Dipole moments of molecules

The leading tenn in the electrostatic interaction between the dipole moment of molecule A and the axial quadnipole moment of a linear, spherical or synunetric top B is... 

The HyperChem log file includes calculated dipole moments of molecules. To set the amount of information collected in the log file, change the value of the QuantumPrintLevel setting in the chem.ini file. Note that the sign convention used in the quantum mechanical calculation of dipoles is opposite to that used in molecular mechanics dipole calculations this reflects the differing sign conventions of physics and chemistry. 

Table 6-3. Comparison of the dipoles of the isolated individual monomers (dipole M) compare to the dipole moments of molecules within the dimer (dipole D) via the interaction, calculated with different functionals. Units are atomic units, and we give as well the difference in length and orientation ...

Quantities that have both magnitude and direction are called vectors. Examples of vectors in chemistry include things like the dipole moments of molecules, magnetic and electric fields, velocities and angular momenta. It is convenient to represent vectors geometrically and the simplest example is... 

W.Liptay, Dipole Moments of Molecules in Excited States and the Eifect of External Electric Fields on the Optical Absorption of Molecules in Solution, in Modem Quantum Chemistiy, ed by O.Subabiglu, Academic Press, New York, 1965. 

When a constant electric field is suddenly applied to an ensemble of polar molecules, the orientation polarization increases exponentially with a time constant td called the dielectric relaxation time or Debye relaxation time. The reciprocal of td characterizes the rate at which the dipole moments of molecules orient themselves with respect to the electric field. 

Geometry of molecules and ions, structural isomerism of simple organic molecules and coordination complexes, dipole moments of molecules, relation of properties to structure... 

In this chapter we will also focus on the dipole moment of molecules. With these, some of the most interesting phenomena are the molecules for which the electric moment is in the wrong direction insofar as the atomic electronegativities are concerned. CO is probably the most famous of these cases, but other molecules have even more striking disagreements. One of the larger is the simple diatomic BF. We will take up the question of the dipole moments of molecules like BF in Chapter 12. In this chapter we will examine in a more general way how various sorts of structures influence electric moments for two simple cases. For some of the discussion in this chapter we restrict ourselves to descriptions of minimal basis set results, since these satisfactorily describe the physics of the effects. In other cases a more extensive treatment is necessary. 

Contribution of Unshared Electron Pairs to the Electric Dipole Moments of Molecules.—In the preceding chapter we discussed the dipole moments of molecules, in relation to the partial ionic character of bonds, without considering the possible contribution of unshared electron pairs. A simple treatment based on hybrid orbitals provides some justification of this procedure. 

The Effect of Resonance on the Electric Dipole Moments of Molecules.—It was pointed out by Sutton27 in 1931 that resonance of the... 

Fig. 12.5 Illustration of the orientation angles used in the Stockmayer intermolecular potential. Molecule j consists of atoms A and B, and molecule i consists of atoms C and D. The vector ry runs from the center of mass of molecule i to the center of mass of molecule j. The vector JTJ gives the orientation and magnitude of the dipole moment of molecule i, with a similar definition for JTj. A ghost copy of molecule j is shifted to left to more easily visualize the orientation angle ifr. See Eqs. 12.11 to 12.13 and accompanying text for definition and description of these angles.

In a ferroelectric material, the dipole moments of molecules remain aligned in the absence of an external field. This alignment gives the material a permanent electric polarization. 

So far we have discussed 2D density modulated phases that are formed by deformation or breaking of the layers. However, there are also 2D phases with more subtle electron density modulations. In some cases additional peaks observed in the XRD pattern (Fig. 10) are related to a double layer periodicity in the structure. As double layer periodicity was observed in the bent-core liquid crystals formed by the asymmetric as well as symmetric molecules [22-25] it should be assumed that the mechanism leading to bilayers must be different from that of the pairing of longitudinal dipole moments of molecules from the neighboring layers, which is valid for smectic antiphases made by asymmetric rod-like molecules. 

The dipole moments of molecules are often treated as being equal to the vector sum of the bond dipoles of the various bonds in the molecules. It is almost impossible to measure the dipole moment of an individual bond within a molecule. For example, molecules such as methane, carbon tetrachloride, and p-dichlorobenzene have no dipole moments, whereas molecules such as methylene chloride and m-dichlorobenzene do. The vector sum treatment could be made to agree quantitatively with all known dipole moments if the bond moments were treated as variables that depend on the nature of the particular molecule in which the bonds were located. 

Again uq(R) is the hard sphere potential. This is necessary to keep the molecules from overlapping. The parameter is the dipole moment of molecule i. The factor D(i, j) is a term that depends on the orientation of the dipoles i and j and need not concern us here. We can call this potential the dipolar hard sphere potential. 

Ct and C2 are constants, pA and Pg represent the electric dipole moments of molecule A and molecule B, respectively. The angular dependence is summarized in the function... 

Now, as experimental estimates of dipole moments of molecules in solution are scarce, we must probe the calculated converged charge density by analyzing results for other properties. One possibility is to analyze the solute-solvent interaction, and that can be obtained by solvent shifts in UV-vis or NMR spectra for example. 

One of the purposes for which static permittivities of aqueous nonelectrolyte solutions have been determined, by Franks and his coUeagues, is for calculation of the dipole moments of molecules. There are molecules for whidi no studies can be made in the vapour phase or in solution in non-polar solvents however, sufficiently good comparisons of determinations of dipole moments in aqueous solution with other determinations can sometimes be made to allow an estimate of the limitations of the method. 

Giant molecules zeolites de-ionized" water., Temporary hardness and permanent hardness methods of softening water. Heat capacity (specific heat). Van der Waals attraction, boiling point, melting point-dependence on molecular size. Electric dipole moments of molecules—effect on boiling point. Ionic dissocia-... 

Among all the methods which have hitherto been brought forward for the determination of the electric dipole moment of molecules, the molecular-beam method occupies a special position, for it enables us to investigate the behaviour of individual free molecules in the electric field directly. It also has the great advantage that the effect of the field on the dipole moment of a single molecule occurs in it as a directly measurable quantity, so that it avoids the uncertainties of all other methods, which are chiefly due to the fact that in their case it is the macroscopic actions between the field and the dielectric (gas or liquid) which are measured and the mutual effects of the molecules can neither be eliminated entirely nor be allowed for accurately. 

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