J. Phys. Chem. 1995, 99, 15023-15027
15023
Ab Initio Molecular Orbital Study of Toluene and Its Derivatives P. C. Chen* and C. W. Wu Department of Applied Chemistry, Chung Cheng Institute of Technology, Tashi, Taoyuan, Taiwan (ROC) Received: January 6, 1995; In Final Form: August 4, 1 9 9 9
Geometries of toluene and eight of its heavy atom derivatives (3-fluorotoluene, 3-chlorotoluene, 2,6difluorotoluene, 2,6-dichlorotoluene, benzotrifluoride, benzotrichloride, 2,6-difluorobenzotrifluoride,and 2,6dichlorobenzotrifluoride)were calculated using ab initio molecular orbital techniques. The sixfold barrier of toluene was calculated using various HF optimizations and single point MP2 and MP4SDTQ methods. Among them, one value of the barrier calculated by HF/6-31+G** method is close to the experimental value observed by microwave spectrum. Except that bond angles are affected by intemal rotation, most bond lengths and angles of the internal rotational isomer are similar to the original geometry. The more the substituents, the more the deformations found in the benzene ring. HF energies of these molecules and their internal rotational isomers show that not only the 2,6-substitution effect affects the stability of the molecules but also the steric effect.
Introduction
The conformation of the methyl group is important for the study of toluene and its related derivative^.'-^ Toluene is a very important solvent and is used extensively in various fields of chemi~try.~ Toluene and its derivatives are fairly large and for theoretical studies it is important to find a level of theory that can correctly predict the molecular structures, particularly the conformation of the intemal rotation of the methyl group and the deformation of the phenyl ring. The microwave spectrum of toluene was reported earlier.’ Isotopic investigations of the microwave spectra2J were carried out to obtain the molecular structure of toluene. Internal rotation of the methyl group about toluene’s axis is almost free, the barrier potential being only about 14 cal/mol. This low internal rotational barrier is very similar to that observed for the molecules CH3N02 (6.03 ~al/mol),~ CH3BFz (13.77 cal/mo1),6-8 and CH3C5H4N (13.51 ~al/mol).~All these compounds exhibit an extremely low sixfold barrier. Although the dynamics of those molecules with low intemal rotational barrier of the methyl top are of much interest, their molecular structures are also of great importance. In addition to the microwave study of toluene, an electron diffraction studylo and some ab initio calculations of the molecular structure of toluene have also been However, the study of the molecular structures of some toluenelike compounds has seldom been done. In order to understand more about these molecules, improved better basis sets of ab initio calcultions are needed for further work. These studies are focused on the geometries by introducing the halogen atoms in different positions of the benzene ring. Also different intemal rotational configurations are obtained. The manner in which the heavy halogen atoms affected the phenyl ring and the stability of ther intemal rotational isomers are investigated in this report. A systematic understanding of toluene and its halogen derivatives is obtained in this particular study. Calculation Method
The calculation tool was the Gaussian 92 package.I6 Geometry of toluene is obtained with some improved better HF basis sets of ab initio method and the comparisons are done. @
Abstract published in Advance ACS Abstracts, September 15, 1995.
k7
Figure 1. Molecular structure of the toluene and its derivatives (X = H, F, or Cl).
Calculation of the intemal rotational barrier of toluene was performed by both planar (the CH3-top and the molecular frame having C2” symmetry about the common axis) and orthogonal (the CH3-top and the molecular frame having C3,-symmetry about the common axis) configurations. In order to compare with experimental data, in addition to ab initio optimizations, single point MP2/6-31G* and MP4SDTQ/6-31G* methods were also performed. After analysis of toluene molecule, two halogen derivatives with para substitution such as 3-fluorotoluene and 3-chlorotoluene were chosen in ab initio HF/6-3 lG* calculations and the substituent effects were investigated. On the basis of experience with this basis,I7 the geometries of these molecules should be in reasonable agreement with experimental results. Then more complicated halogen derivatives with the 2,6disubstitution such as 2,6-difluorotoluene and 2,6-dichlorotoluene were calculated with the same basis set as 3-fluorotoluene and 3-chlorotoluene in order to understand how they affect the geometry of the phenyl ring and the intemal rotation of the methyl group. Finally, four other halogen derivatives with -CF3 or -CCl3 in place of -CH3 and with 2,6-disubstitution (such as C&CF3, C&CC13, C&FzCF3, and Cd33C12CF3) were also studied using HF/6-31G* basis set.
