Umbrella inversions of cyclononatetraenylidenes at ab initio and DFT

Barriers of umbrella inversions for non-planar triplet 2-, 3-, 4-, or 5-X-2,4,6,8-cyclononatetraenylidenes, inverting through planar transition states, appear roughly four times lower in energy than their corresponding singlet states, at ab initio and DFT levels (X = H, F, Cl, Br). Relative activation energies for these racemization (ΔEr#), follow electronegativity for both singlet and/or triplet states of 2-X-2,4,6,8-cyclononatetraenylidenes (F > Cl > Br > H). This trend does not hold for species with halogens further away from the carbenic center: 3-, 4-, or 5-X-2,4,6,8-cyclononatetraenylidenes. Frequency calculations show one negative force constant for all planar species (transition states), while they appear positive for non-planar minima.