J. Phys. Chem. 1994,98, 1087-1089
1087
?r-ElectronStabilization in Benzocyclobutenes Ming-Chiu Ou and San-Yan Chu' Department of Chemistry, National Tsing H u a University, Hsinchu 30043, Taiwan, Republic of China Received: July 20, 1993; In Final Form: November IS, 1993"
A series of benzocyclobutenes with increasing ring distortion were studied by 6-3 lG* Hartree-Fock calculations. The 7~ electrons show increasing stability relative to those in the corresponding methyl-substituted benzene systems. T h e result is in accord with the photoelectron spectroscopy and the notion that ?r electrons in benzene disfavor the symmetric structure.
Introduction
SCHEME 1
The special stability and the regular geometry of benzene ring can be qualitatively explained by the Hiickel theory for A electrons. However, a modified theory using the resonance integral /3 as bond distancedependent found that the a electronsactually prefer a distorted geometry corresponding to a Kekule structure.' On the other hand, a VB modeP5 predicted that the *-electron delocalization in benzene is not a driving force of stability and symmetric geometry. From many recent studies, it is found that the ?r electrons are unstable in the regular geometry and the u frame always favor equal bond length.6s7 Thus, the regular geometry of benzene is the product of two opposing driving forces: a distorting 7r system and a symmetrizing u frame. The latter dominates and forces the delocalization of a electrons. In this work, we would like to show that *-electron energies of some strained aromatic systems would bear out such general theme. The systems we studied are a series of benzocyclobutenes. We are interested in the stabilization effect of s electron with respect to the ring alternating distortion from the strain. The work is focused on the *-electron energy difference between these benzocyclobutenes and the corresponding methyl-substituted benzenes. These two types of compounds have similar ringsubstituent interactions. Therefore, the difference can be attributed mainly to the *-stabilization energy from the ring distortion. We would also compare our calculation with the photoelectron spectroscopy.8-11
Theoretical Methods Ab initio LCAO-MO S C F calculations were performed with 6-31G* basis set using GAUSSIAN 92 program.12 A series of benzocyclobutenes and the corresponding methyl-substituted compounds were fully optimized and harmonic vibrational frequencies were also calculated with the specific symmetry as required (Scheme 1). The benzocyclobutenes are highly strained. Therefore, there is a significant change of carbon hybridization in the ring. We analyze the 2s character of the hybrid orbitals and their relation with the bend bond angles. Because the a electrons of those molecules are strongly delocalized, the resonance keyword has to be used for wave function analysis by the natural bond orbital (NBO)I3J4method.
Results and Discussion The mono-, bis-, and trisannelated benzocyclobutenes have an important, so-called Mills-Nixon ( M N ) effectI5 which gives rise to alternate carbon4arbon distances in the benzene ring. When the benzene ring is fused to a small ring,'G19it results in bend bond strain in both rings and rehybridization of the fused carbon centers. The effect can be studied from the 2s and 2p populations. ~~
@
~~
~
Abstract published in Advance ACS Abstracts, January 1, 1994.
