COMMENT pubs.acs.org/JPCA
Comment on “Hess-Schaad Group Additivity Type Model Predicts Superaromaticity” B. Andes Hess, Jr.* Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, United States
A
recent paper by Fishtik in the Journal of Physical Chemistry A1 purports to have shown that the Hess-Schaad method2 of calculating resonance energies of cyclic conjugated compounds when applied to experimental heats of formation of certain benzenoid hydrocarbons supports Clar’s proposed superaromaticity of these systems.3 Clar’s proposal of superaromaticity in coronene and related compounds is based on NMR studies and a priori should not necessarily correlate with the resonance energies of his proposed superaromatic benzenoid systems. Nevertheless, it is certainly an interesting question whether there is a correlation or not. However, it will be shown here that there are two assumptions made in Fishtik’s scheme that are incorrect and that consequently invalidate his claim of the correlation between Clar’s superaromaticity and resonance energies of certain benzenoid compounds. Prior to the publication of Dewar and de Llano’s seminal work4 on the definition of a new reference structure for computation of resonance energies of cyclic, conjugated hydrocarbons, it was generally agreed that all previous attempts to compute resonance energies had met with failure. The new reference structure was based on the finding that computed Pariser Parr Pople energies of any acyclic polyene could be reproduced in an additive manner based on the numbers of carbon carbon double and single bonds present in the polyene. These could in turn be used to compute the energy of reference structures of cyclic, conjugated hydrocarbons for the calculation of their resonance energies. For example, the resonance energy of benzene could easily be determined as the difference between the energies of “cyclohexatriene” and benzene. This reference structure also nicely accounted for the antiaromaticity of cyclobutadiene and other unstable species. The energy of cyclobutadiene was computed to be less stable than that of its reference structure, which gave a strong basis for the then recently coined term or concept of antiaromaticity.5 We subsequently showed that extending the Dewar reference structure to eight different types of double and single bonds in the acyclic polyenes2 made it possible to correlate Dewar resonance energies computed with the simple H€uckel method (REPE) with the known experimental properties (unusual stability and electrophilic substitution reactivity) of a wide range of cyclic systems, both alternant and nonalternant.6 Due to the lack of experimental heats of formation of acyclic polyenes, Fishtik defined an additional bond type or group, CH3 C, in order to be able to expand the number of reference acyclic polyenes for which heats of formation were known.7 This allowed the determination of all nine bond types, the original eight proposed by Hess and Schaad plus Fishtik’s newly defined ninth one. However, the introduction of this ninth group makes his newly defined reference structure no longer a true Dewar r 2011 American Chemical Society
Table 1. Energies of Acyclic Polyenes 1 3a compound
nonplanar
planar
Δ
1
310.636940
310.628093
5.6
2 3
272.564518 311.852631
272.559672 311.845023
3.0 4.8
a
All energies are in Hartrees except for the differences, which are in kcal/mol.
reference structure, because it involves an sp3 hybridized methyl group; and as a result, his reference structure is likely to lead to results that no longer mimic the original Dewar-de Llano and Hess-Schaad reference structures. In the original H€uckel reference structure, based on acyclic polyenes, all eight bond types (parameters) were obtained with fully conjugated systems, which contained only sp2 hybridized carbons. The energy of the central double bond in 1,3,5-hexatriene is certainly different from the energy of the isolated double bond in 2-butene, however, both compounds are used by Fishtik in the parametrization of the CHdCH bond. Perhaps even more deleterious to the calculation of his reference structure energies using his nine groups is that among these polyenes are three compounds (1 3) that can easily be shown with models or ab initio calculations to be nonplanar. Presented in Figure 1 and Table 1 are their computed8,9 structures and energies as well as their “forced” planar conjugated structures. Because 1 3 include bonds that are very prevalent in benzenoid hydrocarbons, this will have a serious effect on their energies obtained from the presumably nonplanar, experimental structures. In particular, C C single bond energies will be underestimated, which will lead to computed resonance stabilizations greater than that which would be obtained with a true Hess-Schaad reference structure. This might very well account for the divergence of the Fishtik REPEs from the original HessSchaad REPEs, as shown in Table 2. Note that none of these systems is predicted to be more aromatic than benzene with the original Hess-Schaad reference structure. Furthermore, corannulene and the other benzoids in Table 2 all have very similar REPEs. Hence, the superaromaticitiy of coronene, as proposed by Clar, does not correlate with REPE.10 It is also seen from Table 2 that the Fishtik REPEs of all of the systems except for the last two (these in fact are nonbenzenoids) more closely parallel the delocalization energies per π electron (DEPE) than the HessSchaad REPEs. It was shown many years ago that DEPE does not correlate with aromatic character.2,6 Received: January 16, 2011 Revised: March 25, 2011 Published: April 21, 2011 5017
dx.doi.org/10.1021/jp200469v | J. Phys. Chem. A 2011, 115, 5017–5018
The Journal of Physical Chemistry A
COMMENT
Hess-Schaad REPE of benzene. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES
Figure 1. Structures of the three nonplanar acyclic reference polyenes of Fishtik. The structures on the left are those of the nonplanar minima and those on the right are the structures obtained when all carbons were forced to lie in a plane.
