Theoretical study on structures and stability of Si2P2 isomers

The structures, energetics, spectroscopies, and isomerization of possible l ow-lying Si2P2 isomers in both singlet and triplet states are theoretically investigated at the B3LYP/6-311G(d) and CCSD(T)/6-311+G(2df) (single-point ) levels. At the final CCSD(T)/6-311+G(2df)//B3LYP/6-311G(d) level, the low est energy isomer is a singlet butterfly-like SiPSiP structure (1)1 with P- P cross bonding followed by a singlet rhombic SiPSiP isomer (1)2 with Si-Si cross bonding, whereas the cyanogen analogue PSiSiP (1)5 is the highest ly ing of all the singlet isomers. The singlet potential energy surface of Si2 P2 indicates that the rhombic isomer (1)2 is kinetically much more stable t han the butterfly-like isomer (1)1, although isomer (1)2 is 3.2 kcal/mol hi gher in energy than isomer (1)1, while other isomers are kinetically unstab le toward isomerization to isomer (1)1 or (1)2. It is also shown that the t riplet Si2P2 isomers are energetically higher than all the single species e xcept (1)5. Furthermore, for the most relevant singlet Si2P2 isomers and in terconversion transition states, the relative energies obtained at the B3LY P/6-311G(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 t he isomers of Si2P2 and other analogous molecules C2N2, Si2N2 and C2P2 are compared and analyzed.