Solvent-Dependent Cage Dynamics of Small Nonpolar Radicals: Lessons from the Photodissociation and Geminate Recombination of Alkylcobalamins

Abstract

Time-resolved transient absorption spectroscopy was used to investigate the primary geminate recombination and cage escape of alkyl radicals in solution over a temperature range from 0 to 80 degrees C. Radical pairs were produced by photoexcitation of methyl, ethyl, propyl, hexylnitrile, and adenosylcobalamin in water, ethylene glycol, mixtures of water and ethylene glycol, and sucrose solutions. In contrast to previous studies of cage escape and geminate recombination, these experiments demonstrate that cage escape for these radical pairs occurs on time scales ranging from a hundred picoseconds to over a nanosecond as a function of solvent fluidity and radical size. Ultrafast cage escape (<100 ps) is only observed for the methyl radical where the radical pair is produced through excitation to a directly dissociative electronic state. The data are interpreted using a unimolecular model with competition between geminate recombination and cage escape. The escape rate constant, k(e), is not a simple function of the solvent fluidity (T/eta) but depends on the nature of the solvent as well. The slope of k(e) as a function of T/eta for the adenosyl radical in water is in near quantitative agreement with the slope calculated using a hydrodynamic model and the Stokes-Einstein equation for the diffusion coefficients. The solvent dependence is reproduced when diffusion constants are calculated taking into account the relative volume and mass of both solvent and solute using the expression proposed by Akgerman (Akgerman, A.; Gainer, J. L. Ind. Eng. Chem. Fundam. 1972, 11, 373-379). Rate constants for cage escape of the other radicals investigated are consistently smaller than the calculated values suggesting a systematic correction for radical size or coupled radical pair motion.

Hydrodynamic and Nonhydrodynamic Contributions to the Bimolecular Collision Rates of Solute Molecules in Supercooled Bulk Water

“…Caged radical pairs are produced by photodissociation from their parent molecule in a cage. Accounting for the differences between pDTO and the alkyl radicals, the values of τ RE measured in this work are several times smaller than the values of the cage escape time obtained in ref (46). In the same way, we assume that the re-encounters and rotation of pDTO very likely occur in molecular cages made of a dynamic network of tetrahedrally coordinated water molecules.…”

Beyond ferryl-mediated hydroxylation: 40 years of the rebound mechanism and C–H activation

“…A compelling example is the homolytic cleavage of carbon-cobalt bond in 5′-deoxyadenosylcobalamin (coenzyme B 12 ) to form cobalt(II)-cobalamin and an adenosyl radical, which is important for the biological functions of coenzyme B 12 -dependent mutase enzymes. Time-resolved spectroscopic studies showed that the initial radical pair formed after Co–C bond homolysis in adenosylcobalamin undergoes in-cage radical recombination and cage escape both at approximately 10 9  s −1 , clearly indicating a competition between the two processes [124–126]. …”

Excited electronic states and internal conversion in cyanocobalamin

“…Ultrafast broadband transient absorption spectroscopy has been used to study the photochemistry and photophysics of naturally occurring and synthetically prepared cobalamin compounds [9,[15][16][17][18][19][20][21][22][23][24][25][26][27]. Alkylcobalamins are photolabile.…”

“…Alkylcobalamins are photolabile. For most alkylcobalamins that have been studied to date, the primary photolysis quantum yield is on the order of unity [15,21,22,24]. The quantum yield for long-lived radicals is determined by competition between diffusive separation of the radical pair and geminate recombination [15].…”

“…For most alkylcobalamins that have been studied to date, the primary photolysis quantum yield is on the order of unity [15,21,22,24]. The quantum yield for long-lived radicals is determined by competition between diffusive separation of the radical pair and geminate recombination [15]. Methylcobalamin is an exception with a pronounced wavelength dependence to the primary photolysis quantum yield [19,23,25].…”