Mechanism of Orientational Isomerism of Unsymmetrical Guests in a

Apr 6, 2010 - The large electron correlation contributions to the attraction (−27.0 to −31.8 kcal/mol) show that the dispersion interactions are t...
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J. Phys. Chem. B 2010, 114, 5335–5341

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Mechanism of Orientational Isomerism of Unsymmetrical Guests in a Heterodimeric Capsule: Analysis by Ab Initio Molecular Orbital Calculations Seiji Tsuzuki,*,† Tadafumi Uchimaru,† Masuhiro Mikami,† Hitomi Kitagawa,‡ and Kenji Kobayashi‡ National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan, and Department of Chemistry, Faculty of Science, Shizuoka UniVersity, 836 Ohya, Shizuoka 422-8522, Japan ReceiVed: February 4, 2010; ReVised Manuscript ReceiVed: March 16, 2010

The geometries and interaction energies of the heterodimeric capsule [tetrakis(4-hydroxyhphenyl)-cavitand 1 and tetra(4-pyridyl)-cavitand 2] complexes with methyl p-acetoxybenzoate 3, methyl p-ethoxybenzeoate 4, and p-ethoxyiodobenzene 5 were studied by ab initio molecular orbital calculations. The optimized structures and charge distributions of the complexes suggest that the electrostatic interactions of oxygen atoms in the guest molecules with the hydrogen atoms of aromatic rings and methylene-bridge rim in the heterodimeric capsule stabilize the complexes. The calculated relative energies of the two orientational isomers of the complexes well reproduce the experimentally observed orientational selectivity of the guest molecules. The calculated stabilization energies for the major orientational isomers of the heterodimeric capsule (1 · 2) complexes with guest molecules (3, 4, and 5) are -21.6, -19.6, and -19.4 kcal/mol, respectively. Those for the minor orientational isomers are 1.5, 3.5, and 3.7 kcal/mol smaller (less negative), respectively. The magnitude of the calculated energy differences agrees well with the order of the experimental population of the major orientational isomer (3 < 4 ∼ 5). The large electron correlation contributions to the attraction (-27.0 to -31.8 kcal/mol) show that the dispersion interactions are the major source of the attraction in the complexes, while the electrostatic interactions (-4.9 to -12.5 kcal/mol) are also an important source of the attraction. Although the electrostatic interactions are weaker than the dispersion interactions, the highly orientation dependent electrostatic interactions mainly determine the orientation of the unsymmetrical guest molecules in the complexes. The electrostatic interactions in the major orientational isomer are 2.6-3.9 kcal/mol larger (more negative) than those in the minor orientational isomer, while the differences of other energy terms are small (less than 1.1 kcal/mol). The interaction energies calculated for model complexes show that the CH/π interactions are not playing important roles in controlling the orientation of the guest molecules in the complexes. Introduction The creation and use of self-assembling nanospaces attract considerable interest, since a broad range of applications are expected.1-15 The control of the orientation of guest molecules in nanospaces is one of the most exciting challenges.16-24 The stereoisomerism upon a guest encapsulation in self-assembling capsules offers a new concept in physical chemistry as well as supramolecular chemistry.25 A tetrakis(4-hydoxyphenyl)-cavitand 1 and a tetrakis(4-pyridil)-cavitand 2 self-assemble into a heterodimeric capsule 1 · 2 via four ArOH · · · pyridyl hydrogen bonds in CDCl3.26-28 The 1 · 2 expresses the orientational isomerism of an encapsulated unsymmetrical guest with highly orientational selectivity (Figure 1). The selectivity of two orientational isomers was extensively studied using several unsymmetrical guests molecules. The influence of CH/π, CH/ halogen, and halogen/π interactions on the orientational selectivity was discussed.26-28 Despite the extensive studies on the orientatinal selectivity, only little was known on the details of the intermolecular interactions of the heterodimeric capsule complexes with guest molecules, which control the orientational selectivity. A quan* To whom correspondence should be addressed. E-mail: s.tsuzuki@ aist.go.jp. † AIST. ‡ Shizuoka University.

titative evaluation of the intermolecular interactions is necessary for understanding the mechanism of the orientational selectivity. It is still not an easy task to reveal the details of the intermolecular interactions by experimental measurements only. Ab initio molecular orbital calculation is a powerful tool for studying intermolecular interactions. The binding energies calculated for small clusters agree well with the experimental binding energies in the gas phase, if a sufficiently large basis set is used and electron correlation is properly corrected.29,30 Ab initio calculations provide detailed information on the intermolecular interactions (the magnitude and directionality of the interaction energy and the origin of the attraction). Unfortunately, however, accurate ab initio calculations of the intermolecular interactions of the heterodimeric capsule complexes with guest molecules were not reported. An accurate evaluation of the interaction energies for large systems such as the heterodimeric capsule complexes was difficult, since ab initio calculations of large systems are highly computationally demanding. The CPU time for an ab initio calculation with the MP2 level electron correlation correction31,32 is approximately proportional to the fifth power of the number of basis functions. In this work, we will show that ab initio calculation is a very powerful tool for studying the mechanism of the stereoisomerism of guest encapsulation. We studied the interaction energies of the heterodimeric capsule (1 · 2) complexes with guest molecules

