ARTICLE pubs.acs.org/JPCA
DFT Study of [2.2]-, [3.3]-, and [4.4]Paracyclophanes: Strain Energy, Conformations, and Rotational Barriers Steven M. Bachrach* Department of Chemistry, Trinity University, 1 Trinity Place, San Antonio, Texas 78212, United States
bS Supporting Information ABSTRACT: The three smallest symmetrical paracyclophanes, having tethers with two, three, or four methylene groups, have been examined with four density functional methods (B3LYP, M06-2x, B97-D, ωB97X-D). The geometries predicted by functionals accounting for medium-range correlation or longrange exchange and/or dispersion are in close agreement with experiment. In addition, these methods provide similar estimates of the strain energy of the paracylcophanes, which decrease with increasing tether length. [4.4]Paracyclophane is nearly strain-free, reflecting the small out-of-plane distortion of its phenyl rings. Lastly, the barrier for interconversion of the conformers of [3.3]paracylcophane is computed in close agreement with experiment, and an estimate for phenyl rotation in [4.4]paracyclophane of about 19 kcal mol-1 is predicted by the DFT methods employed.
1. INTRODUCTION The paracyclophanes are of interest as they test our notions of strain and aromaticity,1-3 two bedrock concepts of organic chemistry.4,5 When the alkyl chains connecting the phenyl rings are short, the phenyl rings are distorted from planarity, inducing loss of aromaticity. Larger distortions of the phenyl rings are avoided by introducing some strains into the alkyl chains. In addition, the nature of the interaction between the π electrons of the two rings requires careful attention, especially as to how one treats these interactions within an electronic structure computation. The paracyclophanes and their derivatives have found application as ligands in chiral catalysts6 and in optoelectronic materials.7 The smallest member of the family, [2.2]paracyclophane (1) has been prepared8 and extensively studied, especially by theorists.9-17 The solid-state structure of 1 has been subject to some controversy. A reinterpretation of the earliest X-ray data18 indicates a symmetry break with a twisting of the two phenyl rings relative to each other. The degree of twisting is a sensitive test of computational methodologies. Grimme’s recent computational study and re-evaluation of the X-ray structure (obtained from an unpublished analysis of the crystal structure) shows a very nice agreement between the SCS-MP2 optimized geometry and the X-ray structure.13 The twisting within 1 has been disputed by Lyssenko, Antipin, and Antonov, who find that at 100 K the molecule possesses D2h symmetry.19
The next larger symmetric member of the series, [3.3]paracyclophane (2), is also a known compound20 and its X-ray structure has been published; the so-called trans isomer 2t was observed in the solid state.21 Dodziuk et al.22 have examined the solution phase NMR spectra of 2 and interpreted it as resulting from an equilibrium of the trans 2t and cis 2c isomers, with the cis isomer predominant. Line-shape analysis aided by B3LYP-computed NMR shifts allowed for an estimation of the activation barrier for this interconversion of 12.29 ( 0.08 kcal mol-1. This estimate is in good agreement with previous estimates of the free energy of activation determined by NMR coalescence.23,24 Both the NMR spectra and B3LYP computations indicate that the cis isomer is favored over the trans isomer, which agrees with the solution-phase result. The B3LYP structure of 2t is in nice agreement with the experimental X-ray structure.22 One would expect that both 1 and 2 are strained. Kind et al.12 estimated the strain energy of 1 as 39.8 