Hydrogen Bonds and Water

[Qube]

I'm a little confused on hydrogen bonds. Here's a sample question and answer I'm having trouble with:


1) In a single molecule of water, two hydrogen atoms are bonded to a single oxygen atom by
A) hydrogen bonds.
B) nonpolar covalent bonds.
C) polar covalent bonds.
D) ionic bonds.
E) van der Waals interactions.
Answer: C

OK - polar covalent bonds are holding the hydrogen atoms to the oxygen atom. That I can understand.

But couldn't the interaction between the hydrogen and the oxygen also be classified as a hydrogen bond? Hydrogen bond, as I was taught in Chem, is simply the interaction between H and F, O, or N. So couldn't the answer also be A?

[Mike H]

Hydrogen bonding is the interaction of a hydrogen atom with an electronegative atom that comes from a different molecule (or functional group if you're dealing with a large molecule). There's (typically) a clear cut donor and acceptor - for example, in liquid water, the intramolecular OH bond length is one-half the typical hydrogen bond length between water molecules. But if they're both hydrogen bonding - to go with your proposed redefinition - why are they so different, then?

[VincentweZu]

No because Hydrogen bonding is a form of intermolecular force - forces which occur between molecules. The bonding of hydrogen and oxygen in water is an example of intramolecular force - the force within a molecule.

[Qube]

Hydrogen bonds, according to the IUPAC Gold Book, can be an intramolecular force.

http://goldbook.iupac.org/H02899.html

Hydrogen bonds may be inter-molecular or intramolecular.

[Mike H]

That is what I was attempting to (imperfectly, it seems) convey - for example, if you have a very large molecule such as a protein, the amide proton of residue i can hydrogen bond to to the carbonyl oxygen of residue i-4. However, that hydrogen is going to be - on average - closer to one of the atoms than the other.

If you need further clarification, just ask. But I think the protein example is the best way to understand what is meant by an intramolecular hydrogen bond.

P.S. - When I said "intramolecular OH bond length," I was referring to the bond lengths *within* the water molecule, not the distances between hydrogen bonded atoms. Perhaps that wasn't clear - sorry about that.