Experimental determination of the antiaromaticity of cyclobutadiene

Science

Volumn 286, Issue 5442, 1999, Pages 1119-1122

  Deniz, Ashok A ^a,b,c   Peters, Kevin S ^b   Snyder, Gary J ^a,d  

^a UNIVERSITY OF CHICAGO   (United States)

^b UNIVERSITY OF COLORADO   (United States)

^c SCRIPPS RESEARCH INSTITUTE   (United States)

^d UNIVERSITY OF CALIFORNIA   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

1,3 BUTADIENE DERIVATIVE;

ARTICLE; CALORIMETRY; CHEMICAL STRUCTURE; ENTHALPY; PHOTOCHEMISTRY; PRIORITY JOURNAL; THERMODYNAMICS;

EID: 0033527622     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.286.5442.1119     Document Type: Article

References (93)

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13. M. J. S. Dewar, K. M. Merz Jr., J. J. P. Stewart, J. Am. Chem. Soc. 106, 4040 (1984); H. Kollmar, F. Carrion, M. J. S. Dewar, R. C. Bingham, J. Am. Chem. Soc. 103, 5292 (1981); M. J. S. Dewar and H. W. Kollmar, J. Am. Chem. Soc. 97, 2933 (1975); A. Schweig and W. Thiel, J. Am. Chem. Soc. 101, 4742 (1979); see also M. J. S. Dewar, M. C. Kohn, N. Trinajstic, J. Am. Chem. Soc. 93, 3437 (1971).
14. M. J. S. Dewar, K. M. Merz Jr., J. J. P. Stewart, J. Am. Chem. Soc. 106, 4040 (1984); H. Kollmar, F. Carrion, M. J. S. Dewar, R. C. Bingham, J. Am. Chem. Soc. 103, 5292 (1981); M. J. S. Dewar and H. W. Kollmar, J. Am. Chem. Soc. 97, 2933 (1975); A. Schweig and W. Thiel, J. Am. Chem. Soc. 101, 4742 (1979); see also M. J. S. Dewar, M. C. Kohn, N. Trinajstic, J. Am. Chem. Soc. 93, 3437 (1971).
15. M. J. S. Dewar, K. M. Merz Jr., J. J. P. Stewart, J. Am. Chem. Soc. 106, 4040 (1984); H. Kollmar, F. Carrion, M. J. S. Dewar, R. C. Bingham, J. Am. Chem. Soc. 103, 5292 (1981); M. J. S. Dewar and H. W. Kollmar, J. Am. Chem. Soc. 97, 2933 (1975); A. Schweig and W. Thiel, J. Am. Chem. Soc. 101, 4742 (1979); see also M. J. S. Dewar, M. C. Kohn, N. Trinajstic, J. Am. Chem. Soc. 93, 3437 (1971).
16. M. J. S. Dewar, K. M. Merz Jr., J. J. P. Stewart, J. Am. Chem. Soc. 106, 4040 (1984); H. Kollmar, F. Carrion, M. J. S. Dewar, R. C. Bingham, J. Am. Chem. Soc. 103, 5292 (1981); M. J. S. Dewar and H. W. Kollmar, J. Am. Chem. Soc. 97, 2933 (1975); A. Schweig and W. Thiel, J. Am. Chem. Soc. 101, 4742 (1979); see also M. J. S. Dewar, M. C. Kohn, N. Trinajstic, J. Am. Chem. Soc. 93, 3437 (1971).
17. f(CBD) values were derived from homodesmotic reactions or relative to vinylacetylene [V. Hrouda, M. Roeselová, T. Bally, J. Phys. Chem. A 101, 3925 (1997); S. W. Staley and T. D. Norden, J. Am. Chem. Soc. 111, 445 (1989); B. A. Hess Jr., W. D. Allen, D. Michalska, L. J. Schaad, H. F. Schaefer III, J. Am. Chem. Soc. 109, 1615 (1987); R. L. Disch, J. M. Schulman, M. L. Sabio, J. Am. Chem. Soc. 107, 1904 (1985); H. Kollmar, J. Am. Chem. Soc. 102, 2617 (1980); W. J. Hehre and J. A. Pople, J. Am. Chem. Soc. 97, 6941 (1975)].
18. f(CBD) values were derived from homodesmotic reactions or relative to vinylacetylene [V. Hrouda, M. Roeselová, T. Bally, J. Phys. Chem. A 101, 3925 (1997); S. W. Staley and T. D. Norden, J. Am. Chem. Soc. 111, 445 (1989); B. A. Hess Jr., W. D. Allen, D. Michalska, L. J. Schaad, H. F. Schaefer III, J. Am. Chem. Soc. 109, 1615 (1987); R. L. Disch, J. M. Schulman, M. L. Sabio, J. Am. Chem. Soc. 107, 1904 (1985); H. Kollmar, J. Am. Chem. Soc. 102, 2617 (1980); W. J. Hehre and J. A. Pople, J. Am. Chem. Soc. 97, 6941 (1975)].