kcal mol-1 using a homodesmotic reaction, while Galembeck and Laali,14 using an isodesmic reaction, suggest its strain energy is 29.7 kcal mol-1. Dodziuk et al22 indicate that 2 is less strained than 1 but provide no estimate of the magnitude of its strain energy. Given our interest in strained aromatic molecules,25-27 examination of the paracyclophanes is a logical extension. Our approach here, as before, is to employ the group equivalent reaction28 to assess the total strain energy in the three smallest examples of symmetric paracyclophanes 1-3. The group equivalent reaction preserves chemical groups as defined by Benson29 and affords a simple procedure for attempting to isolate the effect under examination by preserving all other molecular features. In Received: December 3, 2010 Revised: February 3, 2011 Published: February 25, 2011
r 2011 American Chemical Society
2396
dx.doi.org/10.1021/jp111523u | J. Phys. Chem. A 2011, 115, 2396–2401
The Journal of Physical Chemistry A
ARTICLE
Table 1. Experimental and Computed Geometric Parameters of [2.2]Paracyclophane (1) parametera rC1-C10
exptb
SCS-MP2c
2.782
2.772 3.080
ωB97X-D
PBE0d
BH&Hd
B3LYP
M06-2x
B97-D
2.832
2.798
2.759
2.825
2.788
2.810
2.799
3.166
3.119
3.067
3.148
3.096
3.099
3.112
17.8
0.0
18.5
9.9
15.4
HF
2.78 rC2-C20
3.097 3.09
dC1-C7-C70 -C10
12.6
17.6
8.8
11.5
0.0 R
12.5
12.2
13.3
12.3
12.2
11.4
12.4
β
12.4 11.0
11.5
10.9
10.9
11.8
12.0
11.6
-0.13
-0.14
11.2 ΔEe
-0.04
0.0f
0.19
All distances are in angstroms and all angles are in degrees. The first values are from ref 18 with revisions reported in ref 13 and the second values are from ref 19. c Reference 13. d Reference 17. e Energy of the D2h structure relative to the D2 structure. f Only the D2h structure is found. a
b
addition, we will assess the performance of some new density functionals that incorporate medium-range electron correlation (the hybrid M06-2x functional30,31), dispersion correction (the B97-D functional32), and long-range corrected exchange (the ωB97X-D functional,33 which also incorporates the dispersion correction), especially regarding the structure of the compounds. Lastly, we compute the activation barriers for the interconversion of the cis and trans conformers of 2, comparing the former with the recent experimental results, along with the barrier for phenyl rotation in 3.
2. COMPUTATIONAL METHODS The geometries of the various conformers of 1-3 were fully optimized within appropriate point groups using four different methods: B3LYP,34 which has been the de facto DFT method in organic chemistry for over a decade but it lacks medium- or longrange exchange and dispersion corrections; M06-2x,30,31 a hybrid method designed to account for medium-range electron correlation; B97-D,32 a dispersion corrected functional; and ωB97XD,33 a functional that contains both dispersion and long-range exchange corrections. In all cases the 6-311G(d,p) basis set has been utilized. Analytical frequencies were obtained to ensure that a local energy minimum or transition state has been located and to correct for zero-point vibrational energy (ZPVE). ZPVEs have been used without any scaling factor. All computations were performed with the Gaussian-09 suite.35 3. RESULTS AND DISCUSSION 3.1. The Structure of [2.2]Paracyclophane. The initial X-ray study of 1 suggested that it has D2h symmetry, but subsequent analysis revealed that the phenyl rings are twisted relative to each other.13,18 A low-temperature X-ray structure, however, suggests the D2h structure.19 The twisting reduces the symmetry to D2 and diminishes the eclipsing interactions along the two ethano bridges. The twist is 12.6° in the X-ray structure.18 This subtle twist turns out to be a sensitive test of electron structure computations. HF computations with small basis sets predict a D2h structure,9,10 as does B3LYP with many basis sets (none give a twist of more than a couple of degrees).11,13,14,17 Two other functionals (PBE0 and BH&H) predict a twist from 10 to 20°.17 Both MP2 and SCS-MP2 predict a twist of around 18°.13 Twisting from D2h to D2 reduces the energy by a very small
