Molecules, polar

In contrast to molar polarisation calculated from optical refractivities, that calculated from relative permittivities observed at lower frequencies is by no means always independent of temperature. Actually, materials tend to fall into one of two classes. Those in one class show a relatively constant molar polarisation in accord with the simple Clausius-Mosotti relation, whilst the members of the other class, which contains materials with high relative permittivities, show a molar polarisation that decreases with increase in temperature. Debye recognised that permanent molecular dipole moments were responsible for the anomalous behaviour. From theories of chemical bonding we know that certain molecules which combine atoms of different electronegativity are partially ionic and consequently have a permanent dipole moment. Thus chlorine is highly electronegative and the carbon-chlorine 

The order of magnitude of such a dipole will be equal to the product of the charge on an electron (1.6 x 10 19 C) and a typical bond length (10 10 m) giving a value of 10 29 Cm. The old electrostatic Debye unit of molecular dipoles was in fact equivalent to 3.335 x 10 30 Cm. Orientation of such molecular dipoles will clearly produce an extra contribution to the molar polarisation in addition to dipoles induced by the applied field, and it is not unreasonable to expect that the equilibrium degree of orientation in a given field will depend on temperature. 

First consider the application of an external electric field to an assembly of rigid molecular dipoles, i.e. let there be no deformational polarisation. If the permanent moment of each dipole is /i, then the potential energy u of a dipole in the applied field will be  

If the electric field is the only orientational force acting on the molecules, the distribution of dipole orientations will be given by Boltzmann s law. Thus the number of dipoles pointing in the directions confined within the element of solid angle dfi will be 

Now the element of solid angle dfi, which includes all the directions in which the dipole lies at an angle in the range 0 to 6 + 6 with respect to the direction of the applied field, is dfi = 2% sin(9d(9 = 2% d(cos 6). Making this substitution and introducing the variable x = iiE jkT, we obtain 

The shared electron pair in HF is distorted toward the F end of the molecule because of the electronegativity difference between H and F atoms. As a result of the electrons not being shared equally, the structure can be shown as 

If the electrons were shared equally, the result would be H F and there would be no charge separation in the molecule. If the electron were completely transferred from the H atom to the F atom, the structure could be shown as H+ FA which represents an ionic bond. 

The dipole moment is a way of expressing nonsymmetrical charge distribution of electrons in a molecule. It is represented as p, which is defined by the equation 

Therefore, the actual value of q is 2.1 x 10-10 esu. The fraction of an electron that appears to have been transferred is 2.1 x 10-10 esu/4.8 x 10-10 esu = 0.44 of the electron charge so it appears that 44% of the electron has been transferred. The following structures illustrate this situation  

Only the middle structure exists, and it is sometimes said that the bond in HF is 44% ionic. What is really meant is that the true structure behaves as though it is a composite of 56% of the nonpolar structure and 44% of the ionic structure. The true structure is a resonance hybrid of these hypothetical structures, neither of which actually exists. 

Source R. J. Gillespie and P. L. A. Popelier, Chemical Bonding and Molecular Geometry, Oxford University Press, New York, 2001, Table 5.3, p. 119 R. J. Gillespie, Coord. Chem. Rev., 2000,197, 51. 

Experimentally, the polarity of molecules is measured indirectly by measuring the dielectric constant, which is the ratio of the capacitance of a cell filled with the substance to be measured to the capacitance of the same cell with a vacuum between the electrodes. Orientation of polar molecules in the electric field partially cancels the effect of the field and results in a larger dielectric constant. Measurements at different temperatures allow calculation of the dipole moment for the molecule, defined as 

Gillespie and R L. A. Popelier, Chemical Bonding and Molecular Geometry, Oxford University Press, New York, 2001, pp. 113-133. 

Source Experimental data, Handbook of Chemistry and Physics, 66th ed., CRC Press, Cleveland, OH, 1985-86, p. E-58 (from NBS table NSRDS-NBS 10) Spartan, see footnote 23. 

Nonpolar molecules, whether they have polar bonds or not, still have intermolec-ular attractive forces acting on them. Small fluctuations in the electron density in such 

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While vapor-phase corrections may be small for nonpolar molecules at low pressure, such corrections are usually not negligible for mixtures containing polar molecules. Vapor-phase corrections are extremely important for mixtures containing one or more carboxylic acids. 

Edeleanu process An extraction process utilizing liquid sulphur dioxide for the removal of aromatic hydrocarbons and polar molecules from petroleum fractions. 

Mutr> Y 1943 Force between non-polar molecules J. Phys. Math. Soc. Japan 17 629... 

Su T, Viggiano A A and Paulson J F 1992 The effect of the dipole-induced dipole potential on ion-polar molecule collision rate constants J. Chem. Phys. 96 5550-1... 

An alternative approach to obtaining microwave spectroscopy is Fourier transfonn microwave (FTMW) spectroscopy in a molecular beam [10], This may be considered as the microwave analogue of Fourier transfonn NMR spectroscopy. The molecular beam passes into a Fabry-Perot cavity, where it is subjected to a short microwave pulse (of a few milliseconds duration). This creates a macroscopic polarization of the molecules. After the microwave pulse, the time-domain signal due to coherent emission by the polarized molecules is detected and Fourier transfonned to obtain the microwave spectmm. 

