Journal of Molecular Structure (Theo&em), 235 (1991) 193-196 Elsevier Science Publishers B.V., Amsterdam

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Short Communication

Homodesmotic reaction energies show little or no aromatic stabilization in 1,6-methano [lo] annulene

Philip George*, Jenny P. Glusker” and Charles W. Bockb aInstitute for Cancer Research, Fox Chase Cancer Center, Philadelphia, PA 19111 (USA) bDepartment of Chemistry, Philadelphia College of Textiles and Science, Philadelphia, PA 19144, and American Research Institute, Upper Darby, PA 19082 (USA)

(Received 10 December 1990)

In a recent computational molecular orbital study [l] of the oxide and ox- epin valence tautomers of naphthalene using the 6-31G basis set, we found the reaction energies for the homodesmotic fission [2-51 of the ring fused to the benzene oxide ring in N-1,2-oxide

N-1,2-oxide+BD+2E+B_oxide+ 2MPD

and the ring fused to the benzene oxepin ring in N-2,3-oxepin

N-2,3-oxepin+BD+2E+B-oxepin+2MPD

to be large and positive (+41.4 kcaI mol-’ and +40.0 kcaI mol-’ respec- tively ) , thereby corroborating the benzenoid character of the fused ring in these structures [ 61. In the above equations and those that follow, the abbreviation N stands for naphthalene, BD for truns-1,3-butadiene, E for ethylene, B for benzene, MPD for 3-methylene-1,4-pentadiene, DVE for divinylether, PD for 1,4-pentadiene, P for propene, IB for isobutene, 1,6-O [ lO]A for 1,6-0x- ido [lo] annulene, and 1,6-M [lo] A for 1,6-methano [lo] annulene.

Instead of large positive values commensurate with the aromatic stabiliza- tion energy of a ten n-electron molecule such as naphthalene [ 4,7], we found the reaction energies for the homodesmotic fission of the entire ring system in 1,6-oxido [lo] annulene

1,6-O[lO]A+5E+DVE+2MPD+BD

and in 1,6-methano [lo] annulene

1,6-M[lO]A+5E+PD+‘ZIMPD+BD

to be small and negative (-4.8 kcaI-’ and -2.2 kcal mol-l respectively),

0166-1280/91/$03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.

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indicative of slight destabilization relative to the bonding energy of the acyclic reference molecules.

These values seem to be at variance with experimental evidence regarding the nature of the ring systems. UV spectroscopic observations, NMR mea- surements and X-ray diffraction studies [ 8-101 have shown unequivocally that 1,6-methano [ lo] annulene is an aromatic compound with a delocalized a-elec- tron structure having Cz, symmetry (see Fig. 1 (A) ). In ab initio calculations at the 6-31G level the geometry is in excellent agreement with the X-ray data [ 111. However, there is a very delicate energy balance between this delocalized x-electron structure and the corresponding localized x-electron structure with polyenic bonding, C, symmetry (see Fig. 1 (B ) ). The inclusion of electron cor- relation at the MP2 level swings the balance in favor of the former in accord- ance with the experimental evidence. X-ray data [ 121 show 1,6-0x- ido [lo] annulene, which is the oxepin valence tautomer of N-4a,8a-oxide, to have a delocalized n-electron structure like the methano derivative and, with- out electron correlation, the polyene structure was likewise found to be a little more stable [ 11.

It is thus important to determine whether this apparent absence of aromatic stabilization in these 1,6-bridged [ lo] annulenes is substantiated by reaction energies derived from experimental data. No m value has been reported for MPD, so we have calculated the energies for the alternative homodesmotic fission reactions in which isobutene provides the C3 carbon atom with no bonded hydrogen atom with propene to balance the methyl groups [ 4 J, and for which all the necessary A,!$’ values are available [ 13,141:

1,6-M[lO]A+4P+5E+PD+2IB+5BD AH”= -9.854.6 kcalmol-’

1,6-0[10]A+4P+5E+DVE+2IB+5BD AH”= -8.744.1 kcal mol-’

Compared with the AEr values calculated using 6-31G total molecular ener- gies, i.e. - 18.6 kcal-l and - 16.1 kcal mol-’ respectively, these experimental AH” values are consistently more positive by about 8 kcal mol-‘. The discrep- ancy is attributable in part to the large uncertainty in the experimental values, and to differences between the summation of zero-point energies and heat- capacity increments for reactants and products which would be accentuated in reactions involving such a large number of molecules. If the calculated values

Fig. 1.1,6-Methano [ lO]annulene: (A) delocalized n-electron structure; (B) polyenic structure.

