This stereo now shows the full symmetry D∞h for the N=N=N (or similar: O=C=O) molecule as experimentally observed and theoretically predicted. The probability clouds of the π electrons form charge cylinders around the molecular axis. The picture is a rough schematic representation with cylinders having the same volume as the “π” clouds they replace. Remember, that π-electrons on the linear, vibrationless, molecule have an (orbital) angular momentum around the molecular axis, hence rotate with + or - /2 (called Λ=1 (lambda) analogous to the l = 1 (L) quantum number of p-electrons in spherical atoms). What happens, when the nuclear rotation of the molecule, perpendicular to the figure axis, becomes excited? The electronic and nuclear rotations combine to a total angular momentum which remains a “good” quantum number, i.e. the orbital electron rotation and the nuclear rotation add vectorially. This produces a precessing top. At higher temperatures vibrations of the three nuclei are turned on, two (degenerate) bending modes at right angles to each other, and two stretching modes along the molecular axis. Since the bending vibrations define a plane, each, including the axis, the cylinder symmetry is broken. Hence, the electronic angular momentum is quenched. The stretching vibrations do not influence these phenomena, they just change the moment of inertia for the nuclear rotation.-
All this is observed with the tools of molecular spectroscopy, and its quantum mechanical theory is completely understood, see e.g. G.Herzberg, “Molecular Spectra and Molecular Structure”, 3 vols., Van Nostrand.

To put this into a larger perspective, we summarize: Only linear molecules with point groups D∞h or C∞v, meaning cylinder symmetry, possess an orbital electronic angular momentum. With diatomic molecules, e.g. N2 or CO, this is persistent, even at high temperature. With triatomic and larger linear molecules, the orbital angular motion is quenched when bending vibrations set in. Examples are linear molecules like CO2, N2O, NCF, NCO(-), N3(-), NO2(+), C3O2, HC≡CH, HC≡N, and others. But, e.g. H2C=C=CH2, “linear” allene (D2d), with cumulated double bonds similar to O=C=O, does not belong to this category! The two CH2 groups are oriented orthogonal to each other, and the off CCC-axis protons would collide with electrons rotating around the figure axis. Of course, formaldehyde H2C=O (C2v), ethene H2C=CH2 (D2h), or benzene (D6h), do not have orbital angular motions. However, benzene’s “π” electrons form stationary rings of delocalized charges above and below the molecular plane. Although there is no permanent angular momentum around the rings, the mobile electrons easily react to an electric or magnetic field. When the latter is applied during an NMR experiment, it excites a “ring-current” producing a measurable diamagnetic shielding effect. I.e. the magnetically induced rotation of the electrons around the C6-ring makes a tiny magnet pointing against the component of the external magnetic field. This is sensed by the nuclear spin system of the molecule, hence observed as enhanced “chemical shift”, e.g. of the proton resonance.