The structures and isomerization mechanisms of eight (H)FNO^＋ isomers have been studied at CCSD(T)/6-311＋G(2d,2p)//B3LYP/6-311＋G(2d,2p) levels. The molecular formation density difference (MFDD) and the topological analysis of electron density theory were used to discuss the electron transfer in the protonated processes of FNO, the results show that expect for HFNO^＋, the electron transfers from N, O atom to H, F atom, the other seven isomers, the electron density of H increased and that of F atom decreased, which means that the electron transfers from F atom to H atom. Among the eight isomers, HFNO^＋, FNOH-cis, FN(H)O^＋and FNOH^＋-trans are the stable species because of their lower energies. The isomerizations of (H)FNO^＋could be achieved by the migration of H atom or F atom, the rotation or vibration of chemical bond. Because the atom migration mechanism has the higher energy barrier, it could not achieve easily. The chemical bonds changes on the reaction paths are described by electron localization function (ELF).

Key words: isomerization, topological analysis of electron density, molecular formation of density difference, electron localization function