Electric field polar molecule

In Chapter 9, you learned that a covalent bond is polar when it joins atoms of different electronegativities because the atoms share the electrons unequally. In diatomic molecules, such as HF, where there is only one bond, the bond polarity causes the molecule itself to be polar. Molecules with a net imbalance of charge have a molecular polarity. In molecules with more than two atoms, both shape ami bond polarity detennine molecular polarity. In an electric field, polar molecules become... 

Molecular polarity Covalently bonded molecules are either polar or nonpolar which type depends on the location and nature of the covalent bonds in the molecule. A distinguishing feature of nonpolar molecules is that they are not attracted by an electric field. Polar molecules, however, are attracted by an electric field. Because polar molecules are dipoles with partially charged ends, they have an uneven electron density. This results in the tendency of polar molecules to align with an electric field. 

Figure 1.10 Polarization of molecules in an electric field. In the absence of an applied electrical field (a), molecules are aligned randomly, with no net dipole. When the field is applied (b), the solvent molecules are polarized and align themselves to reduce the strength of the field...

The dielectric constant is the electrostatic expression of the interaction of atoms and molecules with macroscopic electric fields rather than with the exceedingly strong fields of individual atoms and molecules. The interaction between the homogeneous outside field and electrically asymmetrical (polar) molecules results in a finite effect, since in these molecules the contributions of positive and negative charges do not cancel. 

Dipo/e Moment The evidence comes from an examination of the dielectric constant of the hydrogen halides. In an electric field, say between the plates of a condenser, molecules that have a charge separation within them will tend to orient themselves with the electric field. Such molecules behave like electric dipoles (Fig. 4.3) and are called polar molecules. The extent of orientation is reflected by a change in the dielectric... 

If a material of polar molecules, such as water, is exposed to a fixed or static electric field, the molecules will all rotate in an attempt to orient themselves in the direction of the field. The magnitude of separated charges of a polar molecule is defined as the dipole moment, and determines the strength of interaction with the field. The dipole moment is also a measure of the dielectric constant e. A symmetrical molecule, with no dipole moment, is said to be non-polar and does not react with an electric field. If an electric field impinging upon a polar molecule is alternating, the molecules will rotate, following reversals of field. 

Constitutive Relations. A more general representation of the nonlinear polarization is that of a power series expansion in the electric field. For molecules this expansion is given by... 

In the relations between the macroscopic susceptibilities y , y and the microscopic or molecular properties a, ft, y, local field corrections have to be considered as explained above. The molecule experiences the external electric field E altered by the polarization of the surrounding material leading to a local electric field E[oc. In the most widely used approach to approximate the local electric field the molecule sits in a spherical cavity of a homogenous media. According to Lorentz the local electric field [9] is... 

Vapors dilute and not-so-dilute In a dilute vapor, the external electric field polarizing any one particle—atom or molecule—is unchanged by electric fields emanating from dipoles induced on the other particles. (These dipolar fields drop off as the inverse cube of the distance from the particle.) The total polarization per unit volume of the dilute gas is the sum of individual particle dipoles. If a(a>) denotes the single-particle polarizability and N is the number of particles per unit volume, then for a vapor... 

In the van der Waals attraction, the first important is the dielectric constant, s, dependent on the frequency at which alternating electric field varies. This is the name given to the factor by which the capacitance of a parallel plate condenser is increased on insertion of an insulating materials because net charges appears on the surface of the dielectric between the plates [59]. Under the electric field, dielectric molecules are polarized, so that an electric dipole moment can induce. These polarized charges are referred to the (total)... 

Dipole rotation refers to the alignment, by effect of the electric field, of molecules in the sample that have permanent or induced dipole moments. As the electric field of microwave energy increases, it aligns polarized molecules. As the field decreases, thermally induced disorder is restored. In fact, applied microwave fields cause molecules, on average, to temporarily spend very slightly more time pointing in one direction than in others. Associated with that very small fraction of preferred orientation there is another very small fraction of molecular order imposed and hence a tiny bit of energy. When the... 

For both models, we used atomic trajectories from our room-temperature MD simulations to calculate thioredoxin s THz spectra in a quasi harmonic approximation. The absorption coefficient was calculated for different orientations of the molecule with respect to the electric field polarization. 

When a polar solvent is placed in a changing electrical field, the molecules must realign so that their dipole vectors maintain the orientation corresponding to minimum energy. Because of intermolecular forces, this process does not occur infinitely fast but on a time scale which depends on the properties of the medium and which is usually on the order of 1-100 ps. Dielectric relaxation experiments provide very useful information about molecular motion in polar liquids and the ability of the solvent molecules to respond to changing electrical conditions. 

The coherently pseudorotating molecule then interacts with a phase-locked pair of electronically resonant light pulses. The laser electric field, polarized in the I direction of a space-fixed cartesian frame, gives an interaction of the form... 

It is of interest to compare these results with those for the field dependencies of the relaxation times and for T for the longitudinal and for the transverse polarization components of a polar fluid in a constant electric field Eq. As shown in [52, 55] the relaxation times and T are also given by Eqs. (5.55) and (5.56), where = nEJkT, p. is the dipole moment of a polar molecule and is the Debye rotational diffusion time with = 0. Thus, Eqs. (5.55) and (5.56) predict the same field dependencies of the relaxation times Tj and T for both a ferrofluid and a polar fluid. This is not unexpected because from a physical point of view the behavior of a suspension of fine ferromagnetic particles in a constant magnetic field Hg is similar to that of a system of electric dipoles (polar molecules) in a constant electric field Eg. 

On the other hand, for polar molecules having permanent dipole moments, when subjected to an electric field, these molecules may have their existing permanent dipole moments modified temporarily by the applied field, and the measured dipole moment is the total dipole moment. For this reason, it is important to discriminate between induced dipole moment, fx and permanent dipole moment, fx, for such polar molecules. 

To take an example, the NH molecule possesses a E ground state. Eor this state, ignoring spin, the critical field is of the order 7 x 10 V/cm. This is far smaller than the field required to polarize electrons in an atom or molecule, but still large for laboratory-strength electric fields. Diatomic molecules with smaller rotational constants, such as LiF, would have correspondingly smaller critical fields. In any event, by the time the critical field is applied, it is already a bad approximation to... 

The many-body dynamics of cold polar molecules is thus governed by an interplay between dressing and manipulating the rotational states with dc and ac fields, and strong dipole-dipole interactions. In the absence of electric fields, the molecules prepared in a ground rotational state J = 0 have no net dipole moment, and interact... 

If the vacuum is replaced by a dielectric medium, the capacity of the sphere increases because of polarization of the dielectric. Electric fields polarize matter in two ways by orienting molecules with permanent dipoles and by deforming electron clouds of each molecule. The polarization vector P is related to the characteristics of individual dipoles by the relation ... 

Molecules in the tetrahedral, octahedral, and icosahedral point groups behave like spherical electron distributions in this respect the induced dipole moment is independent of the molecule s orientation in the electric field. Most molecules, however, are more easily polarized along one axis than another. The polarizability in this case is actually represented by a matrix called the polarizability tensor, with elements that describe the polarizability along the molecule s principal inertial axes. 

---