Scheme 1. Angles r and βa
Angle R is defined by the line from midpoints of 3-5 and 2-6 with the line from the midpoint 2-6 to 1. Angle β is defined as the line from the midpoint 2-6 to 1 and the line from 1 to 7. a
amount: 0.2 kcal mol-1 at SCS-MP2/cc-pVTZ.13 While all of the high-level computations (see below as well) indicate a D2 geometry, the low barrier and the apparent temperature dependence of the X-ray structure implicate a dynamic structure. Perhaps the most interesting aspect of 1 is the nature of the interaction between the π-electrons associated with the two rings. The distance between the rings is less than the sum of the van der Waals radii of two carbon atoms. The resulting interaction has been much discussed. Caramori and Galembeck15 employed the atoms-in-molecules analysis of Bader36 and found through-space, closed-shell-closed-shell interactions that stabilize 1. Grimme13 argues that there are HOMO-LUMO double excitations between the rings that lead to a reduction of the Pauli repulsion between the rings. This, he argues, is “strong π-π electron correlation” and is different than van der Waals interactions; Grimme calls it an “overlap-dispersive” interaction. Given the dispersive, longer-range correlation nature of the interaction between the rings of 1, this molecule is an interesting test of the performance of new density functionals that account for medium-range correlation and long-range exchange and dispersion. The geometry of 1 was optimized with M06-2x, B97-D, and ωB97X-D with the 6-311G(d,p) basis set. All three of these functionals predict a D2 structure, with varying twist angles: M06-2x, 18.5°; B97-D, 9.9°; and ωB97X-D, 15.4°. Other geometric parameters are of interest as well, and these are listed in Table 1 for the optimized structures computed with these three functionals, along with some other methods. Angles R and β (see Scheme 1), defined by Grimme,13 assess the extent of out-of-plane deformation of the phenyl rings. All of the DFT methods predict very similar phenyl deformations, with R and β values within a degree of the experimental value. 2397
dx.doi.org/10.1021/jp111523u |J. Phys. Chem. A 2011, 115, 2396–2401
The Journal of Physical Chemistry A
ARTICLE
Table 2. Experimental and Computed Geometric Parameters of [3.3]Paracyclophane (2c and 2t)
Table 3. Computed Geometric Parameters of [4.4]Paracyclophane (3c and 3t)
M06-2x
B97-D
ωB97X-D
parametera
exptb
HF
B3LYP
3.229
3.146
3.142
3.162
rC1-C10
3.978
4.173
3.420 3.436
3.394 3.411
3.291 3.309
3.272 3.287
3.308 3.323
rC2-C20 R
3.990 0.6
5.5
7.3
6.8
6.1
5.4
6.1
β
3.5
4.6
3.8
3.9
3.7
4.1
4.0
0.30
0.19
-0.06
0.37
0.25
parametera
exptb
HF
B3LYP
rC1-C10
3.14
3.246
rC2-C20 rC3-C30
3.29 3.31
R β
M06-2x
B97-D
ωB97X-D
4.220
3.830
4.145
4.029
4.250 3.1
4.300 3.1
3.817 0.6
4.188 1.7
4.060 1.2
3.2
4.0
2.6
4.4
3.3
3.868
Trans
ΔΕc
Trans
Cis rC1-C1
Cis
4.134
4.167
3.830
4.074
rC2-C20
4.2
4.228
3.817
4.092
3.863
0
rC1-C10
3.251
3.229
3.140
3.150
3.159
R
2.6
2.4
0.6
0.7
0.3
rC2-C20
3.336
3.313
3.216
3.210
3.231
β
3.3
3.6
2.6
4.0
2.6
rC6-C60
3.533
3.492
3.371
3.369
3.393
ΔΕc
1.34
1.54
1.63
2.05
1.88
R β
7.3 4.0
6.9 4.0
6.1 3.7
5.5 4.2
6.1 4.0
a
All distances are in angstroms and all angles are in degrees. b Reference 21. c Energy relative to the cis isomer.
