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Theoretica chimica acta

, Volume 20, Issue 3, pp 282–291 | Cite as

Calculation of the energies of the lower excited states of CH3

  • Ruth McDiarmid
Commentationes

Abstract

Ab initio LCAO-MO-SCF calculations have been performed on assorted low-lying configurations of the CH3 radical to assess the adequacy of the Hartree-Fock treatment of this species. The 2E′ configuration of lowest energy in the planar ground state equilibrium geometry is shown not to be the valence configuration. Near its (pyramidal) equilibrium geometry the 2E′ valence configuration is well below the other 2E′ configuration. The two 2E′ configurations must, then, cross as the molecule is distorted. In this region the Hartree-Fock formation is unable to describe 2E′ states. The other low-lying states [2A′'2 (ground), 2A1 (3s Rydberg), and 1A1 (ion)] and the 2E′ states in the pyramidal geometry are satisfactorily determined within the Hartree-Fock formalism.

Keywords

Inorganic Chemistry Organic Chemistry Lower Energy Excited State State Equilibrium 
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.

Zusammenfassung

Die Resultate von ab initio-Rechnungen für das CH3-Radical für die tiefliegenden Konfigurationen werden angegeben. Die Konfiguration des tiefsten 2E′-Zustandes resultiert allerdings bei planarer Geometrie nicht aus der Elektronen-Grundkonfiguration. Letztere ist zwar bei fast pyramidaler Struktur wesentlich tiefer, schneidet aber erstere im Verlauf der Verzerrung des Moleküls. In dieser Region ist die HF-Näherung nicht mehr gültig. — Die anderen tiefliegenden Konfigurationen [2A″2 (Grund-Konfiguration), 2A1 (3s Rydberg) und 1A1 (Ion)] verhalten sich normal.

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References

  1. 1.
    Weiss, A. W.: Astrophysic. J. 138, 1262 (1963).Google Scholar
  2. 2.
    Mulliken, R. S.: J. Amer. chem. Soc. 86, 3183 (1964).Google Scholar
  3. 3.
    Although another valence configuration can be constructed [(1a1)2 (2a1)1 (e′)4 (a′'2)2], since this corresponds to a 2p←2s promotion, which requires 7.9 eV [10] for carbon, it would occur at such high energies in CH3 that it is unlikely a state exists that can be represented by such a configuration. In any case, we are interested in the lower energy states in this investigation.Google Scholar
  4. 4.
    Herzberg, G.: Proc. Roy. Soc. (London) A 262, 291 (1961).Google Scholar
  5. 5.
    Padgett, A., Krauss, M.: J. chem. Physics 32, 189 (1960).Google Scholar
  6. 6.
    Walsh, A. D.: Proc. chem. Soc. 1953, 2296.Google Scholar
  7. 7.
    Roothaan, C. C. J., Bagus, P. S.: In: Methods in computational physics, Vol. 2, ed. B. Alder, S. Fernbach and M. Rotenberg. New York: Academic Press 1963.Google Scholar
  8. 8.
    Herzberg, G.: Molecular spectra and molecular structure, Vol. III. Electronic spectra and electronic structure of polyatomic molecules, p. 411. Princeton, N. J.: D. Van Nostrand Co., Inc., 1966.Google Scholar
  9. 9.
    Huzinaga, S.: J. chem. Physics 42, 1293 (1965).Google Scholar
  10. 10.
    Moore, C. E.: Atomic energy levels, Vol. I., p. 1, 21. Washington, D. C. 20025: U.S. Government Printing Office, 1949.Google Scholar
  11. 11.
    Kari, R. E., Csizmadia, I. G.: (J. chem. Physics 50, 1443 [1969]) have recently shown that in the case of CH3 the calculated equilibrium geometry depends on the quality of the input basis set. Our basis set, which neglects carbon d orbitals, can not give the correct equilibrium geometry of the 2 E state of CH3. It can, however, predict the approximate geometry of this state of the radical.Google Scholar
  12. 12.
    Millie, P., Berthier, G.: Int. J. quant. Chemistry 25, 67 (1968).Google Scholar
  13. 13.
    This point is also made by Millie and Berthier in Ref. [12].Google Scholar
  14. 14.
    The correlation error, the failure of the Hartree-Fock formalism to correctly calculate the interelectron interaction, obviously depends on the number of electrons in a species. Since an ion of a given species has fewer electrons than the corresponding neutral species, its correlation error is smaller, hence its H-F energy better approximates its true energy. The difference in correlation energy between the neutral and ion species is most likely approached stepwise as the molecule is excited from the ground through the various members of a Rydberg series.Google Scholar
  15. 15.
    Ref. [8]. p. 493.Google Scholar
  16. 16.
    Zelikoff, M., Watanabe, K.: J. opt. Soc. America 43, 756 (1953).Google Scholar
  17. 17.
    Ref. [8].p. 619.Google Scholar
  18. 18.
    Although the 2 E Rydberg configuration energy increases greatly on distorting the molecule, there is a parallel increase in the ionization energy. The 3p xy Ryd.-ion energy separation thus remains approximately constant as the molecule is distorted, indicating that the energy obtained for the 3p xy Rydberg state is not unreasonable.Google Scholar
  19. 19.a.
    Robin, M. B., Basch, H., Kuebler, N. A., Kaplan, B. E., Meinwald, J.: J. chem. Physics 48, 5037 (1968).Google Scholar
  20. 19.b.
    Basch, H., Robin, M. B., Kuebler, N. A.: J. chem. Physics 47, 1201 (1967).Google Scholar

Copyright information

© Springer-Verlag 1971

Authors and Affiliations

  • Ruth McDiarmid
    • 1
  1. 1.National Institute of Arthritis and Metabolic DiseasesNational Institutes of HealthBethesda

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