Re: What kind of hydrogen bonding is the strongest?

Date: Sat Jan 1 00:51:34 2000
Posted By: Martin Thomas, Post-doc/Fellow, Phyiscal Chemistry, Quantachrome Corporation
Area of science: Chemistry
ID: 944629157.Ch

Message:

A very good question Matt!

All chemistry students know of the existence of the hydrogen bond, but few are provided with any details in the course of their studies.

I shall refer you to the great Cotton & Wilkinson¹... track down their latest edition as soon as possible!

In their third edition, they say

"...hydrogen bonds vary considerably in nature and involve many contributing factors. To the question "Where does the energy of the hydrogen bond come from?"..."there is still no concise answer that seems to be generally acceptable and there probably never will be."

Having said that, there is a great deal known about the hydrogen bond. You have already ascertained that boiling points of certain substances are related to hydrogen bonds. Higher than expected boiling points are observed in H₂O, HF and NH₃ when compared to their own-group analogs (e.g. H₂S, HCl and PH₃ respectively). Elevated boiling points and heats of vaporization are used as evidence of the special association between certain molecules that we call hydrogen bonding.

The hydrogen bond is termed a secondary interaction between a hydrogen atom already bonded to an electronegative atom and another atom, also generally electronegative. So yes, electronegativity is very important! This is confirmed, since the hydrogen bond is strongest for first-row elements, the extreme case being F-H-F. Electronegativity depends upon the electron configuration of the atom (ionization potential/electron affinity which are atomic number related, so size is a factor, its valence state and its environment, i.e. that is to say the molecule in which it resides.

C&W¹ give these bond energies...

Bond	as in...	Bond Energy (kJ mol^-1)
F-H-F	KHF₂	113
F-H---F	HF_gas	28.6
O-H---O	(HCOOH)₂	29.8
O-H---O	H₂O_solid	21
N-H---N	melamine	25

But do be aware, that the atoms between which the hydrogen atom is located need not be the the same! For example, N₂H₅Cl is hydrogen-bonded thusly: N-H---Cl, and hydrogen cyanide is hydrogen-bonded (C-H---N) such that it can be considered a polymer, (HCN)_n. Hydrogen bonding is extremely important in biological systems. There are even cases of intramolecular hydrogen bonding, wherein the association is made between one very polar group of the molecule and another polarizable part. For example, in C₆H₅(CH₂) ₂OH, there exists a hydrogen bond between the hydrogen of the alcohol group and the aromatic ring of the same molecule.

I hope that I've given you a little more insight, but do rush off to your library to find out some more!

References
1. F.A. Cotton & G Wilkinson, Advanced Inorganic Chemistry 3rd ed, Interscience (John Wiley & Sons, Inc.) (1972) ISBN 0-471-17560-9.
2. W.C. Hamilton & J.A. Ibers, Hydrogen Bonding in Solids, Benjamin (1968)
3. G.C.Pimentel & A.L. McClellan, The Hydrogen Bond, Freeman (1960)
3. A.Rich & N. Davidson, eds., Structural Chemistry and Molecular Biology, Freeman (1968)
4. A.K. Covington & P. Jones, eds., Hydrogen-bonded Solvent Systems, Taylor & Francis (1968)
5. G. Zundel, Hydration and Intermolecular Interaction, Academic Press (1970)

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