Carbon dioxide molecular orbitals

III. MOLECULAR ORBITAL DRAWINGS Carbon Dioxide (Continued)... 

Dimethylfulvene 93 also reacts with sydnone 89, albeit sluggishly, to form the dihydrocyclopenta[c]pyrazole 94 after elimination of carbon dioxide and hydrogen (Equation 10). Molecular orbital energies and coefficients of 3-phenylsydnone 89 and fulvenes 91 and 93 have been calculated (PM3-MNDO), but when orbital symmetries... 

Give the degree of the polynomial equation that arises in calculating the molecular orbitals for the following species in their ground states (cr or n bonding, as indicated) (a) carbon dioxide (cr only) (b) benzene (n bonds only). 

Carbon dioxide is a linear molecule with both a and jt bonds. The coordinate system chosen for CO2 is shown in Fig. 3.4.4. Once again, the molecular axis is taken to be the z axis. The atomic orbitals taking part in the bonding of this molecule are the 2s and 2p orbitals on C and the 2p orbitals on O. There are a total of ten atomic orbitals and they will form ten molecular orbitals. 

Of the 18 systems, some of which are unstable and must be generated in the reaction has been accomplished for at least 15, but not in all cases with a carbon-carbon double bond (the reaction also can be carried out with other double bonds ). Not all aUcenes undergo 1,3-dipolar addition equally well. The reaction is most successful for those that are good dienophUes in the Diels-Alder reaction (15-60). The addition is stereospecific and syn, and the mechanism is probably a one-step concerted process, as illustrated above, " largely controlled by Frontier Molecular Orbital considerations. " In-plane aromaticity has been invoked for these dipolar cycloadditions. " As expected for this type of mechanism, the rates do not vary much with changes in solvent, " although rate acceleration has been observed in ionic liquids. " Nitrile oxide cycloadditions have also been done in supercritical carbon dioxide. There are no simple rules... 

Molecular Orbitals of C02. In most cases of polyatomic molecules, p orbitals cause the orbital analysis to be much more complex than in the case of H3+. Carbon dioxide is such a situation, involving valence p orbitals on both the central and the surrounding atoms. The group orbitals are on the oxygens and are of three types. 

Carbon dioxide, (a) Overlap of carbon sp hybrid orbitals with oxygen 2px orbitals to give a bonds O—C—O. (b) The two tt molecular orbitals of the molecule formed from unused 2py and 2pz orbitals. 

It is difficult to view the carbon dioxide molecule on a molecular orbital approach as, relatively, there are so many electrons to be accommodated. A reasonable picture may be obtained using a valence bond approach. 

The approach used so far can be applied to other linear species—snch as CO2, N3, and BeH2—to consider how molecular orbitals can be constructed on the basis of interactions of group orbitals with central atom orbitals. However, we also need a method to understand the bonding in more complex molecules. We will first illustrate this approach using carbon dioxide, another linear molecule with a more complicated molecular orbital description than FHF . The following stepwise approach permits examination of more complex molecules ... 

Strategy Identify the atomic orbitals involved in tt bonding in carbon dioxide and use them to construct molecular orbitals that fit the symmetry of the molecule. 

The N ion is found to be linear and is isoelectronic with carbon dioxide. It is also synunetiical and on the basis of molecular orbital picture, there are two a and two n bonds. The N-N bond length is the same and is 0.116 nm. 

Note 5.6 (Van der Waals force). Even for nonpolar molecules, the polar character is generated due to instantaneous deviations in the electron orbit. Because of this electric fleld, the neighboring molecules become polarized, and the energy level of the total system becomes lower if the force is attractive rather than repulsive. Frozen carbon dioxide and crystals of iodine h are examples of crystals formed by van der Waals forces, which are known as molecular crystals. Since the van der Waals forces have no orientation, the molecular crystals occur in a closely-packed structure. The van der Waals force V(r) is inversely proportional to the sixth power of the intermolecular distance r V(r) = —C/r . The van der Waals force is extremely small compared to chemical forces such as ionic bonds, covalent bonds and metallic bonds (i.e., less than 1/100). ... 

The bonds along the molecular axis of carbon dioxide can be thought of as forming from combinations of hybrid orbitals on each of the three atoms. Assume the molecule lies on the z axis in the arrangement OaCOb. 

The molecular orbitals most relevant to the chemical reactivity of carbon dioxide are the litg and 2jt orbitals, which play the role of HOMOs and LUMOs, respectively. Although the doubly occupied non-bonding Ijig MOs are mainly localized... 

The Walsh diagram (Fig. 2.1), already discussed in Chap. 1 and recalled here for the reader s benefit, suggests that carbon dioxide (CO2) can in principle act as an acceptor of electrons (through the lowest unoccupied molecular orbital [LUMO] centred at the C atom) and a donor, through the highest occupied molecular orbital... 

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