Hydrogen bonding occurs when a proton Lewis acid, H+, is held between two Lewis bases.
The hydrogen bond is a weak type of complexation deemed responsible for the high boiling points of water, alcohols, carboxylic acids etc. and the high solubility of (low molecular weight) alcohols, carboxylic acids and sugars in water.
There is a long and detailed discussion on the Wikipedia hydrogen bond page, however, while this page gives lots of description and illustration it avoids an explanation of the true nature of hydrogen bonding.
While hydrogen bonding does occur when a proton Lewis acid, H+, is held between two Lewis bases there a severe problem. The simple ‘Lewis base–proton–Lewis base’ model cannot exist on simple LACO MO grounds as there are too many electrons!
Two electrons are provided by each Lewis base, giving a total of four electrons. The presence of four electrons must result in the formation of (at least) two molecular orbitals due the Pauli exclusion principle. The first MO will be bonding but the second MO will be antibonding. As the higher energy antibonding MO will dominate, the net result will be antibonding so no bond will form.
Lewis base-H+ bonds are generally highly polar structures and it is easy to consider the hydrogen bond to result from dipole-dipole electrostatic attraction. This is certainly the model and used by many text books. "The lone pair of electrons on one water's δ– oxygen attracts the δ+ hydrogen on an adjacent water molecule:
There is a clues to the true nature of the hydrogen bond, and it comes from reaction chemistry: All Brønsted acid proton transfer reactions pass through a hydrogen bonded intermediate transition state.
Ammonia reacts with hydrogen chloride to produce ammonium chloride, NH4Cl. However, if the reaction is performed at -269°C, 4K, the NH3/HCl hydrogen bonded complex can be trapped – by matrix isolation – and studied by infrared vibrational spectroscopy. This experimental system shows there to be a structure in which the hydrogen atom rapidly moves, vibrates, between the chloride and amine Lewis base centres:
When the proton vibrates in the hydrogen bond it moves from a state in which it is bonded to one Lewis base to a state in which it is bonded to the other:
The time averaged effect, the superposition, is for the two Lewis bases to be attracted to each other through the hydrogen atom.
A Hydrogen Bonding Analogy From Any Soap Opera
The girl cannot decide between two boys, who hate each other, yet the boys find themselves strangely drawn together because of the girl... they are attracted to each other through the girl...
And so it is with hydrogen bonding: the two Lewis base centres should repel, but are drawn together through the proton Lewis acid...
Because, with the exception of the electron e– and the photon hν, the proton, H+, is smallest and lightest of all chemical entities. In the hydrogen bond the proton quantum tunnels between the two Lewis bases. The proton buzzes between the two Lewis base centres, associating with both, so drawing them together.