Hydroperoxy radicals (HOO•) play a large part in numerous gas and solution phase reactions in the atmosphere. The presence of water strongly influences the rate constant for the reaction HOO• + HOO•:

1) Rate constants were higher in the presence of water vapor than in dry air.

2) The activation energy was negative in the gas phase, yet positive in aqueous solution.

These two effects might seem contradictory; however, consider that in the gas phase, the HOO• + HOO• reaction proceeds through a doubly hydrogen-bonded HOO• dimer. Water can also form strong hydrogen bonds with HOO•. The suspected explanation for observation (1), above, is that when a hydroperoxy radical collides with a H₂O-HOO• cluster in the gas phase, the H₂O may detach, yielding a hydrogen bonded HOO• dimer with much higher efficiency than in the collision of two bare HOO• radicals.

In the solution phase, water-HOO• hydrogen bonding competes with HOO•-HOO• hydrogen bonding, and this may be key to the change in the sign of the activation barrier. We will consider the effects of water on the HOO• + HOO• reaction in three different ways:

• microscopic solvation (by explicit water molecules) of (HOO•)₂ in the gas phase

• bulk solvation of (HOO•)₂, treated with the polarizable continuum model (PCM)

• the combination of explicit solvation of HOO•-HOO• with PCM for bulk solvation.

The comparison of the results of these three approaches may explain the striking differences between the gas and aqueous phase reactions.