0022-3654/95/2099-15023$09.00/0 0 1995 American Chemical Society
Chen and Wu
15024 J. Phys. Chem., Vol. 99,No. 41, 1995
TABLE 1: Molecular Structure of Toluene with Various Basis S e w 3-21G Cl-C2 C2-H3 C2-C4 C4-H5 C4-C6 C6-H7 C6-C8 C8-H9 C8-C10 C10-H11 c1-ClO c1-c12 C12-Hl3 C12-Hl4 C12-Hl5 Cl-C2-H3 Cl-C2-C4 C2-C4-H5 C2-C4-C6 C4-C6-H7 C4-C6-C8 C6-C8-H9 C6-C8-C10 C8-ClO-Hll C8-C10-C1 c2-c1-ClO C2-Cl-Cl2 C10-Cl-C12 Cl-Cl2-Hl3 Cl-Cl2-Hl4 Cl-Cl2-Hl5 a
6-31G
6-31G*
6-3 1lG*
6-3 11+G**
planar
orthogonal
planar
orthogonal
planar
orthogonal
planar
orthogonal
1.388 1.072 1.387 1.071 1.385 1.072 1.385 1.071 1.384 1.073 1.388 1.516 1.084 1.085 1.084 119.5 120.9 119.8 120.2 120.3 119.5 120.1 120.2 119.7 120.9 117.9 121.4 120.7 110.1 110.5 110.0
1.388 1.073 1.384 1.072 1.384 1.072 1.384 1.072 1.384 1.073 1.388 1.518 1.084 1.086 1.084 119.5 120.8 119.8 120.2 120.3 119.5 120.0 120.2 119.6 120.8 118.6 120.7 120.7 110.0 110.4 110.0
1.391 1.074 1.389 1.074 1.386 1.073 1.386 1.073 1.386 1.075 1.391 1.516 1.082 1.084 1.084 119.5 120.9 119.7 120.2 120.3 119.4 120.0 120.2 119.6 121.0 117.7 121.5 120.8 110.0 108.5 108.5
1.392 1.074 1.388 1.074 1.387 1.073 1.387 1.073 1.387 1.075 1.392 1.515 1.083 1.084 1.082 119.5 121.0 119.8 120.2 120.3 119.4 120.0 120.2 119.6 121.0 118.4 120.8 120.8 110.0 108.3 108.4
1.388 1.076 1.388 1.076 1.383 1.075 1.383 1.075 1.383 1.077 1.393 1.516 1.084 1.085 1.085 119.6 121.0 119.7 120.2 120.4 119.4 120.0 120.2 119.5 121.0 117.7 121.5 120.8 110.0 108.5 108.5
1.390 1.076 1.386 1.076 1.385 1.075 1.385 1.075 1.385 1.077 1.390 1.516 1.084 1.086 1.083 119.5 121.0 119.7 120.2 120.3 119.4 120.1 120.2 119.5 121.0 118.4 120.8 120.8 110.0 108.4 108.5
1.387 1.076 1.387 1.076 1.382 1.075 1.387 1.076 1.382 1.077 1.390 1.515 1.084 1.085 1.085 119.6 121.0 119.7 120.3 120.4 119.3 120.0 120.2 119.5 121.0 118.2 121.4 120.4 110.1 108.6 108.6
1.389 1.077 1.385 1.076 1.384 1.075 1.384 1.076 1.385 1.077 1.390 1.515 1.084 1.086 1.084 119.6 121.0 119.7 120.2 120.3 119.3 120.1 120.2 119.5 121.0 118.4 120.8 120.8 110.0 108.5 108.5
planar 1.388 1.077 1.388 1.076 1.383 1.075 1.388 1.076 1.383 1.077 1.393 1.515 1.084 1.085 1.085 119.6 121.0 119.7 120.2 120.4 119.3 120.0 120.2 119.5 121.0 118.2 121.4 120.4 110.0 108.6 108.6
orthogonal 1.390 1.076 1.386 1.076 1.385 1.075 1.385 1.076 1.386 1.077 1.390 1.515 1.084 1.086 1.084 119.6 121.0 119.7 120.2 120.3 119.3 120.1 120.2 119.4 121.0 118.4 120.8 120.8 110.0 108.5 108.6
Bond lengths in A, bond angles in degrees.
