Carbon dioxide bond moments

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... 

In contrast with water, methanol, ammonia, and other substances in Table 2.1, carbon dioxide, methane, ethane, and benzene have zero dipole moments. Because of the symmetrical structures of these molecules, the individual bond polarities and lone-pair contributions exactly cancel. 

A polyatomic molecule may be nonpolar even if its bonds are polar. For example, the two fi+C—Ofi dipole moments in carbon dioxide, a linear molecule, point in opposite directions, and so they cancel each other (25) and C02 is a nonpolar... 

Carbon dioxide is a symmetrical, linear triatomic molecule (0 = C=0) with a zero dipole moment. The carbon-to-hydrogen bond distances are about 1.16A, which is about 0.06A shorter than typical carbonyl double bonds. This shorter bond length was interpreted by Pauling to indicate that greater resonance stabilization occurs with CO2 than with aldehydes, ketones, or amides. When combined with water, carbonic acid (H2CO3) forms, and depending on the pH of the solution, carbonic acid loses one or two protons to form bicarbonate and carbonate, respectively. The various thermodynamic parameters of these reactions are shown in Table I. 

In contrast with water and ammonia, carbon dioxide and tetrachloromethane (CCI4) have zero dipole moments. Molecules of both substances contain individual polar covalent bonds, but because of the symmetry of their structures, the individual bond polarities exactly cancel. 

Carbon dioxide (CO2) has a low supercritical temperature (31°C) and pressure (73 atm). It is nontoxic and nonflammable and is available at high purity. Therefore, CO2 has become the solvent of choice for most SFE applications. Being nonpolar and without permanent dipole moment, supercritical CO2 is a good solvent for the extraction of nonpolar and moderately polar compounds. However, its solvating power for polar solutes is rather poor. Moreover, when the solutes bind strongly to the matrix, the solvent strength of CO2 is often inadequate to break the solute-matrix bond. 

For example, formaldehyde has one strongly polar C=0 bond, and carbon dioxide has two. We might expect C02 to have the larger dipole moment, but its dipole moment is actually zero. The symmetry of the carbon dioxide molecule explains this surprising result. The structures of formaldehyde and carbon dioxide are shown here, together with their electrostatic potential maps. These electrostatic potential maps show the directions of the bond dipole moments, with red at the negative ends and blue at the positive ends of the dipoles. In carbon dioxide, the bond dipole moments are oriented in opposite directions, so they cancel each other. 

Sulfur dioxide has a dipole moment of 1.60 D. Carbon dioxide has a dipole moment of zero, even though C—O bonds are more polar than S —O bonds. Explain this apparent contradiction. 

Carbon dioxide and carbon tetrachloride tell a different story about polar bonds and overall dipole moment. Both carbon dioxide and carbon tetrachloride have polar bonds, as diagrammed by the dipole arrows shown in Figure 5.23. 

In these two cases the dipole arrows cancel each other out because of the shape of the molecules. The linear shape of the molecule of carbon dioxide puts the dipole arrows in opposite directions to counterbalance each other. The same holds true for the tetrahedral molecular geometry found in carbon tetrachloride. Despite having polar bonds, these two molecules are nonpolar. There is no overall dipole moment in these molecules because the dipole arrows are of the same magnitude but lie in opposite directions in the molecule. This counterbalance causes the molecule to be nonpolar. 

A molecule with more than one polar bond in which all the effects of the polar bonds do not cancel out has a dipole moment—an electrical dissymmetry that causes an inter-molecular attraction between this molecule and other similar ones. This attraction is called a dipolar attraction, and it lowers the ability of the substance to exist in the gas phase. However, if the polar bonds in a molecule are oriented so that their effects are canceled out, as in carbon dioxide, then a molecule with no dipole results (Section 13.5). 

Problem 2.5 Carbon dioxide, CO, has zero dipole moment even though carbon-oxygen bonds are strongly polarized. Explain. 

The molecules OCO, SCS, and OCS are linear. Bond lengths are given in Tables 21.5 and 21.6. In carbon dioxide the carbon-oxygen bond is intermediate in length between a double and triple bond. The studies of CO2 and CS2 by high resolution infrared spectroscopy provide an example of the use of this method for molecules which cannot be studied by the microwave method because they have no permanent dipole moment. The structure of the CS2 molecule has also been studied in the crystalline state. (C—S, 1-56 A). Under a pressure of 30 kbar CS2 polymerizes to a black solid for which a chain structure has been suggested. ... 

Carbon dioxide, CO2, is a three-atom molecule in which each carbon-oxygen bond is polar because of the electronegativity difference between C and O. But the molecule as a whole is shown by experiment (dipole moment measurement) to be nonpolar. This tells us that the polar bonds are arranged in such a way that the bond polarities cancel. Water, H2O, on the other hand, is a very polar molecule this tells us that the H—O bond polarities do not cancel one another. Molecular shapes clearly play a crucial role in determining molecular dipole moments. We will develop a better understanding of molecular shapes in order to understand molecular polarities. 

The environmentally benign nature of carbon dioxide comes from its very stable molecular bonds, which in turn do not provide high polarity. In fact, a carbon dioxide molecule has only a weak quadrupole moment, due to minor charge separation on oxygen and carbon atoms. Hence, the molecular interaction with most polar and heavy substances of interest is minor, providing only a weak solvent power. If needed, a small amount of cosolvent (also termed as entrainer or modifier) is added to enhance polarity and affinity with solutes. In many applications, however, the design limitation is the solubility of the substance in supercritical carbon dioxide. Therefore, the solubility data are essential both for the initial feasibihty study and final process design. 

Now let s examine the miscibility situation of supercritical CO2 with the same liquids at 400 bar and 40°C the solubility parameter of CO2 is about 7.3. Carbon dioxide at 400 bar is miscible with hexane, so again we might be pleased with the fact that 82- 81 = 1 predicts miscibility behavior. But CO2 is also miscible with methanol, DMF, NMP, and DMSO, and the solubility parameters for the respective four liquids are clearly different by more than 1 H. How do we explain that CO2 is miscible with the liquids Although CO2 is strictly a nondipolar molecule, i.e., its dipole moment is zero, it has strong bond dipoles or, equivalently, a large quadrupole, which can interact with other polar molecules. 

Diatomic molecules containing atoms of different elements (for example, HCl, CO, and NO) have dipole moments and are called polar molecules. Diatomic molecules containing atoms of the same element (for example, H2, O2, and F2) are examples of nonpolar molecules because they do not have dipole moments. For a molecule made up of three or more atoms both the polarity of the bonds and the molecular geometry determine whether there is a dipole moment. Even if polar bonds are present, the molecule will not necessarily have a dipole moment. Carbon dioxide (CO2), for example, is a triatomic molecule, so its geometry is either linear or bent ... 

A molecular electric potential exists because some types of atoms in a molecule attract electrons better than others. For instance, it is well known that fluorine attracts electrons more than hydrogen in HF, resulting in a nonzero dipole moment for this molecule. In the linear molecule carbon dioxide, electronic charge migrates toward the oxygens giving the molecule a measurable quadrupole moment. Even tetrahedral methane has a measurable octupole moment due to charge separation in its C —H bonds its dipole and quadrupole moments are zero by symmetry. 

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