10.1021/jp101111n  2010 American Chemical Society Published on Web 04/06/2010

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Tsuzuki et al. Edef was calculated at the HF/6-31G* level. Eint was calculated according to eq 2

Eint ) EHF + Ecorr

(2)

where EHF denotes the calculated interaction energy between the heterodimeric capsule and the guest molecule at the HF level. Ecorr is the electron correlation contribution (the difference between the MP2 and HF level interaction energies) to the interaction energy, which was calculated according to eq 3

Ecorr ) Ecorr(1) + Ecorr(2)

Figure 1. (a) Orientational isomerism of an unsymmetrical guest within a heterodimeric capsule. (b) Self-assembly of 1 and 2 into a heterodimeric capsule. (c) Unsymmetrical guest molecules.

(p-acetoxybenzoate 3, methyl p-ethoxybenzeoate 4, and pethoxyiodobenzene 5) by ab initio calculations. The relative stability of the two orientational isomers was evaluated. The contributions of the electrostatic and dispersion interactions to the attraction were studied for understanding the origin of the orientational selectivity. We also discussed the roles of the CH/π interactions in controlling the orientation of the guest molecules in the complexes.

(3)

where Ecorr(1) denotes the electron correlation contribution to the interaction energy between 1 and the guest molecule and Ecorr(2) denotes that between 2 and the guest molecule.43-45 The electrostatic energy was calculated as the interactions between distributed multipoles44,46 of the interacting heterodimeric capsule and the guest molecule using ORIENT version 3.2.47 Distributed multipoles up to dipole on hydrogen atoms and hexadecapole on all other atoms were obtained from the HF/6-31G* level wave functions of the isolated heterodimeric capsule and the isolated guest molecules using the GDMA program.48 The distributed multipoles were used only to estimate the electrostatic energies. The intermolecular interaction energies for the model complexes shown in Figure 2 were calculated using the MP2/6311G** level optimized geometries of isolated molecules. The interacting C-H bond was on the C6 axis of the benzene. The interaction energy potentials were calculated by changing the intermolecular distances without further geometry optimizations. The intermolecular distance (R) is the distance between the centroid of benzene and the carbon atom of the interacting C-H bond. The MP2 level interaction energy at the basis set limit [EMP2(limit)] was estimated by Helgaker’s method49 from the MP2 level interaction energies calculated using the aug-cc-pVDZ and aug-cc-pVTZ basis sets. The CCSD(T) level interaction energy at the basis set limit [ECCSD(T)(limit)] was

Computational Method The Gaussian 03 program33 was used for the ab initio molecular orbital calculations. The geometries of the heterodimeric capsule complexes with guest molecules, an isolated heterodimeric capsule, and isolated guest molecules were optimized at the HF/6-31G* level. The DGDZVP basis set34,35 was used for iodine atoms.36 The intermolecular interaction energy between the heterodimeric capsule and the guest molecule (Eint) was calculated by the supermolecular method using the same basis set. The basis set superposition error (BSSE)37 was corrected by the counterpoise method38 in the interaction energy calculations. Electron correlation was accounted for by the second-order Mφller-Plesset perturbation (MP2) method.30-32,39-42 The stabilization energy by the formation of the complex from an isolated heterodimeric capsule and a guest molecule (Eform) was calculated according to eq 1

Eform ) Eint + Edef

(1)

where Edef denotes the sum of the increases of energies of the heterodimeric capsule and guest molecules by the deformation of molecular geometries associated with the complex formation.

Figure 2. Geometries of model complexes for studying CH/π interactions.

Orientational Isomerism of Unsymmetrical Guests

J. Phys. Chem. B, Vol. 114, No. 16, 2010 5337 TABLE 1: Distance between the Oxygen Atom of the Guest Molecule and the C-H Hydrogen Atom of the Heterodimeric Capsule in Geometries Optimized by Ab Initio Calculationsa major b Ar CsHd benzoate CdO benzoate sOs acetoxy CdO acetoxy sOs

2.206 2.691 2.457 2.603

rim CsHe

p-acetoxybenzoate 2.249 2.270 2.801 2.994 2.512 2.428 3.098 2.932

Ar CsHd 3 2.648 2.845 2.318 2.549

rim CsHe

2.759 3.172 2.447 2.657

2.267 3.214 2.362 3.136

4 2.497 2.813 2.916 3.134 2.506 2.667

2.174 3.253 3.165

p-ethoxyiodobenzene 5 2.916 2.934 3.377 2.588 2.588

3.302

p-ethoxybenzoate benzoate CdO 2.301 2.722 2.159 benzoate sOs 2.608 2.701 3.186 ethoxy sOs 2.801 2.853 3.134 ethoxy sOs

minor c

a Distance in Å. Geometries for complexes were optimized by ab initio calculations. See text and Figure 3. b Distance in the major orientational isomer. See text. c Distance in the minor orientational isomer. See text. d Distance between the oxygen atom and β-position hydrogen atoms of two aromatic rings (pyridine or phenol rings). e Distance between the oxygen atom and inner hydrogen atom of the methylene-bridge rim.

