Pair Sites in Water Monomer and Dimer

Lone Pair Sites in Water Monomer and Dimer 

Hydrogen bonding in water dimer has been extensively studied both theoretically and experimentally. The empirical form for a hydrogen bond potential energy function V b, which is compatible with usual van der Waals functions, is not well established. A clear separation of the coulombic component would 

Vim is to assume that the van der Waals potential of hydrogen in the bond is zero. The nonexistent repulsion of the hydrogen allows close approach of the hydrogen-bonded atoms. The electrostatic potential between net atomic charges further defines the directionality of the bond. With this type of model it is obvious that reliable values for net atomic charges are necessary. 

Singh and Kollman found that a 6-31G PD net atomic charge model for water yielded rrms= 10.5%. Thus the fit is not as good as expected. Water 

The five site model of Singh and Kollman allowed the site on the 2-fold axis to split into two sites above and below the molecular plane. They also considered a four site model where the oxygen charge was set to zero. This simplification of the model led to tjn = 0.537 with opposite sign for the inverted lone pair sites. The oxygen to lone-pair distance was 0.279 A, the angle 65.5°, and the rrms fit to the electric potential 7.5%. 

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