Results and Discussion
Ab initio SCF calculations of the geometry of toluene were carried out using the HF/3-21G, 6-31G, 6-31G*, 6-311G*, and 6-31 l+G** basis sets. Two configurations of toluene (named planar and orthogonal) were obtained by full optimizations of all bond lengths, bond angles, and torsional angles. The computed bond distances and bond angles are given in Figure 1 and Table 1. Our calculated results are consistent with those reported by George et al.I4 who performed HF/6-31G and HFI 6-31G* calculations on toluene. From Table 1, although different basis sets were chosen, bond lengths and bond angles do not differ significantly. The split-valence 3-21G basis set, which can save considerable computational time, appears to be choice for this type of molecule. However, in order to understand the relative energies such as the ground and excited states and other applications such as rotational barrier, higher level calculations are necessary. Although we chose two intemal rotational isomers to do the calculation, their molecular structures were quite similar. Hence the C2-Cl-Cl2 and C10C1-C12 bond angles are almost equal in the orthogonal form, but in the planar form these angles are different. This difference was assigned by Ghosh and co-workers as the effect on the rotation-intemal rotation coupling term.I5 For nitromethane,'* its eclipsed and staggered configurations also have similar molecular structures. This suggests that molecules having sixfold intemal rotational barriers may have similar bond lengths and bond angles. Although the methyl group is not a strong electron acceptor, a substituent effect to the benzene ring still exists, causing a slight distortion in the phenyl ring. However, in some molecules such as the phenolic compounds or nitro compo~nds,~~ the - *substituent ~ effects are greater. Since toluene has a sixfold intemal rotational barrier, our calculations of toluene by various HF basis sets find that the
TABLE 2: Calculations of the Rotational Barrier in Toluene theoretical model HF16-31G//HFl6-3 1G HFl6-3 1G*//HF/6-3lG* HF16-31 lC**I/HF/6-311G** HFl6-31 l+G**I/HF/6-31 l+G** MP2/6-3 lG*//HF/6-3 lG*
E(p1anar)
-269.645 -269.739 -269.799 -269.802 -270.626 MP4SDTQ/6-31G*//HF/6-31G* -270.714 a
310 1 784 9 615 5 434 9 640 2 954 2
E(orthogona1) -269.645 -269.139 -269.799 -269.802 -270.626 -270.715
AE
324 0 8.72 7.97 797 6 686 6 7.09 452 6 11.10 819 1 112.30 021 5 42.20
Energies in hartrees, rotational barriers in cal/mol.
orthogonal configuration is more stable than that of the planar form. George et aLl4 and Ghosh et al.I5 also obtained similar results. The 4-21, STO-3G, 4-31G, 6-31G, and 6-31G* basis set calculations underestimated the barrier.11-12,14Ghosh et al.I5 overestimated the intemal rotational barrier of toluene as 19.36 cal/qol (the experimental value from microwave spectroscopy is 13.94 cal/m0l'9~~) by ab initio (HF/4-31G) harmonic force field for the geometry and semiempirical nonbonded energies. The results of our calculations for intemal rotational barrier of toluene using various basis sets are given in Table 2. Clearly, the values of the intemal rotational barrier depend on the basis set employed in the calculation. All our HF optimizations underestimate the intemal rotational barriers. Among them, the 6-31 l+G** basis set, the most complete basis set we employed at the HF level, gives the best result; see Table 2. In order to assess the effects of electron correlation, we performed the Moller-Plesset perturbation theory. Hence, two single point calculations (MP2/6-3 lG* and MP4SDTQ/6-31G*) were chosen, and the results are shown in Table 2. From these calculations, it was found that the inclusion of electron correlation overestimated the intemal rotational barrier of toluene. The same result was also found for the Moller-Plesset perturbation theory studied for nitrobenzene, but the error was not so high.24 Some report^",^^-^^ suggested that quite modest
J. Phys. Chem., Vol. 99, No. 41, 1995 15025
Ab Initio Study of Toluene and Its Derivatives
