Free Radicals
A. Miyoshi

FREE RADICALS

  FREE RADICALS are active and short-lived intermediates which play important roles in chain-reaction systems such as combustion and atmospheric chemical reaction processes.   Our research studies are dedicated to the elucidation of the complex reaction processes through the direct observation of the free radicals and labile intermediates.

Elucidation of Chein-Reaction Systems

  A chain-reaction system consisting of multiple elementary reaction steps of free radicals shows different behavior from those expected from a simplefied one-step net reaction or from each constituent reaction step.   For example, a chlorine atom (Cl) produced by a photolysis of a fleon molecule in the stratosphere destroys ozone (O3) molecules by the chain-reaction system consisting of the following two reactions [1].
Cl + O3 → ClO + O2   (1)
ClO + O → Cl + O2   (2)
O + O3 → 2 O2   (net reaction)
The chlorine atom (Cl) reacts with an ozone molecule (1) and produces a ClO radical, which eventually reproduces a chlorine atom by the reaction (2) [2].   Consequently, a Cl atom produced by a photolysis of a fleon molecule destroys a number of ozone moleclues by this chain-reaction system.   Under a typical stratospheric condition, one Cl atom is estimated to destroy about 104 ozone molecules.
  Such a behavior cannot be captured from either from the net reaction nor from the reaction (1) or (2) only.   To understand the free radical kinetics is to elucidate the behavior of such a reaction network (or system) consisting of multiple elementary steps.

free radical = radical ?

  The original meaning of the term radical is a group of atoms in chemical coumpounds. For example, 'methyl radical' (-CH3) and 'hydroxy radical' (-OH) in organic coumpounds are radicals in its original meaning. (But they would rather be called 'methyl group' and 'hydroxy group' in recent terminology.) When chemists started to use the word radical, a group called radical had never been known to exist in isolation either directly or indirectly, though some might imagine it. Thus, when the isolated radicals came into observation of chemists, they needed to be distinguished from groups in compounds and started to be called 'free radicals'. As the development of invesigation on free radicals, the term radical became to mean 'free radicals'. For example, in recent years, 'methyl (CH3) radical' means free methyl radical rather than the methyl (-CH3) in compounds. For this reason, the methyl in comnounds would better be denoted as 'methyl group' rather than 'methyl radical'.

[1]  M. J. Molina and F. S. Rowland, Nature, 249, 810 (1974).
[2]  Like the ozone molecules, the oxygen atoms (O) always present in the photo-stationary state stratosphere, though their concentration is low.