Figure 3. Geometries optimized for major and minor orientational isomers of three complexes. (a, b) Major and minor orientational isomers of (3)@(1 · 2). (c, d) Major and minor orientational isomers of (4)@(1 · 2). (e, f) Major and minor orientational isomer of (5)@(1 · 2).

obtained as the sum of EMP2(limit) and a CCSD(T) correction term [∆CCSD(T)].42 The ∆CCSD(T) is the difference between the CCSD(T) and MP2 level interaction energies. The ∆CCSD(T) was obtained using the cc-pVDZ basis set, since it was reported that the basis set dependence of ∆CCSD(T) for CH/π clusters is not large, and sufficiently accurate ECCSD(T)(limit) can be obtained from the ∆CCSD(T) calculated with a medium-size basis set.42,50 The electrostatic energies in the model complexes were calculated using the distributed multipoles obtained from the MP2/6-311G** level wave functions of isolated molecules. Results and Discussion Optimized Geometries for Heterodimeric Capsule Complexes with Guest Molecules. The geometries optimized for the major and minor orientational isomers of the three complexes were analyzed. The carbonyl oxygen atom of the benzoate group in 3 has close contact with the C-H bonds at β-positions of two pyridine rings and the inner C-H bond of a methylene-

bridge rim (O-CHinHout-O) of 2 in the optimized geometry of the major orientational isomer of the complex (3)@(1 · 2), as illustrated in Figure 3a. The three C-H bonds point toward the oxygen atom. The O · · · H distances are 2.2-2.3 Å, which are substantially shorter than the sum of the van der Waals radius of oxygen and hydrogen atoms, as summarized in Table 1. The O · · · H distances of the carbonyl oxygen atom of the benzoate are longer (2.2-2.8 Å) in the minor orientational isomer (Figure 3b). The other three oxygen atoms of 3 also have close contact with the two C-H bonds on the β-positions of pyridine or phenol rings and the inner C-H bond of a methylene-bridge rim in the complexes (Figure 3a and b). The O · · · H distances are 2.3-3.2 Å. Similar O · · · H contact was also found in the geometries optimized for the complexes of 4 and 5 (Figure 3c-f). The O · · · H contact was also observed in the heterodimeric capsule complex of 4 in the crystal.27 The short contact suggests that the attraction between the oxygen atoms and the C-H bonds stabilizes the complexes. The atomic charge distributions of the heterodimeric capsule and the guest molecules were calculated by electrostatic potential fitting using Kollman’s scheme.51-53 The atomic charge distributions suggest that the attractive electrostatic interactions of oxygen atoms in guest molecules with the C-H bonds of the heterodimeric capsule stabilize the complexes. Stabilization Energies by Formation of Complexes. The relative stability of the two orientational isomers calculated for the complexes well reproduces the experimentally observed orientational selectivity of the unsymmetrical guest molecules in the complexes. The stabilization energies by formation of the complexes (Eform) from an isolated heterodimeric capsule and guest molecules were calculated for the two orientational isomers. The Eform values for the major orientational isomers are always larger (more negative) than those for the minor orientational isomers, as shown in Table 2.54 The ∆Eform values (the difference between the Eform values for the two orientational isomers) calculated for the three complexes well explain the experimental population of the two orientational isomers. The ∆Eform values calculated for the complexes of 3, 4, and 5 are 1.5, 3.5, and 3.7 kcal/mol, respectively, as summarized in Table 3. The experimentally

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TABLE 2: Interaction Energy between the Heterodimeric Capsule and the Guest Molecule and Stabilization Energy by Formation of a Complexa Eintb

orientation 3 3 4 4 5 5

major minor major minor major minor

EHFc

Eesd

Erepe

Ecorr f

-27.0 3.2 -12.5 15.7 -30.3 -24.4 6.7 -8.6 15.3 -31.1 -24.2 7.6 -7.8 15.4 -31.8 -20.6 10.7 -5.2 15.9 -31.3 -21.6 6.3 -8.0 14.3 -27.9 -18.0 9.0 -4.9 13.9 -27.0

Edef g

Eformh

5.4 4.4 4.6 4.5 2.2 2.3

-21.6 -20.1 -19.6 -16.1 -19.4 -15.7

a Energy in kcal/mol. Geometries for complexes are shown in Figure 3. b Total interaction energy. Eint ) EHF + Ecorr. c Interaction energy calculated at the HF level. BSSE was corrected by the counterpoise method. d Electrostatic energy. See text. e Erep ) EHF Ees. Erep is mainly exchange-repulsion energy. f Electron correlation contribution to total interaction energy. Ecorr is mainly dispersion energy. Ecorr ) Ecorr(1) + Ecorr(2). Ecorr(1) and Ecorr(2) are shown in Table 1S in the Supporting Information. g Sum of deformation energies of the heterodimeric capsule and guest molecule associated with complex formation. h Stabilization energy by formation of a complex. Eform ) Eint + Edef.

