A theoretical calculation of the bond orders and bond lengths in thiophthen

Using a molecular-orbital method, which involves the d orbitals of the sulfur atom, as developed by Longuet-Higgins for thiophen, we have calculated the bond orders in thiophthen and related them to bond lengths. The calculated bond lengths are compared with the observed values reported by Cox, Gillot & Jeffrey; taking into account the approximate character of the calculations, the agreement is very satisfactory. The value calculated for the central bond in the molecule is in greatest error. This `central-bond discrepancy' has also appeared in molecular-orbital calculations not involving d orbitals on molecules not containing heterocentres, and appears to be a more general feature which requires further investigation.