Structures and stability of HNS₂ isomers

Potential energy surface of HNS₂ is investigated by means of second-order Moller-Plesset perturbation theory (MP2) and QCISD(T) (single-point) methods. At final QCISD (T)/6-311++G(3df,2p)//MP2/6-311++G(d,p) level with zero-point vibrational energies included, cis-HNSS is found to be global minimum on the potential energy surface, followed by low-lying trans-HNSS, HN(S)S(C₂,), cis-HSNS, cis-HSSN, trans-HSNS, trans-HSSN, and HN(S)S(C_s,) by 13.46, 66.92, 78.25, 80.38, 81.22, 81.38 and 86.40 kJ/mol, respectively. A new high-lying HS(N)S isomer with C_s symmetry is located on the potential energy surface. The kinetic stabilities of all isomers are predicted. Comparisons are made for HNS₂ with its analogues, HNO₂, HFS₂ and HPO₂. The causes that lead to the differences between HNS₂ and its analogues are hypervalent capacity of phosphorus and distinct electronegativities of hydrogen, nitrogen and phosphorus.