TABLE 3: Difference of Each Energy Term of the Minor Oritntational Isomer from That of the Major Orientational Isomera complex

∆Eintb

∆Eesc

∆Erepd

∆Ecorre

∆Edef f

∆Eformg

3 4 5

2.6 3.6 3.6

3.9 2.6 3.1

-0.4 0.5 -0.4

-0.9 0.5 0.9

-1.1 -0.1 0.1

1.5 3.5 3.7

a Energy in kcal/mol. b Difference of Eint (total interaction energy). c Difference of Ees (electrostatic energy). d Difference of Erep (mainly exchange-repulsion energy). e Difference of Ecorr (mainly dispersion energy). f Difference of Edef (deformation energy). g Difference of Eform (stabilization energy by formation of a complex).

observed population of the major and minor orientational isomers for the complex of 3 in CDCl3 at 23 °C is 1:0.11. The population of the minor isomer of the complex of 4 is less than 5% of the population of the major isomer. Only the major isomer was found in the complex of 5.26 The observed population of the two orientational isomers shows that the ∆Eform values for the complexes of 4 and 5 are larger than that for 3. The relative stability of the orientational isomers of the complexes can be explained by the HF level interaction energies (EHF). The EHF values for the major orientational isomers are always lower than those for the minor orientational isomers. This suggests that the electrostatic interactions are mainly responsible for the orientational selectivity of the unsymmetrical guest molecules, since the HF level interaction energies are mainly the exchange-repulsion and electrostatic energies. Figure 4 shows the orientation of the guest molecules in the major orientational isomers of the complexes. The dipole moment of the heterodimeric capsule and those of the guest molecules are shown by arrows. The dipole moments of the heterodimeric capsule and guest molecule always have an antiparallel orientation in the major orientational isomers of the complexes, which suggests that the electrostatic interactions mainly control the orientation of the guest molecules in the complexes. The Eform values calculated for the complexes are -15.7 to -21.6 kcal/mol, which shows that strong attraction exists between the heterodimeric capsule (1 · 2) and the guest molecules (3-5) in the complexes. Although the Eform values calculated for the complexes in the gas phase are significant, the stabilization energies for the formation of complexes in CDCl3 solution should be substantially smaller due to the contributions of the

Figure 4. Orientation of unsymmetrical guest molecules in major orientational isomers. Dipole moments of the heterodimeric capsule and guest molecules are shown by arrows.

desolvation energies of the heterodimeric capsule and guest molecules (the interaction energies with solvent molecules). The interaction energy between a benzene and a CHCl3 is significant. The binding energy for the benzene-CHCl3 cluster measured in the gas phase is 5.2 ( 0.2 kcal/mol.55 The binding energy obtained by ab initio calculations is 5.0 kcal/mol.55,56 The Eform value calculated for the major orientational isomer of the complex of 3 (-21.6 kcal/mol) is larger than those for the complexes of 4 and 5 (-19.6 and -19.4 kcal, respectively), which shows that the complex of 3 is more stable than the complexes of 4 and 5 in the gas phase. Effects of Basis Set and Electron Correlation. The effects of basis set used for the geometry optimizations of the complexes on the calculated interaction energies of the complexes were studied for evaluating the accuracy of the calculated interaction energies. In our preliminary calculations, the geometries of the complexes were optimized at the HF level using a smaller mixed basis set, which is the 3-21G basis set for carbon and hydrogen atoms, the 6-31G* basis set for nitrogen and oxygen atoms, and the DGDZVP basis set for iodine atoms. The HF/6-31G* level interaction energies of the complexes were calculated (the DGDZVP basis set was used for iodine atoms) using the optimized geometries. The Eint values calculated for the major orientational isomers of the complexes of 3, 4, and 5 are -26.8, -23.8, and -21.9 kcal/mol, respectively. Those for the minor orientational isomers are -24.4, -20.6, and -17.9 kcal/mol, respectively. These values are close to the Eint values obtained using the HF/6-31G* level optimized geometries (-27.0, -24.2, -21.6, -24.4, -20.6, and -18.0 kcal/mol, respectively). The ∆Eint values for the three complexes obtained from the optimized geometries using the mixed basis set are 2.3, 3.2, and 4.0 kcal/mol, respectively, which are also close to those obtained from the HF/6-31G* level optimized geometries (2.6, 3.6, and 3.6 kcal/mol, respectively). The very small differences (less than 0.4 kcal/mol) show that the effects of basis set used for the geometry optimizations on the calculated interaction energies of the complexes are small and suggest that further improvement of the basis set used for the geometry optimizations does not largely change the ∆Eint values. The geometry for the major orientational isomer of the complex of 3 was optimized at the PW91/6-31G* level for evaluating the effect of electron correlation on the geometry optimization. EHF was calculated at the HF/6-31G* level for the PW91/6-31G* level optimized geometry. EHF (3.4 kcal/mol) is very close to that calculated using the HF/6-31G* level optimized geometry (3.2 kcal/mol). These results suggest that the HF/6-31G* level optimized geometries are sufficiently accurate for evaluating the interaction energies of the complexes.

