Studies on the singlet and triplet states of unsaturated carbenes

Quantum chemical calculations and studies have been made on the different spin states of unsaturated carbenes X₂C?C: and X₂C?C?C: (X?H, Li, F). The geometries and relative stabilities of these carbenes with various groups X have been outlined. The ground states of unsaturated carbenes are all singlet. The energy splittings between the ground states and first excited states ΔE (¹A₁-³B₁) are generally within the value of 60 kcal/mol and change greatly with the electronegativities of groups X, but little with the sizes of the cumulidenes. The equilibrium conformations of ¹A₁ and ³B₁ are different.