The photoelectron spectra (PES) of ClSCN and BrSCN have been recorded and Outer Valence Green's Function (OVGF) calculations give quite good results in ionization potential energies. The ionization potential energies of ClNCS and BrNCS have been predicted by OVGF calculations. The isomerization reactions XNCS→XSCN (X=Cl, Br) have been computed by B3LYP/6-311++G(2df) and the four-membered ring transition states are optimized. The breakage and formation of the chemical bonds in the reactions have been discussed by the topological analysis method of electronic density, and the calculated results show that there is a transitional structure of three-membered ring on both of the isomerization reaction paths. The “energy transition state” and the “structure transition state” in both of the studied reactions have been found.

Key words: halogen thiocyante, photoelectron spectroscopy(PES), topological analysis of electronic density, structure transition state