The experimental (X-ray crystal structure) distance between the rings, measured here as rC1-C10 and rC2-C20 , is 2.782 and 3.097 Å13 (2.78 and 3.09 Å at 100 K19), respectively. The Hartree-Fock method predicts values that are too long, understandably so since it does not include any sort of dispersion or HUMO-LUMO effect. B3LYP and PBE0 decrease these distances only slightly. B97-D, which includes a dispersion correction, shortens the distances further, but the rC1-C10 distance is still too long, and the twist is too small. Interestingly, the BH&H functional provides a geometry that is also consistent with experiment.17 The other two functionals, which include medium(M06-2x) or long-range corrections (ωB97X-D), predict a structure in very nice agreement with experiment and with the SCS-MP2 optimized geometry. Either of these two functionals— M06-2x or ωB97X-D—appear to be appropriate for computing the paracyclophanes. It should be noted that HF, M06-2x, B97-D and ωB97X-D with the 6-311G(d,p) basis set all find the D2h structure to have one imaginary frequency, while B3LYP predicts it to be the local energy minimum. However, while the D2 structures have lower electronic energies, when ZPVE is included, only M06-2x predicts that the D2 structure is lower in energy that the D2h one; even so all four DFT methods predict the energy difference between the two forms to be less than 0.2 kcal mol-1. Therefore, the potential energy surface for twisting about the D2h geometry is very flat, certainly a factor at play in the contentious interpretation of the X-ray structure. 3.2. The Structure of [3.3]Paracyclophane. The X-ray structure of [3.3]paracyclophane has been reported, and the only isomer identified is 2t.21 Optimization of the cis and trans structures of 2 with the different functionals reveals similar trends as with 1 (see Table 2). The HF optimized structure has an interring distance that is too long. This is decreased somewhat with B3LYP, but a reasonable agreement between the experimental and computed structures is only had when medium-range correlation or long-range exchange and/or dispersion corrections are included. Here as with 1, the ωB97X-D gives very close agreement with the experimental structure, though both M06-2x and B97-D are almost as good. The geometries of the cis and
a
All distances are in angstroms and all angles are in degrees. b Reference 37. c Energy relative to the trans isomer.
trans conformers of 2 differ little in terms of bond and inter-ring distances, angles, and phenyl deformation. Dodziuk et al.22 noted that the B3LYP energy of 2t is higher than that of 2c, in contrast to the fact that only the trans isomer is identified in the solid state but in concert with the preference for 2c found in solution. B97-D and ωB97X-D also predict that 2c is more stable than 2t, though M06-2x predicts the reverse ordering. With all density functionals examined, however, the energy difference is very small, no larger than 0.4 kcal mol-1. The point groups of the cis and trans isomers of 2 are C2v and C2h, respectively. It should be noted that both the M06-2x and B97-D methods predict these structures to have a single imaginary frequency. Reducing their symmetry to Cs for 2c and Ci for 2t gave true local minima. However, when ZPVE is included the higher symmetry structures are in fact lower in energy than their symmetry-broken forms. Once again, the potential energy surfaces are very flat near the local minima. 3.3. The Structure of [4.4]Paracyclophane. We examined four conformations of [4.4]paracyclophane, having D2, C2h, Cs, and C1 symmetry. The two lowest energy conformers have D2 and C2h symmetry and are designated as 3t and 3c, respectively. Some important computed geometric parameters of these two structures are listed in Table 3. The X-ray crystal structure of [4.4]-paracylophane identifies only the 3t isomer.37
Similar geometric trends with the functionals are again seen here as with the other two paracyclophanes. Both isomers of 3 have similar geometric features regarding the inter-ring distances and out-of-plane bending. HF and B3LYP predict inter-ring distances that are too long, while the three functionals that include dispersion and/or medium-range correlation or longrange exchange show shorter distances. Interestingly, these three functionals also indicate very little deviation from planarity of the phenyl rings. 2398
dx.doi.org/10.1021/jp111523u |J. Phys. Chem. A 2011, 115, 2396–2401
The Journal of Physical Chemistry A
ARTICLE
Table 4. Computed Strain Energies (kcal mol-1) of 1-3 Evaluated Using Reactions 2-4 compound
HF
B3LYP
M06-2x
B97-D
ωB97X-D
1
45.6
36.7
30.4
27.0
30.8
2c 3t
18.2 10.8
14.4 8.6
5.7 0.3
4.6 0.1
5.8 1.6
Table 5. Estimated Strain Energy (kcal mol-1) of the Phenyl and Alkyl Fragmentsa phenyl
alkyl
total
1
10.2
5.6
31.6
2 3
1.7 0.3
3.4 2.5
10.2 5.6
a
3.4. Ring Strain Energy of 1-3. Evaluation of the ring strain
energy of the paracyclophanes requires selecting appropriate reference compounds. With the isodesmic reaction of Galembeck and Laali,14 the reference is p-dimethylbenzene and the resulting strain energy is 29.7 kcal mol-1. The homodesmotic reaction employed by Kind et al.12 (reaction 1) gives a strain energy of 39.8 kcal mol-1. Our approach is to use the group equivalent reaction which conserves chemical groups and is also homodesmotic. This approach utilizes p-diethylbenzene as a reference, and reactions 2-4 provide the strain energies of 1, 2, and 3, respectively. These computed strain energies are listed in Table 4.