0022-3654/94/2098- 1087$04.50/0
6 la
Ib
SCHEME 2
According to the M N effect, more p character is associated with the hybrid orbital at the annelated carbon directed to the endo C-C bond and more s character is associated with the carbon directed to the exo C-C bond (Scheme 2). This rehybridization of the carbon atom of the benzene ring is expected to cause C-C bond alternation; i.e., one bond is lengthened and the adjacent ones are shortened. Calculated C-C bond distances of the benzene ring in benzocyctobutenes la-3a are displayed in Figure 1. The C-C bond distances a t HF/6-31G* level agree well with the experimental data.20 For example, in la, the calculated bond distances vs the experimental ones shown in parentheses are 1.380 ~(1.391)forR~2,1.518~(1.518)forR~7,and 1.572A(1.576) for R78 in the cyclobutene ring (see Scheme 1 for the numbering system). For R23, R34, and R45 in the benzene ring, the bond lengthsare 1.378 A (1.385), 1.394A (1.400) and 1.392A (1.399), respectively. The calculated endo C-C bond distance is longer than the neighboring exo C-C distances in all three systems without exception and it is useful to rationalize the M N effect. The alternation in 3a reaches a maximum due to the presence of the largest numbers of fused cyclobutenes. We define a quantity AR to show the importance of the bond alternation. hR = (R12 + R34 + R56) - (R23 + R45 + R61) (l) As displayed in Table 1, l a with only monoannelation has a small value of AR 0.0203 A; 2a with bisannelation has a larger value of AR 0.0351 A, and 3a with trisannelation has the largest value of AR 0.0457 A. The distortion of benzene ring with methyl substitution is different from that in benzocyclobutenes (Table 1). The methyl substitutent gives the same bond-lengthening effect on the two adjacent C - C bonds in the ring. The fused cyclobutene group 0 1994 American Chemical Society
Ou and Chu
1088 The Journal of Physical Chemistry, Vol. 98, NO. 4, 1994
TABLE 1: Calculated Total Energies (hartree), Bond Lengths (angstroms), and a-Orbital Energies (hartree) at the HF/631G* Level for Optimized Structure benzene la lb 2a 2b 3a 3b symmetry D6h C2" C7.O D3h C7.O C7.O C6h Etotal
Ra ARb
E, I Er2 ET3
ETC
A&
I
-230.7031 1.3862 0.0000 -0.33063 -0.33063 -0.49196 -2.31844
-307.5654 1.3858 0.0203 -0.3 1416 -0.32740 -0.48369 -2.25050
-308.7762 1.3883 0.0187 -0.3 129 1 -0.32266 -0.47753 -2.22620 0.00125 0.02430
-386.8428 1.3920 0.0036 -0.30267 -0.30909 -0.45866 -2.14084
-384.4272 1.3852 0.0351 -0.30562 -0.31768 -0.4 7023 -2.18706
464.8993 1.3979 0.0000 -0.29446 -0.29446 -0,44005 -2.05794
0.00870 0.06934
0.00295 0.04622
a R is the averaged - C-C bond distances in benzene ring. AR: see eq 1 for definition. E , = 2(E,1 = E,(b) - &(a).
la
-461.2884 1.3846 0.0457 -0.30316 -0.30316 -0.45732 -2.12728
+ E,z + E,3),
AE,1 = E,l(b) - E,l(a), AE,
-'-I
.
-2.44
I
1
0
.
I
2
.
I
3
.
I
4
n
Figure 2. The total
electron energies (E,) for the series a and b, respectively. The difference, AE,, of the ?r stabilizationenergies for the ?r
seriesaisdefinedasE,(b)-E&). R=1.3979
Figure 1. Carbon-carbon bond length in benzene ring shown by the upper number and s character of the two bonding atoms shown by the lower italic number. has the opposite effect on endo and exo C-C bond as illustrated in Scheme 2 for benzocyclobutenes. From NBO analysis, the s character for the u hybrid orbitals of C2 atom is 0.31 and 0.38 along the endo and exo direction respectively for the system l a (seescheme 1). Incontrast, thescharacter forthehybridorbitals at the methyl-substituted site is the same constant 0.35 along both C-C bond directions in the series 1C3b. The s character of hybrid orbital correlates well with the bond length. The larger the s characters, the shorter the bond length. Therefore, the distribution of s character is well correlated with AR. The benzocyclobutene series has more important alternative distributions of s character and thus compounds in this series have larger AR values than those in the methyl-substituted series. Furthermore, the average C-C bond lengths around the ring, E,is quite different between the two series. In the a series, R is nearly constant equal to 1.385. It means a true bond alternation occurs in the benzene ring. On the other hand, R in the b series increases slightly with increasing number of methyl substituents. It indicates some intramolecular steric effect between the methyl substituents and the benzene ring. This may explain the fact that there is one imaginary out-of-plane vibrational frequency for the planar 3b molecule. The a-orbital energies are also listed in Table 1. There are two types of perturbations for the a-electron energy for the a series when compared with benzene; one is the substituent effect from H to CH2, and the other is the ring alternation effect. We would like to make a comparison between the series a and series b to obtain the information of the second effect in the series a.
