# Are bond angles constant?

I'd imagine the answer to the question posed in the title is no - bond angles are not constant. Covalent bonds are not infinitely rigid; hence IR as a spectroscopic technique.

If so, how can we accurately determine bond angles? What margin of error might we expect from an experimentally measured bond angle?

No, bond angles are not constant. For example, the low-energy IR absorption of water (at $1595 \:\mathrm{cm^{-1}}$) is from the H-O-H bend