Strongest Hydrogen Bond?

[theyellowking]

So a BR question from Chemistry I asked which is the strongest Hydrogen bonding:

1) Free pair of electrons on an oxygen, and a hydrogen covalently bonded to an oxygen
2) Free pair... on an oxygen, and a hydrogen... to a nitrogen
3) Free pair... on a nitrogen, and a hydrogen... to an oxygen
4) Free pair... on a nitrogen, and a hydrogen... to a nitrogen

Now I knew that the choices were narrowed down to 1) and 3), since the high electronegativity would essentially "strip" the electron from the hydrogen, making it more available for hydrogen bonding, but I was confused as to whether the nitrogen or oxygen's electrons would form a stronger hydrogen bond. In other words, what factor decides this? Electronegativity? Size?

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[Czarcasm]

So a BR question from Chemistry I asked which is the strongest Hydrogen bonding:

1) Free pair of electrons on an oxygen, and a hydrogen covalently bonded to an oxygen
2) Free pair... on an oxygen, and a hydrogen... to a nitrogen
3) Free pair... on a nitrogen, and a hydrogen... to an oxygen
4) Free pair... on a nitrogen, and a hydrogen... to a nitrogen

Now I knew that the choices were narrowed down to 1) and 3), since the high electronegativity would essentially "strip" the electron from the hydrogen, making it more available for hydrogen bonding, but I was confused as to whether the nitrogen or oxygen's electrons would form a stronger hydrogen bond. In other words, what factor decides this? Electronegativity? Size?

It's essentially the electronegativity in either case that determines the strength of the hydrogen bond. The more electronegative the atom (F, O, N), the more electrons it pulls toward itself and therefore the greater partial negative charge that resides on the atom. In terms of the lone pair for F, O, or N, Fluorine being the most electronegative would have the greatest partial negative charge, nitrogen (the lowest electronegativity of the 3) would have the smallest. Likewise, the most partial positive hydrogen is caused by the greatest pull of electron density due to the electronegative atom (F, O, N). Again, in this scenario, the H attached to an F is most positive, the N-H the least positive. Given the choices above, the lone pair of an oxygen and the hydrogen of an oxygen would form the strongest hydrogen bond because the partial charge for each is greatest. They would therefore have the greatest attraction to each other forming the strongest hydrogen bond.

 

[theyellowking]

It's essentially the electronegativity in either case that determines the strength of the hydrogen bond. The more electronegative the atom (F, O, N), the more electrons it pulls toward itself and therefore the greater partial negative charge that resides on the atom. In terms of the lone pair for F, O, or N, Fluorine being the most electronegative would have the greatest partial negative charge, nitrogen (the lowest electronegativity of the 3) would have the smallest. Likewise, the most partial positive hydrogen is caused by the greatest pull of electron density due to the electronegative atom (F, O, N). Again, in this scenario, the H attached to an F is most positive, the N-H the least positive. Given the choices above, the lone pair of an oxygen and the hydrogen of an oxygen would form the strongest hydrogen bond because the partial charge for each is greatest. They would therefore have the greatest attraction to each other forming the strongest hydrogen bond.

That's what I thought as well, but here's what they gave me for an answer:

The strongest hydrogen bond comes from the more basic lone pair donor (found on the Nitrogen, which is less electronegative than oxygen) being donated to the most protic hydrogen found covalently bonded to the oxygen.

 

[Czarcasm]

That's what I thought as well, but here's what they gave me for an answer:

The strongest hydrogen bond comes from the more basic lone pair donor (found on the Nitrogen, which is less electronegative than oxygen) being donated to the most protic hydrogen found covalently bonded to the oxygen.

Seems odd. Not sure if I'd agree with that reasoning. Would probably be true if we're considering the strongest covalent bond (ie. the least electronegative atom would have the strongest bond between two atoms). In this case, it's an intermolecular bond and so the partial charges are the main thing we're considering here. Would be interested to get others input.

 

[Concrete Jungle]

Hydrogen bonds are both dipole-dipole electrostatic interactions and partial covalent bond. The more electronegative the donor atom is (the one bound to the protic hydrogen) the more covalent character it has. The accepter atom, with the lone pair, has less electron density to contribute/overlap with the hydrogen as you increase electronegativity. Remember, electronegativity is an atom's attraction to electrons not protons.

I bet if you looked at an electrostatic map of the two possible accepter atoms, the nitrogen would have more electron density in lone pair orbital compared to oxygen because it is less electronegative.

 

[Czarcasm]

Hydrogen bonds are both dipole-dipole electrostatic interactions and partial covalent bond. The more electronegative the donor atom is (the one bound to the protic hydrogen) the more covalent character it has.

Increasing electronegativity would result in more ionic character, not covalent. The electrons in the bond being shared results in an uneven pull of electrons to the more electronegative atom resulting in an unequal distribution of e-density.

The accepter atom, with the lone pair, has less electron density to contribute/overlap with the hydrogen as you increase electronegativity.

A more electronegative atom will have a greater electron density and therefore more negative character. The hydrogen bond itself is formed between the electrostatic interactions between the two molecules. There is no overlap. An overlap would imply electrons are being shared.

