Moment of Polar Molecules in Solution

When a substance is placed in an electric field, such as exists between the plates of a charged capacitor, it becomes to some extent electrically polarized. The polarization results at least in part from a displacement of electron clouds relative to atomic nuclei polarization resulting from this cause is termed electronic polarization. For molecular substances, atomic polarization may also be present, owing to a distortion of the molecular skeleton. Taken together, these two kinds of polarization are called distortion polarization. Finally, when molecules possessing permanent dipoles are present in a liquid or gas, application of an electric field produces a small preferential orientation of the dipoles in the field direction, leading to orientation polarization. 

The permanent dipole moment /r of a polar molecule, as a solute molecule in a liquid solution in a nonpolar solvent or as a molecule in a gas, can be determined experimentally from measurements of the dielectric constant k. This quantity is the ratio of the electric permittivity s of the solution or gas to the electric permittivity sq of a vacuum (8.854 X 10- Fm )  

For that reason the dielectric constant is also known as the relative permittivity (with symbol s,). The dielectric constant is determined with a capacitance cell, incorporating a capacitor of fixed dimensions in a suitable contaimnent vessel. If C is the capacitance of 

The present experiment deals with polar molecules in solution in a nonpolar solvent. Experiment 30 deals with polar molecules in a gas. The basic theoretical framework is the same for both. 

Either of two systems can be studied in this experiment (1) o- and m-dichlorobenzene, or (2) succinonitrile and propionitrile. hi the first case, the results can be compared with the vector sum of C—Cl bond moments obtained from the known dipole moment of monochlorobenzene. In the second case, one obtains direct information about internal rotation in a simple 1,2-disubstituted ethane 

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Onsager 0 equations (Section 5.10), one can extract the dipole moments of polar molecules and the polarizability of any solute molecule. One needs a capacitance cell whose electrodes are as close to each other as practical (for higher capacitances) and reasonable solubilities. If the shape of the solute is very different from the sphere used in the Debye model, then the ellipsoidal cavity has been treated theoretically [13] and applied to hypsochromism [14]. 

The linear and non-linear polarizabilities of a molecule in solution differ from those of the isolated molecule in the gas phase since the molecular properties are modified by solute-solvent interactions. Some of these interactions are present even in the absence of externally applied static or optical fields. For molecules with a non-zero dipole moment fj in the electronic ground state the dominant interaction is usually due to the reaction field contribution The molecular dipole moment polarizes the solvent environment and thus generates a polarization field which interacts with the solute. This field is given by (88) (Boettcher, 1973 Wortmann and Bishop, 1998). 

Simple electrostatic models can be used to interpret the activity coefficients of polar molecules in terms of just three parameters a radius, the dipole moment of the solute, and the dielectric constant of the solvent. The continuum model of the solvent can be used to deduce a value for the free energy of solvation of a spherical molecule of radius r containing a point dipole at its center. The value obtained by Kirkwood from electrostatic theory is... 

Dipole moments may also be derived by a consideration of the dielectric constant data themselves. Since amino acids and proteins are soluble only in polar solvents, the treatment which is applicable to dilute solutions of polar molecules in a non-polar medium cannot be applied here. However, the general theory of polar liquids developed by Onsager (S7) and Kirkwood (67) [see also Kirkwood in Cohn and Edsall [16), Chapter 12], is applicable here. According to Kirkwood s treatment, the dipole moment (/z) of an individual molecule in the liquid is in general different from its moment in the gaseous state because the attractions... 

COOH group, tends to have approximately the same dipole moment independently of the molecule of which it is a constituent part. Equation (9.16), which applies to dilute solutions of polar molecules in a non-polar solvent, can be used to find fi for a range of different molecules. This is done by plotting against /T the values calculated for the left-hand side of the equation from values of e measured at various temperatures T. The gradient of the straight line plot is Njs,iJ /(9sgk), from which /x is easily... 

Here a = Spafi is the average value of the polarization tensor of the molecule, / = a —la. being its anisotropy, and fi the dipole moment of the molecule. We assume that the concentration of active molecules in the gas mixture or liquid solution is so small that intermolecular coupling may be neglected. 

Polypeptides are electrically polar, carrying permanent dipoles at the planar CO-NH groups of the backbone chain and generally at some atomic groups of the side-chains. Because of the vector nature of dipoles, we must speak of the mean-square dipole moment, averaged over all possible conformations of the backbone chain and all accessible orientations of the side-chains when the dipolar nature of a polypeptide in solution is considered. The of a polypeptide thus may depend on what conformation the molecule assumes in a given solvent. 

As you might expect, the favored orientation of a polar molecule in the presence of ions is one where the positive end of the dipole is near an anion and the negative end of the dipole is near a cation. The magnitude of the interaction energy E depends on the charge on the ion z, on the strength of the dipole as measured by its dipole moment /x, and on the inverse square of the distance r from the ion to the dipole E = z/x/r2. Ion-dipole forces are particularly important in aqueous solutions of ionic substances such as NaCl, in which polar water molecules surround the ions. We ll explore this point in more detail in the next chapter. 

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