Asnincreaseswithmoresubstitution,
the bond alternation (AR)for the a series is more important, as shown in Table 1. The assumption is that the substituent (CH2)2 in a has a similar effect as (CH& in b. The general observation is that the a series with larger AR has a more stable a-electron energies (E,) than those in the b series. The E, here is simply approximated by the sum of a orbital energies. We expect that the driving force behind the a-electron stabilization is the ring alternation. The AE,,the a-stabilization energy, which is defined as the difference between E,(b) and the corresponding &(a), is 0.024 30 hartree for monoannelated benzene, 0.04622 for bisannelated benzene, and 0.069 34 for trisannelated benzene. As the number of the fused ring increases, so does the a-stabilization energy AE, (Figure 2). We can approximate the amount of AE, per one fused cyclobutene to be 0.024hartree relative to the dimethyl substitution (Table 1). Since there is a slight C-C bond lengthening in the b series, a question about the meaning of AE, arises. Does it AE, reflect the destabilization of E,(b) due to C-C bond stretching or the stabilization of E&) due to ring alternation? To answer this question, we have performed the calculation for the unsubstituted benzene ring with the C-C bond distance of 3b. We find the E, difference between the optimized and the stretched one is 0.0189 hartree, which is small in comparison with the value of AE, for 3a (0.06934 hartree). The result supports our expectation that AE, is mainly caused by the a-electron stabilization from the ring distortion in the series 8.21 It is also interesting to find that the H O M O orbital energy (E,,) is more stable in the a series than in the b series in all cases. It is qualitatively consistent with the observed ionization potentials (IP) in photoelectron spectroscopy.*-ll In the a series, IPequals to 8.66,8.35,and 8.15 eV for la-3a, respectively. The corresponding values in the b series are 8.56,8.29,and 7.85 eV. The IP difference for each pairbetweenaandbis0.1,0.06,and0.30eVor0.003 68,0.00551, and 0.011 03 hartree. Our calculated values, listed as AE,1, are 0.001 25,0.00295,and 0.008 70 hartree. The trend is in general agreement with the experimental results. However, the more
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1089
*-Electron Stabilization in Benzocyclobutenes
SCHEME 3
Conclusion
A
2a
2a’
TABLE 2 Comparison of Calculated Results for 2a and 2a‘ Shown in Scheme 3. 2a
2a’
C2C
D2h
-384.4271632 1.3852 0.0351 -0.30562 -0.31768 -0.47023 -2.18706
-386.4306072 1.3858 0.0000 -0.29894 -0.32444 -0,46975 -2.18626 0.00668 0.00080
(1
See Table 1 for definitions.
important conclusion is that the ?r-electronenergies, either HOMO or the sum of *-orbital energies, are more stable in the alternate C-C bond environment of series a than in series b. We note that the magnitude of *-electron stabilization energy defined here AE, = E,(b) - &(a) is considerably higher than the values obtained from the distorted benzene calculations with a more elaborate definition of E,. For example, our AE, for the 3a system is0.069 34 hartreeor 43.5 kcal mol-’ with AR = 0.0457 A. The corresponding AE,, AR values of Star1ger,2~.~~ Jug,6and Hiberty’ are 9.0 kcal mol-l,O.l77 A; 12.6 kcal mol-l,O.l6 A; and 9.7 kcal mol-’, 0.12 A, respectively. We suspect the fact that fewer hydrogen atoms in the a series relative to b series may contribute some extra .rr stabilization beside the effect from the ring distortion effect. Unfortunately, it is difficult to estimate this contribution directly. We suggest to compare the pair (2a, 2a’) (Scheme 3) instead of (2a, 2b) for the ring distortion effect. The two systems in the former pair have the same number of hydrogen atoms. The E,, AR values (Table 2) for 2a and 2a’ are -2.18706 hartree, 0.0351 A and -2.18616 hartree, 0.00 A, respectively. The bond alternation is more important in 2a than 2a’. The E, of 2a is 0.5 kcal mol-’ lower than that of 2a’. The E,1 values of 2a and 2a’are 8.31 and 8.13 eV, respectively, with the difference of 4.2 kcal mol-’. The observed IP for 2a and 2a’ are 8.35 and 8.22 eV, respectively, with thedifference of 3.5 kcal mol-’. From this comparison, we see that the picture that ?r electrons favor bond fixation still persists. However, the driving force, the E, difference between 2a and 2a’ vs AR,is smaller than the AE,, the value obtained from 2a and 2b. Such reduced value from 2a and 2a’ appears to be more in accord with the results of the other authors.
