Photolysis and the Hydroxyl Radical


What happens when light is absorbed by a molecule? Let's look at the absorption of a photon of light by a hydrogen molecule. If the photon of light contains just enough energy to promote an electron from the lower energy level to the upper one, the bond order decreases from 1 to 0. The bond is broken and two hydrogen atoms are formed. If the photon is more energetic, the hydrogen atoms will have excess kinetic energy.

Light breaks chemical bonds when the energy of the photon is greater than the bond dissociation energy.

What is the wavelength required to break the H-H bond?

What about molecular oxygen? The O=O bond is stronger than the H-H bond. By the same analysis, it would require light of 240 nm to break it.

However, this doesn't ususally happen in the troposphere. The 240 nm light is very energetic. Very little of light in this frequency range reaches the troposphere because most of it is absorbed in the stratosphere.

What frequency of light would be needed to cleave the N2 molecule? The bond dissociation energy is 941 kJ/mol. Is this photolytic reaction likely to occur in the troposphere?

Bond Dissociation Energy

The energy required to break a particular bond in a specified molecule is the bond dissociation energy. There is a table of average bond energies below. Be careful! Bond dissociation energy and bond energy are not the same. It takes 493 kJ/mol of bond dissociation energy to break the first O-H bond in water and 424 kJ/mol to cleave the remaining O-H bond. The average bond energy of the O-H bonds in water is 459 kJ/mol. The O-H bond energy will vary a little from molecule to molecule. The average is about 464 kJ/mol.

Methane has 4 C-H bonds and the bond dissociating energies are 435 kJ/mole for D(CH3-H), 444 kJ/mole for D(CH2-H), 444 kJ/mole for D(CH-H) and 339 kJ/mole for D(C-H). The average bond energy is 414 kJ/mole. The C-H bond energy changes depending on the structure of the molecule. The average for many molecules is shown in the table.

Photolysis of Molecules in the Troposphere

There are molecules in the troposphere that are easier to cleave than H2, O2, or N2. For example, ozone has weaker O-O bonds and can be cleaved by light of 330 nm. Reaction of O with water, abundant in the troposphere, forms 2 hydroxyl radicals that are more stable than oxygen atoms.

Molecules with O-O single bonds, such has hydrogen peroxide, are even easier to cleave.

= h c N/ (204 x 103 J) = 587 nm

Here are some other common photolysis reactions of the troposphere. Note that the aldehyde C-H bond is much weaker (~368 kJ/mol) than other C-H bonds in organic molecules.

Hydroxyl Radicals

Hydroxyl radicals are the most important oxidizing agents in the troposphere and are produced in a small, steady state concentration.

Radicals are atoms or molecules with unpaired electrons. They are always electron deficient and highly reactive. In the Lewis structure of the OH radical, for example, there are only 7 electrons around the electronegative oxygen atom.

Radicals can react with other molecules in several ways. The reactions lead to more stable radicals or to non-radical molecules.
  1. Dimerization
    This has a low probability because the concentration of HO is low. The dimer, hydrogen peroxide, is more stable than the hydroxyl radicals but the O-O bond is very weak.

  2. Reactions with other radicals

  3. Addition to unsaturated molecules

  4. Abstraction of an atom

Professor Patricia Shapley, University of Illinois, 2010