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Why cannot anti and gauche forms of 1,2-dideuterioethane usually be separated?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — isomerism Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)

The anti and gauche forms of 1,2-dideuterioethane are conformational isomers that cannot be separated under normal conditions because the energy barrier required to interconvert them is too low.1

In 1,2-dideuterioethane, these staggered forms are interconverted through rotation around the central carbon-carbon bond. The energy barrier separating these isomers is approximately 3 kcal/mol. Because this barrier is so small, the molecules undergo rapid rotation, preventing the isolation of the individual anti and gauche forms. In general, for conformational isomers to be stable enough to be isolated, the molecular structure must impose a significantly higher barrier to rotation.12