Orientational Isomerism of Unsymmetrical Guests The EHF values for the major and minor orientational isomers of the complex of 3 were also calculated at the HF/6-311G* and HF/6-311G** levels using the optimized geometries at the HF/6-31G* level for evaluating the effects of basis sets used for the interaction energy calculations. The EHF values for the two isomers calculated at the HF/6-311G* level are 2.0 and 5.9 kcal/mol, respectively. Those at the HF/6-311G** level are 1.7 and 5.5 kcal/mol, respectively. The ∆EHF values (the energy of the minor isomer relative to that of the major isomer) calculated at the HF/6-311G* and HF/6-311G** levels are 3.9 and 3.8 kcal/mol, respectively. These values are close to that calculated at the HF/6-31G* level (3.5 kcal/mol). The very small differences suggest that further improvement of the basis set used for the interaction energy calculations does not largely change the ∆EHF values and that the relative stability of the two isomers can be evaluated sufficiently accurately by the HF/ 6-31G* level interaction energies. Role of Electrostatic and Dispersion Interactions. The contributions of the electrostatic and dispersion interactions were evaluated for understanding their roles in the stabilization of the complexes and the orientational selectivity of the guest molecules in the complexes. The electrostatic (Ees) and repulsion (Erep) energies and the electron correlation contributions to the interaction energies (Ecorr) in the heterodimeric capsule (1 · 2) complexes with the guest molecules (3-5) (Figure 3) are summarized in Table 2. Ecorr is mainly dispersion energy. Erep () EHF - Ees) is mainly exchange-repulsion energy, but it also includes other terms such as induction energy. The large (negative) Ecorr values (-27.0 to -31.8 kcal/mol) show that the dispersion interactions are the major source of the attraction in the gas phase. Ab initio calculations for the benzene-CHCl3 cluster show that the dispersion interaction between the CHCl3 and benzene is significant (about -6.5 kcal/mol),55,56 which shows that the dispersion interactions of the heterodimeric capsule and guest molecules with solvent CDCl3 molecules are significant. The magnitude of the stabilization energy by the formation of the complex in solution is determined by balance between the host-guest interaction energy and the desolvation energy. Therefore, the dispersion contributions to the stabilization of the complexes in CDCl3 solution are substantially smaller than those in the gas phase. The electrostatic interactions are also important for stabilizing the complexes, although the Ees values (-4.9 to -12.5 kcal/ mol) are smaller than Ecorr. The Ees values for the complexes are about 100-250% of the hydrogen bonding energy of the water dimer (about -5 kcal/mol). The Coulombic interactions between the oxygen atoms of the guest molecules and the positively charged atoms of the heterodimeric capsule contribute to the attractive electrostatic interactions. The Ees value for the major orientational isomer of the complex of 3 (-12.5 kcal/ mol) is substantially larger than those for the major isomers of the complexes of 4 and 5 (-7.8 and -8.0 kcal/mol, respectively), which shows that the electrostatic interactions are responsible for the large Eform value for the complex of 3. The electrostatic interactions mainly determine the orientation of the guest molecules in the complexes. Table 3 summarizes the difference of each energy term between the two orientational isomers. The ∆Ees values (the difference of the electrostatic energies) for the complexes of 3, 4, and 5 are substantial (3.9, 2.6, and 3.1 kcal/mol, respectively), while the differences of other terms (∆Erep, ∆Ecorr, and ∆Edef) are small (less than 1.1 kcal/mol). Although the magnitude of the electrostatic interactions is substantially smaller than the dispersion interactions, the electrostatic interactions are highly orientation dependent.

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Figure 5. Interaction energy potentials calculated for model complexes shown in Figure 2 at the MP2/cc-pVTZ level.

