Question

Experiments at very low temperatures have shown that there are two distinct isomers of 1,2-dideuteriocyclobutadiene, with the molecular formula C4H2D2. These compounds have been spectroscopically demonstrated to be rectangular rather than square. As part of your explanation provide the structures of both compounds.

Answer 1

2 Isomers of 1,2-Dideuterocyclobutadiene are shown in image file.

And very low temperature cyclopentadiene compound exists. At very low temperature the resonance also ceased. Hence the degeneracy of all C is removed. And two sets of C-C bonds come into the picture. One C-C single bond and C=C double bond. As bond lengths for single and double bond are different molecule takes rectangular shape.

Also 2 Deuterium at C=C bond is one isomer and when they are at C-C is another isomer.