Water O-H Bond Length: Evidence & Measurement

[zincshow]

Not sure where this goes, but here goes:

Water O-H bond is usually thought of as 0.942 angstrom when looked at as H2O molecules.

This very nice site on water http://www.lsbu.ac.uk/water/evidnc.html suggests that x-ray data O···H peaks at 1.85 Å and 3.3 Å with no peak less then 1.85, suggesting there are not 0.942 H-O bond lengths in liquid water. Does this sound correct?

 

[jthechemist]

Intramolecular H-O bond lengths are approximately 0.9 - 1.0 Angstroms in liquid water at 25 C; intermolecular (hydrogen) bond lengths are much longer, quite possibly around 1.8 Angstroms as your source claims. Of course, all that changes for various phases at different temperatures. Crystal formation in ice should lead to H-O bond length changes to accommodate for hexagonal crystal packing, for example.

 

[zincshow]

Intramolecular H-O bond lengths are approximately 0.9 - 1.0 Angstroms in liquid water at 25 C; intermolecular (hydrogen) bond lengths are much longer, quite possibly around 1.8 Angstroms as your source claims. Of course, all that changes for various phases at different temperatures. Crystal formation in ice should lead to H-O bond length changes to accommodate for hexagonal crystal packing, for example.

I agree, but from searching defraction data for liquid water it seems to agree with their statement:

"However the model gives H···H peaks at 2.35 Å, 3.9 Å and 4.6 Å with a small peak at 2.9 Å and O···H peaks at 1.85 Å and 3.3 Å with smaller peaks at 4.55 Å and 5.25 Å similar to published data [17, 35, 37]."

Which makes me wonder why do they not see any O...H peaks under 1 Angstrom? Why do people think there are "Intramolecular H-O bond lengths are approximately 0.9 - 1.0 Angstroms in liquid water at 25 C"?

 

[t!m]

Radial distribution functions are useful for studying surrounding structure; the intramolecular OH bonds are *always* there and thus do not contribute any key insight. Thus the peaks arising from intramolecular bonds are often removed. See point (c) on the website you referenced: http://www1.lsbu.ac.uk/water/evidnc.html#c

 

[zincshow]

Thank you. My mistake.