Biradicals

A biradical is a molecule that contains two unpaired electrons located in two separate orbitals, often described as a diradical. In such species, the unpaired electrons can occupy near-degenerate orbitals and the overall electronic state may be a singlet or a triplet, depending on the relative spin alignment of the two electrons. In a singlet biradical the electrons have opposite spins, while in a triplet biradical their spins are parallel. Biradicals are typically highly reactive and short-lived, though some exhibit substantial diradical character that makes them more persistent or “biradicaloid.”

Structural patterns vary. The unpaired electrons may reside on different atoms (acentric biradicals) or on adjacent centers (vicinal biradicals), and they can be delocalized over conjugated frameworks. The electronic structure of biradicals is often described using concepts such as diradical character and exchange coupling, which influence their magnetic and reactive properties.

Formation and detection: biradicals can be produced by homolytic bond cleavage, photolysis, or certain redox processes, and they frequently appear as reactive intermediates in cyclizations, rearrangements, and photochemical reactions. They are typically detected and studied by spectroscopic methods such as ESR/EPR, UV–visible absorption, and time-resolved techniques, often complemented by computational analyses that estimate occupation numbers of natural orbitals and the diradical character index.

Reactivity and applications: the spin state of a biradical strongly influences its chemistry. Triplet biradicals tend to participate in radical-type reactions with specific regio- and stereochemical patterns, while singlet biradicals may rapidly cyclize or form new bonds to yield closed-shell products. Biradical character is also of interest in theoretical chemistry and materials science, including studies of organic electronic and magnetic materials.

Examples: oxygen (O2) is a classic diradical in its triplet ground state; cyclobutadiene is often cited as a simple singlet biradical in discussions of biradical character.