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astrosnapper@'s thorough answer to the question What is precipitable water vapor in millimeter-wave radioastronomy and how is it measured? links to the paper Observing Conditions for Submillimeter Astronomy which discusses spectral windows of atmospheric transmission in the 100 ~ 1500 GHz range. These windows are "delineated" or limited by strong absorption lines of molecular water vapor in the atmosphere.

Figure 2 is shown below. The caption reads

Fig. 2. Energy levels of the lowest rotational states of water with transitions labeled by the frequency in GHz. The lines at 557, 752, 988, 1113, and 1229 GHz delineate the principal submillimeter windows.

and displays ortho- and para- water separately.

Curious to find out what those were, I went to Martin Chapin's (also here) site Water Structure and Science which discusses the physics of ortho- and para- water, and describes experiments on single water molecules trapped and isolated in C60 fullerene "cages" (Endohedral fullerenes):

Question: Have caged molecules ever been observed in space?

These could be trapped in fullerenes (which I believe have been observed) or otherwise.

ortho- or para- water trapped in a fullerene cage

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    $\begingroup$ Define "in space". Do you just mean extraterrestrial, or do you mean interstellar? $\endgroup$ – Rob Jeffries Apr 27 at 7:22
  • $\begingroup$ @RobJeffries I'll stick with Von Karman's definition; anything higher than 100 km will do. If you suspected that I was originally thinking only of interstellar molecules, you are absolutely right, but that's because I didn't think it through. If there's a caged molecule "out there" somewhere, I'd love to hear about it! $\endgroup$ – uhoh Apr 27 at 7:27
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    $\begingroup$ Clathrates have been found I think. $\endgroup$ – Rob Jeffries Apr 28 at 0:01
  • $\begingroup$ @RobJeffries I'd say those would count. See "...or otherwise" in last sentence. $\endgroup$ – uhoh Apr 28 at 0:04
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Fullerenes have been detected in space, including C60, C60+ and C70 and appear to be responsible for the diffuse interstellar bands (DIBs), which are absorption features caused by the interstellar medium. The question of endohedral fullerenes is an interesting one, it is discussed in the paper "Interstellar fullerene compounds and diffuse interstellar bands" by Omont (2016).

In section 3.1.2, the situation for metallofullerenes M@C60 is discussed:

Analysis of the optical spectra of metallofullerenes thus presents the same difficulties as do C60 or C60-. It is therefore not surprising that even theoretical studies of their energy levels and excitation are lacking, incomplete or ambiguous, and we are still waiting for laboratory spectra. The accuracies of wavelength determinations are still far from being useful in constraining possible associations of visible DIB features with metallofullerenes. Estimates of line strengths are practically always lacking.

From this it would appear that more lab work is necessary before we are able to identify metallofullerenes from astronomical spectra.

In section 7, Omont describes the situation for hydrogen molecules in fullerenes (H2@C60), with section 7.2 discussing the prospects for finding interstellar H2@C60:

It seems that the presence of endohedral H2 would not change the basic properties of interstellar fullerenes much, making it difficult to reveal its presence, except thanks to specific spectroscopic features. However, even if it were significant, the presence of endohedral H2 should be difficult to confirm because the abundance of H2@C60+, for instance, could hardly be expected to exceed or even reach 10−9. There is thus practically no chance to ever detect its near-IR H2 2 µm absorption. UV spectroscopy is more sensitive by orders of magnitude than absorption of weak IR lines. It can detect H2 column densities lower than ∼1014 cm−2 (e.g. Gillmon et al. 2006). But even so, the UV detection of H2@C60 seems out of reach.

A quick check of the papers citing Omont (2016) doesn't turn up any detections of endohedral fullerenes, so it looks like the situation remains as described.

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  • $\begingroup$ Wow, thanks for digging in and writing such a thorough answer. The situation sounds pretty challenging. I'll start reading Omont 2016 this morning at arxiv.org/abs/1511.01250 $\endgroup$ – uhoh Apr 27 at 23:51
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    $\begingroup$ Yikes, this is quite a densely written review! $\endgroup$ – uhoh Apr 28 at 1:20

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