The small lithium Li" and beryllium Be ions have high charge-radius ratios and consequently exert particularly strong attractions on other ions and on polar molecules. These attractions result in both high lattice and hydration energies and it is these high energies which account for many of the abnormal properties of the ionic compounds of lithium and beryllium. 

Bashin A A and K Namboodiri 1987. A Simple Method for the Calculation of Hydration Enthalpies c Polar Molecules with Arbitrary Shapes. Journal of Physical Chemistry 91 6003-6012. 

This technique has not been used as widely as transition state theory or trajectory calculations. The accuracy of results is generally similar to that given by pTST. There are a few cases where SACM may be better, such as for the reactions of some polyatomic polar molecules. 

Table 1 3 lists the dipole moments of various bond types For H—F H—Cl H—Br and H—I these bond dipoles are really molecular dipole moments A polar molecule has a dipole moment a nonpolar one does not Thus all of the hydrogen halides are polar molecules To be polar a molecule must have polar bonds but can t have a shape that causes all the individual bond dipoles to cancel We will have more to say about this m Section 1 11 after we have developed a feeling for the three dimensional shapes of molecules... 

Both water and carbon dioxide have polar bonds but water is a polar molecule and carbon dioxide is not... 

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

Induced dipole/induced dipole forces are the only intermolecular attractive forces available to nonpolar molecules such as alkanes In addition to these forces polar molecules engage m dipole-dipole and dipole/mduced dipole attractions The dipole-dipole attractive force is easiest to visualize and is illustrated m Figure 4 3 Two molecules of a polar substance experience a mutual attraction between the positively polarized region of one molecule and the negatively polarized region of the other As its name implies the dipole/induced dipole force combines features of both the induced dipole/mduced dipole and dipole-dipole attractive forces A polar region of one mole cule alters the electron distribution m a nonpolar region of another m a direction that produces an attractive force between them... 

In many addition reactions the attacking reagent unlike H2 is a polar molecule Hydro gen halides are among the simplest examples of polar substances that add to alkenes... 

Neither bromine nor ethylene is a polar molecule but both are polarizable and an induced dipole/mduced dipole force causes them to be mutually attracted to each other This induced dipole/mduced dipole attraction sets the stage for Br2 to act as an electrophile Electrons flow from the tt system of ethylene to Br2 causing the weak bromine-bromine bond to break By analogy to the customary mechanisms for electrophilic addition we might represent this as the formation of a carbocation m a bimolecular elementary step... 

Ozone (O3) IS the triatomic form of oxygen It is a neutral but polar molecule that can be represented as a hybrid of its two most stable Lewis structures... 

Ethers like water and alcohols are polar molecules Diethyl ether for example has a dipole moment of 1 2 D Cyclic ethers have larger dipole moments ethylene oxide and tetrahydrofuran have dipole moments m the 1 7 to 1 8 D range—about the same as that of water (1 8D)... 

The carbonyl group makes aldehydes and kefones rafher polar molecules dipole momenfs fhaf are subsfanfially higher fhan alkenes... 

Section 17 2 The carbonyl carbon is sp hybridized and it and the atoms attached to It are coplanar Aldehydes and ketones are polar molecules Nucleophiles attack C=0 at carbon (positively polarized) and electrophiles especially protons attack oxygen (negatively polarized)... 

Aryl halides are polar molecules but are less polar than alkyl halides... 

Extensive intercalation of polar molecules takes place in this substance in an irreversible manner, and marked hysteresis results (Fig. 4.28). The driving force is thought to be the interaction between the polar molecules and the exchange cations present in the montmorillonitic sheets, since non-polar molecules give rise to a simple Type B hysteresis loop with no low-pressure hysteresis. 

Type III (and Type V) isotherms may originate through the adsorption of either nonpolar or polar molecules, always provided that the adsorbent-adsorbate force is relatively weak. 

Sorption of nonionic, nonpolar hydrophobic compounds occurs by weak attractive interactions such as van der Waals forces. Net attraction is the result of dispersion forces the strength of these weak forces is about 4 to 8 kj/mol ( 1 2 kcal/mol). Electrostatic interactions can also be important, especially when a molecule is polar in nature. Attraction potential can develop between polar molecules and the heterogeneous sod surface that has ionic and polar sites, resulting in stronger sorption. 

The dissolution of polar molecules in water is favored by dipole—dipole interactions. The solvation of the polar molecules stabilizes them in solution. Nonpolar molecules are soluble in water only with difficulty because the relatively high energy cost associated with dismpting and reforming the hydrogen-bonded water is unfavorable to the former occurring. 

Molecular Interactions. Various polysaccharides readily associate with other substances, including bile acids and cholesterol, proteins, small organic molecules, inorganic salts, and ions. Anionic polysaccharides form salts and chelate complexes with cations some neutral polysaccharides form complexes with inorganic salts and some interactions are stmcture specific. Starch amylose and the linear branches of amylopectin form inclusion complexes with several classes of polar molecules, including fatty acids, glycerides, alcohols, esters, ketones, and iodine/iodide. The absorbed molecule occupies the cavity of the amylose helix, which has the capacity to expand somewhat to accommodate larger molecules. The starch—Hpid complex is important in food systems. Whether similar inclusion complexes can form with any of the dietary fiber components is not known. 

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