Dipole moment, and molecular geometry

Any molecule that has a net separation of charge, as in HCl, has a dipole moment A molecule in which the distribution of electric charge is equivalent to charges +S and —8 separated by a distance d has a dipole moment equal to 8d. Dipole moments are usually measured in units of debyes (D). In SI units, dipole moments are measured in coulomb-meters (C m), and 1 D = 3.34 X 10 ° C m. 

You can sometimes relate the presence or absence of a dipole moment in a molecule to its molecular geometry. For example, consider the carbon dioxide molecule. Each carbon-oxygen bond has a polarity in which the more electronegative oxygen atom has a partial negative charge. 

CHAPTER 10 Molecular Geometry and Chemical Bonding Theory 

We will denote the dipole-moment contribution from each bond (the bond dipole) by an arrow with a positive sign at one end ( - ). The dipole-moment arrow points from the positive partial charge toward the negative partial charge. Thus, you can rewrite the formnla for carbon dioxide as 

Each bond dipole, like a force, is a vector quantity that is, it has both magnitude and direction. Like forces, two bond dipoles of equal magnitude but opposite direction cancel each other. (Think of two groups of people in a tug of war. As long as each group puUs on the rope with the same force bnt in the opposite direction, there is no movement—the net force is zero.) Because the two carbon-oxygen bonds in CO2 are equal but point in opposite directions, they give a net dipole moment of zero for the molecule. 

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The semiempirical MNDO SCF-MO and AMI SCF-MO models introduced by Dewar et al. (77JA4899 85JA3902) have proved to be suitable tools for reproducing experimental data for several examples of heterocyclic betaines of general type 1, such as dipole moments and molecular geometries. Relevant results are collected in Table IX and C). 

Relationship between dipole moment and molecular geometry is given in table 1.8. 

Relating dipole moment and molecular geometry State what geometries of a molecule AX are consistent with the information that the molecule has a nonzero dipole moment. (EXAMPLE 10.3)... 

St.-Amant, A., W. D. Cornell, P. Kollman, and T. A. Halgren. 1995. Calculation of Molecular Geometries, Relative Conformation Energies, Dipole Moments, and Molecular Electrostatic Potential Fitted Charges of Small Organic Molecules of Biochemical Interest by Density Functional Theory. J. Comp. Chem. 16, 1483. 

St.-Amant A, Cornell WD, Kollman PA, Halgren TA (1995) Calculation of molecular geometries, relative conformational energies, dipole-moments, and molecular electrostatic potential fitted charges of small organic-molecules of biochemical interest by density-functional theory, J Comput Chem, 16 1483-1506... 

Calculation of Molecular Geometries, Relative Conformational Energies, Dipole Moments, and Molecular Electrostatic Potential Fitted Charges of Small Organic Molecules of Biochemical Interest by Density Functional Theory. 

Relationship Between Molecular Geometry and Dipole Moment Formula Molecular Geometry Dipole Moment ... 

Energy, geometry, dipole moment, and the electrostatic potential all have a clear relation to experimental values. Calculated atomic charges are a different matter. There are various ways to define atomic charges. HyperChem uses Mulliken atomic charges, which are commonly used in Molecular Orbital theory. These quantities have only an approximate relation to experiment their values are sensitive to the basis set and to the method of calculation. 

The group centred around M. J. S. Dewar has used a combination of (2) and (3) for assigning parameter values, resulting in a class of commonly used methods. The molecular data used for parameterization are geometries, heats of formation, dipole moments and ionization potentials. These methods are denoted modified as their parameters have been obtained by fitting. 

Contributions in this section are important because they provide structural information (geometries, dipole moments, and rotational constants) of individual tautomers in the gas phase. The molecular structure and tautomer equilibrium of 1,2,3-triazole (20) has been determined by MW spectroscopy [88ACSA(A)500].This case is paradigmatic since it illustrates one of the limitations of this technique the sensitivity depends on the dipole moment and compounds without a permanent dipole are invisible for MW. In the case of 1,2,3-triazole, the dipole moments are 4.38 and 0.218 D for 20b and 20a, respectively. Hence the signals for 20a are very weak. Nevertheless, the relative abundance of the tautomers, estimated from intensity measurements, is 20b/20a 1 1000 at room temperature. The structural refinement of 20a was carried out based upon the electron diffraction data (Section V,D,4). 

Semiempirical methods, on the other hand, utilize minimum basis sets to speed up computations, and the loss in rigor is compensated by the use of experimental data to reproduce important chemical properties, such as the heats of formation, molecular geometries, dipole moments, and ionization potentials (Dewar, 1976 Stewart, 1989a). As a result of their computational simplicity and their chemically useful accuracy, semiempirical methods are widely used, especially when large molecules are involved (see, for example, Stewart, 1989b Dewar et al., 1985 Dewar, 1975). 

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