Quantum Mechanical Description of a Hydrogen Bond

The polarities of the molecules concerned are increased. The dipole moment of the hydrogen-bonded complex is larger than the vectorial sum of the monomer moments. 

The hnearity of the A-H. .. B segment tends to increase as the strength of the hydrogen bond increases. 

The protons of hydrogen bond donors usually tend to be situated on or near the hybrid lone-pair orbital axes of proton acceptors. 

EX is the electron exchange repulsion energy. This is the interaction energy caused by the exchange of electrons between monomers Ma and Mb. The interaction is a short-range repulsion due to the overlap of electron clouds of Ma and Mb. 

CT is the charge transfer or electron delocalization interaction energy. It results from the interaction caused by electron transfer from the highest occupied molecular orbital (HOMO) of Ma to the lowest unoccupied molecular orbital (LUMO) of Mb, and from the HOMO of Mb to the LUMO of Ma, and higher-order coupled interactions. This interaction is always attractive and highly directional. In hydrogen-bonded structures, a net electron transfer always occurs from the molecule with the nonbonded (lone-pair) electrons, or weakly bonded (tt) electrons to the molecule with a highly polarized A-H bond. 

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