See text for description of the model system employed for these ωB97X-D/6-311G(d,p) computations.
Figure 1. Geometry of 2ts at ωB97X-D/6-311G(d,p).
As might be expected, HF overestimates the strain energy since it does not account for any of the favorable π-π interactions between the phenyl rings. B3LYP also gives estimates of the strain energy of the three paracylcophanes that are too large for the same reason. The three other functionals (M06-2x, B97-D, and ωB97X-D), which do account for dispersion and/or medium- or long-range corrections, provide similar strain estimates of 1-3. The strain of 1 is about 30 kcal mol-1, consistent with the estimate of Galembeck and Laali,14 but appreciably lower than that of Kind.12 The strain energy of 2 is substantially lower than that of 1, only about 6 kcal mol-1. This reflects a significant decrease in the out-of-plane distortions of the phenyl rings; R and β are about 12° in 1 but only 6° and 4°, respectively, in 2. Since the outof-plane distortions of the phenyl rings in 3 are negligible, one might anticipate that its strain energy is very small; in fact, the three functionals predict the strain energy of 3 to be 1.6 kcal mol-1 or less, nearly strain-free. The strain energy can be associated with two regions of the molecule: the phenyl rings and the alkyl linkers. Distortion of the phenyl ring from planarity results in a loss of aromatic stabilization energy. The alkyl chains are strained mostly by potential eclipsing interactions. For example, in 2 the twist angle, the dihedral about the C7-C70 bond, ranges from about 10 to 20°, far
from the ideal value of 60° in a pure staggered arrangement. In 3, the C1-C7-C9-C90 and C7-C9-C90 -C70 dihedrals are about 75° and -140°, respectively, reflecting a much less strained chain. We estimate the magnitude of the two sources of strain in the following way. For the strain in the phenyl ring, we optimized 1,4dimethylbenzene with the phenyl ring fixed as it is in 1-3 along with the β angle. The energy of this constrained molecule is then compared with fully relaxed 1,4-dimethylbenzene. This approach is similar to the one we took in estimating the strain energy of nanohoops.27 To estimate the strain in the alkyl chain, the structure of butane, pentane, and hexane was optimized holding the central two, three, or four carbon atoms fixed as they are in 1, 2, and 3, respectively. These energies are then compared back to the all-trans conformation of butane, pentane, or hexane. Representative of all of the functionals, we performed these computations at ωB97X-D/6-311G(d,p), and the results are listed in Table 5. As expected the strain energy associated with both the phenyl group and the alkyl chain decreases in the series 1 > 2 > 3. This reflects the diminished out-of-plane bending of the phenyl group and more favorable dihedral angles in the alkyl chain as one progresses along this series. The total strain energy of the three paracyclophanes can be estimated by adding twice the strain energy of the phenyl groups to twice the strain energy of the alkyl chains. This estimate for 2 (31.6 kcal mol-1) is quite close to the computed value using reaction 2 (30.8 kcal mol-1). The agreement for the other two paracyclophanes is less good, due to an overestimation of the strain of the alkanes. Nonetheless, it is clear that as the tethers lengthen, the strain in both the phenyl and alkyl fragments decreases, leading to a less strained macrocycle. 3.6. Barrier for Conversion of Conformers of 2. NMR analysis of the cis to trans isomerization of [3.3]paracyclophane provides the Arrhenius activation barrier of 12.29 ( 0.08 kcal mol-1.22 We have located the transition structure (2ts) for this process with all four functionals. The ωB97X-D optimized structure, representative of three computations, is shown in Figure 1. The propyl chain on the left side of this 2399
dx.doi.org/10.1021/jp111523u |J. Phys. Chem. A 2011, 115, 2396–2401
The Journal of Physical Chemistry A
ARTICLE
Table 6. Activation Energy (kcal mol-1) for the Conversion 2c f 2t method
a
Table 7. Phenyl Rotation Barrier (kcal mol-1) for 3 method
ΔEa
ΔEa(3tRot)
ΔEa(3cRot)
B3LYP
20.0
20.4
B3LYP
12.4
M06-2x
17.6
18.5
M06-2x B97-D
11.6 11.6
B97-D
17.6
18.0
ωB97X-D
18.7
20.0
ωB97X-D
12.5
expta
12.29
Reference 22.