TABLE 4: Geometry of Toluene, 2,6-Dffluorotoluene,and 2,6-DichlorotolueneUsing HF/6-31G* Basis SeP
TABLE 3: Geometry of Toluene, Fluorotoluene, and Chlorotoluene Using HF/6-31G* Basis SeP toluene
fluorotoluene
chlorotoluene
toluene
olanar orthoeonal olanar orthoeonal Dlanar orthoeonal Cl-C2 C2-H3 C2-C4 C4-H5 C4-C6 C6-X7 C6-C8 C8-H9 C8-C10 C10-HI1 c1-c10 c1-c12 C12-Hl3 C12-Hl4 C12-Hl5 Cl-C2-H3 Cl-C2-C4 C2-C4-H5 C2-C4-C6 C4-C6-X7 C4-C6-C8 C6-C8-H9 C6-C8-C10 C8-ClO-Hll C8-c10-Cl C2-C1-C10 C10-Cl-C12 C2-Cl-Cl2 Cl-Cl2-Hl3 CI-Cl2-Hl4 Cl-Cl2-Hl5
1.388 1.076 1.388 1.076 1.383 1.075 1.383 1.075 1.383 1.077 1.393 1.516 1.084 1.085 1.085 119.6 121.0 119.7 120.2 120.4 119.4 120.0 120.2 119.5 121.0 117.7 120.8 121.5 110.1 108.5 108.5
1.390 1.076 1.386 1.076 1.385 1.075 1.383 1.075 1.385 1.077 1.390 1.516 1.084 1.086 1.083 119.5 121.0 119.7 120.2 120.3 119.4 120.1 120.2 119.5 121.0 118.4 120.8 120.8 110.0 108.4 108.5
1.388 1.076 1.388 1.074 1.375 1.332 1.379 1.074 1.383 1.077 1.393 1.516 1.084 1.085 1.085 119.7 121.5 121.6 118.6 119.2 121.3 119.7 118.6 119.0 121.4 118.6 121.3 120.4 110.0 108.5 108.5
1.390 1.076 1.385 1.074 1.377 1.332 1.377 1.074 1.385 1.076 1.390 1.516 1.084 1.085 1.083 119.6 121.5 121.7 118.6 119.1 121.8 119.7 118.6 118.9 121.5 118.0 120.9 120.9 109.9 108.3 108.5
1.387 1.076 1.387 1.074 1.380 1.746 1.384 1.074 1.382 1.076 1.392 1.515 1.083 1.085 1.085 119.8 121.4 120.6 119.3 119.7 120.7 120.1 119.2 120.4 121.3 118.1 121.6 119.8 110.0 108.5 108.5
1.389 1.076 1.385 1.074 1.382 1.746 1.382 1.074 1.384 1.076 1.393 1.515 1.084 1.085 1.083 119.8 121.4 120.6 119.3 119.7 120.7 120.1 119.3 118.9 121.4 118.0 120.9 121.0 110.0 108.3 108.5
planar orthogonal planar orthogonal planar orthogonal Cl-C2 C2-X3 C2-C4 C4-H5 C4-C6 C6-H7 C6-C8 C8-H9 C8-C10 c10-x11 c1-ClO c1-c12 C12-HI3 C12-Hl4 C12-HI5 Cl-C2-X3 c 1 -c2-c4 C2-C4-H5 C2-C4-C6 C4-C6-H7 C4-C6-C8 C6-C8-H9 C6-C8-C10 C8-ClO-Xl1 C8-C10-Cl C2-Cl-C10 c2-c1 -c12 c1o-c1-c12 Cl-Cl2-Hl3 Cl-Cl2-Hl4 Cl-Cl2-Hl5
Bond lengths in A, bond angles in degrees; X = H, F, or C1 atom.
levels of ab initio theory perform very well for the study of methyl or ethyl rotation and our calculations agree with those reports. Molecular structures of 3-fluorotoluene and 3-chlorotoluene are given in Table 3 using HF/6-31G* basis set. Both the fluorine and the chlorine atoms are strong electron acceptors. From Figure 1 and Table 3, it can be seen that the bond lengths C4-C6 and C6-C8 of the 3-fluorotoluene and 3-chlorotoluene decreased when compared with the usually adopted bond length of C-C (1.396 A) for the benzene ring27and toluene. It was also found that the bond angle C4-C6-C8 of the 3-fluorotoluene and 3-chlorotoluene are larger than that of toluene. Although the phenyl rings of these two compounds are still planar, their bond lengths and bond angles are distorted. The electron-withdrawing strength of the F atom is stronger than that of the C1 atom. Hence compared with the substituent effect of these two molecules, 3-fluorotoluene has shorter C4-C6 and C6-C8 bond lengths and a larger C4-C6-C8 bond angle. These results had the same trends as the study of the molecular geometry of substituted benzene derivatives by Domenicano et alS2*Like the toluene molecule, there was no significant change in the geometries of 3-fluorotoluene and 3-chlorotoluene with their internal rotational isomers. The effect on the rotationinternal rotation coupling term also existed in these compounds. It was also found that the configuration with the 30" torsional angle of the methyl group was still the most stable molecule. The 2,6-disubstituted effect is also important to understand how they influence the internal rotational isomers as well as the phenyl ring. In this study, two toluene derivatives, 2,6difluorotoluene and 2,6-dichlorotoluene, were chosen. Their geometries are given in Table 4 using HF/6-31G* basis set. For 2,6-difluorotoluene, because of the substituent effect, larger Cl-C2-C4 and Cl-ClO-C8 bond angles, a smaller C2-C1C10 bond angle, and shorter Cl-C2, C2-C4, C8-Cl0, and C1-C10 bond lengths as compared to toluene were obtained.
