Molecular polarity dipole moment

The polarization properties of single-molecule fluorescence excitation spectra have been explored and utilized to detennine botli tlie molecular transition dipole moment orientation and tlie deptli of single pentacene molecules in a /7-teriDhenyl crystal, taking into account tlie rotation of tlie polarization of tlie excitation light by tlie birefringent... 

Generally, the solubihty characteristics of organic compounds depend on several properties of the participating components. For the solute, these properties are the molecular size and structure, polarity, dipole moment, va-por/sublimation pressure, and, in the case of a sohd solute, also its melting characteristics. When using SCCO2 as the solvent, mainly its dipole moment and quadrupole moment influence the solvatation process (Sect. 2.2). 

Here d is the molecular electric dipole moment, E0 is the amplitude of the drivitt field whose polarization direction defines the z direction, / is the moment of inertiai j of the molecule about an axis perpendicular to the symmetry axis, and A(t/T — ) i the pulse shape function of the form... 

Potentially important discriminants of P450 selectivity include size (diameter), molecular mass or length of the molecule, planarity (aid1) or rectangularity (II w) of the molecule, lipophilicity (log P or log D 7.4), basicity/acidity of the compound (pAa), polarity (dipole moment) of the molecule, and compound lipophilicity for substrate binding to human P450s in drug metabolism. 

Electric polarization, dipole moments and other related physical quantities, such as multipole moments and polarizabilities, constitute another group of both local and molecular descriptors, which can be defined either in terms of classical physics or quantum mechanics. They encode information about the charge distribution in molecules [Bbttcher et al, 1973]. They are particularly important in modelling solvation properties of compounds which depend on solute/solvent interactions and in fact are frequently used to represent the -> dipolarity/polarizability term in - linear solvation energy relationships. Moreover, they can be used to model the polar interactions which contribute to the determination of the -> lipophilicity of compounds. 

Polarity is a molecular property. For polyatomic species, the net molecular dipole moment depends upon the magnitudes and relative directions of all the bond dipole moments in the molecule. In addition, lone pairs of electrons may contribute significantly to the overall value of ji. We consider three examples below, using the Pauling electronegativity values of the atoms involved to give an indication of individual bond polarities. This practice is useful but must be treated with caution as it can lead to spurious results, e.g. when the bond multiplicity is not taken into account when assigning a value of x - Experimental values of molecular electric dipole moments are determined by microwave spectroscopy or other spectroscopic methods. 

The theoretical formula of Onsager (J. Amer. Chem. Soc. 1936, 58, 1486) relating the molecular electric dipole moment to the dielectric constant e and refractive index w of a pure polar liquid and to the molar volume V is... 

I Cl isotopomer in the A Hi electronic state employing LIF in a molecular beam. Takei et al. [97Tak] were able to deduce the direction of the molecular electric dipole moment in the A II] state relative to that in theX E ground state by simulating measured Stark effect modulation line shapes. They found both directions to be parallel, in agreement with previous literature results [94Fri, 94Dur]. Thus the polarity in the two states is CF. 

Dipole Moments In a diatomic molecule the difference in the electronegativities of bonding atoms results in a polar bond and a dipole moment. The dipole moment of a molecule made up of three or more atoms depends on both the polarity of the bonds and molecular geometry. Dipole moment measurements can help us distinguish between Afferent possible geometries of a molecule. 

Figure 9.36 shows the variation of the spontaneous electric polarization AP of NP/4-MPU with temperatures between 10 K and the melting point. AP amounts to about 15% of the electric polarization, which can be estimated from the volume density of molecular electric dipole moments of about 3 Debye (10 Cm). 

Equations (14)-(17) apply to isotropic media. In an orientationally ordered material the extinction coefficient becomes dependent on the angle between the alignment axis and the polarization direction of the incident light, and has the characteristics of a second rank tensor. At a microscopic level, the optical absorption depends on the angle between the molecular transition dipole moment iJ.j for the particular absorption band, and the electric field of the light wave. Restricting attention to uniaxial systems, an effective order parameter (5op) for optical absorption can be defined as ... 

Let us assume that we obtain a two-photon image induced in the near field (similar to Fig. 9.12 from the nanoscaled organic fibers that are shown in Fig. 8.19 as far-field images. If we measure two sets of data under different polarization combinations of exciting and detecting light we might obtain near-field information about the orientation of molecular transition dipole moments. 

We introduce a pseudo polarization vector P = sample, which replaces the magnetization vector in NMR. The components and Py are the vector sums of the x- and y-components of the molecular transition dipole moments y 2> while the z-component of P is proportional to the population difference aN = - N2. It can be shown (see for example... 

C. Foroutan-Nejad, to be published. The structures of all molecules were optimized under and C2v symmetry at the B3LYP/def2-TZVPPD level using the Gaussian 09 rev D suite of programs [33]. Atomic polar tensor (APT) atomic charges were computed as one third of the sum of the first derivatives of the molecular electric dipole moment with respect to the three coordinates of each nucleus [34]. 

An experimental determination of the polarization and molecular electric dipole moments... 

In order to describe the second-order nonlinear response from the interface of two centrosynnnetric media, the material system may be divided into tlnee regions the interface and the two bulk media. The interface is defined to be the transitional zone where the material properties—such as the electronic structure or molecular orientation of adsorbates—or the electromagnetic fields differ appreciably from the two bulk media. For most systems, this region occurs over a length scale of only a few Angstroms. With respect to the optical radiation, we can thus treat the nonlinearity of the interface as localized to a sheet of polarization. Fonnally, we can describe this sheet by a nonlinear dipole moment per unit area, -P ", which is related to a second-order bulk polarization by hy P - lx, y,r) = y. Flere z is the surface nonnal direction, and the... 

The unequal distribution of charge produced when elements of different electronegativities combine causes a polarity of the covalent bond joining them and, unless this polarity is balanced by an equal and opposite polarity, the molecule will be a dipole and have a dipole moment (for example, a hydrogen halide). Carbon tetrachloride is one of a relatively few examples in which a strong polarity does not result in a molecular dipole. It has a tetrahedral configuration... 

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