19. f(CBD) values were derived from homodesmotic reactions or relative to vinylacetylene [V. Hrouda, M. Roeselová, T. Bally, J. Phys. Chem. A 101, 3925 (1997); S. W. Staley and T. D. Norden, J. Am. Chem. Soc. 111, 445 (1989); B. A. Hess Jr., W. D. Allen, D. Michalska, L. J. Schaad, H. F. Schaefer III, J. Am. Chem. Soc. 109, 1615 (1987); R. L. Disch, J. M. Schulman, M. L. Sabio, J. Am. Chem. Soc. 107, 1904 (1985); H. Kollmar, J. Am. Chem. Soc. 102, 2617 (1980); W. J. Hehre and J. A. Pople, J. Am. Chem. Soc. 97, 6941 (1975)].
20. f(CBD) values were derived from homodesmotic reactions or relative to vinylacetylene [V. Hrouda, M. Roeselová, T. Bally, J. Phys. Chem. A 101, 3925 (1997); S. W. Staley and T. D. Norden, J. Am. Chem. Soc. 111, 445 (1989); B. A. Hess Jr., W. D. Allen, D. Michalska, L. J. Schaad, H. F. Schaefer III, J. Am. Chem. Soc. 109, 1615 (1987); R. L. Disch, J. M. Schulman, M. L. Sabio, J. Am. Chem. Soc. 107, 1904 (1985); H. Kollmar, J. Am. Chem. Soc. 102, 2617 (1980); W. J. Hehre and J. A. Pople, J. Am. Chem. Soc. 97, 6941 (1975)].
21. f(CBD) values were derived from homodesmotic reactions or relative to vinylacetylene [V. Hrouda, M. Roeselová, T. Bally, J. Phys. Chem. A 101, 3925 (1997); S. W. Staley and T. D. Norden, J. Am. Chem. Soc. 111, 445 (1989); B. A. Hess Jr., W. D. Allen, D. Michalska, L. J. Schaad, H. F. Schaefer III, J. Am. Chem. Soc. 109, 1615 (1987); R. L. Disch, J. M. Schulman, M. L. Sabio, J. Am. Chem. Soc. 107, 1904 (1985); H. Kollmar, J. Am. Chem. Soc. 102, 2617 (1980); W. J. Hehre and J. A. Pople, J. Am. Chem. Soc. 97, 6941 (1975)].
22. f(CBD) values were derived from homodesmotic reactions or relative to vinylacetylene [V. Hrouda, M. Roeselová, T. Bally, J. Phys. Chem. A 101, 3925 (1997); S. W. Staley and T. D. Norden, J. Am. Chem. Soc. 111, 445 (1989); B. A. Hess Jr., W. D. Allen, D. Michalska, L. J. Schaad, H. F. Schaefer III, J. Am. Chem. Soc. 109, 1615 (1987); R. L. Disch, J. M. Schulman, M. L. Sabio, J. Am. Chem. Soc. 107, 1904 (1985); H. Kollmar, J. Am. Chem. Soc. 102, 2617 (1980); W. J. Hehre and J. A. Pople, J. Am. Chem. Soc. 97, 6941 (1975)].
23. f(CBD) can be derived from reported isodesmic and homodesmotic reaction enthalpies. These range from 103 to 122 kcal/mol, with the lower values obtained at the higher levels of theory [M. N. Glukhovtsev, R. D. Bach, S. Laiter, J. Mol. Struct. (Theochem) 417, 123 (1997); J. S. Murray, J. M. Seminario, P. Politzer, Int. J. Quantum Chem. 49, 575 (1994); P. Politzer, M. E. Grice, J. S. Murray, J. M. Seminario, Can. J. Chem. 71, 1123 (1993); R. C. Haddon, Pure Appl. Chem. 58, 129 (1986); Pure Appl. Chem. 54, 1129 [1982; see also (8)].
24. f(CBD) can be derived from reported isodesmic and homodesmotic reaction enthalpies. These range from 103 to 122 kcal/mol, with the lower values obtained at the higher levels of theory [M. N. Glukhovtsev, R. D. Bach, S. Laiter, J. Mol. Struct. (Theochem) 417, 123 (1997); J. S. Murray, J. M. Seminario, P. Politzer, Int. J. Quantum Chem. 49, 575 (1994); P. Politzer, M. E. Grice, J. S. Murray, J. M. Seminario, Can. J. Chem. 71, 1123 (1993); R. C. Haddon, Pure Appl. Chem. 58, 129 (1986); Pure Appl. Chem. 54, 1129 [1982; see also (8)].
