Novel quintet and triplet (nitrenoethynyl)halomethylenes at theoretical levels

Coupling of a local triplet carbene with a local triplet nitrene through an acetylene linkage gives a new brand of high spin quintet minima (X-C-C C-N, where X = H, F, Cl, Br), which are rather experimentally unreachable. Placing the same linkage between the local open-shell singlet carbene (sigma(1)pi(1)) and the local triplet nitrene (p 1 p 1) gives triplet minima which are 54-56 kcal/mol more stable than their corresponding quintets. The carbenic angles in both quintets and triplets follow electropositivity of X (H > Br > Cl > F), with every divalent angle in quintet being smaller than the corresponding one in the triplet. Finally no reactive intermediate is observed through connecting singlet states of carbene and nitrene subunits which gives a neutral linear molecule with X-C C-C N formula, and show about 70 kcal/mol more stability than the corresponding triplet states. Our results are compared at B3LYP, HF, MP2, MP4(SDTQ), CCSD(T), and QCISD(T) levels using 6-311++G** basis set.