Figure 2. Geometry of 2tRot, 3tRot, and 3cRot at ωB97X-D/6311G(d,p).
displayed structure is undergoing the inversion. The C1-C7C9-C70 dihedral angles (from the top ring) and C10 -C70 -C9C7 (from the bottom ring) in this inverting chain are 16.8° and 18.0°, indicating the near eclipsed arrangement in the TS. The values of these dihedral angles in the noninverting chain are 58.9° and 74.2°, quite similar to the corresponding dihedral in 2t of 67.5°. Similar geometries are found with the other functionals. The computed inversion barriers are listed in Table 6. These barriers are computed in the gas phase at 0 K, so direct comparison to the experimental value must be treated with caution. Nonetheless, these computed barriers are within a kcal mol-1 of experiment, providing additional support for the interpretation of the NMR data. 3.7. Barrier for Phenyl Rotation in 3. Rotation of the phenyl ring through the interior of the macrocycle is impossible for the small paracyclophanes; the macrocycle is simply too small for the phenyl group to pass through. For example, the barrier for rotation (computed at ωB97X-D/6-311G(d,p)) of the phenyl ring in 2, passing through the transition state 2tRot (see Figure 2), is 70.8 kcal mol-1 (71.1 through 2cRot), enough energy to start rupturing bonds. This high barrier is due in part to the steric conflict between the phenyl hydrogens of the rotating ring with the second phenyl ring; the distance between these hydrogens and the nearest phenyl carbon is only 1.863 Å. However, such a phenyl rotation is feasible for 3. We have located two transition states for this rotation, one having C2 symmetry that comes about by rotation from 3t and one with Cs symmetry coming from 3c. Representative (ωB97X-D) structures 3tRot and 3cRot are shown in Figure 2.
The phenyl rotation within 3 is a tight squeeze. The closest distance between a hydrogen of the rotating phenyl and a carbon of the other phenyl ring is 2.215 Å in 3tRot and 2.212 Å in 3cRot at ωB97X-D (distances are similar in the structures optimized with the other functionals). The C1-C10 distance increases by about 0.3 Å in going from 3t to 3tRot, with a similar change for 3c to 3cRot. Additional distortion occurs at the nonrotating phenyl: the values of R and β for 3tRot increase to 5.0° and 4.4° from 1.2° and 3.3°, respectively, in 3t. All of these distortions are to accommodate the perpendicular phenyl ring in the interior. The barriers for rotation through either 3tRot or 3cRot are listed in Table 7. This barrier is much lower than the one for 2, reflecting the larger size of the macrocycle of 3 than of 2. This barrier would only be overcome at fairly high temperatures. Phenyl rotation through the macrocycle of the paracyclophanes would only become possible at nominal temperatures when the tethers are pentyl chains or longer.