On the other hand, the orientation dependence of the dispersion interactions (∆Ecorr) is weak. Therefore, the electrostatic interactions mainly determine the orientation of the guest molecules in the complexes. Role of CH/π Interactions. The geometries of the complexes and the calculated charge distributions suggest that the attractive electrostatic interactions of the oxygen atoms of the guest molecules with the hydrogen atoms of the heterodimeric capsule stabilize the complexes. The interaction energies calculated for the complexes show that the electrostatic interactions mainly determine the orientation of the guest molecules in the complexes. On the other hand, the CH/π interaction (the attraction between a C-H bond and an aromatic ring) was often regarded as one of the important driving forces for molecular recognition.57 The C-H bonds of methyl groups of the guest molecules (3-5) are close to the aromatic rings of the heterodimeric capsule (1 · 2) in the complexes.26 However, the influence of the CH/π interactions on the orientational selectivity was unclear. Therefore, we analyzed the CH/π interactions in model complexes (Figure 2) for understanding the nature of the CH/π interactions in the heterodimeric capsule complexes. The interaction energies in the benzene complexes with propylbenzene (6), methoxymethylbenzene (7), methylbenzoate (8), and acetoxybenzene (9) were studied. The MP2/cc-pVTZ level interaction energy potentials for the four complexes are compared with that for the benzene-methane complex, as shown in Figure 5. The interactions of the benzene with 6-9 are larger than those in the benzene-methane complex.50 The potentials for the complexes of 8 and 9 are deeper than those for the complexes of 6 and 7, which suggests that the -COO- group enhances the attraction in the complexes. The potentials of the four complexes have their minima when R ) 3.6 Å, while the benzene-methane potential has its minimum when R ) 3.8 Å. Apparently, the large attraction in the complexes of 6-9 is the cause of the short equilibrium distances compared with the benzene-methane complex. The CCSD(T) level interaction energies at the basis set limit [ECCSD(T)(limit)] were estimated for the four complexes at the potential minima (R ) 3.6 Å). The total interaction energies [Eint ) ECCSD(T)(limit)], electrostatic energies (Ees), and electron correlation contribution to the interaction energies (Ecorr ) Eint - EHF) are summarized in Table 4. The Eint values for the complexes of 6-9 are -2.2 to -2.9 kcal/mol, which are 0.7-1.5 kcal/mol larger than that for the benzene-methane complex. The Eint values for the complexes of 8 and 9 are close. The dispersion interactions are mainly responsible for the large

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TABLE 4: Electrostatic and Dispersion Energies in Model Complexesa complex

Eintb

Eesc

Erepd

Ecorre

C6H6-CH4f (R ) 3.8 Å) C6H6-CH3CH2CH2Ph 6 (R ) 3.6 Å) C6H6-CH3OCH2Ph 7 (R ) 3.6 Å) C6H6-CH3OCOPh 8 (R ) 3.6 Å) C6H6-CH3COOPh 9 (R ) 3.6 Å)

-1.47 -2.21 -2.35 -2.74 -2.94

-0.19 -0.23 -0.49 -0.89 -0.66

1.21 1.92 1.70 1.72 1.59

-2.50 -3.91 -3.55 -3.58 -3.87

a

Energy in kcal/mol. Geometries for model complexes are shown in Figure 2. b Total interaction energy. Estimated CCSD(T) interaction energy at the basis set limit. See text and Table 2S in the Supporting Information. c Electrostatic energy. See text. d Erep ) EHF - Ees. (EHF is the interaction energy calculated at the HF/ aug-cc-pVTZ level.) Erep is mainly exchange-repulsion energy. e Ecorr ) Eint - EHF. Electron correlation contribution to total interaction energy. Ecorr is mainly dispersion energy. f Reference 58.

Eint values of the four complexes compared with the benzenemethane complex. The Ecorr values for the four complexes (-3.6 to -3.9 kcal/mol) are 1.1-1.4 kcal/mol larger than that for the benzene-methane complex (-2.5 kcal/mol).58 The electrostatic interactions in the complexes of 6 and 7 (-0.2 and -0.5 kcal/ mol, respectively) are very small, as in the case of the benzenemethane complex (-0.2 kcal/mol). The electrostatic interactions in the complexes of 8 and 9 (-0.9 and -0.7 kcal/mol, respectively) are slightly larger. However, they are still significantly smaller than the dispersion interactions. The calculations show that the dispersion interactions are mainly responsible for the attraction between the methyl group and the benzene (CH/π interaction) in the model complexes, as in the case of the benzene-methane complex. The electrostatic interactions are weak compared with the dispersion interactions. The comparison of the interactions in the major and minor orientational isomers of the complexes shows that the electrostatic interactions have a paramount importance in determining the orientation of the guest molecules in the complexes. These results show that the CH/π interactions in the complexes, which are mainly dispersion interactions, are not playing important roles in controlling the orientation of the guest molecules in the complexes. Conclusion The geometries of the heterodimeric capsule complexes with unsymmetrical guest molecules and the interaction energies in the complexes were studied by ab initio calculations. The oxygen atoms of the guest molecules have close contact with the hydrogen atoms in pyridine and phenol rings and methylenebridge rim of the heterodimeric capsule in the optimized geometries. The geometries and the calculated charge distributions suggest that the attractive electrostatic interactions between the oxygen atoms and the hydrogen atoms stabilize the complexes. The orientational selectivity of the guest molecules obtained by the calculations agrees with the experimental measurements. The stabilization energy by the complex formation calculated for the major orientational isomer is always larger than that for the minor orientational isomer. The calculations show that the dispersion interactions are the major source of the attraction in the complexes. Although the electrostatic interactions are substantially smaller than the dispersion interactions, the electrostatic interactions are also playing important roles for stabilizing the complexes. The comparison of each energy term for the major and minor orientational isomers shows that the electrostatic interactions, which are highly orientation dependent, mainly determine the orientation of the unsym-

