Dipole moment electrical properties

The electrical properties of materials are important for many of the higher technology applications. Measurements can be made using AC and/or DC. The electrical properties are dependent on voltage and frequency. Important electrical properties include dielectric loss, loss factor, dielectric constant, conductivity, relaxation time, induced dipole moment, electrical resistance, power loss, dissipation factor, and electrical breakdown. Electrical properties are related to polymer structure. Most organic polymers are nonconductors, but some are conductors. 

The dipole moments just discussed are permanent dipole moments, intrinsic properties of a molecule. A net separation of charge may also be induced in any molecule by application of an external electric field. The induced dipole moment /x(ind) so created is approximately proportional to the strength of the applied field. Thus, for molecule, /x,4(ind) = where E is the applied field strength and a a is the polarizability of A. If the source of the electric field is a pemiaiient dipole in a neighboring molecule B, then the contribution to U from the permanent dipole/induced dipole interaction is ... 

Electrical properties will be discussed, and a new correlation will be presented for the dielectric constant at room temperature, in Chapter 9. The molar polarization, dipole moment, electrical losses and dielectric strength, will also be considered in Chapter 9. 

The discussion to this point has been limited to static electric and magnetic fields. However, molecules are often exposed to time-dependent fields, as for example in the interaction with electromagnetic radiation. Some of the properties introduced in this chapter, hke the frequency-dependent polarizabihty are generalizations to time- or frequency-dependent fields of the properties introduced in Chapters 4 and 5. Other spectral properties hke the vertical excitation energies, transition dipole moments and properties derived from them, are a completely different type of property as they cannot be defined as derivatives of the groimd-state energy. 

Moreover, MM does not give any information about the electronic distribution of the system, and about its changes under the influence of some external agent. Therefore MM is not able to compute molecular properties relative to the electronic distribution, usually classified as primary, derivative, and primary induced properties (e.g., dipole moment, electric polarizability, chemical shift), or electronic excitation and photoionization processes. In standard MM methods it is not possible to use all the tools elaborated by theoretical chemistry... 

Dipole in a uniform electric field arising from two oppositely charged parallel plates. A dipole moment, a property of a separated positive and negative charge, interacts differently with the field at the three orientations shown since each end of the dipole is attracted toward the oppositely charged plate. 

Essentially all experimentally measured properties can be thought of as arising through the response of the system to some externally applied perturbation or disturbance. In turn, the calculation of such properties can be formulated in terms of the response of the energy E or wavefunction P to a perturbation. For example, molecular dipole moments p are measured, via electric-field deflection, in terms of the change in energy... 

The perturbation V = H-H appropriate to the particular property is identified. For dipole moments ( i), polarizabilities (a), and hyperpolarizabilities (P), V is the interaction of the nuclei and electrons with the external electric field... 

Equations (6.5) and (6.12) contain terms in x to the second and higher powers. If the expressions for the dipole moment /i and the polarizability a were linear in x, then /i and ot would be said to vary harmonically with x. The effect of higher terms is known as anharmonicity and, because this particular kind of anharmonicity is concerned with electrical properties of a molecule, it is referred to as electrical anharmonicity. One effect of it is to cause the vibrational selection mle Au = 1 in infrared and Raman spectroscopy to be modified to Au = 1, 2, 3,. However, since electrical anharmonicity is usually small, the effect is to make only a very small contribution to the intensities of Av = 2, 3,. .. transitions, which are known as vibrational overtones. 

The central role of the concept of polarity in chemistry arises from the electrical nature of matter. In the context of solution chemistry, solvent polarity is the ability of a solvent to stabilize (by solvation) charges or dipoles. " We have already seen that the physical quantities e (dielectric constant) and p (dipole moment) are quantitative measures of properties that must be related to the qualitative concept of... 

Theoreticians did little to improve their case by proposing yet more complicated and obviously unreUable parameter schemes. For example, it is usual to call the C2 axis of the water molecule the z-axis. The molecule doesn t care, it must have the same energy, electric dipole moment and enthalpy of formation no matter how we label the axes. I have to tell you that some of the more esoteric versions of extended Hiickel theory did not satisfy this simple criterion. It proved possible to calculate different physical properties depending on the arbitrary choice of coordinate system. 

To look ahead a little, there are properties that depend on the choice of coordinate system the electric dipole moment of a charged species is origin-dependent in a well-understood way. But not the charge density or the electronic energy Quantities that have the same value in any coordinate system are sometimes referred to as invariants, a term borrowed from the theory of relativity. 

This is not a unique way of classifying molecular properties. For example, Dykstra et al. (1990) concentrate on the response of a system to an apphed external field the electric dipole moment can be defined as the first derivative of the energy with respect to the field, and so on. I will stick with the Boys and Cook nomenclature as a broad basis for discussion. 

At the molecular level, electric dipole moments are important because they give information about the charge distribution in a molecule. Examination of the experimental data for a few simple compounds reveals that the electric dipole moment is also a property associated with chemical bonds and their polarity. The... 

If the perturbation is a homogeneous electric field F, the perturbation operator P i (eq. (10.17)) is the position vector r and P2 is zero. As.suming that the basis functions are independent of the electric field (as is normally the case), the first-order HF property, the dipole moment, from the derivative formula (10.21) is given as (since an HF wave function obeys the Hellmann-Feynman theorem)... 

The linear polarizability, a, describes the first-order response of the dipole moment with respect to external electric fields. The polarizability of a solute can be related to the dielectric constant of the solution through Debye s equation and molar refractivity through the Clausius-Mosotti equation [1], Together with the dipole moment, a dominates the intermolecular forces such as the van der Waals interactions, while its variations upon vibration determine the Raman activities. Although a corresponds to the linear response of the dipole moment, it is the first quantity of interest in nonlinear optics (NLO) and particularly for the deduction of stracture-property relationships and for the design of new... 

Dipole Moments, Polarography, and Other Electrical Properties... 

Vibrational Properties. Figures 4 and 5 show the variation of the energy Eg and the electric dipole moment p as a function of the relevant geometrical variables for H2O and NHg respectively. For the Internal variables, the curves corresponding to the Isolated molecules are also shown (dashed lines) for comparison lhe20Sclllatlon frequencies v and dipole matrix elements <1, sre also... 

Typical properties of the charge distribution are summarized by its various electric multipole moments. The electric dipole moment p. induced in the system by the external field is obviously... 

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