4. CONCLUSIONS The small paracyclophanes provide an interesting test for the density functional attempting to account for medium-range correlation and long-range exchange and dispersion interactions. On the basis of comparison to experimental geometries, the M06-2x, B97-D, and ωB97X-D all provide quite reasonable optimized geometries of the three smallest symmetric paracylophanes 1-3. In contrast, the widely utilized B3LYP functional which lacks terms that deal with these weaker interactions predicts geometries with inter-ring distances that are too large. Due to this inability to account for the attraction between the phenyl rings of the paracyclophanes, B3LYP overestimates the strain energies of these compounds. The other three functionals, however, give very similar strain energies. The strain energy decreases in the series 1 > 2 > 3, reflecting two sources of strain. First, as the macrocycle gets larger, the out-of-plane bending of each phenyl ring decreases, thereby restoring some of the aromatic stabilization energy. Second, as the alkyl tethers get longer, these chains can adopt better conformations, reducing the eclipsing interactions and thereby reducing their strain. In fact, 3 is nearly strain-free. Clearly, B3LYP is inappropriate for dealing with this type of system and calls into question its use in any system dealing with π-π stacking, an argument made by many others as well.38-43 The other three functionals perform about equally well in predicting geometries and strain energies of the paracyclophanes. The barrier for conversion of the two conformers of 2 was recently determined by NMR analysis to be 12.29 kcal -1.22 All four DFT methods reproduce this barrier with 1 kcal mol-1, including B3LYP, supporting the experimental analysis. Lastly, the phenyl group of 3 can rotate through the macrocycle. The barrier for this rotation is estimated by the DFT methods to be 18-20 kcal mol-1, too large to observe at ambient temperature. The barrier is however greatly reduced from that in 2, where the estimate is over 70 kcal mol-1. The next larger 2400
dx.doi.org/10.1021/jp111523u |J. Phys. Chem. A 2011, 115, 2396–2401
The Journal of Physical Chemistry A symmetric paracyclophane, [5.5]paracyclophane, is likely to have a phenyl rotational barrier that is observable at normal temperatures.
’ ASSOCIATED CONTENT
bS
Supporting Information. Full citation of ref 35 and optimized coordinates and energies of all structures (ground and transition) for 1-3 computed with the 6-311G(d,p) basis set using HF, B3LYP, M06-2x, B97-D and ωB97X-D. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the Welch Foundation (Grant W-0031) for financial assistance used to purchase the computers utilized in this study. ’ REFERENCES (1) Cram, D. J.; Allinger, N. L.; Steinberg, H. J. Am. Chem. Soc. 1954, 76, 6132–6141. (2) Tsuji, T. Adv. Strained Interesting Org. Mol. 1999, 7, 103–152. (3) Modern Cyclophane Chemistry; Gleiter, R., Hopf, H., Eds.; WileyVCH: Weinheim, Germany, 2004. (4) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry. Part A, Structure and Mechanisms, 4th ed.; Kluwer Academic/Plenum: New York, 2000. (5) Bachrach, S. M. Computational Organic Chemistry; Wiley-Interscience: Hoboken, NJ, 2007. (6) For recent examples, see:(a) Xin, D.; Ma, Y.; He, F. Tetrahedron: Asymmetry 2010, 21, 333–338. (b) Cheemala, M. N.; Gayral, M.; Brown, J. M.; Rossen, K.; Knochel, P. Synthesis 2007, 2007, 3877–3885. (c) Zhang, T.-Z.; Dai, L.-X.; Hou, X.-L. Tetrahedron: Asymmetry 2007, 18, 1990–1994. (d) Wu, X.-W.; Yuan, K.; Sun, W.; Zhang, M.-J.; Hou, X.-L. Tetrahedron: Asymmetry 2003, 14, 107–112. (e) Dahmen, S.; Brase, S. Chem. Commun. 2002, 26–27. (f) Zanotti-Gerosa, A.; Malan, C.; Herzberg, D. Org. Lett. 2001, 3, 3687–3690. (7) Bartholomew, G. P.; Bazan, G. C. Acc. Chem. Res. 2001, 34, 30–39. (8) Winberg, H. E.; Fawcett, F. S. Org. Synth. 1962, 42, 83. (9) Canuto, S.; Zerner, M. C. Chem. Phys. Lett. 1989, 157, 353–358. (10) Shen, T. L.; Jackson, J. E.; Yeh, J. H.; Nocera, D. G.; Leroi, G. E. Chem. Phys. Lett. 1992, 191, 149–156. (11) Walden, S. E.; Glatzhofer, D. T. J. Phys. Chem. A 1997, 101, 8233–8241. (12) Kind, C.; Reiher, M.; Roder, J.; Hess, B. A. Phys. Chem. Chem. Phys. 2000, 2, 2205–2210. (13) Grimme, S. Chem.—Eur. J. 2004, 10, 3423–3429. (14) Caramori, G. F.; Galembeck, S. E.; Laali, K. K. J. Org. Chem. 2005, 70, 3242–3250. (15) Caramori, G. F.; Galembeck, S. E. J. Phys. Chem. A 2007, 111, 1705–1712. (16) Pelloni, S.; Lazzeretti, P.; Zanasi, R. J. Phys. Chem. A 2007, 111, 3110–3123. (17) Kamya, P. R. N.; Muchall, H. M. J. Phys. Chem. A 2008, 112, 13691–13698. (18) Hope, H.; Bernstein, J.; Trueblood, K. N. Acta Crystallogr., Sect. B 1972, 28, 1733–1743. (19) Lyssenko, K. A.; Antipin, M. Y.; Antonov, D. Y. ChemPhysChem 2003, 4, 817–823.