2,6-difluorotoluene 2,6-dichlorotoluene
1.388 1.076 1.388 1.076 1.383 1.075 1.383 1.075 1.383 1.077 1.393 1.516 1.084 1.085 1.085 119.6 121.0 119.7 120.2 120.4 119.4 120.0 120.2 119.5 121.0 117.7 121.5 120.8 110.1 108.5 108.5
1.390 1.076 1.386 1.076 1.385 1.075 1.383 1.075 1.385 1.077 1.390 1.516 1.084 1.086 1.083 119.5 121.0 119.7 120.2 120.3 119.4 120.1 120.2 119.5 121.0 118.4 120.8 120.8 110.0 108.4 108.5
1.383 1.331 1.381 1.073 1.383 1.074 1.386 1.073 1.376 1.331 1.386 1.512 1.080 1.083 1.083 118.5 123.6 119.3 118.6 119.8 120.4 122.1 118.3 118.5 124.0 115.1 123.5 121.4 109.6 109.0 109.0
1.384 1.331 1.379 1.073 1.384 1.074 1.384 1.073 1.378 1.331 1.389 1.512 1.081 1.085 1.081 118.1 123.8 119.4 118.5 119.8 120.4 122.1 118.5 118.2 123.5 115.3 122.3 122.4 109.6 108.5 109.2
1.393 1.750 1.385 1.073 1.380 1.074 1.383 1.073 1.380 1.748 1.396 1.514 1.078 1.083 1.083 120.9 122.7 119.5 119.7 120.1 119.8 121.0 119.3 117.4 123.1 115.5 123.8 120.7 110.5 109.0 109.0
1.394 1.749 1.383 1.073 1.381 1.074 1.382 1.073 1.382 1.749 1.395 1.514 1.079 1.085 1.079 120.3 122.9 119.6 119.5 120.2 119.7 120.9 119.5 116.9 122.9 115.4 122.3 122.3 110.1 108.7 108.7
Bond lengths in A, bond angles in degrees; X = H, F, or C1 atom.
TABLE 5: Geometry of Toluene, Benzotrifluoride, and Benzotrichloride Using HF/6-31G* Basis SeP ~~
toluene
benzotrifluonde
benzotnchlonde
planar orthogonal planar orthogonal planar orthogonal Cl-C2 1.388 C2-H3 1.076 C2-C4 1.388 C4-H5 1.076 C4-C6 1.383 C6-H7 1.075 C6-C8 1.383 C8-H9 1.075 C8-C10 1.383 C10-H11 1.077 c1-ClO 1.393 c1-c12 1.516 C12-Xl3 1.084 C12-Xl4 1.085 C12-Xl5 1.085 Cl-C2-H3 119.6 Cl-C2-C4 121.0 C2-C4-H5 119.7 C2-C4-C6 120.2 C4-C6-H7 120.4 C4-C6-C8 119.4 C6-C8-H9 120.0 C6-C8-C10 120.2 C8-ClO-HI 1 119.5 C8-ClO-C1 121.0 C2-C1-ClO 117.7 C2-Cl-Cl2 121.5 C10-C1-C12 120.8 Cl-Cl2-Xl3 110.1 Cl-Cl2-Xl4 108.5 Cl-Cl2-Xl5 108.5 a
1.390 1.076 1.386 1.076 1.385 1.075 1.383 1.075 1.385 1.077 1.390 1.516 1.084 1.086 1.083 119.5 121.0 119.7 120.2 120.3 119.4 120.1 120.2 119.5 121.0 118.4 120.8 120.8 110.0 108.4 108.5
1.383 1.073 1.388 1.075 1.383 1.075 1.388 1.075 1.381 1.075 1.390 1.511 1.323 1.321 1.321 120.2 119.8 119.7 120.2 120.0 120.0 120.2 120.0 120.1 119.9 120.1 121.4 118.5 110.9 107.9 107.9
1.386 1.074 1.385 1.075 1.385 1.075 1.386 1.075 1.384 1.074 1.392 1.511 1.324 1.321 1.320 120.1 119.9 119.8 120.1 120.0 120.0 120.2 120.1 120.0 119.9 120.1 120.0 119.9 110.5 107.5 107.9
1.384 1.070 1.390 1.075 1.379 1.075 1.389 1.075 1.378 1.074 1.398 1.533 1.778 1.782 1.782 121.0 120.1 119.2 120.5 120.3 119.5 120.3 120.2 119.4 120.6 119.1 123.1 117.8
112.5 108.0 108.0
1.389 1.071 1.385 1.075 1.383 1.075 1.385 1.075 1.383 1.072 1.391 1.534 1.782 1.778 1.782 120.7 120.5 119.3 120.4 120.3 119.4 120.3 120.4 118.9 120.5 118.9 120.9 120.2 111.4 109.0 106.8
Bond lengths in A, bond angles in degrees; X = H, F, or C1 atom.