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c10.7 c2,5

1,6methanoP0]annukne

cydoheptatriene

1,&0tido(10]annulene

benzeneoxepin

Fig. 2. The folding angles LX, /I and y (in degrees) in l,&methano[lO]annulene [15] and 1,6- oxido [lo] annulene [ 1 ] shown as a cross-section in the plane of symmetry passing through the midpoints of the lines joining C, and C,, Cs and Ce, and the methano carbon atom and the oxygen atom, respectively. The corresponding folding angles in cycloheptatriene [ 161 and benzene oxepin [ 171 are also shown.

for the MPD reactions were to be made more positive by 8 kcal mol-‘, the “predicted” AH values for these reactions would become positive, but only to the extent of 5 kcal mol- ‘, thereby confirming that there is little or no aromatic stabilization in these 1,6-bridged [ lo] annulene structures.

Upon further consideration, in view of the existence of polyenic structures almost equal in energy for which the homodesmotic fission reactions would be the same and for which no aromatic stabilization would be expected, it follows that these delocalized 1,6-bridged [ lo] annulenes must also lack any significant aromatic stabilization.

Evidently, factors that are destabilizing in nature offset the stabilization due to electron delocalization, which would be maximal if the Cl0 ring system were planar. The folding angles p and especially y, calculated using the 6-31G basis set, show there to be a marked departure from planarity in both the oxido and methano derivatives (see Fig. 2). The major source of destabilization is prob- ably the deformation of the folding angle cy, which defines the position of the oxygen atom of the oxido group and carbon atom of the methano group with respect to the adjacent plane of four carbon atoms. In the oxido derivative this angle is decreased by some 30” with respect to the value in benzene oxepin, and in the methano derivative by some 20” with respect to the value in cycloheptatriene.

ACKNOWLEDGMENTS

We thank the Advanced Scientific Computing Laboratory, NCI-FCRF, for providing time on the Cray XMP supercomputer. This work was also sup-

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ported in part by grants CA-10925 from the National Institutes of Health and CN-1OM from the American Cancer Society.

REFERENCES AND NOTES

1 C.W. Bock, P. George and J.P. Glusker, J. Mol. Struct. (Theochem), 234 (1991) 227. 2 In homodesmotic reactions there is conservation of both bond type and connectivity type

[3-51. 3 P. George, M. Trachtman, C.W. Bock and A.M. Brett, Theor. Chim. Acta, 38 (1975) 121. 4 P. George, M. Trachtman, C.W. Bock and A.M. Brett, J. Chem. Sot., Perkin Trans. 2, (1976)

1222. 5 P. George, C.W. Bock and M. Trachtman, J. Chem. Educ., 61 (1984) 225. 6 D.M. Jerina, H. Yagi and J.W. Daly, Heterocycles, 1 (1976) 267, and referencescited therein. 7 The reaction energy for the homodesmotic ring fission of naphthalene, N + 5Ed2MPD + 2BD,

evaluated from total molecular energies obtained using the 6-31G basis set, is +56.2 kcal mol-‘.

8 E. Vogel, in Aromaticity, Special Publication No. 21, The Chemical Society, London, 1967, pp. 113-147, and references cited therein.

9 H. Gunther and H. Schmickler, Pure Appl. Chem., 44 (1975) 807, and references therein. 10 R. Bianchi, T. Pilati and M. Simonetta, Acta Crystallogr., Sect. B, 36 (1980) 3146. 11 R.C. Haddon and K. Raghavachari, J. Am. Chem. Sot., 107 (1985) 289. 12 N.A. Bailey and R. Mason, Chem. Commun., (1967) 1039. 13 J.B. Pedley, R.D. Naylor and S.P. Kirby, Thermochemical Data of Organic Compounds, 2nd

edn., Chapman and Hall, London, 1986. 14 W. von Bremser, R. Hagen, E. Heilbronner and E. Vogel, Helv. Chim. Acta, 52 (1969) 418. 15 Calculated from the coordinates given in the Supplementary Material in ref. 11. 16 The calculation on cycloheptatriene at the 6-31G level was carried out specifically for this

paper. 17 C.W. Bock, P. George, J.J. Stezowski and J.P. Glusker, Struct. Chem., 1 (1989) 33.