How is bond angle of a molecule measured? Simply, how are (average?) bond angles for simple (or complex) molecules such as methane or ethene measured (determined)?
I'm guessing that X Ray crystallography can be used to determine bond angles for, well, materials that have a regular crystalline form. However, is there some other technique for determining bond angles in non crystalline  molecules?
I have researched on Chemistry SE and on the wider web and I can't find this question addressed - which I find strange as it seems to me an elementary question. Apologies if this question has already been addressed in Chem SE - I just wasn't able to find it using my search terms. I can find many resources about predicting bond angle and none on measuring them.
The most obvious method is single crystal xray crystallography, of course, and for simple molecules, IR and microwave spectroscopy in the gas phase can be used.
For many complex (e.g. bioorganic) molecules, this fails, but there are two (afaik) last hopes to help solve/prove structures if they don´t crystallise and the usual NMR spectroscopy with added computational chemistry is not enough.

Single molecule xray diffraction, a very new approach that can only just be done on the one most brilliant xray &quot;laser&quot; source in the world, the XFEL in Hamburg. It cost 1.2 billion euros to build, and this is the &quot;killer&quot; application it is supposed to prove feasible.

RDC (residual dipolar coupling) measurements with NMR, a very fancy method where the interesting compound is trapped in an oriented (sort of &quot;stretched&quot;) gel. If the molecules of the compound have a preferred orientation in that gel (they usually do, because very few biomolecules are spherical), you observe dipolar splittings in the spectra, which would normally be averaged out due to the free tumbling of the molecules. These couplings can be used to calculate relations between distances and angles between the magnetic field and the direct connecting line between the atoms involved.


No 2 of course relies heavily on additional input from conventional (spectroscopic) methods and computer simulations.