The distortion of the phenyl ring is substantial. However, for 2,6-dichlorotoluene, in addition to the substituent effect the steric effect is also important. Because of the steric effect, longer Cl-C2 and C1-C10 bond lengths, and larger Cl-C2-C13
Chen and Wu
15026 J. Phys. Chem., Vol. 99, No. 41, 1995
Cl-ClO-C8, and C2-C1-C10 bond angles have several degrees of change and the reason is that the -CF3 group is a heavy atom and strong electron acceptor. Similar geometry was found by the internal rotation of this -CF3 group of the benzotrifluoride. Second, the geometry of benzotrichloride was analyzed. The molecular structure of benzotrichloride is mainly controlled by the steric effect of the C1 atoms. Hence, the C 1C12 bond length was longer than that of toluene and benzotrifluoride. Like benzotrifluoride, the geometry of the intemal rotational isomer of benzotrichloride has no significant change. For the orthogonal configuration of benzotrichloride, the C2C1-C12 and C10-C1-C12 bond angles were not the same as those of toluene and its derivatives. This difference is due to the strong -CCl3 group and steric effect. The inclusion of the F atoms or C1 atoms in the 2,6-position of benzotrifluoride changes the bond lengths and the bond angles of the phenyl ring, which are shown in Table 6. For each molecule, internal rotational isomer has similar geometry with its original molecule. Comparing the geometry of 2,6-difluorobenzotrifluoride with 2,6-difluorotoluene (or 2,6-dichlorobenzotrifluoride with 2,6-dichlorotoluene), the heavy atom of -CF3 (or -CCl3) group indeed has some effects on the deformation of the benzyl ring. Energies and some physical parameters of toluene and its derivatives are listed in Table 7 using the HF/6-3 lG* basis set. For each molecule, the planar and the orthogonal configurations were calculated. For molecules without the 2,6-disubstitutions, the orthogonal structures were the most stable compounds. One exception is benzotrichloride which has more stable planar configuration. Steric factors may be the reason for this molecule to adopt this particular configuration. Similar results were observed by Ghosh et al. for toluene and fluorotoluene molecule^.'^ However, for the other four molecules, 2,6difluorotoleune, 2,6-dichlorotoluene, 2,6-difluorobenzofluoride, and 2,6-dichlorobenzofluoride, the substitutions of the heavy atoms at the 2,6-positions has a strong steric effect to prevent the intemal rotation of the -CH3, -CF3, or -CCl3 group. Hence these orthogonal configurations have unstable structures. Data on the dipole moments are useful for the study of the solvent chemistry. The calculated dipole moments of toluene and 3-fluorotoluene using I-IF/6-3 lG* basis set were smaller than the experimental values;29 see Table 7. This result has the same trend as that reported by Marriott and co-workers for some monosubstituted benzenes.30 It was found that the substitutions change the dipole moments to larger values, but the dipole moments of each intemal rotational isomer were similar. Ionization potentials were calculated by Koopmans’ theory. Intemal rotation of methyl or other heavy top did not change the ionization potential of each molecule. The values of the ionization potentials increased with the heavy atom substitutions.
TABLE 6: Geometry of Benzotrifluoride, 2,6-Difluorobenzotrifluoride, and 2,6-DichlorobenzotrifluorideUsing HF/6-31G* Basis SeP benzotrifluoride
benzotrifluoride
benzotrifluoride
planar ortthgonal planar orthogonal planar orthogonal Cl-C2 C2-X3 C2-C4 C4-H5 C4-C6 C6-H7 C6-C8 C8-H9 C8-C10 ClO-XI1 Cl-c10 c1-c12 C12-Fl3 C12-Fl4 C12-Fl5 Cl-C2-X3 CI-C2-C4 C2-C4-H5 C2-C4-C6 C4-C6-H7 C4-C6-C8 C6-C8-H9 C6-C8-C10 C8-ClO-Xll C8-ClO-C1 c2-Cl-ClO c2-c1 -c12 C10-Cl-C12 Cl-Cl2-Fl3 Cl-Cl2-Fl4 Cl-Cl2-Fl5 a
1.383 1.073 1.388 1.075 1.383 1.075 1.388 1.075 1.381 1.075 1.390 1.511 1.323 1.321 1.321 120.2 119.8 119.7 120.2 120.0 120.0 120.2 120.0 120.1 119.9 120.1 121.4 118.5 110.9 107.9 107.9
1.386 1.074 1.385 1.075 1.385 1.075 1.386 1.075 1.384 1.074 1.392 1.511 1.324 1.321 1.320 120.1 119.9 119.8 120.1 120.0 120.0 120.2 120.1 120.0 119.9 120.1 120.0 119.9 110.5 107.5 107.9
1.385 1.320 1.382 1.073 1.380 1.074 1.386 1.073 1.374 1.323 1.389 1.516 1.316 1.318 1.318 120.4 122.4 118.8 119.1 119.7 120.7 122.3 118.2 118.6 123.1 116.5 125.0 118.5 111.7 107.9 107.9
1.389 1.321 1.378 1.073 1.382 1.074 1.383 1.073 1.378 1.321 1.389 1.519 1.319 1.315 1.315 119.6 123.0 119.0 118.8 119.7 120.6 122.2 118.7 117.5 123.1 115.9 122.2 121.9 110.8 108.0 107.3
1.396 1.742 1.388 1.072 1.376 1.074 1.382 1.072 1.379 1.741 1.403 1.523 1.313 1.321 1.321 124.4 121.3 118.9 120.3 120.0 119.9 121.0 119.6 115.8 122.1 116.8 124.9 118.3 114.2 106.7 106.7
1.401 1.743 1.384 1.072 1.378 1.074 1.378 1.072 1.384 1.742 1.404 1.526 1.316 1.321 1.317 123.7 121.8 119.1 120.1 120.1 119.8 120.8 120.1 114.6 121.8 116.5 122.0 121.5 112.8 107.8 105.4
Bond lengths in A, bond angles in degrees; X = H, F, or C1 atom.