 

[BerkReviewTeach]

If the hydrogen bond were purely an ionic interaction, the there would be no preferred spatial alignment. If it's covalent in nature, then the orbitals would have to overlap in a linear fashion (much like a transition state that transfers an atom): O-H---N. So which is it?

We know from millions of examples in organic chemistry and biology (such as anti-parallel and parallel alignments in beta-pleated sheets) that the H-bond must be linear for the O-H---N (or any H-bond) to exist, so the natural conclusion is that the H-bond is in essence a very weak covalent bond. If the three atoms are not linear, the H-bond will not form, which is why folding in a protein so specific.

So now for the best H-bond. The reasoning that H on an O will be more electron poor than an H on an N is well reasoned in all the responses here, so that is accepted. Where trouble seems to fall is with the strength of the electron pair donor of the H-bond. Electron density is not pertinent, because even with all sorts of e-density, if the atom won't share its electrons, then it won't form a good H-bond. The greater willingness of N to donate e-density than O allows it to form a stronger H-bond.

If you still feel uncertain, consider glycine at a very high pH where it exists as H2N-CH2-CO2-. You have a neutral N and O- on that molecule, yet when you add H+ it favors going to the neutral N over the negatively charged O. Even if you wish to invoke the idea of resonance, the you still have two O atoms with -0.5 charges competing with and getting beaten out by a neutral N. N is a better donor atom because of its lower electronegativity.

 

[Concrete Jungle]

A more electronegative atom will have a greater electron density and therefore more negative character. The hydrogen bond itself is formed between the electrostatic interactions between the two molecules. There is no overlap. An overlap would imply electrons are being shared.

The more electronegative atom by definition attracts its electrons more. How could you have more stabilization in the transition state with a more tightly held lone pair. It is like saying the best nucleophile is the most electonegative one because .... its electron pair is more tightly held by the atom. The lone pair on nitrogen are better nucleophiles than oxygen because they are more polarizable.

Your first quote is literally you arguing over semantics.

 

[Czarcasm]

If the hydrogen bond were purely an ionic interaction, the there would be no preferred spatial alignment. If it's covalent in nature, then the orbitals would have to overlap in a linear fashion (much like a transition state that transfers an atom): O-H---N. So which is it?

We know from millions of examples in organic chemistry and biology (such as anti-parallel and parallel alignments in beta-pleated sheets) that the H-bond must be linear for the O-H---N (or any H-bond) to exist, so the natural conclusion is that the H-bond is in essence a very weak covalent bond. If the three atoms are not linear, the H-bond will not form, which is why folding in a protein so specific.

So now for the best H-bond. The reasoning that H on an O will be more electron poor than an H on an N is well reasoned in all the responses here, so that is accepted. Where trouble seems to fall is with the strength of the electron pair donor of the H-bond. Electron density is not pertinent, because even with all sorts of e-density, if the atom won't share its electrons, then it won't form a good H-bond. The greater willingness of N to donate e-density than O allows it to form a stronger H-bond.

If you still feel uncertain, consider glycine at a very high pH where it exists as H2N-CH2-CO2-. You have a neutral N and O- on that molecule, yet when you add H+ it favors going to the neutral N over the negatively charged O. Even if you wish to invoke the idea of resonance, the you still have two O atoms with -0.5 charges competing with and getting beaten out by a neutral N. N is a better donor atom because of its lower electronegativity.

Great explanation. Kind of feels silly now... must be the sleep deprivation . See, the way I reasoned this was that the hydrogen bonding was purely due to electrostatic attraction of charges: the more electronegative atom having a large cloud of electrons would have a highly negative/dense region of negative charge. This dense region of negative charge would have the greatest coulombic attraction for the electropositive hydrogen atom. But what I over looked was that it's the availability of the lone pair that's involved here instead. The lone pair on a nitrogen is less tightly held and therefore, more readily available "to donate".

 

[theyellowking]

If the hydrogen bond were purely an ionic interaction, the there would be no preferred spatial alignment. If it's covalent in nature, then the orbitals would have to overlap in a linear fashion (much like a transition state that transfers an atom): O-H---N. So which is it?

We know from millions of examples in organic chemistry and biology (such as anti-parallel and parallel alignments in beta-pleated sheets) that the H-bond must be linear for the O-H---N (or any H-bond) to exist, so the natural conclusion is that the H-bond is in essence a very weak covalent bond. If the three atoms are not linear, the H-bond will not form, which is why folding in a protein so specific.

So now for the best H-bond. The reasoning that H on an O will be more electron poor than an H on an N is well reasoned in all the responses here, so that is accepted. Where trouble seems to fall is with the strength of the electron pair donor of the H-bond. Electron density is not pertinent, because even with all sorts of e-density, if the atom won't share its electrons, then it won't form a good H-bond. The greater willingness of N to donate e-density than O allows it to form a stronger H-bond.

If you still feel uncertain, consider glycine at a very high pH where it exists as H2N-CH2-CO2-. You have a neutral N and O- on that molecule, yet when you add H+ it favors going to the neutral N over the negatively charged O. Even if you wish to invoke the idea of resonance, the you still have two O atoms with -0.5 charges competing with and getting beaten out by a neutral N. N is a better donor atom because of its lower electronegativity.

Ah, this makes a lot of sense. Thank you!