Bonding dipole moments

In stead, the electrostatic con tribn tion conies from definin g a set of bond dipole moments associated woth polar bonds. These bond moments are defined in the m m psir.LxL(dbf) file along with the bond stretching parameters and are given in units of Debyes. The cen ter of th e dipole Is defined to be th e m Idpoint of the bond an d two dipoles p. and pj. separated by Rjj. as shown beltnv ... 

Bond Dipole moment D Bond Dipole moment D... 

We can combine our knowledge of molecular geometry with a feel for the polarity of chemical bonds to predict whether a molecule has a dipole moment or not The molec ular dipole moment is the resultant of all of the individual bond dipole moments of a substance Some molecules such as carbon dioxide have polar bonds but lack a dipole moment because their geometry causes the individual C=0 bond dipoles to cancel... 

FIGURE 1 7 Contri bution of individual bond dipole moments to the mo lecular dipole moments of (a) carbon tetrachloride (CCy and (b) dichloro methane (CH2CI2)... 

BOND AND GROUP DIPOLE MOMENTS Table 4.12 Bond Dipole Moments... 

Now the overall effects due to hydrogen bonding, dipole moment, acid-base properties, and molecular configuration can be expressed as... 

Charge asymmetry can be associated with a particular bond in a molecule and gives rise to what is called a bond dipole moment or, simply, bond moment. One use of bond moments is that they can be transferred, to a fair degree of approximation, from one molecule to another. In this way the dipole moment of a molecule can sometimes be estimated from a vector sum... 

Molecular dipole moment (Section 1.11) The overall measured dipole moment of a molecule. It can be calculated as the resultant (or vector sum) of all the individual bond dipole moments. 

Since the elementary quantum-mechanical treatment does not seem to give a high enough barrier, various treatments of the problem have been proposed which use empirical data such as bond dipole moments and steric repulsive forces. These treatments do not introduce any new forces which would not be included in a proper quantum-mechanical analysis, but they attempt to short-circuit these difficult and uncertain calculations. 

As we have seen by comparing C02 and H20, the shape of a polyatomic molecule affects whether or not it is polar. The same is true of more complicated molecules. For instance, the atoms and bonds are the same in c/s-dichloroethene (28) and frans-dichloroethene (29) but, in the latter, the C—Cl bonds point in opposite directions and the dipoles (which point along the C—Cl bonds) cancel. Thus, whereas c/s-dichloroethene is polar, traws-dichloroethene is nonpolar. Because dipole momenrs are directional, we can treat each bond dipole moment as a vector. The molecule as a whole will be nonpolar if the vector sum of the dipole moments of the bonds is zero. 

Using a combination of techniques 1,3,4-thiadiazole was shown to have a dipole moment of 3.0 D directed from the sulfur atom toward the center of the N—N bond. Dipole moment measurements were useful for proving the mesoionic structure and the large negative charge of an exocyclic sulfur atom for certain 1,3,4-thiadiazole derivatives <84CheC-I(4)545>. 

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