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Hydrogen Bonding

Proton Held Between Two Lewis Bases: Lewis Base/Proton/Lewis Base Complex

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.

But why?

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.

Water, oxygen hydride, is a liquid at room temperature yet all of the other main group hydrides close to water in the periodic table are gases at 25°C and 1.0 atm pressure:

While NH₃ and HF do exhibit hydrogen bonding and elevated boiling points, it seems that H₂O is ideally suited to exhibit hydrogen bonding.

Another strange manifestation of hydrogen bonding is that water has its maximum density at 4°C. Thus, water ice is less dense than liquid water and it floats. Most solids have greater density than the liquid. Crystal structure of hexagonal ice. Gray dashed lines indicate hydrogen bonds (Wikipedia):

Water hydrogen bonds with ammonia, and either molecule can behave as the H⁺ donor or acceptor. More complicated molecules can have different types of hydrogen bonding function (from Wikipedia):

Hydrogen bonding is seen with all molecules possessing -OH functions, including alcohols, carboxylic acids and sugars such as glucose. Carboxylic acids such as acetic acid exist as gas phase dimers:

β-Diketones partially exist the hydrogen bonded cyclic-enol form:

Hydrogen bonding is of immense importance in molecular biology as it constitutes the glue which holds together the twin strands of the DNA double helix and is responsible for secondary, alpha–helix & beta-sheet, and tertiary protein structure.

Side view of an α-helix of alanine residues in atomic detail. Two hydrogen bonds to the same peptide group are highlighted in magenta:

Diagram of a section of β-pleated sheet with H-bonding between protein strands:

Chemical structure of DNA. Hydrogen bonds between A=T and between C≡G are shown as dotted lines: