An interacting segment model of molecular electric tensor properties: Theory and application to electric dipole moments of the halogenated methanes

A model is developed to describe trends in molecular properties such as electric dipole moment, polarizabilities, and hyperpolarizabilities for series of structurally related molecules. The model takes account of intramolecular electrostatic interactions, which give rise to induced electric moments in the various segments of the molecule. For this purpose each molecular segment is ascribed a set of "bare" electric tensor properties, which describe the hypothetical situation where intersegment interactions are absent, and a corresponding set of "dressed" properties, which include the effects of the intramolecular field. Any overall molecular electric tensor property is then obtained by summing the appropriate dressed segment properties. After theoretical development the model is used to fit electric dipole moment data for the 12 halogenated methanes CX_nY_4-n(X,Y = H,F,Cl;n = 0-4), for which a simple bond additivity model is known to be inadequate and which severely test any theory of induced dipole moments. The results of such an analysis are highly satisfactory and indicate that the model should also give an adequate description of higher-order electric tensor properties.