Dipole Moments and Molecular Polarity

The exact expression for the dipole moment does n( consider atoms as point charges, but rather as nuclei (eat with a positive charge equal to the atomic number) ar electrons (each with unit negative charge). Atoms wii lone pairs may contribute to the dipole moment, even the atom is neutral, as long as the lone pair electrons a not symmetrically placed around the nucleus. 

Draw a Lewis structure for singlet methylene, CH2 (all the electrons in singlet methylene are spin-paired). Ho many electrons remain after all bonds have been formei Where are the extra electrons located, in the plane the molecule or perpendicular to the plane Examine t highest-occupied molecular orbital (HOMO) of methyle to tell. 

Hydrocarbons normally have very small dipole momen Why (Hint Consider the relationship betwe electronegativity differences and dipole momer established above for hydrogen halides.) Does sing methylene possess a small dipole moment Explain. W1 direction do you expect singlet methylene s dipole point Explain. In what direction does it point  

Partial charges (6+ and 6—) are generally on the order of 10 °esu (electrostatic units) and the distances are generally on the order of 10 cm. Therefore, for a polar compound, the dipole moment (p) will generally have an order of magnitude of around 10 esu cm. The dipole moment of chloromethane, for example, is 1.87 x 10 esu cm. Since most compounds will have a dipole moment on this order of magnitude (10 ), it is more convenient to report dipole moments with a new unit, called a debye (D), where 

Using these units, the dipole moment of chloromethane is reported as 1.87 D. The debye unit is named after Dutch scientist Peter Debye, whose contributions to the fields of chemistry and physics earned him a Nobel Prize in 1936. 

Measuring the dipole moment of a particular bond allows us to calculate the percent ionic character of that bond. As an example, let s analyze a C—Cl bond. This bond has a bond length of 1.772 X 10 cm, and an electron has a charge of 4.80 x 10 ° esu. If the bond were 100% ionic, then the dipole moment would be 

The molecular dipole moment of dichloromethane is the net sum of all dipole moments in the compound. 

In this way, the lone pairs contribute significantly to the magnitude of the molecular dipole moment, although they do not contribute to its direction. That is, the direction of the molecular dipole moment would be the same with or without the contribution of the lone pairs. 

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Some molecular properties of polar solvents are summarized in table 4.3. The dipole moment and molecular polarizability are the molecular parameters which lead to the solvent permittivity. The other parameters listed are the molecular diameter and the Lennard-Jones interaction energy, Elj. These are of interest in assessing the role of van der Waals forces in determining the properties of a polar liquid. 

The important point illustrated by this discussion of HC1 is that the net molecular dipole moment is a vector sum of the bond moment and the lone-pair moments. There is clearly no necessary relationship between molecular dipole moments and bond polarity alone. Naturally, an entirely analogous situation prevails with polar polyatomic molecules, as illustrated by NH3 and NF3. 

LD spectroscopy is a technique based on the interaction between linearly polarized radiation and chemical species [3]. Such a technique has no relation with molecular chirality and circular dichroism. It relies on the fact that the probability of inducing an electronic transition with electromagnetic radiation is proportional to the cosine square of the angle between the transition dipole moment and the polarization direction of the radiation (which coincides with the direction of oscillation of the electric field). Therefore, if a linearly polarized radiation is employed, the probability of a transition to occur is maximum when the corresponding dipole moment is parallel to the polarization direction, while the probability is zero when the transition dipole moment is oriented perpendicularly to the polarization direction. The dependence of the absorption of a sample on the polarization direction of the light is called linear dichroism. 

Knowing the shape of a molecule and the polarity of its various bonds allows the presence or absence of a molecular dipole moment and its direction to be predicted... 

For both polar and nonpolar nonhydrocaihon gaseous mixtui es at low pressui es, the most accurate viscosity prediction method is the method of Brokaw. The method is quite accurate but requires the dipole moment and the Stockmayer energy parameter (e/A ) for polar components as well as pure component viscosities, molecular weights, the normal boding point, and the hq-uid molar volume at the normal boding point. The Technical Data Manual should be consulted for the fidl method. 

This is not an SCRF model, as the dipole moment and stabilization are not calculated in a self-consistent way. When the back-polarization of the medium is taken into account, the dipole moment changes, depending on how polarizable the molecule is. Taking only the first-order effect into account, the stabilization becomes (a is the molecular polarizability, the first-order change in the dipole moment with respect to an electric field, Section 10.1.1). 

Net molecular polarity is measured by a quantity called the dipole moment and can bethought of in the following way assume that there is a center of mass of all positive charges (nuclei) in a molecule and a center of mass of all negative charges (electrons). If these two centers don t coincide, then the molecule has a net polarity. 

Polar groups, such as carbonyl (C=0) and hydroxyl (OH), have a strong ground state dipole moment and show strong IR absorptions at characteristic frequencies. The IR spectrum can thus be used as a fingerprint of molecular... 

Generally, the solubihty characteristics of organic compounds depend on several properties of the participating components. For the solute, these properties are the molecular size and structure, polarity, dipole moment, va-por/sublimation pressure, and, in the case of a sohd solute, also its melting characteristics. When using SCCO2 as the solvent, mainly its dipole moment and quadrupole moment influence the solvatation process (Sect. 2.2). 

The C-O bonds In ethers are polar and thus, ethers have a net dipole moment. The weak polarity of ethers do not appreciably affect their boiling points which are comparable to those of the alkanes of comparable molecular masses but are much lower than the boiling points of alcohols as shown in the following cases ... 

This description of antennas may seem more appropriate to a discussion of radio or television waves. We must realize, however, that at the molecular level dipoles behave exactly like antennas. Since molecules are made up of charged parts, a dipole moment /x is induced by the electric field of the radiation in any material through which radiation passes. In this discussion, the dipole moment equals the product of the effective charge displaced by the field and its distance of separation from the opposite charge. In SI, pi has units C m. We consider isotropic materials characterized by a polarizability a. As the name implies, this property measures the ease with which charge separation —polarity —is induced in a molecule by an electric field. For isotropic substances, the dipole moment and the field are related by the expression... 

The various factors affecting the magnitude of the dipole moment in a polar molecule were discussed in previous chapters. For the present discussion it is sufficient to picture a molecular dipole as two equal and opposite charges (q = ) separated by a distance r. The dipole moment, n. is given by... 

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