Radical cage effects: A method for measuring recombination efficiencies of secondary geminate radical cage pairs using pump-probe transient absorption methods

Abstract

A method is reported for measuring the recombination efficiency of secondary geminate radical cage pairs. The procedure involves measuring the recombination efficiency for primary geminate recombination (F(c1)) using pump-probe laser methods and measuring the "apparent" (or net) recombination efficiency (F(cP)) for all geminate pairs (primary and secondary) using steady-state irradiation methods. A mathematical relationship between F(cP), F(c1), and F(c2) (where F(c2) is the recombination efficiency for secondary geminate recombination) is derived and demonstrated using the photolysis reactions of the [(CpR)Mo(CO)(3)](2) molecules, where CpR = eta(5)-C(5)H(4)CH(3) and eta(5)-C(5)H(4)(CH(2))(2)C(O)NCH(3)(CH(2))(n)CH(3) (n = 3, 8, 13, 18). As an example of the results obtained using the new method, it was found that F(c1) = 0.43 and F(c2) = 0.68 for the molecule with CpR = eta(5)-C(5)H(4)CH(2)CH(2)N(CH(3))C(O)(CH(2))(18)CH(3). The value of F(c2) decreased as the side-chain on the Cp ring got shorter; F(c2) is equal to 0.0 for the molecules with n = 3 and for CpR = eta(5)-C(5)H(4)CH(3). It is hypothesized that a longer side-chain prevents facile diffusion of the radicals out of the secondary cage, whereas the smaller side-chains permit more facile diffusion apart of the radicals. A general conclusion is that the reactions of large radicals in particular may be especially impacted by secondary geminate cage recombination.

Direct Detection of the Oxygen Rebound Intermediates, Ferryl Mb and NO2, in the Reaction of metMyoglobin with Peroxynitrite

“…Typically, the relative efficiency of in-cage ligand capture to cage escape (k BA /k BC ) are within an order of magnitude of each other 40,41. Similar results and conclusions have been reported to describe the behavior of small molecules in solution 55,56. Ligand recombination in deoxyMb (Fe II ) is relatively slow for CO, resulting in diffusive release to the medium after photolysis 57.…”

Dinuclear PhotoCORMs: Dioxygen-Assisted Carbon Monoxide Uncaging from Long-Wavelength-Absorbing Metal–Metal-Bonded Carbonyl Complexes

“…During irradiation at 659 nm, new bands characteristic of Re 2 (CO) 10 appeared at 1968, 2009, and 2070 cm –1 and remained unchanged upon further photolysis (see the SI, Figure S9b). Synchronously, two lower-frequency ν CO bands appeared at 1857 and 1934 cm –1 , which are close to bands previously observed in 2-methyltetrahydrofuran for a species identified in situ as 5 . The lower-frequency ν CO bands in the homonuclear dimer relative to 1 may be attributed to rhenium having a higher electronegativity than manganese, leading to polarization of the Re–Mn bond, which draws the electron density away from the manganese center in 1 .…”

“…The M−M′ bond is formed by radical coupling after halide loss from the latter intermediate. Consistent with this pathway was the isolation of M 2 (CO) 10 and M′ 2 (CO) 6 L 2 coproducts in addition to the desired heteronuclear complex M-(CO) 5 M′(CO) 3 L. The observation that heteronuclear product yields are often quite high was explained by Tyler and coworkers 50,51 in terms of the solvent structure inhibiting escape of the heteronuclear radical pair generated by electron transfer, thereby favoring formation of the M−M′ bond. In the present case, chromatography of the crude product indicated minor byproducts that may be the result of some homonuclear coupling.…”

“…Synchronously, two lower-frequency ν CO bands appeared at 1857 and 1934 cm −1 , which are close to bands previously observed in 2-methyltetrahydrofuran for a species identified in situ as 5. 51 The lower-frequency ν CO bands in the homonuclear dimer relative to 1 may be attributed to rhenium having a higher electronegativity than manganese, 55 leading to polarization of the Re−Mn bond, which draws the electron density away from the manganese center in 1. The latter two ν CO bands did eventually disappear upon standing, an observation attributed to the imperfect seal of the IR cell and slow leakage of air into the system.…”

“…After column chromatography, product yields generally exceeded 50%. The mechanism of M–M′ bond formation in such reactions has been the subject of some debate. − On the basis of earlier work, Morse and Wrighton proposed electron transfer leading to two metal radicals, one a 17 e – complex and the other a 19 e – complex. The M–M′ bond is formed by radical coupling after halide loss from the latter intermediate.…”