Bond order-bond length relationships in conjugated hydrocarbons–the microwave spectrum and structure of 3,4-dimethylenecyclobutene

Abstract

The microwave spectrum of the benzene isomer 3,4-dimethylenecyclobutene including spectra of all possible single ¹³C-substituted and sufficient singly and doubly D-substituted species to give a complete r₅ geometry, have been measured and analysed. An estimate of the r_e geometry has also been derived. The additional precise CC bond lengths obtained for an unsubstituted, conjugated hydrocarbon enable us to examine bond order-bond length relationships more thoroughly than has previously been possible. The CC bond lengths exhibit a noticeably better correlation with SCFMO bond orders than with simple Hückel bond orders.

Further confirmatory measurements of the dipole moment of dimethylenecyclobutene have been made. Ab initio molecular orbital calculations using a 6-31G basis set give an optimized geometry with CC bond lengths within 2 pm of the r₅ values. The computed dipole moment agrees almost exactly with experiment but a corresponding calculation on fulvene is discrepant with experiment by 0.16 D, which is probably a more typical error.