Lecture 11: Carbon Oxides

Read section 13.9 (partial) from your textbook.

Structure and Bonding

We have already discussed the structure and molecular orbital diagram of CO. How about CO2? Let's look at the Lewis structure:



VSEPR tells us that the molecule is planar. Because there is an inversion center, the molecule is in the Dh symmetry point group. Here is the character table:



We can compare this with another triatomic molecule in that group, BeH2. The sigma bonding framework will be the similar in these two molecules but the energies of the orbitals will be different. See the chart on atomic orbital ionization energies for this.

The 2 bonded atoms (H in BeH2 or O in CO2) will each contribute a s atomic orbital. These combine to form 2 group orbitals. Let's look at the symmetry of the central atom valence orbitals and these group orbitals. In this symmetry point group, only the 2s and the 2pz orbitals on Be or C have the correct symmetry for a sigma overlap. The 2s orbital has the same symmetry as the low energy group orbital and the 2pz has the same symmetry as the higher energy group orbital.



The diagrams below show the combination of the central atom 2s orbital with the A1g symmetry group orbital and the central atom 2pz orbital with the A1u symmetry group orbital.



There are no other orbitals on H to combine with the remaining Be 2p orbitals, so the 2px and 2py become non-bonding molecular orbitals.

In CO2, some of the oxygen 2p orbitals can combine to form group orbitals that have pi symmetry. Two of these can combine with the carbon 2px and 2py orbitals. These form 2 bonding and 2 antibonding orbitals. The remaining 4 oxygen 2p orbitals become non-bonding molecular orbitals.



Here is the complete MO diagram with electrons. How does this correlate with the Lewis structure?



CO, CO2 in the Atmosphere

Concentration:
    CO: 1.0 x 102 ppb

    CO2: 3.5 x 105 ppb


Global Warming

Carbon dioxide and other gases absorb IR radiation from the Earth's surface and act as a greenhouse to increase the temperature of the planet.



The radiation excites vibrational modes in the molecules. The vibrational absorption spectrum of CO2 is shown below.



The concentration of carbon dioxide in the atmosphere is increasing rapidly due to our use of fossil fuels. The changes in atmospheric concentration of CO2 have been monitored by the observatory on Mauna Loa, Hawaii and through antarctic ice cores.



Chemistry of Carbon Oxides

CO Chemistry

  • CO as a Lewis base



  • CO as a reducing agent



CO2 Chemistry

  • Reaction with nucleophiles



  • Reaction with basic oxides