25. f(CBD) can be derived from reported isodesmic and homodesmotic reaction enthalpies. These range from 103 to 122 kcal/mol, with the lower values obtained at the higher levels of theory [M. N. Glukhovtsev, R. D. Bach, S. Laiter, J. Mol. Struct. (Theochem) 417, 123 (1997); J. S. Murray, J. M. Seminario, P. Politzer, Int. J. Quantum Chem. 49, 575 (1994); P. Politzer, M. E. Grice, J. S. Murray, J. M. Seminario, Can. J. Chem. 71, 1123 (1993); R. C. Haddon, Pure Appl. Chem. 58, 129 (1986); Pure Appl. Chem. 54, 1129 [1982; see also (8)].
26. f(CBD) can be derived from reported isodesmic and homodesmotic reaction enthalpies. These range from 103 to 122 kcal/mol, with the lower values obtained at the higher levels of theory [M. N. Glukhovtsev, R. D. Bach, S. Laiter, J. Mol. Struct. (Theochem) 417, 123 (1997); J. S. Murray, J. M. Seminario, P. Politzer, Int. J. Quantum Chem. 49, 575 (1994); P. Politzer, M. E. Grice, J. S. Murray, J. M. Seminario, Can. J. Chem. 71, 1123 (1993); R. C. Haddon, Pure Appl. Chem. 58, 129 (1986); Pure Appl. Chem. 54, 1129 [1982; see also (8)].
27. f(CBD) can be derived from reported isodesmic and homodesmotic reaction enthalpies. These range from 103 to 122 kcal/mol, with the lower values obtained at the higher levels of theory [M. N. Glukhovtsev, R. D. Bach, S. Laiter, J. Mol. Struct. (Theochem) 417, 123 (1997); J. S. Murray, J. M. Seminario, P. Politzer, Int. J. Quantum Chem. 49, 575 (1994); P. Politzer, M. E. Grice, J. S. Murray, J. M. Seminario, Can. J. Chem. 71, 1123 (1993); R. C. Haddon, Pure Appl. Chem. 58, 129 (1986); Pure Appl. Chem. 54, 1129 [1982; see also (8)].
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40. K. S. Peters, Angew. Chem. Int. Ed. Engl. 33, 294 (1994); K. S. Peters, in Kinetics and Spectroscopy of Carbenes and Biradicals. M. S. Platz, Ed. (Plenum, New York, 1990), pp. 37-49; S. E. Braslavsky and G. E. Heibel, Chem. Rev. 92, 1381 (1992).
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43. R. Pettit and J. Henery, Org. Synth. Coll. 6, 310, (1988); Org. Synth. Coll. 6, 422 (1988); H-D. Martin and M. Hekman, Synthesis 1973, 667. The use of an improved procedure for ether formation [Mitsunobu reaction; J. T. Carlock and M. P. Mack, Tetrahedron Lett. 52, 5153 (1978), with some modifications] raised the yield from 21% to 55%. The sample of P used for photolysis experiments was >99.8% pure by gas chromatography (GC) (39).
44. R. Pettit and J. Henery, Org. Synth. Coll. 6, 310, (1988); Org. Synth. Coll. 6, 422 (1988); H-D. Martin and M. Hekman, Synthesis 1973, 667. The use of an improved procedure for ether formation [Mitsunobu reaction; J. T. Carlock and M. P. Mack, Tetrahedron Lett. 52, 5153 (1978), with some modifications] raised the yield from 21% to 55%. The sample of P used for photolysis experiments was >99.8% pure by gas chromatography (GC) (39).
45. R. Pettit and J. Henery, Org. Synth. Coll. 6, 310, (1988); Org. Synth. Coll. 6, 422 (1988); H-D. Martin and M. Hekman, Synthesis 1973, 667. The use of an improved procedure for ether formation [Mitsunobu reaction; J. T. Carlock and M. P. Mack, Tetrahedron Lett. 52, 5153 (1978), with some modifications] raised the yield from 21% to 55%. The sample of P used for photolysis experiments was >99.8% pure by gas chromatography (GC) (39).