Table 2. Resonance Energies and Resonance Energies per π Electron, Computed with the Original Hess-Schaad Method (RE and REPE), Delocalization Energies per π Electron (DEPE), and Those with Fishtik’S Newly Proposed Reference Structure (RE(F) and REPE(F)) compound
a
EHMOa REa REPEa DEPEa RE(F)b REPE(F)b
benzene
8.00
0.39 0.065
0.33
20.75
3.46
naphthalene anthracene
13.68 19.31
0.55 0.055 0.66 0.047
0.37 0.38
45.18 67.12
4.52 4.79
phenanthrene
19.45
0.77 0.055
0.39
73.53
5.25
benzo[c]phenanthrene 25.19
0.96 0.053
0.40
94.14
5.23
triphenylene
25.27
1.01 0.053
0.40
101.64
5.65
perylene
28.25
0.97 0.048
0.41
120.19
6.01
pyrene
22.51
0.81 0.051
0.41
96.34
6.02
coronene
34.57
1.26 0.053
0.44
175.79
7.32
corannulene C60 fullerenec
28.74 59.61
0.99 0.050 0.72 0.036
0.44 0.49
95.70 260.64
4.79 4.34
(1) Fishtik, I. J. Phys. Chem. A 2010, 114, 11017. (2) Hess, B. A., Jr.; Schaad, L. J. J. Am. Chem. Soc. 1971, 93, 305. (3) Clar, E. The Aromatic Sextet; John Wiley & Sons: New York, 1970. (4) Dewar, M. J. S.; de Llano, C. J. Am. Chem. Soc. 1969, 91, 789. (5) There is some uncertainty who actually coined the term “antiaromaticity”, Ron Breslow or Michael Dewar. (6) Hess, B. A., Jr.; Schaad, L. J. Chem. Rev. 2001, 101, 1465. (7) Fishtik, I J. Phys. Org. Chem. 2010, DOI: 10.1002/poc.1751 (8) All calculations reported here were at the DFT [mPW1P91/6311_G(2d,p)//mPW1P91/6-311_G(2d,p)] level computed with Gaussian 03 (ref 9). All energies reported contain zero point energy corrections. (9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; 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.; and Pople, J. A. Gaussian 03, Revisions B.02 and C.01; Gaussian, Inc.: Wallingford, CT, 2004. (10) This assumes that based on REPE a benzenoid hydrocarbon must have a significantly larger REPE than benzene has for one to consider it to be “superaromatic”. (11) Hess, B. A., Jr.; Schaad, L. J. J. Am. Chem. Soc. 1983, 105, 7500.
In units of β. b Fishtik's RE (kcal/mol). c Fishtik's B26 compound.
Finally, it should be pointed out that, to our knowledge, because of the nonplanarity of all but the simplest acyclic polyenes, there is no straightforward way to obtain the eight Hess-Schaad bond energy parameters from either experimental or computational data. However, it is possible to obtain such values for the annulenes.11 We have previously obtained the ab initio REPE of benzene at the SCF level (23 kcal/mol), using the series of planar “linear” acyclic polyenes. We report here that a much more sophisticated method (DFT) with a much larger basis set yields a calculated resonance energy of benzene of 18 kcal/mol.8
’ ASSOCIATED CONTENT
bS
Supporting Information. Summaries of the results of all DFT calculations and the plot of the DFT linear acyclic energies from which the value of the sum of the CHdCH and CH CH bond energy terms was obtained for use in computing the DFT 5018
dx.doi.org/10.1021/jp200469v |J. Phys. Chem. A 2011, 115, 5017–5018