and C1 -ClO-Clll bond angles as compared to that of toluene and 2,6-difluorotoluene, were observed. Comparing the substituent and steric effects of the 2,6-dichlorotoluene, it can be seen that the steric effect is more important than the substituent effect. Internal rotation studies of the geometries of 2,6difluorotoluene and 2,6-dichlorotoluene yielded similar bond lengths and bond angles to their original geometries. The C2C1-C12 and C10-C1-C12 bond angles were still almost equal in the orthogonal form as in toluene molecule, but the difference of these two bond angles in the planar configuration was up to several degrees and this difference was far more than that observed for toluene molecule. This change was probably to avoid the steric crowd during the torsion of the methyl top. Benzotrifluoride (C6H5CF3) and benzotrichloride (CsHsCC13), two heavy top derivatives of toluene, are useful solvents and intermediates for dyes and pharmaceutical^.^ Their molecular structures are given in Table 5 using HF/6-31G* basis set. First, we compared the geometry of benzotrifluoride with toluene. It was found that the bond lengths of the phenyl ring were similar to those of the toluene molecule. However, the Cl-C2-C4,
TABLE 7: Energies and the Physical Constants of Toluene and Its Derivatives Using HF/6-31G* Basis SeP planar E C6&CH? C6H4FCH3 C6&C1C& C6H3FzCH3 C6H3C12CH3 C6H5CF3 C6H5CC13 C6H3FzCF3 C6H3ClzCF3 a
-269.739 -368.590 -728.640 -467.443 -1187,535 -566.323 -1646.407 -764.015 -1484.099
785 903 202 393 482 582 056 375 369
orthogonal P
IP
E
P
IP
0.276 1.974 2.515 1.162 1.391 2.489 2.689 3.887 4.100
0.319 0.322 0.324 0.340 0.341 0.349 0.348 0.378 0.355
-269.739 798 -368.590 91 1 -728.640 231 -467.443 350 -1187.535 207 -566.323 705 - 1646.406 544 -764.014 584 -1484.097 195
0.275 (0.3)b 1.973 2.517 1.157 1.388 2.505 (2.8)b 2.701 3.883 4.081
0.319 0.322 0.322 0.340 0.340 0.349 0.350 0.380 0.355
Energies ( E ) and ionization potentials (IP)in hartrees, dipole moments (u) in debyes. * Experimental value.29
Ab Initio Study of Toluene and Its Derivatives Conclusions
1. Geometries of toluene and its sixfold intemal rotational isomer were calculated by various high level ab initio HF calculations. Results showed no significant difference in the two intemal rotational isomers. Rotational barrier was studied by both HF and post-HF levels, and the results showed that a precise rotational barrier can be obtained by the HF/6-31l+G** basis set. 2. For other toluene-like molecules in this study, its two intemal rotational configurations were similar with the exception of the tilts of the C1 -C12 bond. The rotation-internal rotation coupling term has an effect on all these intemal rotational isomers. 3. Single substitution by the heavy atom at the para-position in the phenyl ring did not change the phenyl ring significantly, but two heavy atom substitutions on the 2,6-position of the phenyl ring had a substantial deformation of the phenyl ring. 4. Stability of two intemal rotational isomers of nine toluene derivatives determined not only the heavy atom substitutions on the 2,6-position but also the steric effects between the substitution and the methyl or heavy tops. Acknowledgment. This work was supported by the National Science Foundation (NSC 83-0208-M-014-005). The calculation facility was supported by National Center for Highperformance Computing. Supporting Information Available: Tables listing, the Z-matrices of toluene and its derivatives (4pages). Ordering information is given on any current masthead pages. References and Notes (1) Rudolph, H. D.; Dreizler, H.; Jaeschke, A,; Wendling, P. 2. Natulforsch. 1967, 22a, 940. (2) Kreimer, W. A.; Rudolph, H. D.; Tan, B. T. J . Mol. Spectrosc. 1973, 48, 86. (3) Amir-Ebrahimi, V.; Choplin, A.; Demaison, J.; Roussy, G . J . Mol. Spectrosc. 1981, 89, 42. ( 4 ) Hawley, G. G. The Condensed Chemical Dictionary, loth ed.; Mei Ya Publications, Inc.: Taipei, 1981. ( 5 ) Tannenbaum, E.; Myers, R. J.; Gwinn, W. D. J . Chem. Phys. 1956, 25, 42. (6) Naylor, R. E.; Wilson, Jr. E. B. J . Chem. Phys. 1957, 26, 1057.
