Dipole molecular compounds

Molecular Compounds Ex h2o, nh3 - Van der Waals - Dipole - dipole - Hydrogen bond Polar molecules. (partially negative and positive atoms) - soft - low melting point - nonconductors or poor conductors of electricity... 

Molecules of 1,3,5-trinitrobcnzene or p- dinitrobenzene have no electric dipole moments but they have moments in solutions where molecular compounds are formed. For example they have no moments in carbon tetrachloride or chloroform, but they do have moments in benzene, naphthalene, or dioxane. 

The chance of finding molecular complexes in the solid state seems quite high if one or both of the partner molecules already carry a permanent dipole moment in the gaseous state. A number of such molecular complexes was investigated by NQR Table V.2 gives the NQR results on molecular compounds,... 

In the first group of molecular compounds both partners possess a dipole moment and the compound can be considered as being produced thanks to the possibility of attaining a favourable mutual orientation of the dipoles (Keesom forces). Looked at in this way the numerous cases of association also belong to this group. These phenomena in which hydrogen dipoles, especially such as FH, OH, NH and (GH), play a part are discussed under the hydrogen bond . 

In addition to the molecular compounds which we have so far discussed and in which the dipole moment of one of the molecules is the primary factor in causing the association, intermolecular compounds exist in which,... 

Table CXXX. Dipole Moments of Molecular Compounds and their Organic Components...

In materials that contain molecular or ionic dipoles, orientational polarization can occur. This polarization depends on the part of interaction of the dipoles with their surroundings that can be represented by an effective viscosity. It is attributed, in molecular compounds, to the restricted rotation of the whole molecule or of a part of it and, in ionic crystals, to ions jumping between neighboring sites (ion-vacancy pair). The criteria adopted to identify the large number of dipolar relaxation phenomena are ... 

According to Kitaigorodskii s close-packing principle, molecules will pack in a manner that minimizes void space or, in other words, in a way to maximize van der Waals interactions [20]. Hence, effects of both molecular shape and size are important in crystal engineering. For example, since racemic crystals tend to pack into centrosymmetric space groups, this statistical preference was utilized to prepare cocrystal g in Appendix 2, which is a rare molecular compound called a quasiracemate. The formation of cocrystal g was made possible due to the isosteric nature of the isoptopenyl and dimethylamino substituents, indicating that shapes rather than dipoles can be in fact the dominant factor (cf. Ref. 31). 

There are two mechanisms by which microwaves interact with reaction mixtures [7]. Polarization of dielectric material arises when the distribution of an electron cloud is distorted or physical rotation of molecular dipoles occurs. For generation of heat on irradiation with microwaves, at least one component of a reaction mixture must have a dipole moment. Compounds with high dipole moments also have large dielectric constants, e. The selectivity of microwave irradiation is clear when comparing the heating of water and hexane. Water, a polar solvent, has a high dielectric constant and therefore heats rapidly on microwave irradiation whereas hexane, a nonpolar solvent, heats very slowly. 

That is exactly the competition between different forms of orientation interaction, which determines the composition and chemical properties of ground water. In very fresh water dominates orientation interaction between H O dipoles. As salinity and concentration of the dissociated ions, i.e., the simplest anions and cations (Na+, Ca +, Cl", COj ", etc.) grows, also increases the role of interaction between water and ions. At relatively high salinity ions have to interact between themselves, forming more complex associated ions (CaHCOj+, NaHCO ", HSiO ", HCO, etc.), capable of decomposing into simpler ones with freshening water. At this, the number of associated ions increases. Thus form complex super-molecular compounds with relatively weak bonds (PbClj, AlF, Fe OH ", etc.). Moreover, one and the same dissociated ion may be part of the composition of different associated ones. For instance, calcium in the sea water may exist simultaneously as Ca, [CaHCOj], [CaCOj, [CaSOj, etc. 

As the size and mass of similar types of molecular compounds increase, there are more electrons that produce stronger temporary dipoles. In general, larger nonpolar molecules with increased molar masses also have higher boiling points. As the molar mass of similar compounds increases, the dispersion forces also increase due to the increase in the number of electrons. We see this trend in the boiling points of the first four alkanes shown in Table 10.7. 

Attractive forces called dipole-dipole attractions and hydrogen bonds hold the solid and liquid states of polar molecular compounds together. 

Molecular solids are held together by dispersion forces, dipole-dipole forces, or hydrogen bonds. Most molecular compounds that are solid at room temperature, such as sugar, have a large molar mass and are poor conductors of heat and electricity. 

Examine the molecular models of H2 HE CH4 CH3F and CF4 Find the calculated dipole moment of each compound and examine their electrostatic potential maps... 

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

Absorption, metaboHsm, and biological activities of organic compounds are influenced by molecular interactions with asymmetric biomolecules. These interactions, which involve hydrophobic, electrostatic, inductive, dipole—dipole, hydrogen bonding, van der Waals forces, steric hindrance, and inclusion complex formation give rise to enantioselective differentiation (1,2). Within a series of similar stmctures, substantial differences in biological effects, molecular mechanism of action, distribution, or metaboHc events may be observed. Eor example, (R)-carvone [6485-40-1] (1) has the odor of spearrnint whereas (5)-carvone [2244-16-8] (2) has the odor of caraway (3,4). 

Oxirane (1) and methyloxirane (3) are miscible with water, ethyloxirane is very soluble in water, while compounds such as cyclopentene oxide and higher oxiranes are essentially insoluble (B-73MI50501) (for a discussion of the solubilities of heterocycles, see (63PMH(l)l77)). Other physical properties of heterocycles, such as dipole moments and electrochemical properties, are discussed in various chapters of pmh. The optical activity of chiral oxiranes has been investigated by ab initio molecular orbital methods (8UA1023). 

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