Diradical character of the cope rearrangement transition state

Journal of the American Chemical Society

Volumn 122, Issue 1, 2000, Pages 186-187

  Staroverov, Viktor N ^a   Davidson, Ernest R ^a  

^a INDIANA UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ARTICLE; CALCULATION; CHEMICAL REACTION; CHEMICAL STRUCTURE; DENSITY; ELECTRON TRANSPORT; GEOMETRY;

EID: 0034639422     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja993375x     Document Type: Article

References (26)

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15. Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic Resonance: Elementary Theory and Practical Applications; John Wiley & Sons: New York, 1994.
16. In this connection, Dewar and Healy used the term "diradicaloid", defined as "a closed shell species derived from a singlet diradical by a weak interaction between the radical centers": Dewar, M. J. S.; Healy, E. F. Chem. Phys. Lett. 1987, 141, 521-524.
17. This approach is based on the fact that, for a single Slater determinant description of a singlet diradical, the unrestricted method gives a much better PES than the restricted one. However, it also yields a totally wrong (nonzero) spin density. Moreover, the range of stability of a Slater determinant to spin-symmetry breaking on the PES strongly depends on the type of the unrestricted method: Davidson, E. R. Chem. Phys. Lett. 1998, 284, 301-307.
18. (a) Mayer, I. Int. J. Quantum Chem. 1986, 29, 73-84.
19. (b) Mayer, I. Int. J. Quantum Chem. 1986, 29, 477-483.
20. More generally, we define the corresponding reduced density matrix, u(r|r′) = 2ρ(r|r′) - ∫ρ(r|r″)ρ(r″|r′) dr″.
21. Staroverov, V. N.; Davidson, E. R. Int. J. Quantum Chem., accepted.
22. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S., Ochterski, J.; Petersson, G. A..; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L., Gonzalez, C.; Head-Gordon, M.; Kepiogle, E. S.; Pople, J. A. Gaussian 98, Revision A.6; Gaussian, Inc.: Pittsburgh, PA, 1998.
23. McMurchie, L. E.; Elbert, S. T.; Langhoff, S. R.; Feller, D.; Rawlings, D. C.; Davidson, E. R. The MELD series of electronic structure codes.
24. Dupuis, M.; Marquez, A.; Davidson, E. R. HONDO 99.6; IBM Corp.: Neighborhood Road, Kingston, NY 12401; 1999.
25. Contributions of the hydrogen atoms to the total n are relatively small even in the tightest structure and decrease with larger R. In the short-R structures, where the hydrogen atoms bonded to C1, C3, C4, and C6 can be classified as equatorial and axial, the axial ones consistently have larger unpaired electron populations because of hyperconjugation: hydrogens at C2 and C5 bear the smallest populations.
26. For comparison, in the much lighter structure of benzene, a similar CASSCF/6-31G* calculation gives 0.15 unpaired electron on each carbon.