J. Phys. Chem., Vol. 99, No. 41, I995 15027 (7) Cheng, C. S . ; Beaudet, R. A. J . Mol. Spectrosc. 1970, 36, 337. (8) Wollrab, J. E.; Rinehart, E. A.; Reinhart, P. B.; Reed, Jr. R. R. J. Chem. Phys. 1971,55, 1998. (9) Rudolph, H. D.; Dreizler, H.; Seiler, H. 2. Naturforsch. 1967, 22a, 1738. (10) Seip, R.; Schultz, G. Y.; Haittai, I.; Szabo, G. Z. Naturforsch., Teil a, 1977,32, 1178, 1977. Iijima, T. Z . Naturforsch., Teil a 1977, 32, 1063. (1 1) Pang, F.; Boggs, J. E.; Pulay, P.; Forgarasi, G. J. Mol. Struct. 1980, 66, 281. (12) Gough, K. H.; Henry, B. R.; Wildman, T. A. J . Mol. Struct. 1985, 124, 71. (13) Bock, C. W.; Trachtman, M.; George, P. J . Mol. Struct. (THEOCHEM) 1985, 122, 155. (14) George, P.; Bock, C. W.; Stezowski, J. J.; Hildenbrand, T.; Glusker, J. P. J . Phys. Chem. 1988, 92, 5656. (15) Ghosh, P. N.; Ha, Tae-kyu. J . Mol. Struct. (THEOCHEM) 1990, 206, 287. (16) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S . ; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92 User’s Guide; Gaussian Inc.: Pittsburgh, PA, 1992. (17) Here, W. J.; Radom, L.; Schelyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; John Wiley & Sons: New York, 1986. (18) Bock, C. W.; Krasnoshchiokov, S . V.; Khristenko, L. V.; Panchenko, Yu. N.; Pentin, Yu. A. J . Mol Struct. (THEOCHEM) 1987, 149, 201. (19) Chen, P. C.; Huang, C. C. J . Mol Struct. (THEOCHEM) 1993,282, 287. (20) Pandarese, F.; Ungaretti, L.; Coda, A. Acta Crystallogr., Sect. E 1975. 31, 2671. (21) Thompson, M.J.; Zeegers, P. J. Tetrahedron 1989, 45, 191. (22) Kagawa, T.; Kawai, R.; Kashimo, S . ; Haisa, M. Acta Crystallogr., Sect. B 1976, 32, 3171. (23) Gorgy, W.; Cook, R. L. Microwave Molecular Spectra; John Wiley & Sons: New York, 1984. (24) Head-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 173, 585. (25) Radom, L.; Baker, J.; Gill, P. M. W.; Nobes, R. H.; Riggs, N. V. J . Mol. Struct. 1985, 126, 271. (26) Wiberg, K. B.; Martin, E. J. Am. Chem. SOC. 1985, 107, 5035. (27) Sutton, L. E. Tables of Interatomic Distances; The Chemical Society: London, 1958. (28) Domenicano, A,; Vaciago, A.; Coulson, C. A. Acta Crystallogr., Sect. E 1975, 31, 221. (29) Nelson, R. D.; Lide, D. R.; Maryott, A. A. Selected values of electric dipole moments for molecules in the gas phase. Natl. Stand. Re$ Data Ser., Natl. Bur. Stand. 1967, 10. McClennan, A. L. Tables of Experimental Dipole Moments, Vol. 1 ; Freeman: San Francisco, 1963; Vol. 2. (30) Marriott, S . ; Silverstro, T.; Topsom, R. D.; Bock, C. W. J . Mol. Struct. (THEOCHEM) 1987, 151, 15. Jp950102P