ARTICLE
(20) Longone, D. T.; Kusefoglu, S. H.; Gladysz, J. A. J. Org. Chem. 1977, 42, 2787–2788. (21) Gantzel, P. K.; Trueblood, K. N. Acta Crystallogr., Sect. B 1965, 18, 958–968. (22) Dodziuk, H.; Szymanski, S.; Jazwinski, J.; Marchwiany, M. E.; Hopf, H. J. Phys. Chem. A 2010, 114, 10467–10473. (23) Anet, F. A. L.; Brown, M. A. J. Am. Chem. Soc. 1969, 91, 2389–2391. (24) Sako, K.; Meno, T.; Shinmyozu, T.; Inazu, T.; Takemura, H. Chem. Ber. 1990, 123, 639–642. (25) Bachrach, S. M. J. Org. Chem. 2009, 74, 3609–3611. (26) Bachrach, S. M. J. Phys. Chem. A 2008, 112, 7750–7754. (27) Bachrach, S. M.; St€uck, D. J. Org. Chem. 2010, 75, 6595–6604. (28) Bachrach, S. M. J. Chem. Educ. 1990, 67, 907–908. (29) Benson, S. W. Thermochemical Kinetics, 2 ed.; Wiley-Interscience: New York, 1976. (30) Zhao, Y.; Truhlar, D. G. Org. Lett. 2006, 8, 5753–5755. (31) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241. (32) Grimme, S. J. Comput. Chem. 2006, 27, 1787–1799. (33) Chai, J.-D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2008, 10, 6615–6620. (34) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5650. (35) Frisch, M. J.; et al. Gaussian-09, rev. A.02; Gaussian, Inc.: Pittsburgh, PA, 2003. (36) Bader, R. F. W. Atoms in Molecules—A Quantum Theory; Oxford University Press: Oxford, 1990. (37) Jones, P. G.; Hopf, H.; Pechlivanidis, Z.; Boese, R. Z. Kristallogr. 1994, 209, 673–676. (38) Seiji, T.; Hans, P. L. J. Chem. Phys. 2001, 114, 3949–3957. (39) Swart, M.; van der Wijst, T.; Fonseca, C.; Bickelhaput, F. M. J. Mol. Model. 2007, 13, 1245–1257. (40) Leverentz, H. R.; Truhlar, D. G. J. Phys. Chem. A 2008, 112, 6009–6016. (41) Hohenstein, E. G.; Chill, S. T.; Sherrill, C. D. J. Chem. Theory Comput. 2008, 4, 1996–2000. (42) Pitonak, M.; Neogrady, P.; Rexac, J.; Jurecka, P.; Urban, M.; Hobza, P. J. Chem. Theory Comput. 2008, 4, 1829–1834. (43) Sherrill, C. D.; Takatani, T.; Hohenstein, E. G. J. Phys. Chem. A 2009, 113, 10146–10159.
2401
dx.doi.org/10.1021/jp111523u |J. Phys. Chem. A 2011, 115, 2396–2401
