Naresh D. Ghatlia

Exchange Interaction in Micellized Radical Pairs

The recombination probabilities of primary geminate triplet radical pairs, separation of 13C/12C isotopes due to the magnetic isotope effect in the recombination reactions, the magnetic field dependence of 13C chemically induced dynamic nuclear polarization in the recombination products of micellized radical pairs, magnetic field effects and stimulated nuclear polarization have been investigated in alkyl sulfate micelles of different sizes. The four characteristics that are relevant to the discussion of the behaviour of the radical pair are: (1) the micelle size, (2) the penetrability of the micelle boundary, (3) the distance dependence of the electron-electron exchange interaction and (4) the dependence of the microviscosity of the micellar core on the micellar size. Computer simulations unambiguously show that only an explicit consideration of the distance dependent electron spin exchange allows for a qualitative and quantitative reproduction of the experimental data. However, the geometric factors including the penetrability of the micelle boundary and the dependence of the viscosity of the micellar

Diastereoselective induction in radical coupling reactions: photolysis of 2,4-diphenylpentan-3-ones adsorbed on faujasite zeolites

Abstract The photochemistry of d, l and meso isomers of 2,4-diphenylpentan-3-one (DPP), adsorbed onto various cation-exchanged X and Y faujasites, was investigated. LiY and NaY show greater diastereoselectivity for the formation of the d,l isomer of diphenylbutane over the meso isomer than LiX and NaX. KY displays a significantly attenuated selectivity relative to LiY and NaY. The magnitude of the diastereoselectivity is a function of the substrate configuration and the difference between the magnitude of diastereoselection exhibited by LiY and NaY over LiX and NaX is greater when the substrate is d,l-DPP rather than meso-DPP. All the zeolites studied show substantially more diastereomerization relative to decarbonylation, as measured by the s parameter (vide infra), when compared with other supercage media such as micelles and porous glass. However, substrate configuration is irrelevant when comparing the s parameters. The different cation-exchanged Y zeolites all show similar s parameters, whereas the X zeolites display a non-monotonic behavior with respect to the s parameter and cation size. The stereoselectivities for NaX and NaY are independent of loading and these media display cage effects of 100% for d,l-DPP.

Magnetic probing of transition states

Abstract The concept of dynamic trajectories on the potential energy surface of magnetic and exchange interactions for cage reactions in a radical pair is formulated. The magnetic effects induced by static as well as by a combination of static and radiowave magnetic fields are considered as tools for magnetic probing of the exchange potential along the dissociating bond. Transition state radiospectroscopy as an analogue to the transition state femtosecond spectroscopy, developed by Zewail, is suggested.