Question

How are hydrogen bonds differentiated from run-of-the-mill dipole-dipole bonds?

Answer 1

They are significantly quantum, whereas the ordinary dipole-dipole interaction is classical.

The best way to understand this is to realize the H-bond is just like the electron-bond between two H nuclei in H2+. Instead of two positive nuclei being attracted to the same electron (the source of the covalent bond in H2+) you have two negative oxygen cores being attracted to the same positive proton. The proton is significantly delocalized between the two oxygen cores, and the bond has many of the same quantum characteristics of a covalent bond, e.g. it is directional and saturable (you can't have more than H-bonds than you X-H-X setups).

Answer 2

I have never come across the term 'oxygen core' before, and can't find a definition online. What is it?

Also- what is the difference between HCl -HCl attraction (not classed as H-bonding), and H-F -H-F attraction (it is)? Is it trivial or is there a physical meaning for the cutoff point?

Answer 3

I should probably just say oxygen atom. I mean the oxygen nucleus plus as many of its electrons as aren't involved in the H-bond. It's tricky because you're solving a quantum many-body problem. Simplifing perspectives are necessary for any real insight. Here I'm simplifying by saying let's treat the oxygen atoms as negative objects sharing a positive object (the proton). That leads us to see it as very similar to two positive atoms sharing a valence electron -- and there are deep similarities.

I wouldn't say there's a difference -- I think you're thinking of partitioning intermolecular forces where an H-atom is involved, saying these are classical dipole forces, and these over here are quantum H bonds. That's a mistake, I think. (Which doesn't mean lots of gen chem instructors and books won't make it. In case it hasn't been clear to you already, neither of those is fallable.) I think a better way to look at is that the classical dipole forces are ALWAYS exerted, so both HCl and HF experience them. But whether a situation makes quantum H-bonding possible is another story. It requires (of course) a proton, but it also requires sufficient electronegativity on the X atom, and sufficient localization of the lone pairs on the X atom to form a nice directional bond to the proton. You certainly get H-bonds in HF, because the F is very electronegative and small. In HCl they would be weaker, because the Cl is less electronegative, and much larger. Whether you would say the H bonds in HCl contribute significantly to its intermolecular interactions, I do not know. It probably depends on your point of view -- what you're trying to understand.

Sorry to be vague. A great deal of chemistry at the gen chem level must be simplified and made arbitrary, because the complete reality -- a quantum many body problem -- is fiercesomly complex, mathematically speaking.

Answer 4

Thank you. So, in short, I should take with a pinch of salt my chemistry book where it says only O,N and F can H-bond.

Answer 5

Yes. It's just drawing an arbitrary line, giving you a rule of thumb.