Association of dipoles

It is important to keep in mind that the ability of dipoles to associate is influenced by their environment. Many studies on association of dipoles have been carried out in solutions containing the polar molecules. If the molecules of the solvent are polar or can have polarity induced in them (see Section 6.3), association of the solute molecules will be hindered. The solvent molecules will surround the polar solute molecules, which will inhibit their interaction with other solute molecules. The solute... 

Dipole assembly Association of dipoles of the same energy variety but different natures. 

With this method of chromatography the actual process takes place on a thin layer of adsorbent fixed on a glass or aluminum plate, or on a plastic foil. The separation takes place on the basis of a partition mechanism, adsorption, or ion exchange, etc. Individual mechanisms are often combined or overlap, and it is not easy to determine the boundaries between individual processes. Basic forces playing a decisive role during the separation are ionic forces, further coordination forces, chelate formation, association of dipoles, hydrogen-bond formation, and dispersion (van der Waals forces). 

The dipole moments of the hydrogen halides decrease with increasing atomic number of the hydrogen, the largest difference occurring between HF and HCl, and association of molecules is not an important factor in the properties of FICl, HBr and HI. This change in dipole moment is reflected in the diminishing permittivity (dielectric constant) values from HF to HI. 

A study of the effect of substitution patterns in oxadiazoles and isoxazoles and their effect on the UV spectra in the lO -lO M concentration range was performed. Hypso-chromic effects and deviations from Beer s law were observed and were believed to be associated with antiparallel, sandwich-type self-association via dipole-dipole interactions. Beer s law is followed when the molecular dipole moments are small or when self-association is sterically hindered. 

The dipole moment of a molecule is another additive property since it arises from the difference in electronegativity of two atoms connected by a double bond. It should therefore be possible to associate a dipole moment with every linkage. Eucken and Meyer" have suggested the following moments for various linkages (in units of 10 e.s.cm)... 

Polar interactions can occur when a molecule contains a dipole or a number of dipoles which take the form of localized charges situated on different parts of the molecule. Each charge has an equal and opposite charge situated elsewhere on the molecule and, thus, the molecule has no net charge associated with it. Interactions occur between the charges on different molecules but are always accompanied by dispersive... 

There are specific associations of various types of dipoles with the four major classes of heterocyclic mesomeric betaines, which have implications in providing a rational foundation for correlating the chemical reactions of these compounds (85T2239). Eight dipole types, systematically generated by union of the heterocations H2C = with carbanions and... 

Obviously, this shift implies the self-association of DMSO. Further frequency shifts to even lower wave numbers (1050-1000 cm " ) are observed in both aprotic polar and protic solvents. In aprotic solvents such as acetonitrile and nitromethane, the association probably takes place between the S—O bond of DMSO and the —C=N or the —NOz group in the molecules by dipole-dipole interaction as shown in Scheme 331,32. Moreover, the stretching frequency for the S—O bond shifts to 1051 cm 1 in CHC13 and to 1010-1000 cm -1 in the presence of phenol in benzene or in aqueous solution33. These large frequency shifts are explained by the formation of hydrogen bonds between the oxygen atom in the S—O bond and the proton in the solvents. Thus, it has been... 

Fig. 3. a Schematic representation of mesogenic molecules possessing steric dipoles, b Schematic representation of mesogenic molecules possessing steric quadrupoles. c The formation of spatial associates of dipolar (1) and steric (2, 3) origin... 

Table 9 3 summarizes the relationships among steric number, electron group geometry, and molecular shape. If you remember the electron group geometry associated with each steric number, you can deduce molecular shapes, bond angles, and existence of dipole moments. 

For highly exothermic SN2 reactions, which have a central barrier significantly lower in energy than that of the reactants, association of the reactants may be the rate controlling step in TST.1 The SN2 rate constant can then be modeled by a capture theory9 such as VTST,10 average dipole orientation (ADO) theory,11 the statistical adiabatic channel model (SACM),12 or the trajectory capture model.13... 

Typical examples are the conversion of the neutral form of an amino acid into its zwitterionic form, the helix-coil transitions in polypeptides and polynucleotides, and other conformational changes in biopolymers. Reactions of higher molecularity in which reactants and products have different dipole moments are subject to the same effect (association of the carboxylic acids to form hydrogen-bonded dimers). Equilibrium involving ions are often more sensitive to the application of an electric field ... 

Theoretical calculations on the dithiazolyl radical 4 (R=CF3) have recently shown that n -n dimerisation was unfavourable but association of two such dimers via electrostatic interactions generated a thermodynamically stable tetramer consistent with single crystal X-ray studies. Thus while the value of [AE-P ] may favour (or disfavour) dimer formation, the van der Waals, dipole contributions and electrostatic interactions to the lattice enthalpy should not be underestimated in assessing the thermodynamic stability or instability of these... 

Related to this is the volume change associated with dipole development in transition states. This has been investigated theoretically for a model substance of molecules with a size similar to that of water (Morild and Larsen, 1978). The calculations show that the volume changes are very pressure-dependent in this case. A change in dipole moment from 0 to 1 x 10-3 C-m gives a volume decrease of about 30 cm3 mol-1 at 350 bar and about 20 cm3 mol-1 at 750 bar. However, this may not be typical for molecules as large as enzyme-substrate complexes. 

Some obscure facets of this intricate picture have been unveiled by Filippi and Speranza who investigated the stereochemistry and the intimate mechanism of a model solvolytic reaction taking place in an ion-dipole pair in the gaseous phase. ° Adducts (7 )-56 and R)-51 are obtained in the gas phase by association of the relevant chiral alcohols, i.e., (/ )-(- -)-l-phenyl-ethanol ((I )-44) and (/f)-(- -)-l-(pentafiuorophenyl)ethanol K)-55), with the CHs OHJ ion, generated by y-radiolysis of CH3F/H2 0 mixtures (Scheme 24). As mentioned above, the absence of neutral nucleophile molecules, i.e., CH3 OH, in the reaction medium ensures that the 0-labeled ethers 45 and 58 of Scheme 24 arise exclusively from the intracomplex solvolysis of R)-56 and R)-51, respectively. 

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