# Average bond energy of H−F and H−O hydrogen bonds

Which of the following hydrogen bonds (shown by the dotted line) has the highest average bond energy?

A) $$\ce{F-H\bond{...}F}$$
B) $$\ce{F-H\bond{...}O}$$

I am thinking the answer is $$A$$ since the covalently bonded F atom, being highly electronegative, would induce a high partially positive polarity on the H atom. But on the other side, the more electronegative atom F will form a stronger Hydrogen bond than the comparatively less electronegative O atom.

Any flaw in my reasoning?

So fluorine does have the ability to hydrogen bond. However, they are usually transient and depend significantly on the species that is donating the H-bond along with some other factors (i.e. what phase the molecules are in, temperature, etc...)

Fluorine is so electronegative that it holds its electrons much "tighter" then other atoms, making it a poor H-bond acceptor. Oxygen, being less electronegative, is a better H-bond acceptor since the electrons aren't held onto as tightly, allowing them to interact more with other atoms.

Since bond energy is "the average value of the gas-phase bond-dissociation energy for all bonds of the same type within the same chemical species. The larger the average bond energy, per electron-pair bond, of a molecule, the more stable and lower-energy the molecule" (IUPAC WEBSITE), I would suggest that the H-bond with oxygen has a higher bond energy, as it is the stronger interaction of the two.

EDIT

The OP requested some sources, in which I failed to do initially, so I apologize for that but here they are.

Here is a quote from reference 2.

Theoretical calculations variously estimate 5 the strength of a F...H bond to be between 2 to 3.2 kcal mo1-1. This can be compared 6 to an O...H hydrogen bond which is typically between 5 - 10 kcal mo1-1. Consistent with this the electrostatic influence of fluorine is approximately half that of oxygen 7. Thus the greater electronegativity and lower polarisability of fluorine over oxygen, suppresses its electrostatic influence and renders it a poorer hydrogen bond acceptor.

The last reference discusses hydrogen bonding in halides, not too sure how much it compares them to oxygen bonding but it is certainty worth taking a quick look.

• I've searched on the internet and at many reliable sites, I found that the strength of the hydrogen bond depends on the difference of electronegativity between Hydrogen and the Hydrogen bond acceptor atom, "H...F>H...O>H...N" specifically written. Oct 3 '20 at 17:44
• @AryanVerma Please reference the websites you are referring too. In the meantime I will edit some references into my answer. Just so we are clear, you are suggesting that fluorine is a stronger H-Bond acceptor then oxygen? Oct 3 '20 at 17:51
• nimysart.com/hydrogen-bond courses.lumenlearning.com/introchem/chapter/hydrogen-bonding/… ('Key Point' number 2) chemistryguru.com.sg/factors-affecting-hydrogen-bond (Under the heading "Polarity") I personally think that larger the electronegativity of the Hydrogen acceptor atom, larger will be the attractive force exerted by it on the electron cloud of the Hydrogen atom.. leading to greater bond strength. Therefore, I guess H...F Hydrogen bond is stronger Oct 3 '20 at 17:59
• @AryanVerma In your reference, it is talking about the polarity of the molecule, not its ability to form strong hydrogen bonds. Because fluorine is SO electronegative, its electron shell is not very polarizable and it cannot form interactions as strong as oxygen. However, I can see how that can be misinterpreted, as the author does not make it that clear. Additionally, sometimes supporting ones claims requires primary sources, rather than online tutoring services. That is just some advice from someone who has done the same thing. Oct 3 '20 at 18:20
• In gas phase rotational spectroscopic experiments, the H-bond force constant has been related to bond strength (JACS, v109,356,1987.) via 'electrophilicity' of donor and 'nucleophilicity' of acceptor. This shows that FH-OH2 has approx twice the strength of FH-FH hydrogen bonds. Oct 4 '20 at 8:55