Substituent Effect on the Ground State Spin-Multiplicity of Tetramethyleneethane (TME) Derivatives

Abstract

The substituent (X = H, SiH₃, F, OH) effect on the ground state spin-multiplicity and the singlet-triplet energy gap of tetramethyleneethane (TME) derivatives have been computed at CASSCF(6,6)/6-31G(d) level of theory. The singlet ground state is predicted for all the TME derivatives, and the singlet-triplet energy gap (δE_ST = E_S-E_T) is largely dependent on the substituents |δE_ST| = 6.3 (X = F) >5.0 (X = OH) >2.5 (X = SiH₃) >2.4 (X = H). The substituent effect on the ground state spin-multiplicity and energy gap can be explained by through bond interaction between the symmetric non-bonding molecular orbital Ψ_S with the C-X σ or σ* orbital.