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

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Title Singlet-triplet excitation energies of R1R2Si=Si silylene derivatives: A G4/W1BD theoretical study
Authors/Editors* S Rayne, K Forest
Where published* Nature Precedings
How published* Other
Year* 2011
Publisher Nature Publishing Group
Keywords Silylenes, Singlet-triplet excitation energies, Theoretical estimates, Gaussian-4, W1BD
Well-to-well (WWES-T) and adiabatic (AES-T) singlet-triplet excitation energies were calculated at the Gaussian-4 (G4) and W1BD levels of theory for a suite of mono- and disubstituted R1R2Si=Si silylene derivatives (where R1/R2=H, CH3, NH2, OH, and F), as well as H2C=Si and HN=Si. Reasonable agreement was obtained with prior ES-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 ES-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 H2C=Si, H2Si=Si, HN=Si, (H3C)HSi=Si, (H3C)2Si=Si, (H2N)HSi=Si, (H2N)2Si=Si, (HO)2Si=Si, and F2Si=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.
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