Concise Physical Chemistry

c21 JWBS043-Rogers September 13, 2010 11:30 Printer Name: Yet to Come

346 PHOTOCHEMISTRY AND THE THEORY OF CHEMICAL REACTIONS

ground state

intermediate state

excited state

photon in photon out

heat

FIGURE 21.1 Mechanism for fluorescent and phosphorescent light emission.

photoinduced chain reaction. The process starts with the production of two

free radicals Cl•by absorption of a photonhν. The first step is called the

chain-initiating step:

Cl 2 +hν→2Cl•

The chain initiating step is followed by thechain-propagating steps:

Cl•+H 2 →HCl+H•

H•+Cl 2 →HCl+Cl•

These steps constitute the heart of the chain reaction and continue indefinitely,

which explains the high quantum yield. Ultimately, however, a free radical

collides with another free radical

Cl•+Cl•→Cl 2 +heat

or loses its energy by some other transfer mechanism. These are thechain-

terminating steps.

21.2.1 Lipid Peroxidation

Inlipid peroxidationorautooxidation, the chain-initiating step is the production of a

lipid free radical R•by removal of a hydrogen atom from the ground state molecule

by light, heat, or enzymatic initiation. Lipid peroxidation follows a chain sequence.

The chain-propagating steps are

R•+O 2 →ROO•

ROO•+RH→ROOH+R•

and the chain-terminating step is

ROO•+ROO•→molecular products