Charge calculations in molecular mechanics. V. Silicon compounds and π bonding

A previously published scheme for the calculation of partial atomic charges has been extended to include silicon, and has been parameterized for a range of Si? X bonds (X?C,H,O,F,Cl,Br). For the silicon–halogen and silicon–oxygen bonds, a comparison is made between charges calculated with and without the inclusion of π-bonding. An extensive data set consisting of experimental geometries and dipole moments for the silicon compounds considered is presented and this leads to the selection of standard Si? X bond lengths. The calculated dipole moments for the above compounds are in good agreement with those obtained experimentally only when the π charges are included. A comparison has also been made between the partial charges from this scheme and those obtained from computational methods using the Mulliken population analysis. There is considerable disagreement between the methods. Finally, the implications of the charges and structural data are considered in terms of application to zeolite systems.