KK (σ_2s)²(σ_2s*)²(σ_2pz)²(π_2px)²(π_2py)²(π_2px*)¹(π_2py*)¹

with the electrons in the π* orbitals with parallel spins (a triplet ³Σ_g state), in agreement with Hund’s first rule, the first excited state (94.72 kJmol^–1 above ground state) has the configuration

KK (σ_2s)²(σ_2s*)²(σ_2pz)²(π_2px )²(π_2py)²(π_2px*)²(π_2py*)⁰

KK (σ_2s)²(σ_2s*)²(σ_2pz)²(π_2px )²(π_2py)²(π_2px*)¹(π_2py*)¹

with the electrons in the π* orbitals with opposite spins (a singlet ¹Σ_g state) (2–4). The two excited states are interchanged in reference (1), as well as in some inorganic chemistry textbooks (5–7), while other textbooks contain the correct ordering (8–12).

In the excellent textbook by Greenwood and Earnshaw (5), it is correctly stated that the first excited state of the O₂ molecule corresponds to the singlet ¹Δ_g state, but it is incorrectly assigned to a configuration with the π* electrons occupying separate orbitals. It is important that the mistake is corrected in further editions of the textbooks (5–7) in order to avoid the disturbing discrepancies that we put forward a few years ago (4). The very interesting discussions in reference (1) are quite valid just by exchanging the configurations of the singlet-1 and singlet-2 states in Figure 1.

Literature Cited

1. Nyasulu, F.; Macklin, J.; Cusworth III, W. J. Chem. Educ. 2002, 79, 356.
2. Ebsworth, E. A. V.; Connor, J. A.; Turner, J. J. In Comprehensive Inorganic Chemistry; Trotman-Dickenson, A. F., Ed.; Pergamon: Oxford, 1973, Vol 2; p 702.
3. (a) Bland, J. J. Chem. Educ. 1976, 53, 274; (b) Nardello, V.; Marti, M. J.; Pierlot, C.; Aubry, J. M. J. Chem. Educ. 1999, 76, 1285.
4. Tudela, D.; Fernández, V. Educ. Chem. 1994, 31, 150.
5. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, UK, 1997; p 605.
6. Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, 2nd ed.; Prentice-Hall: Upper Saddle River, NJ, 1999; p 260.
7. Housecroft, C. E.; Sharpe, A. G. Inorganic Chemistry; Pearson Education: Harlow, UK, 2001; p 355.
8. Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry, 3rd ed.; Wiley: New York, 1995; p 440.
9. Lee, J. D. Concise Inorganic Chemistry, 5th ed.; Chapman and Hall: London, 1996; p 541.
10. Shriver, D. F.; Atkins, P. W. Inorganic Chemistry, 3rd ed.; Oxford University Press: Oxford, UK, 1999; p 390.
11. Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999; p 452.
12. Rayner-Canham, G.; Overton, T. Descriptive Inorganic Chemistry, 3rd ed.; Freeman: New York, 2003; p 372.