Origin of dipole moment enhancement in the formation of SiF4–NH3 dimer

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Abstract
The dimerization of SiF4 and NH3 is accompanied by a substantial 1.63 a.u. increase in dipole moment. Theoretical analysis of the charge distribution of the SiF4–NH3 system using the theory of atoms in molecules shows that the origin of this increase is primarily the large geometrical change which occurs within the SiF4 molecule when the dimer is formed, rather than the changes in atomic polarization or interatomic or intermolecular electron transfer. The relative displacement of the highly charged fluorine atoms of SiF4 away from NH3 upon dimer formation accounts for nearly 90% of the dipole moment enhancement. This result is in agreement with predictions made recently based on experimental results.