(R1-R2) homolytic cleavage 3(R1R2) ISC 1(R1R2)• • • •

recombinations

disproportionations

“Cage” Reactions Involving Radical Pairs

Model System

1

The States of a Spin Correlated Radical Pair

vector model of electron spin

3(R1R2)• • • •

1(R1R2)

2

ISC in Triplet Radical Pairs 3

Spin Rephasing Mechanism for ISC

T0 S

spin clock(top-down view, projection in the xy-plane)

J < a

4

Spin-orbit coupling, g rate, kTS

Typical organic radical, g ~ 0.000-0.001Earth’s H ~ 1G 3 X 103 HzLab H ~ 100,000G 3 X 108 HzHyperfine coupling, aTypical organic radical, a ~ 10-100G 3 X 107 - 3 X 108 Hz

kTS ~ (3 X 106)(gH ±Ms a1 )

T0 S

Timescales for ISC in Radical Pairs

2 mechanisms: Spin Rephasing (g) and Spin Flipping (a)

5

kTS ~ (3 X 106)(gH ±Ms a1 )

T0 S

Power of Spin in Product Distribution

Spin RephasingNo nuclear spin flip involved Spin Flipping

Sort magnetic nuclei from nonmagnetic nuclei!then kTS

+1/2 > kTS-1/2

Sort nuclear spin states! Polarized (non-Boltzmann) product distribution!

Ms = 0

Ms = ± 1/2 , a1 is positive

a >>>H

6