Azide (N-3(-)) reacts with the triplet excited states of acetophenone (AP) benzophenone (BP) and benzil (Bz) by electron transfer in acetonitrile, generating the corresponding radical anion of the ketone and the azidyl radical (N-3(.)) UV-visible and time-resolved infrared (TRIR) nanosecond laser flash photolysis techniques have shown that the azidyl radicals combine with excess azide at the diffusion controlled limit (k = 1.2 x 10(10) M(-1) s(-1)) in an equilibrium reaction forming N-6(.-). This species is characterized by a broad, featureless visible absorption centered at 700 nm (epsilon(700) = 8150 +/- 920 M(-1) cm(-1)) and an IR band at 1842 cm(-1) The equilibrium constant for formation of N-6(.-) at room temperature was determined to be 200 M(-1) in acetonitrile, ca. 1000 times greater than in water. The temperature dependence of the equilibrium constant yielded a stabilization energy of 4.4 kcal mol(-1) for N(6)(.-)relative to N-3(.) + N-3(-). Quantum-chemical calculations were carried out to provide some insight into the equilibrium structure of N-6(.-) as well as its associated electronic and vibrational transitions. Excellent agreement between the theoretical and experimental IR frequency was obtained. The theoretical results in conjunction with the experimental observations have allowed for a tentative assignment of the structure of this unusual species to a cyclic, "dimeric" structure of D-2h symmetry With two long N-N bonds and the azidyl monomeric units essentially intact.