SemiexperimentalEquilibrium Structures for cis,cis- and trans,trans-1,4-Difluorobutadieneby the Mixed Estimation Method and Definitive Relative Energies ofthe Isomers.

Equilibriummolecular structures accurate to 0.001 Å and 0.2° have beendetermined for cis,cis- and trans,trans-1,4-difluorobutadiene by thesemiexperimental mixed estimation method. In this method, structuresare fitted concurrently to equilibrium rotational constants and bondparameters obtained from an intermediate level of electronic structuretheory. The effect of fluorine substitution on the carbon backboneof butadiene is surprisingly small. Definitive energy differencesfor the ground states were computed, employing the focal-point analysis(FPA) technique, between the trans,trans and cis,cis isomers (ΔH°0= 5.6(3) kJ mol–1)and the cis,trans and cis,cis isomers (ΔH°0= 3.2(2) kJ mol–1) of 1,4-difluorobutadiene.These differences confirm the exceptional relationship that the trans,transisomer has the highest energy and the cis,cis isomer the lowest energy,endorsing what was reported earlier on the basis of experimental observationsin benzene solution. [ABSTRACT FROM AUTHOR]