metrical guest molecules in the complexes. The orientation dependence of other energy terms is not large. The attraction between the methyl groups of the guest molecules and the aromatic rings of the heterodimeric capsule, which could be called the CH/π interactions, is one of the sources of the significant stabilization energies by the complex formation in the gas phase. The electrostatic interactions have a paramount importance in controlling the orientation of the unsymmetrical guest molecules in the complexes. This shows that the CH/π interactions in the complexes, which are mainly dispersion interactions, are not playing important roles in determining the orientation of the guest molecules. Acknowledgment. We thank Tsukuba Advanced Computing Center for the provision of the computational facilities. Supporting Information Available: The HF and MP2 interaction energies for the complexes and the electron correlation contributions to the interaction energies of 1 with the guest molecules and those of 2 with the guest molecules, the MP2 and CCSD(T) interaction energies for the model complexes shown in Figure 2, and the Cartesian coordinates and calculated energies for the optimized geometries. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) MacGillivray, L. R.; Atwood, J. L. Angew. Chem., Int. Ed. 1999, 38, 1018. (2) Hof, F.; Craig, S. L.; Nuckolls, C.; Rebek, J., Jr. Angew. Chem., Int. Ed. 2002, 41, 1488. (3) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853. (4) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Angew. Chem., Int. Ed. 2001, 40, 2382. (5) Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.; Kusukawa, T.; Biradha, K. Chem. Commun. 2001, 509. (6) Koh, K.; Araki, K.; Shinkai, S. Tetrahedron Lett. 1994, 35, 8255. (7) Chapman, R. G.; Sherman, J. C. J. Am. Chem. Soc. 1995, 117, 9081. (8) Vreekamp, R. H.; Verboom, W.; Reinhoudt, D. N. J. Org. Chem. 1996, 61, 4282. (9) MacGillivray, L. R.; Atwood, J. L. Nature 1997, 389, 469. (10) Heinz, T.; Rudkevich, D. M.; Rebek, J., Jr. Nature 1998, 394, 764. (11) Kobayashi, K.; Shirasaka, T.; Yamaguchi, K.; Sakamoto, S.; Horn, E.; Furukawa, N. Chem. Commun. 2000, 41. (12) Higler, I.; Grave, L.; Breuning, E.; Verboom, W.; de Jong, F.; Fyles, T. M.; Reinhoudt, D. N. Eur. J. Org. Chem. 2000, 1727. (13) Fox, O. D.; Drew, M. G. B.; Beer, P. D. Angew. Chem., Int. Ed. 2000, 39, 136. (14) Corbellini, F.; Knegtel, R. M. A.; Grootenhuis, P. D. J.; CregoCalame, M.; Reinhoudt, D. N. Chem.sEur. J. 2005, 11, 298. (15) Koblenz, T. S.; Dekker, H. L.; de Koster, C. G.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Chem. Commun. 2006, 1700. (16) Timmerman, P.; Verboom, W.; van Veggel, F. C. J. M.; van Duynhoven, J. P. M.; Reinhoudt, D. N. Angew. Chem., Int. Ed. Engl. 1994, 33, 2345. (17) Tucci, F. C.; Rudkevich, D. M.; Rebek, J., Jr. J. Am. Chem. Soc. 1999, 121, 4928. (18) Shivanyuk, A.; Rebek, J., Jr. J. Am. Chem. Soc. 2002, 124, 12074. (19) Scarso, A.; Shivanyuk, A.; Rebek, J., Jr. J. Am. Chem. Soc. 2003, 125, 13981. (20) Shivanyuk, A.; Rebek, J., Jr. Angew. Chem., Int. Ed. 2003, 42, 684. (21) Yamanaka, M.; Shivanyuk, A.; Rebek, J., Jr. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 2669. (22) Kobayashi, K.; Ishii, K.; Sakamoto, S.; Shirasaka, T.; Yamaguchi, K. J. Am. Chem. Soc. 2003, 125, 10615. (23) Kobayashi, K.; Ishii, K.; Yamanaka, M. Chem.sEur. J. 2005, 11, 4725. (24) Ihm, C.; Jo, E.; Kim, J.; Paek, K. Angew. Chem., Int. Ed. 2006, 45, 2056. (25) Rebek, J., Jr. Angew Chem. Int. Ed. 2005, 44, 2068. (26) Kobayashi, K.; Kitagawa, R.; Yamada, Y.; Yamanaka, M.; Suematsu, T.; Sei, Y.; Yamaguchi, K. J. Org. Chem. 2007, 72, 3242. (27) Kitagawa, H.; Kawahata, M.; Kitagawa, R.; Yamada, Y.; Yamanaka, M.; Yamaguchi, K.; Kobayashi, K. Tetrahedron 2009, 65, 7234.