46. 3CN, 150°C, 1 to 2 hours; H.-D. Martin and M. Hekman, Chimia 28, 12 (1974)].
47. 3CN, 150°C, 1 to 2 hours; H.-D. Martin and M. Hekman, Chimia 28, 12 (1974)].
48. 3CN, 150°C, 1 to 2 hours; H.-D. Martin and M. Hekman, Chimia 28, 12 (1974)].
49. Quantum yields were measured according to literature methods [C. G. Hatchard and C. A. Parker, Proc. R. Soc. London Ser. A 235, 518 (1956); W. D. Bowman and J. N. Demas, J. Phys. Chem. 80, 2434 (1976); S. L. Murov, I. Carmichael, G. L. Hug, Handbook of Photochemistry (Dekker, New York, ed. 2, 1993), p. 299], We directed 266-nm laser light (21) into a sample cuvette containing a heptane solution of P and a long-chain alkane as an internal GC standard (39); 15 to 20% of the light was split off using a quartz flat and directed into a matched reference cuvette containing the ferrioxalate solution. Our final quantum yields include a small (<1%) correction for absorption by T. For additional experimental details, see (40).
50. Quantum yields were measured according to literature methods [C. G. Hatchard and C. A. Parker, Proc. R. Soc. London Ser. A 235, 518 (1956); W. D. Bowman and J. N. Demas, J. Phys. Chem. 80, 2434 (1976); S. L. Murov, I. Carmichael, G. L. Hug, Handbook of Photochemistry (Dekker, New York, ed. 2, 1993), p. 299], We directed 266-nm laser light (21) into a sample cuvette containing a heptane solution of P and a long-chain alkane as an internal GC standard (39); 15 to 20% of the light was split off using a quartz flat and directed into a matched reference cuvette containing the ferrioxalate solution. Our final quantum yields include a small (<1%) correction for absorption by T. For additional experimental details, see (40).
51. Quantum yields were measured according to literature methods [C. G. Hatchard and C. A. Parker, Proc. R. Soc. London Ser. A 235, 518 (1956); W. D. Bowman and J. N. Demas, J. Phys. Chem. 80, 2434 (1976); S. L. Murov, I. Carmichael, G. L. Hug, Handbook of Photochemistry (Dekker, New York, ed. 2, 1993), p. 299], We directed 266-nm laser light (21) into a sample cuvette containing a heptane solution of P and a long-chain alkane as an internal GC standard (39); 15 to 20% of the light was split off using a quartz flat and directed into a matched reference cuvette containing the ferrioxalate solution. Our final quantum yields include a small (<1%) correction for absorption by T. For additional experimental details, see (40).
52. -3.
53. 2. Sample and calibrant PAC waves were acquired by averaging the results of 50 laser pulses and subtracting the (dark) baseline. Amplitudes were measured at the first crest (Fig. 2) after small absorbance and incident energy corrections.
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62. Implemented by the SPARTAN computational chemistry package (Wavefunction Inc., Irvine, CA). AM1: M. J. S. Dewar, E. G. Zoebisch, E. F. Healy, J. J. P. Stewart, J. Am. Chem. Soc. 107, 3902 (1985); PM3: J. J. P. Stewart, J. Comp. Chem. 10, 221 (1989).
63. Implemented by the SPARTAN computational chemistry package (Wavefunction Inc., Irvine, CA). AM1: M. J. S. Dewar, E. G. Zoebisch, E. F. Healy, J. J. P. Stewart, J. Am. Chem. Soc. 107, 3902 (1985); PM3: J. J. P. Stewart, J. Comp. Chem. 10, 221 (1989).
