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The Molecular Orbital-Valence Bond Theory of Excited States

  • Nicolaos Demetrios Epiotis
Part of the Lecture Notes in Chemistry book series (LNC, volume 34)

Abstract

The original monograph introducing qualitative Valence Bond (VB) and Molecular Orbital Valence Bond (MOVB) theory to the chemical community was entitled “Unified Valence Bond Theory of Electronic Structure”.1,2 In this work, we begin to justify the use of the adjective “unified” by showing how the same MOVB concepts that are applicable to ground state chemistry can be applied to excited state chemistry. In particular, we shall use the MOVB theory and the accessory conceptual tools developed before in order to elucidate the energetic interrelationships of the low lying excited states of a given system and discover ways in which the energy ordering of these states can be altered. After reading the chapter describing the Induced Deexcitation model and this one, it is hoped that the reader will have no difficulty seeing that the energy ordering of different molecular states at fixed geometry and the energy ordering of different ground state geometrical structures are analogous problems which can be handled by the same MOVB concepts.

Keywords

Valence State Valence Bond Electron Bond Core Excitation Valence Bond Theory 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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References

  1. 1.
    Epiotis, N.D., Larson, J.R., Eaton, H., “Unified Valence Bond Theory of Electronic Structure” in Lecture Notes in Chemistry, Vol. 29; Springer-Verlag: New York and Berlin, 1982.Google Scholar
  2. 2.
    Epiotis, N.D., Pure Appl. Chem., in press.Google Scholar
  3. 3.
    For lucid discussion of the MO and VB theory of such a primordial system, the reader is referred to the work of J. C. Slater.Google Scholar
  4. 4.
    Dauben, W.G.; Salem, L.; Turro, N.J. Acc. Chem. Res. 1975, 8, 41.Google Scholar
  5. 5.
    The following clarifying comments with regards to the overlap rule are in order. Since there are two bonds in 1V (3,1) and 3V (3,1), a one and a three electron bond due to Фc - ФL and Фc* - ФL* overlap respectively, and since there is only one bond, a two electron bond, in 1V (2,z) and 1V (2,r) due to ФC - ФL overlap, neither of the two overlap integrals by itself is an appropriate index of the relative energies of 3V (3,1) and 1V (2,z) and of 3V (3,1) and 3V (2,r). Only the ratio of the two overlap integrals constitutes a valid index.Google Scholar
  6. 6.
    Huckel MO., (HMO) theory tends to emphasize two-electron bonding over one- or three-electron bonding, thus, unreasonably favoring 1V (2,z) over 2V (3,1) and 3V (2,r) over 3V (3,1). The reason for this is that the HL CW’s are correctly perceived as degenerate but the ionic CW’s are incorrectly perceived as degenerate due to the integral approximation of HMO theory. As a result, the contributions of the interaction of the HL and the ionic CW’s to three-electron and one-electron bond formation are relatively well reproduced while the contribution of the interaction to two electron bond formation is exaggerated.Google Scholar
  7. 7.
    For details on the calculation of HVB wavefunctions, see Reference 1.Google Scholar
  8. 8.
    See Chapter 1 and ref. 1.Google Scholar
  9. 9.
    Swope, W.C.; Schaefer, III, H.F. J. Am. Chem. Soc. 1976, 98, 7962.CrossRefGoogle Scholar
  10. 10.
    Upton, T.H.; Goddard, III, W.A. J. Am. Chem. Soc. 1978, 100, 321.CrossRefGoogle Scholar
  11. 11.
    Chandrasekhar, J., Pople, J.A., Seeger, R., Seeger, U., Schleyer, P.V.R., J. M-Chem. Soc., in press.Google Scholar
  12. 12.
    Laidig, W.D.; Shaefer, III, H.F. J. Am. Chem. Soc. 1978, 100, 5972.Google Scholar
  13. 13.
    The ground state of gaseous C2 is the singlet E state with the triplet 3 Hu state lying 1.74 kcal higher in energy: (a) Ballik, E.A.; Ramsay, D.A. J. Chem. Phys. 1958, 29, 1418. (b) Ballik, E.A.; Ramsey, D.A., Astrophys. J. 1963, 137, 61, 84. The ground state of gaseous Si2 is a 3Eg triplet with all indications being consistent that the lowest energy singlet lies significantly above the ground triplet state. For review, see: Burger, H.; Enjen, R., Top. Curr. Chem. 1974, 50, 1.Google Scholar
  14. 14.
    Winstein, S.; Robinson, G.C., J. Am. Chem. Soc. 1958, 80, 175.Google Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 1983

Authors and Affiliations

  • Nicolaos Demetrios Epiotis
    • 1
  1. 1.Department of ChemistryUniversity of WashingtonSeattleUSA

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