Orientational Isomerism of Unsymmetrical Guests (28) Kitagawa, H.; Kobori, Y.; Yamanaka, M.; Yoza, K.; Kobayashi, K. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 10444. (29) Dunning, T. H., Jr. J. Phys. Chem. A 2000, 104, 9062. (30) Tsuzuki, S.; Fujii, A. Phys. Chem. Chem. Phys. 2008, 10, 2584. (31) Mφller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618. (32) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988, 153, 503. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E. ; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (34) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J. Chem. 1992, 70, 560. (35) Sosa, C.; Andzelm, J.; Elkin, B. C.; Wimmer, E.; Dobbs, K. D.; Dixon, D. A. J. Phys. Chem. 1992, 96, 6630. (36) The geometry optimizations of the complex by ab initio calculations are essential for an accurate evaluation of the relative stability of the two orientational isomers. (37) Ransil, B. J. J. Chem. Phys. 1961, 34, 2109. (38) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (39) An accurate evaluation of intermolecular interaction energies of some systems requires CCSD(T) level calculations. On the other hand, recent ab initio calculations show that the MP2 method provides sufficiently accurate interaction energies of many systems. The MP2 level interaction energies between aliphatic hydrocarbon molecules and those for hydrogen bonded systems are usually very close to the CCSD(T) level interaction energies.40,41 The difference between the CCSD(T) and MP2 interaction energies for CH/π complexes are not large.30,42 Therefore, the interaction energies of the complexes were calculated at the MP2 level for evaluating the relative stability of orientational isomers in this work. (40) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M. J. Chem. Phys. 2006, 124, 114304. (41) Tsuzuki, S.; Luthi, H. P. J. Chem. Phys. 2001, 114, 3949. (42) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M.; Tanabe, K. J. Am. Chem. Soc. 2000, 122, 3746. (43) The electron correlation contribution to the interaction energy is mainly the dispersion energy. Although the dispersion energy is non-additive interaction, it is well known that the non-additivity of the dispersion energy

J. Phys. Chem. B, Vol. 114, No. 16, 2010 5341 is very small.44,45 The MP2/6-31G* level calculation of the entire complex requires about 2500 basis functions, which is too computationally demanding and not practical at present. Therefore, we evaluated the dispersion contribution assuming the additivity. (44) Stone, A. J. The theory of intermolecular forces; Clarendon Press: Oxford, U.K., 1996. (45) Tsuzuki, S.; Klopper, W.; Luthi, H. P. J. Chem. Phys. 1999, 111, 3846. (46) Stone, A. J.; Alderton, M. Mol. Phys. 1985, 56, 1047. (47) Stone, A. J.; Dullweber, A.; Hodges, M. P.; Popelier, P. L. A.; Wales, D. J. Orient: a program for studying interactions between molecules Version 3.2; University of Cambridge: 1995; http://www-stone.ch.cam.ac.uk/ programs.html#Orient. (48) Stone, A. J. J. Chem. Theory Comput. 2005, 1, 1128. (49) Helgaker, T.; Klopper, W.; Koch, H.; Noga, J. J. Chem. Phys. 1997, 106, 9639. (50) Shibasaki, K.; Fujii, A.; Mikami, N.; Tsuzuki, S. J. Phys. Chem. A 2006, 110, 4397. (51) Singh, U. C.; Kollman, P. A. J. Comput. Chem. 1984, 5, 129. (52) Besler, B. H.; Mertz, K. M.; Kollman, P. A. J. Comput. Chem. 1990, 11, 431. (53) The atomic charge distributions were calculated from the MP2/6311G** level wave functions of the isolated heterodimeric capsule and guest molecules. The hydrogen atoms at the β-positions of the pyridine ring and that of the phenol ring in the heterodimeric capsule have positive charges (0.21 and 0.18 e, respectively, 1 e ) 1.602 × 10-19 C). The methylene rims also have positive charges (0.50 e). The charges on the oxygen atoms of the benzoate CdO, benzoate sOs, acetoxy CdO, and acetoxy sOs in 3 are-0.62,-0.42,-0.60, and-0.55 e, respectively. The charges on the oxygen atoms of the benzoate CdO, benzoate sOs, and ethoxy sOs in 4 are-0.62,-0.41, and-0.53 e, respectively. The charge on the oxygen atom of 5 is-0.51 e. (54) The HF level interaction energies in the complexes are always positive (repulsive). The HF level interaction energy is mainly the exchangerepulsion and electrostatic energies. The calculations indicate that the exchange-repulsion between the guest and host molecules is stronger than the attractive electrostatic interactions in the complexes. The negative (attractive) MP2 level interaction energies and positive HF level interaction energies indicate that the complexes are stabilized by the dispersion interactions. (55) Fujii, A.; Shibasaki, K.; Kazama, T.; Itaya, R.; Mikami, N.; Tsuzuki, S. Phys. Chem. Chem. Phys. 2008, 10, 2836. (56) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M.; Tanabe, K. J. Phys. Chem. A 2002, 106, 4423. (57) Nishio, M.; Hirota, M.; Umezawa, Y. The CH/π interaction; WileyVCH: New York, 1998. (58) Shibasaki, K.; Fujii, A.; Mikami, N.; Tsuzuki, S. J. Phys. Chem. A 2007, 111, 753.

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