64. MM3(96) was obtained from the Quantum Chemistry Program Exchange, Indiana University, Bloomington, IN [N. L. Allinger, Y. H. Yuh, J.-H. Lii, J. Am. Chem. Soc. 111, 8551 (1989); J.-H. Lii and N. L. Allinger, J. Am. Chem. Soc. 111, 8566 (1989); N. L. Allinger, F. Li, L. Yan, J. Comp. Chem. 11, 848 (1990); N. L. Allinger, F. Li, L. Yan, J. C. Tai, J. Comp. Chem. 11, 868 (1990); N. L. Allinger, M. Rahman, J.-H. Lii, J. Am. Chem. Soc. 112, 8293 (1990)].
65. MM3(96) was obtained from the Quantum Chemistry Program Exchange, Indiana University, Bloomington, IN [N. L. Allinger, Y. H. Yuh, J.-H. Lii, J. Am. Chem. Soc. 111, 8551 (1989); J.-H. Lii and N. L. Allinger, J. Am. Chem. Soc. 111, 8566 (1989); N. L. Allinger, F. Li, L. Yan, J. Comp. Chem. 11, 848 (1990); N. L. Allinger, F. Li, L. Yan, J. C. Tai, J. Comp. Chem. 11, 868 (1990); N. L. Allinger, M. Rahman, J.-H. Lii, J. Am. Chem. Soc. 112, 8293 (1990)].
66. MM3(96) was obtained from the Quantum Chemistry Program Exchange, Indiana University, Bloomington, IN [N. L. Allinger, Y. H. Yuh, J.-H. Lii, J. Am. Chem. Soc. 111, 8551 (1989); J.-H. Lii and N. L. Allinger, J. Am. Chem. Soc. 111, 8566 (1989); N. L. Allinger, F. Li, L. Yan, J. Comp. Chem. 11, 848 (1990); N. L. Allinger, F. Li, L. Yan, J. C. Tai, J. Comp. Chem. 11, 868 (1990); N. L. Allinger, M. Rahman, J.-H. Lii, J. Am. Chem. Soc. 112, 8293 (1990)].
67. MM3(96) was obtained from the Quantum Chemistry Program Exchange, Indiana University, Bloomington, IN [N. L. Allinger, Y. H. Yuh, J.-H. Lii, J. Am. Chem. Soc. 111, 8551 (1989); J.-H. Lii and N. L. Allinger, J. Am. Chem. Soc. 111, 8566 (1989); N. L. Allinger, F. Li, L. Yan, J. Comp. Chem. 11, 848 (1990); N. L. Allinger, F. Li, L. Yan, J. C. Tai, J. Comp. Chem. 11, 868 (1990); N. L. Allinger, M. Rahman, J.-H. Lii, J. Am. Chem. Soc. 112, 8293 (1990)].
68. MM3(96) was obtained from the Quantum Chemistry Program Exchange, Indiana University, Bloomington, IN [N. L. Allinger, Y. H. Yuh, J.-H. Lii, J. Am. Chem. Soc. 111, 8551 (1989); J.-H. Lii and N. L. Allinger, J. Am. Chem. Soc. 111, 8566 (1989); N. L. Allinger, F. Li, L. Yan, J. Comp. Chem. 11, 848 (1990); N. L. Allinger, F. Li, L. Yan, J. C. Tai, J. Comp. Chem. 11, 868 (1990); N. L. Allinger, M. Rahman, J.-H. Lii, J. Am. Chem. Soc. 112, 8293 (1990)].
69. o values reproduced these values to within ∼2 kcal/mol (41).
70. f data (Fig. 5) taken from (42), except as noted in the supplementary material (41).
71. Z. Li, D. W. Rogers, F. J. McLafferty, M. Mandziuk, A. V. Podosenin, J. Phys. Chem. A 103, 426 (1999); D. W. Rogers, F. J. McLafferty, A. V. Podosenin, J. Phys. Chem. A 102, 1209 (1998); J. Phys. Chem. A 101, 4776 (1997); J. Org. Chem. 63, 7319 (1998).
72. Z. Li, D. W. Rogers, F. J. McLafferty, M. Mandziuk, A. V. Podosenin, J. Phys. Chem. A 103, 426 (1999); D. W. Rogers, F. J. McLafferty, A. V. Podosenin, J. Phys. Chem. A 102, 1209 (1998); J. Phys. Chem. A 101, 4776 (1997); J. Org. Chem. 63, 7319 (1998).
73. Z. Li, D. W. Rogers, F. J. McLafferty, M. Mandziuk, A. V. Podosenin, J. Phys. Chem. A 103, 426 (1999); D. W. Rogers, F. J. McLafferty, A. V. Podosenin, J. Phys. Chem. A 102, 1209 (1998); J. Phys. Chem. A 101, 4776 (1997); J. Org. Chem. 63, 7319 (1998).
74. Z. Li, D. W. Rogers, F. J. McLafferty, M. Mandziuk, A. V. Podosenin, J. Phys. Chem. A 103, 426 (1999); D. W. Rogers, F. J. McLafferty, A. V. Podosenin, J. Phys. Chem. A 102, 1209 (1998); J. Phys. Chem. A 101, 4776 (1997); J. Org. Chem. 63, 7319 (1998).
