ESR Studies of Heisenberg Spin Exchange. II. Effects of Radical Charge and Size

The Heisenberg spin exchange behavior of several radical–solvent systems has been studied. This work has shown that k, the second‐order rate constant for bimolecular encounters, and | J | τ1 (where | J | is twice the exchange integral and τ1 is the lifetime of the collision pair) depend markedly on the ionic strength of the solution and upon the size and charge of the radical. For the peroxylamine disulfonate radical in a 5 × 10−2M solution of 1:2 electrolyte, our simple analysis yields | J | τ1 ∼ 0.95 and k = 2.0 × 10+9M−1·sec−1 at 24°C; increasing the electrolyte concentration to 6.1 × 10−1M gives | J | τ1 ∼ 1.3 and k = 3.2 × 109M−1·sec−1. These changes in k and | J | τ1 are attributed to screening by the electrolyte of the charge of the dianion radical; this is discussed in terms of the Debye theory. The di‐tertiarybutyl nitroxide (DTBN) radical undergoes apparent strong exchange (| J | τ1 ≫ 1) in pure water (k = 2.4 × 109M−1·sec−1 at 24°C). The dependence of the exchange frequency (ωHE) on added 1:2 e...