Singlet-triplet excitation energies of R1R2Si=Si silylene derivatives: A G4/W1BD theoretical study

Abstract

Well-to-well (WWE~S-T~) and adiabatic (AE~S-T~) singlet-triplet excitation energies were calculated at the Gaussian-4 (G4) and W1BD levels of theory for a suite of mono- and disubstituted R~1~R~2~Si=Si silylene derivatives (where R~1~/R~2~=H, CH~3~, NH~2~, OH, and F), as well as H~2~C=Si and HN=Si. Reasonable agreement was obtained with prior E~S-T~ estimates at the CCSD(T)/6-311++G(d,p)//QCISD/6-31G(d) and B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G(d) levels of theory. The G4/W1BD E~S-T~ are systematically higher than these prior estimates by between 1 to 5 kcal/mol, averaging positive deviations of about 1-2 and 3-4 kcal/mol from the CCSD(T) and B3LYP estimates, respectively. Qualitative ground state multiplicity agreement between the four levels of theory was found for H~2~C=Si, H~2~Si=Si, HN=Si, (H~3~C)HSi=Si, (H~3~C)~2~Si=Si, (H~2~N)HSi=Si, (H~2~N)~2~Si=Si, (HO)~2~Si=Si, and F~2~Si=Si. However, there is disagreement as to the ground state multiplicity for (HO)HSi=Si and FHSi=Si using the different theoretical methods. For (HO)HSi=Si, G4 and W1BD methods predict either a slightly energetically favored ground state singlet (G4) or an energetic degeneracy between the two multiplicities (W1BD). For FHSi=Si, both the G4 and W1BD methods predict a clear ground state singlet, whereas the CCSD(T)/6-311++G(d,p)//QCISD/6-31G(d) method predicts effective energetic degeneracy, and the B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G(d) method predicts a clear ground state triplet. In light of the current high-level calculations, the ground state multiplicities of (HO)HSi=Si and FHSi=Si should be considered uncertain due to disagreement among various levels of theory. Resolution of the actual ground state multiplicities of these compounds will likely need to await experimental data.