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Why are there no distinguishable conformations in cyanogen and butadiyne?

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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.)

In molecules such as cyanogen (N≡C–C≡N) and butadiyne (H–C≡C–C≡C–H), there are no distinguishable conformations because all of the atoms in these molecules lie directly along the axis of the central single bond.1

Because these atoms are collinear with the bond axis, rotating the constituent groups about that bond does not change the spatial arrangement of the atoms relative to one another. Consequently, no distinct rotational states exist, and the molecule does not exhibit conformational isomerism.1