Biradicals and Biradicaloids

In the beginning of this chapter it was mentioned that biradicaloid minima are of great importance in photochemical reactions. Due to their very short lifetime, molecules in biradicaloid minima are usually not as easy to observe as molecules in spectroscopic minima with ordinary geometries. This is particularly true in the singlet state, whose decay is not slowed down by the need for electron spin inversion. Much of the knowledge of reactive minima is therefore based primarily on theoretical arguments, some of which will be considered in this section. (Cf. BonaCi -Koutecky et al., 1987 Michl 1991.) 

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The concept of biradicals and biradicaloids was often used in attempts to account for the mechanism of photochemical reactions [2,20,129-131]. A biradical (or diradical) may be defined as [132] an even-electron molecule that has one bond less than the number permitted by the standard rules of valence. 

I am grateful to the National Science Foundation for support (CHE 9000292). I thank the faculty of the Chemistry Department at the Technion in Haifa, Israel, for their warm hospitality during the initial stages of my work on this project (Manson Visiting Professorship). It is a pleasure to express my gratitude to Prof. Vlasta Bonacic-Koutecky of the Free University, Berlin, Germany, for years of collaboration on the theory of biradicals and biradicaloids, and to acknowledge a useful discussion with Prof. Joe Paldus of the University of Waterloo, Ontario, Canada. 

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