Lone pair sites

Williams and Weller " made a systematic investigation of lone pair site models for pyridine, pyridazine, pyrimidine, pyrazine, s-triazine, and s-tetrazine. They found that the 6-3IG electric potential of these molecules was significantly better fitted when lone pair sites were included. The population analysis (PA) charge model for the electric potential of these molecules is notably poor, as is shown in Table 8. For pyridine, PA charges have a relative error of fit to the electric... 

With added lone pair site(s) at 0.25 A radial distance from N. kj/mol,... 

Lone Pair Sites in Water Monomer and Dimer... 

Lone-pair site extended along C —F bond 0.25 A from F. Bond-pair site at center of C —F bond. 

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%. 

Nada and van der Eerden [60] proposed a six-site model of rigid H2O for the simulation of ice and water near the real at 1 atm. In this model, an H2O molecule has six interaction sites an O atom, two H atoms, two lone-pair sites (L), and a site M located on the bisector of ZHOH (Figure 17.11). A positive charge is placed on each H atom, and a negative charge is placed on each of the L and M sites. The intermolecular interaction between a pair of H2O molecules is calculated as the sum of the Coulomb potentials between the charges plus the sum of the LJ potentials between the atoms. Parameters in the six-site model were optimized by assuming truncation of the intermolecular interaction at the intermolecular distance around 1 nm (Table 17.1). 

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