The T electrons of benzocyclobutene series show increasing stability relative to those in the corresponding methyl-substituted benzene system with increasing number of substituents. The trend of this calculation agrees with the photoelectron spectroscopy. The implication of our results is consistent with the notion that the T electron in benzene disfavors the symmetric structure. The NBO analysis reveals significant strain in the u frame as evidenced by strong rehybridization at the annelated carbon centers.
Acknowledgment. We thank for the National Science Council for research funds. The computing facilities were supported by the Computer Center of National Tsing Hua University. References and Notes (1) (a) Longuet-Higgins, H. C.; Salem, L. Proc. R . SOC.London, Ser. A 1959,A251,172.Salem, L. The Molecular Orbital Theory of Conjugated System; W. A. Beniamin, Inc.: Reading MA, 1972;pp 103-106 and 494505. (b) Heilbronner, E. J . Chem. Ed. 1989,66,471. (2) Shaik, S. S.; Hiberty, P. C.; Lefour, J.-M.; Ohanessian, G. J. Am. Chem. SOC.1987,109,363. (3) Shaik, S. S.;Hiberty, P. C.; J. Am. Chem. SOC.1985,107,3089. (4) Ohanessian, G.; Hiberty, P. C.; Lefour, J.-M.; Flament, J.-P.; Shaik, S. S . Inorg. Chem. 1988,27,2219. (5) Burdett, J. K. Chemtracts-Inorg. Chem. 1991,57. (6) Jug, K.; Kbster, A. M. J . Am. Chem. Soc. 1990,112,6772. (7) Hiberty, P. C.; Ohanessian, G.; Shaik, S. S.; Flament, J. P. Pure Appl. Chem. 1993,65,35. (8) Santiago, C.; Gangour, R. W.; Houk, K. N.; Nutakul, W.; Cravey, W.E.; Thummel, R. P. J . Am. Chem. SOC.1978,100,3730. (9) Brogli, F.; Giovannini, E.; Heibronner, E.; Schurter, R. Chem. Ber. 1973,106,961. (10) Davies, A. G.; Ng, K. M. J . Chem. SOC.,Perkin Trans. 2 1992,1857. (11)Avila, D. A.; Davies, A. G.; Li, E. R.; Ng, K. M. J . Chem. Soc., Perkin Trans. 2 1993,355. (12) 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.; Replogle, E. S.; Gomperts, R.; Andress, J. L.; Raghavachari, K.; Binkley, J. B.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92;Gaussian, Inc.: Pittsburgh, PA 15213. (13) Foster, J. P.; Weinhold, F. J. Am. Chem. SOC.1980,102, 7211. (14)Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Reu., 1988,88,899. (15) Mills, W. H.; Nixon, I. G. J. Chem. SOC.1930,2510. (16) Eckert-MaksiC, M.; Kovacek, D.; Hodoscek, M.; MitiC, D.; Poljanec, K.; MaksiC, Z. B. J. Mol. Srruct. 1990,206,89. (17) Eckert-MaksiC, M.; MaksiC, Z. B.; Skancke, A,; Skancke, P. N. J . Phys. Chem. 1987,91,2786. (18) Eckert-MaksiC, M.; MaksiC, Z. B.; Skancke, A.; Sckancke, P. N. J . Mol. Struct. 1988,164,25. (19) Baldridge, K. K.; Siegel, J. S. J . Am. Chem. SOC.1992,114,9583. (20) (a) Boese, R.; Blber, D. Angew. Chem. l988,100,293;AngewChem., Int. Ed. Engl. 1988, 27, 304. (b) The experimental structure for 2e is unavailable. (c) The experimental structure for 3a will be published; see footnote Table I1 in ref 19. (21) Faust, R.; Glendening, E. D.; Streitwieser, A.; Vollhardt, K. P. C. J . Am. Chem. SOC.1992,114,8263. (22) Stanger, A.; Vollhard, K. P. C. J. Org. Chem. 1988,53, 4889. (23)Stanger, A. J. Am. Chem. SOC.1991,113,8277.
