This is the gas-phase structure of hypofluorous acid:

enter image description here

The solid-phase structure however has a bond angle of 101 degrees. Why is there a difference in the solid and gas phase bond angles?

Is this due to packing in the solid-phase? In the solid-phase the molecules should be much closer together than in the gas-phase, so I expect more intermolecular forces and interactions. However, what exactly would be responsible for the angle expansion in the solid phase? From one book I read that bond lengths often differ among the phases.

So what exactly is behind the bond angle expansion in the solid phase?

up vote 4 down vote accepted

It is due to hydrogen bonding in the solid but not gas phase, in analogy to what occurs for the water bond angle.

From a VSPER point of view, the oxygen atom of one molecule would form a hydrogen bond with the hydrogen of another molecule, decreasing the repulsion of the lone pairs to be closer to that of the covalently bonded pairs, so the geometry would be closer to tetraherdral in the solid phase than in the gas phase.

  • Thank you; I had suspected hydrogen bonding or some kind of intermolecular force. So how does hydrogen bonding decrease lone pair repulsion? I'm guessing that the partial positively charged hydrogen on one molecule, by forming a "bond" with a lone pair on another molecule, will "dilute" the strength of the lone pair to repel; I think of physics and point charges and vector addition of all the forces on a point charge. Also couldn't the partially positively charged hydrogen bond with a lone pair on fluorine too? I know this isn't strictly hydrogen bonding but this is another factor, right? – Dissenter May 20 '14 at 18:20
  • 1
    The crystal structure is solved in this reference: onlinelibrary.wiley.com/doi/10.1002/anie.198803921/abstract The hydrogen bond does go between two oxygen atoms. Fluorine can certainly participate in hydrogen bonds, but doesn't in this case. There is discussion of both possibilities in the reference. – DavePhD May 20 '14 at 18:41
  • Thank you very much! One of my former chem profs had a near obsession with bond angles and now we both do (hence my question). I'll enjoy the paper (edit: once I figure out how to access it). – Dissenter May 20 '14 at 18:42
  • You can download the document from this link : booksc.org/book/479656 (however i'm not sure if it is the complete document) – LievenB May 21 '14 at 13:08

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