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Helsinki-3  Direct Estimate of Conjugation, Hyperconjugation and Aromaticity With an Energy Decomposition Analysis Gern

Helsinki-3  Direct Estimate of Conjugation, Hyperconjugation and Aromaticity With an Energy Decomposition Analysis Gernot Frenking Fachbereich Chemie, Philipps-Universität Marburg.

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Helsinki-3  Direct Estimate of Conjugation, Hyperconjugation and Aromaticity With an Energy Decomposition Analysis Gern

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  1. Helsinki-3  Direct Estimate of Conjugation, Hyperconjugation and Aromaticity With an Energy Decomposition Analysis Gernot Frenking Fachbereich Chemie, Philipps-Universität Marburg

  2. “Conjugated compounds are usually more stable than bond-shifted isomers in which the double (triple) bonds are isolated from each other by more than on single one bond” (b) conjugated double bonds (a) isolated double bonds Conjugation What is conjugation? IUPAC Definition: “In a topological sense, the indication that each pair of multiple (double or triple) bonds in a polyunsaturated molecule is separated by one single bond.” Pure Appl. Chem. 1999, 71, 1919. b-carotene, the red pigment in carrots and other vegetables

  3. Conjugation • Geometry • Reactivity • Properties Short C–C distance: 1.453 Å

  4. Hyperconjugation What is hyperconjugation? It is the interaction between orbitals having p symmetry where at least one of the p orbitals is located at an atom that does not have a multiple bond p  π* (sat) π*←π(sat) π(sat) π* (sat) π(sat)←π* (sat)

  5. A-B A + B (A-B)s Three Steps: (A-B)p [r(A) + r(B)](rA-B) r(A) + r(B) 1. DEelstat NÂ 0=NÂ(A,B) (A,B) 2. DEPauli 3. (A-B) 0(A,B) DEOrb 1. + 2. + 3. = DEint DEint + DEprep = DE(BDE) Energy Decomposition Analysis (EDA) Extended Transition State Method (ETS) K. Morokuma, J. Chem. Phys.1971, 55, 1236 T. Ziegler, A. Rauk, Theor. Chim Acta1977, 46, 1

  6. D. Cappel, S, Tüllmann, A. Krapp, F. Frenking, Angew. Chem. Int. Ed. 2005, 117, 3683.

  7. Conjugation BP86/TZ2P. Energy Values in kcal/mol

  8. Conjugation BP86/TZ2P. Energy Values in kcal/mol

  9. Conjugation BP86/TZ2P. Energy Values in kcal/mol

  10. Conjugation BP86/TZ2P. Energy Values in kcal/mol

  11. Conjugation BP86/TZ2P. Energy Values in kcal/mol

  12. Conjugation BP86/TZ2P. Energy Values in kcal/mol

  13. Conjugation

  14. r = 0.95, SD=2.60 r = 0.97, SD=2.15 Conjugation BP86/TZ2P. Energy Values in kcal/mol

  15. Conjugation BP86/TZ2P. Energy Values in kcal/mol r = 0.98, SD=2.92

  16. o Cmeta Conjugation in Cyanoethynylethenes, CEEs, (Diederich) BP86/TZ2P. Energy Values in kcal/mol o Cipso r = 0.98, SD= 0.62 r = 0.99, SD= 0.43 1 7

  17. I. Fernández, G. Frenking, Chem. Commun. 2006, 5030. r = 0.99, SD= 0.36 Calculated quinoid character: dr = (((c + c’)/2 – (b + b’)/2) + ((a + a’)/2 – (b + b’)/2))/2.

  18. No clear correlation between hyperconjugation and C–C bond distances Hyperconjugation H2C=CH–CX3 p(H2C=CH)→p*(CX3) p*(H2C=CH)←p(CX3) HC≡C–CX3 p(H2C≡CH)→p*(CX3) p*(H2C≡CH)←p(CX3) BP86/TZ2P. Energy Values in kcal/mol

  19. Hyperconjugation BP86/TZ2P. Energy Values in kcal/mol

  20. Hyperconjugation r = 0.97, SD= 0.29

  21. Hyperconjugation X3C–CY3 BP86/TZ2P. Energy Values in kcal/mol

  22. C–C > C–H C–H > C–C Hyperconjugation X3C–CY3 (p* deleted) black: total hyperconj blue: p(X3C)→p*(CY3) red: p* (X3C)←p(CY3) BP86/TZ2P. Energy Values in kcal/mol the relative p donor strengths of two groups may beinverted depending on the acceptor moiety

  23. D.L. Thorn, R. Hoffmann, Nouv. J. Chim. 1979, 3, 39.

  24. I. Fernandez, G. Frenking, Chem. Eur. J., 2007, 13, 5873.

  25. Conjugation and Hyperconjugation CONCLUSIONS • The calculated DEp values taken from the EDA can be used to estimate the strength of the relative contributions of p interactions that come from conjugation/hyperconjugation • The EDA method has a predictive value. The trend of the calculated DEp values is in very good agreement with NMR chemical shifts and experimentally derived Hammett constants. • The comparison of DEp values of cyclic system with a suitable reference compound makes it possible to estimate the stabilization due to aromaticity. • Bond energies and lengths should not be used as indicators of the strength of hyperconjugation because the effect of s interactions and electrostatic forces may compensate for the hyperconjugative effect.

  26. D. Cappel, S, Tüllmann, A. Krapp, F. Frenking, Angew. Chem. Int. Ed. 2005, 117, 3683.

  27. r = 0.95, SD=2.60 r = 0.97, SD=2.15 I. Fernández, G. Frenking, J. Org. Chem. 2006, 71, 2251. Conjugation BP86/TZ2P. Energy Values in kcal/mol

  28. o Cmeta I. Fernández, G. Frenking, Chem. Commun. 2006, 5030 Conjugation in Cyanoethynylethenes, CEEs, (Diederich) BP86/TZ2P. Energy Values in kcal/mol o Cipso r = 0.98, SD= 0.62 r = 0.99, SD= 0.43 1 7

  29. I, Fernández, G. Frenking, Faraday Discuss., 2007, 135, 403

  30. I. Fernández, G. Frenking, Chem. Eur. J., in print.

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