Initial Radical Separation after Photolysis of 2,2′-Azobis(isobutyronitrile) (AIBN) in Solution: Modeling the Primary Cage Effect for Polar Radicals
✍ Scribed by Anton N. Savitsky; Henning Paul; Anatoly I. Shushin
- Publisher
- John Wiley and Sons
- Year
- 2006
- Tongue
- German
- Weight
- 148 KB
- Volume
- 89
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
Abstract
There is a contradiction as to the initial spatial separation r~i~ of the two transient 2‐cyanoprop‐2‐yl radicals (Me~2~$\rm{{{\dot C}}}$CN) formed by flash photolysis of 2,2′‐azobis(isobutyronitrile) (AIBN) in solvents of various viscosities. The cage effect, expressed in terms of the in‐cage termination probability of the resulting radicals, is predicted correctly by classical Langevin models assuming a decrease of r~i~ with increasing viscosity. However, the electron‐spin polarization of the radicals escaping the primary cage clearly indicates that the initial separation distance r~i~ is independent of the solution viscosity. This obvious discrepancy can be reconciled by accounting for the strong electric dipole moments of these radicals and the resulting inter‐radical dipoledipole interaction potential.
We propose a primary‐caging model for polar radicals in solution based on an attractive inter‐radical mean‐force potential. The model is applied to the flash photolysis of AIBN and shown to describe properly the viscosity dependence of both the in‐cage termination probability (cage effect) and the electron‐spin polarization of the escaping 2‐cyanoprop‐2‐yl radicals.