Local dipoles

That way, the Distributed Electrostatic Moments based on the ELF Partition (DE-MEP) allows computing of local moments located at non-atomic centres such as lone pairs, a bonds and n systems. Local dipole contributions have been shown to be useful to rationalize inductive polarization effects and typical hydrogen bond interactions. Moreover, bond quadrupole polarization moments being related to a n character enable to discuss bond multiplicities, and to sort families of molecules according to their bond order. 

Table 6-4. Local dipole contributions and magnitude of the first and second moments of some basins of typical basins involved in the main chain...

What about dichlorobenzenes Substitution of another hydrogen by chlorine creates another local dipole and a more polar molecule. Certainly 1,2 and 1,3-dichlorobenzene are more polar but 1,4-dichlorobenzene poses a quandary. It is what chemist call a polar molecule but the opposing chlorines result in a net molecular dipole of zero. Right Perhaps it is best for chemists to think in terms of bond-dipoles rather than molecular dipoles and in many cases this is the case in chemical separation discussions. (Reader should look up the properties of the dichlorobenzenes, such as melting and boiling point and liquid density.)... 

In these models, the local dipole moment derivative contribution becomes... 

An analysis of the Wannier functions in CPMD simulations of one dimethyl sulfoxide (DMSO) molecule dissolved in water was carried out by us in 2004 in order to gain more insight into the unusual properties of the DMSO-water mixture (72). In this special case, we have utilized MLWCs to calculate molecular dipole moments of the DMSO molecule in gas phase and aqueous solution. Comparing those two a large increase of the local dipole... 

Figure 3. The arrows indicate how the local dipoles of the H-bonds might interact with the local dipoles of the p- and m-nitroanilines to give head to tail and head to head interactions. The + and - signs indicate centers of charge. Unmarked carbon atoms are neutral.

The net strength of a molecule s Debye interactions is a function of its polarizability and the number and magnitude of its local dipole moments. Since induced dipoles tend to be aligned for maximum attraction, the energy of interaction resulting from Debye forces is very nearly additive. 

Keesom forces are a function of the number and magnitude of a molecule s local dipole moments, but since they are dependent upon the positioning of a molecule with respect to its neighbors, they may not always be strictly additive. Flowever, since the molecules in most crystals are aligned for maximum dipolar interaction, the group interactions are often roughly additive. 

Conformations of primary, secondary, and tertiary amides of (R,R)-tartaric acid, both symmetrically and asymmetrically substituted, have been studied ciys-tallographically [22, 24, 29, 30-40] Moreover, ab initio studies up to MP2 / 6-31G //RHF/6-31G level [41] for both the diamide and N,N,N ,N -tctramcthyl-diamide of (/ ,/ [-tartaric acid have been carried out [20, 22]. X-ray results have shown that primary and secondary amides of (R,R [-tartaric acid tend to adopt a conformation with the extended carbon chain - the Taa structure. In this Taa conformation both the a-hydroxy-amide moieties form planes and the structure gains stabilization from hydrogen bonding between donors, the NH, and acceptors, the proximal OH groups. Moreover, the Taa structure is favorably stabilized by the attraction of antiparallel local dipoles formed along distal C H and Csp2=0 bonds [18, 21, 22],... 

The lowest-energy conformer in water solution of (R,R (-tartaric acid diamide - the Taa one (see Figure 3b) - closely resembles the structure found in crystals (see Figure 3c) and in polar solvents. It is stabilized by hydrogen bonds, each closing a five-membered ring with NH as donor and proximal OH as acceptor. Moreover, this Taa structure is stabilized by the attraction of antiparallel local dipoles formed along C -H and distal C=0 bonds. 

Po denotes the contribution from the ideal dielectric arising from the deformation of electron clouds and the displacements of nuclei. Pi denotes contributions from local dipoles which can rotate but not diffuse. P2 denotes contributions from charges which diffuse over macroscopic distances. P3 denotes contributions from surface charges. 

CDA was tested with KBr single crystals and alkali silicate soft glass, a microscope slide. The KBr crystals behaved as expected for a dielectric containing local dipoles which reorient in an externally applied field. The soft glass showed a monotonous (E-l) vs. T behavior with a decreasing positive surface charge. 

The treatment of the hydration interaction follows the model employed earlier by Schiby and Ruckenstein,7based on the mutual interactions of dipoles. In this model, it is considered that the dipoles of the surface polarize the water molecules of the first layer of water and the polarization propagates from layer to layer. When two surfaces approach each other, the polarized layers will increasingly overlap. As a result, the local dipole moment of the water molecules will be decreased. This increases the free energy of the system and thus generates a repulsive force. 

Huraux and co-workers, in a series of papers 67 "70), and more recently, Sheppard 7I), have attempted quantitative interpretation of er during an epoxy cure in terms of the changing concentrations of constituent polar groups. They used a theory by Onsager which improves Debye s original theory to account for local dipole fields [see, for example, Ref. 23)]. The Onsager theory, expressed below, requires some explanation ... 

Jensen, L., Swart M., Duijnen P.Th. van and Snijders J.G., Medium perturbations on the molecular polarizability calculated with a localized dipole interaction model. J.Chem.Phys. (2002) 117 3316-3320. 

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