Dipole moment electrostatic properties

Once the multipole analysis of the X-ray data is done, it provides an analytical description of the electron density that can be used to calculate electrostatic properties (static model density, topology of the density, dipole moments, electrostatic potential, net charges, d orbital populations, etc.). It also allows the calculation of accurate structure factors phases which enables the calculation of experimental dynamic deformation density maps [16] ... 

Many molecular properties can be related directly to the wave function or total electron density. Some examples are dipole moments, polarizability, the electrostatic potential, and charges on atoms. 

In addition to total energy and gradient, HyperChem can use quantum mechanical methods to calculate several other properties. The properties include the dipole moment, total electron density, total spin density, electrostatic potential, heats of formation, orbital energy levels, vibrational normal modes and frequencies, infrared spectrum intensities, and ultraviolet-visible spectrum frequencies and intensities. The HyperChem log file includes energy, gradient, and dipole values, while HIN files store atomic charge values. 

Figure 4 shows the measured angle of 105° between the hydrogens and the direction of the dipole moment. The measured dipole moment of water is 1.844 debye (a debye unit is 3.336 x 10 ° C m). The dipole moment of water is responsible for its distinctive properties in the Hquid state. The O—H bond length within the H2O molecule is 0.96 x 10 ° m. Dipole—dipole interaction between two water molecules forms a hydrogen bond, which is electrostatic in nature. The lower part of Figure 4 (not to the same scale) shows the measured H-bond distance of 2.76 x 10 ° m or 0.276 nm. 

Many physical properties such as the electrostatic potential, the dipole moment and so on, do not depend on electron spin and so we can ask a slightly different question what is the chance that we will find the electron in a certain region of space dr irrespective of spin To find the answer, we integrate over the spin variable, and to use the example 5.2 above... 

In the present work, we shall investigate the problem of the amount of correlation accounted for in the DF formalism by comparing the molecular electrostatic potentials (MEPs) and dipole moments of CO and N2O calculated by DF and ab initio methods. It is indeed well known that the calculated dipole moment rf these compounds is critically dependent on the level of theory implemented and, in particular, that introduction of correlation is essential for an accurate prediction [13,14]. As the MEP property reflects reliably the partial charges distribution on the atoms of the molecule, it is expected that the MEP will exhibit a similar dependence and that its gross features correlate with the changes in the value of dipole moment when switching from one level of theory to the other. Such a behavior has indeed been reported recently by Luque et al. [15], but their study is limited to the ab initio method and we found it worthwhile to extend it to the DF formalism. Finally, the proton affinity and the site of protonation of N2O, as calculated by both DF and ab initio methods, will be reported. 

Hartree-Fock, DFT or CCSD levels. Because they can reproduce such quantities, APMM procedures should account for an accurate description of the interactions including polarization cooperative effects and charge transfer. They should also enable the reproduction of local electrostatic properties such as dipole moments an also facilitate hybrid Quantum Mechanical/Molecular Mechanical (QM/MM) embeddings. 

Considering that, roughly speaking, the electrostatic component of the solvation free energy varies as the cube of the molecular dipole moment, it becomes obvious that the corrective term (13.1) should be taken into account in the determination of differential solvation properties of very polar solutes. In the computation of transfer free energies across an interface, it has been suggested that equation (13.1) be expressed as a function of the number density of one of the two media, so that the correction is zero in solvent 1 and zl,l lsl lll in solvent 2 [115]. 

It is clear that A—H bonds make no claim to having distinctively large dipole moments or other electrostatic properties that would account for their unique chemical behavior from a classical electrostatic viewpoint. Indeed, numerous examples cited above illustrate the rather striking indifference of H-bonding to the magnitude (or even the orientation) of monomer dipoles. 

The earliest approach to explain tubule formation was developed by de Gen-nes.168 He pointed out that, in a bilayer membrane of chiral molecules in the Lp/ phase, symmetry allows the material to have a net electric dipole moment in the bilayer plane, like a chiral smectic-C liquid crystal.169 In other words, the material is ferroelectric, with a spontaneous electrostatic polarization P per unit area in the bilayer plane, perpendicular to the axis of molecular tilt. (Note that this argument depends on the chirality of the molecules, but it does not depend on the chiral elastic properties of the membrane. For that reason, we discuss it in this section, rather than with the chiral elastic models in the following sections.)... 

According to the electrostatic model the solvation is due to electrostatic interaction between the charged ions and the dipolar solvent molecules. Thus the solvating and ionizing properties of a solvent are considered as being due primarily to the dipole moment of the solvent molecules. Thus, ionic compounds such as sodium chloride are insoluble in non-polar solvents such as carbon tetrachloride. Actually, rather than the dipole moment the field action of the dipoles should be considered. This approach might explain why acetonitrile (p = 3.2) is poor in its ionizing properties compared to water (p = 1.84). However, no numerical values are available for this quantity. 

The Hirshfeld functions give an excellent fit to the density, as illustrated for tetrafluoroterephthalonitrile in chapter 5 (see Fig. 5.12). But, because they are less localized than the spherical harmonic functions, net atomic charges are less well defined. A comparison of the two formalisms has been made in the refinement of pyridinium dicyanomethylide (Baert et al. 1982). While both models fit the data equally well, the Hirshfeld model leads to a much larger value of the molecular dipole moment obtained by summation over the atomic functions using the equations described in chapter 7. The multipole results appear in better agreement with other experimental and theoretical values, which suggests that the latter are preferable when electrostatic properties are to be evaluated directly from the least-squares results. When the evaluation is based on the density predicted by the model, both formalisms should perform well. 

The electrostatic properties of the molecule may be used as a criterion for judging the MEM enhancement. Using the uniform prior density, the MEM molecular dipole moment derived by the discrete boundary partitioning of space (chapter 6) is only 1.3 D, compared with 9.1 D based on the experimental density,... 

The dielectric constants play a particular role in the characterization of solvents. Their importance over other criteria is due to the simplicity of electrostatic models of solvation and they have become a useful measure of solvent polarity. Since both the dielectric constant r and the dipole moment p are important complementary solvent properties, it has been recommended that organic solvents should be classified according to their electrostatic factor EF (defined as the product of 8 and p). 

An averaged solvent electrostatic potential, obtained by averaging over the solvent confignrations, is included in the solnte Hamiltonian and electric and energy properties are obtained. The method provides resnlts for the dipole moment and solnte-solvent interaction which agree with the experimental valnes and with the resnlts obtained by other workers (Mendoza et al., 1998). 

The conception of the electrostatic bond has been found to be most valuable in the field of inorganic chemistry and has helped to clarify a great many phenomena. It has frequently made it possible to predict properties both of unknown compounds and of those which have been little investigated. Nevertheless, a number of difficulties have been encountered. For example, in Section 43, it was shown that the theory did not really provide a satisfactory explanation of the low symmetry of molecules. Again, the theory failed to explain the volatility and small dipole moment of the compounds CO, NO, C120, etc., and lastly, it threw no light whatever on the reason for the existence of molecules of elements such as H2, Cl2, Oa, N2, etc., especially as dipole measurements show that the formulae H+H, Ci+ci are excluded. 

The conformational properties of trimer molecules modeling PVDB (S 100) and PVDF are analyzed by the molecular mechanics method of Boyd and Kesner [J. Chem. Phys. 1980, 72, 21791, which takes into account both steric and electrostatic energy. Total conformational energies are used to calculate a set of intramolecular interaction energies that, by means of the RIS model, allowed estimation of the characteristic ratios and dipole moment ratios of PVDB and PVDF under unperturbed conditions. 

---