Theoretical investigations on stability of pyridylpentazoles, pyridazylpentazoles, triazinylpentazoles, tetrazinylpentazoles, and pentazinylpentazole searching for a replacement of phenylpentazole as N5− source

Stabilities of pyridylpentazoles, pyridazylpentazoles, triazinylpentazoles, tetrazinylpentazoles, and pentazinylpentazole were studied using density functional theory to assess their potentials as the source of pentazole anion (N5−) for replacement of phenylpentazole (PhN5). Replacing the aryl group of PhN5 by six-member heterocycle weakens pentazole ring. Compared to PhN5, title molecules have longer N-N bonds and lower activation energy (Ea,1) needed for the N5 ring breaking. Ea,1 decreases with the increasing number of nitrogen atoms of heterocycle. The ortho nitrogen of heterocycle most obviously lowers the stability of pentazole. The central C-N bond dissociation energies (BDEs) of title molecules are lower than that of PhN5. For the molecule with 0~1 ortho-nitrogen, H rearrangement happens during the central C-N bond breaking. The energy (Ea,2) required for H rearrangement is considerably smaller than the corresponding BDE. ΔEa,2 (Ea,2(PhN5) - Ea,2 = 7.5~35.7 kJ mol-1) is larger than ΔEa,1 (Ea,2(PhN5) - Ea,2 = 4.6~15.5 kJ mol-1), while ΔEa,2/Ea,2(PhN5) (2~9.5 %) is smaller than ΔEa,1/Ea,1(PhN5) ( 4.4~15.0 %). The larger ΔEa,1/Ea,1(PhN5) suggests that title molecules can not be the better N5− than PhN5.