Question

Bond characteristics of hydrogen bonding.

Answer 1

I will start with a quote:

“The [hydrogen] bond was for some time though to result from the formation of two covalent bonds by the hydrogen atom (...). It is now recognized that the hydrogen atoms, with only one stable orbital (the 1s orbital) , can form only one covalent bond, that the hydrogen bond is largely ionic in character, and that it is formed only between the most electronegative atoms” - The nature of the chemical bond, Linus Pauling.

Unfortunately Pauling was not quite right.

Through the use of nuclear magnetic resonance (NMR), it is possible to use J-coupling to provide information regarding the connectivity in proteins. Latest it has come to my attention I could do J-coupling between non-covalent linked amino acid residues. Remember that J-coupling are mediated through chemical bonds connecting two spins (AKA not through space). This means that there MUST be electrons between the donor and acceptor in hydrogen bonding.

This provides evidence of hydrogen bonding must be considered an example of delocalized molecular orbital formation, in which the donor, acceptor and hydrogen contribute to the formation of a single atomic orbital.

The question is just how much this formation happens.

Answer 2

@Kainui @aaronq @sweetburger @Photon336