, where the dotted line is the energy curve in which about 20% of the templating effect of H3O+ bound to CB[n] is taken into account, in consideration of the mole percent of the acid in the experimental
Conclusion Relative stabilization energies of the homologues of CB[n] and DCB[n] have been inspected in order to explain the predominant preference for CB[6] and DCB[5] in the experiments. With our method employing building units and their energies, we were able to find out the relative stabilities of the corresponding macromolecular systems. As it turned out in calculations, CB[6] and CB[7] are the most stable homologues among various CB[n] and DCB[5] among various DCB[n], followed by DCB[6]. The structural analysis explains why the macrocyclic rings smaller than the naturally occurring CB[6] and DCB[5] are unstable by ∼5 and ∼10 kcal/mol, respectively. The structural strain is the largest factor in the energetic barrier in the formation of these smaller supramolecular systems. Moreover, energetically and structurally, CB[6] and DCB[5] were very similar to their singly higher homologues, CB[7] and DCB[6], respectively. This explains why CB[7] was isolated with about third of the amount isolated for CB[6] in the experiments. From this, the isolation of DCB[6] can also be expected from our results. In addition, our investigation on the templating role of H3O+ shows that it effectively leads to the formation of smaller ring structures. From this result, together with the relative stabilization energies of the final products, it is concluded that CB[6] and DCB[5] is the most favored ones among their respective homologues. Also, considering the experiments where no positively charged species other than H3O+ is available, we believe that H3O+ plays the role of templating in the formation of DCB.
Cucurbit[n]uril and Decamethylcucurbit[n]uril Acknowledgment. This work was supported by MOST(KISTEP)/CRI and the Korean Ministry of Education (Brain Korea 21 Program). References and Notes (1) Freeman, W. A.; Mock, W. L.; Shih, N.-Y. J. Am. Chem. Soc. 1981, 103, 7368. (2) Moran, J. R.; Karbach, S.; Cram, D. J. J. Am. Chem. Soc. 1982, 104, 5826. (3) Mock, W. L. Top. Curr. Chem. 1994, 17, 205 and references therein. (4) Hoffmann, R.; Knoche, W.; Fenn, C.; Buschmann, H.-J. J. Chem. Soc., Faraday Trans. 1994, 90, 1507. (5) Buschmann, H.-J.; Cleve, E.; Schollmeyer, E. Inorg. Chim. Acta 1992, 193, 93. (6) Buschmann, H.-J.; Jansen, K.; Schollmeyer, E. Thermochim. Acta 1998, 317, 95. (7) Stoddart, J. F.; Nepogodiev, S. A. Chem. ReV. 1998, 98, 1959. (8) Tuncel, D.; Steinke, J. H. J. Chem. Soc., Chem. Commun. 1999, 1509. (9) Roh, S.-G.; Park, K.-M.; Park, G.-J.; Sakamoto, S.; Yamaguchi, K.; Kim, K. Angew. Chem., Int. Ed. Engl. 1999, 38, 638. (10) Mock, W. L.; Shih, N.-Y. J. Org. Chem. 1983, 48, 3618. (11) Mock. W. L.; Irra, T. A.; Wepsiec, J. P.; Manimaran, T. L. J. Org. Chem. 1983, 48, 3619. (12) Busch, D. H. J. Incl. Phenom. 1992, 12, 389. (13) Anderson, S.; Anderson, H. L.; Sanders, J. K. M. Acc. Chem. Res. 1993, 26, 469.
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