Terry S. Carlton, Professor Emeritus, Chemistry.
Why the lower-energy term of singlet dioxygen has a doubly occupied [pi]* orbital. Journal of Chemical Education vol. 83, no. 3, March 2006: 477-479.
"This article [presents] a simple physical explanation for the relative erergies of the lowest two singlet terms of O2. The explanation invokes [pi positve] and [pi negative] molecular orbitals, which are eigenfunctions of orbital angular momentum about the molecular axis."
A copy of this article is in the science faculty publications file in the copying/printing room, and may also be read in the print issue in the library or online at the publisher's Web site (from computers on campus).