Molecular Vibrations and the Greenhouse Effect

 

Solar Energy:                9% UV, 46% visible, 45% IR

                                    30% reflected back into space

                                    20% absorbed by the atmosphere

                                    50% absorbed by the earth’s surface

 

The earth can absorb light in the UV-visible range, but the heated earth emits light in the IR (infrared) range, the spectral region where molecular vibrational changes are observed.  Some molecules in the atmosphere can increase their vibrational energy by absorbing some of this infrared light, and thus increasing the temperature of the atmosphere.  This is the molecular basis of the “greenhouse effect” and its connection to global warming. A particular molecule with N atoms has 3N-6 vibrational modes if it is nonlinear, and 3N-5 vibrational modes if it is linear.  A molecule can absorb a photon of IR light and increase the vibrational energy of one of its vibrational modes.  However, not all vibrational modes can increase in energy by absorbing a photon in the IR region. For a molecular vibrational mode to be IR active, the dipole moment of the molecule must change during the vibrational mode. 

 

Dry air is approximately 78.0% N2, 21.0% O2, and 0.9% Ar.  Argon gas consists of individual atoms, which cannot undergo molecular vibration.  Oxygen and nitrogen are diatomic molecules with one vibrational mode, involving a stretching and compression of the bond length.  However, the dipole moment remains zero during the vibration, so the molecules will not absorb in the infrared. This leaves mainly H2O, CO2, and other trace gases as contributors to the greenhouse effect.  Water has three normal modes of vibration, all of which are IR active.  Carbon dioxide, a linear molecule, has 4 normal modes of vibration.  Even though it does not have a permanent dipole moment, the dipole moment changes during 3 of the 4 modes, so carbon dioxide can absorb in the IR.

 

                   asymmetric stretch                    asymmetric stretch

                / = 3756 cm-1                      / = 2349 cm-1 

             IR active                                  IR active

 

 

symmetric stretch                  symmetric stretch 

/ = 3657 cm-1                    / = 1388 cm-1                     

            IR active                                   not IR active

 

 

bending vibration                                                          bending vibration

/ = 1545 cm-1                                           / = 667 cm-1

IR active                                      IR active

 

                                                            Note: the bending mode of CO2 is degenerate.

                                                                                                    There is another bending mode at right

    angles to the one shown above.