Theoretical study on structures and stability of SiaPa isomers

The structures, energetics, spectroscopies, and isomerization of possible low-lying Si₂P₂ isomers in both singlet and triplet states are theoretically investigated at the B3LYP/6-31 IG(d) and CCSD(T)/6-31 l+G(2df) (single-point) levels. At the final CCSD(T)/6-311+G(2df)//B3LYP/6-311G(d) level, the lowest energy isomer is a singlet butterfly-like SiPSiP structure ¹1 with P-P cross bonding followed by a singlet rhombic SiPSiP isomer ¹2 with Si-Si cross bonding, whereas the cyanogen analogue PSiSiP ¹5 is the highest lying of all the singlet isomers. The singlet potential energy surface of Si₂P₂ indicates that the rhombic isomer ¹2 is kinetically much more stable than the butterfly-like isomer ¹1, although isomer ¹2 is 3.2 kcal/mol higher in energy than isomer 'l, while other isomers are kinetically unstable toward isomerization to isomer ₁1 or ¹2. It is also shown that the triplet SI2P2 isomers are energetically higher than all the single species except ¹5. Furthermore, for the most relevant singlet Si₂P₂ isomers and interconversion transition states, the relative energies obtained at the B3LYP/6-31 IG(d) level are in excellent agreement with the values calculated at the single-point CCSD-(T)/6-311+G(2df) level within 2 kcal/mol. Finally, the structural, energetic and kinetic similarities and discrepancies between the isomers of Si₂P₂ and other analogous molecules C2N2, SI2N2 and C2?2 are compared and analyzed.