75. d-(C)(H)] is 8.59 kcal/mol.
76. d-(C)(H)] is 8.59 kcal/mol.
77. 2 C [see (40), pp. 51-52].
78. R. Breslow and E. Mohacsi, J. Am. Chem. Soc. 85, 431 (1963); M. J. S. Dewar and C. de Llano, J. Am. Chem. Soc. 91. 789 (1969); L. J. Schaad and B. A. Hess Jr., J. Chem. Educ. 51, 640 (1974).
79. R. Breslow and E. Mohacsi, J. Am. Chem. Soc. 85, 431 (1963); M. J. S. Dewar and C. de Llano, J. Am. Chem. Soc. 91. 789 (1969); L. J. Schaad and B. A. Hess Jr., J. Chem. Educ. 51, 640 (1974).
80. R. Breslow and E. Mohacsi, J. Am. Chem. Soc. 85, 431 (1963); M. J. S. Dewar and C. de Llano, J. Am. Chem. Soc. 91. 789 (1969); L. J. Schaad and B. A. Hess Jr., J. Chem. Educ. 51, 640 (1974).
81. P. George, C. W. Bock, M. Trachtman, J. Chem. Educ. 61, 225 (1984); P. George, M. Trachtman, C. W. Bock, A. M. Brett, J. Chem. Soc. Perkin Trans. 2 1976, 1222; Theor. Chim. Acta 38, 121 (1975).
82. P. George, C. W. Bock, M. Trachtman, J. Chem. Educ. 61, 225 (1984); P. George, M. Trachtman, C. W. Bock, A. M. Brett, J. Chem. Soc. Perkin Trans. 2 1976, 1222; Theor. Chim. Acta 38, 121 (1975).
83. P. George, C. W. Bock, M. Trachtman, J. Chem. Educ. 61, 225 (1984); P. George, M. Trachtman, C. W. Bock, A. M. Brett, J. Chem. Soc. Perkin Trans. 2 1976, 1222; Theor. Chim. Acta 38, 121 (1975).
84. W. J. Hehre, R. Ditchfield, L. Radom, J. A. Pople, J. Am. Chem. Soc. 92, 4796 (1970).
85. f = 33.5 kcal/mol. One can then apply a homodesmotic scheme (reaction 5 in Fig. 6) that avoids most of the ambiguity inherent to the isodesmic treatment (reaction 6), Correcting for ring strain, CBD's antiaromatic destabilization is 40 kcal/ mol, as compared to benzene's aromatic stabilization of 43 kcal/mol [P. George, M. Trachtman, C. W. Bock, A. M. Brett, Tetrahedron 32, 1357 (1976)].
86. f = 33.5 kcal/mol. One can then apply a homodesmotic scheme (reaction 5 in Fig. 6) that avoids most of the ambiguity inherent to the isodesmic treatment (reaction 6), Correcting for ring strain, CBD's antiaromatic destabilization is 40 kcal/ mol, as compared to benzene's aromatic stabilization of 43 kcal/mol [P. George, M. Trachtman, C. W. Bock, A. M. Brett, Tetrahedron 32, 1357 (1976)].
87. G. B. Kistiakowsky, J. R. Ruhoff, H. A. Smith, W. E. Vaughan, J. Am. Chem. Soc. 58, 146 (1936).
88. S. Shaik, A. Shurki, D. Danovich, P. C. Hiberty, J. Mol. Struct. (Theochem) 398-399, 155 (1997).
89. GC was carried out using a Hewlett Packard 5890 Series 2 capillary gas chromatograph equipped with a flame ionization detector and an HP 3396 integrator. The carrier gas was He, and the columns used were J&W DB-5 (0.25 mm inner diameter × 30 m) and DB-17 (0.25 mm i.d. × 15m).
90. A. A. Deniz, thesis, University of Chicago (1996), pp. 68-73.
91. Supplementary data are available at www. sciencemag.org/feature/data/1043622.shl.
92. J. B. Pedley, R. D. Naylor, S. P. Kirby, Thermochemical Data of Organic Compounds (Chapman & Hall, New York, ed. 2, 1986).
93. We thank J. F. Liebman for helpful discussions and Y. Tor for the use of his Spartan computational chemistry software and SGI computer. Supported by NSF grants CHE-9816540 (K.S.P.) and CHE-9157704 (G.J.S.) and in part by the donors of the Petroleum Research Fund, administered by the American Chemical Society.