We have compared CNDO/2 and ab initio STO-3G calculations for furan and thiophene with and without the inclusion of 3d-orbitals on oxygen and on sulphur. Comparison of localized molecular orbitals indicates that d-orbital participation is over-estimated within the CNDO/2 approximation, although trends are reflected correctly. Enhanced aromaticity of thiophene compared to furan is primarily due to the delocalization of C2C3 π-bonds to the vacant d-orbitals of sulphur. On the other hand, d-orbitals are unimportant in describing σ-bonds. Energies of canonical molecular orbitals, as obtained with the CNDO/2 and ab initio STO-3G* methods and divided into separate σ and π groups, run parallel with each other within both groups.
ASJC Scopus subject areas
- Condensed Matter Physics
- Physical and Theoretical Chemistry