Direct observation of f-pair magnetic field effects and time-dependence of radical pair composition using rapidly switched magnetic fields and time-resolved infrared methods

Abstract

A rapidly switched (<10 ns) magnetic field was employed to directly observe magnetic fields from f-pair reactions of radical pairs in homogeneous solution. Geminate radical pairs from the photoabstraction reaction of benzophenone from cyclohexanol were observed directly using a pump-probe pulsed magnetic field method to determine their existence time. No magnetic field effects from geminate pairs were observed at times greater than 100 ns after initial photoexcitation. By measuring magnetic field effects for fields applied continuously only after this initial geminate period, f-pair effects could be directly observed. Measurement of the time-dependence of the field effect for the photolysis of 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone in cyclohexanol using time-resolved infrared spectroscopy revealed not only the presence of f-pair magnetic field effects but also the ability of the time dependence of the MARY spectra to observe the changing composition of the randomly encountering pairs throughout the second order reaction period.

Magnetic Field Effects on Radical Pairs in Homogeneous Solution

“…52,53,95 The concept is similar to the TROA technique, but instead uses strong IR absorptions in a radical to monitor its concentration dependence. To date this technique has been employed to examine RPs containing benzoyl radicals using the carbon-oxygen double bond stretching frequency close to 1800 cm À1 .…”

“…[95][96][97][98] Modern MOSFET transistors have made the possibility of field switching on the timescale of RP dynamics a reality. This technique was first used in a method called SEMF-CIDNP (switched external magnetic field chemically induced dynamic nuclear polarization).…”

“…A magnetic field can be rapidly switched on or off at any point in time after RP formation (with rise times of less than 10 ns possible for fields of up to 40 mT). 95 By measuring the product yield as the timing of the field pulse is varied relative to laser excitation, the time dependence of RP concentration can be determined directly. This allows the problem of overlapping absorptions to be readily overcome and has been used in both the investigation of rapid dephasing processes in micellized RPs 57,98,99 and in the study of f-pair MFEs 95 (see later).…”

“…100 Recent experiments have employed multiple techniques to show unequivocally distinct MFEs from g-and f-pairs and also to investigate the MARY dependence of f-pairs. 95 …”

“…This has recently been demonstrated for the first time for the photolysis of 2-hydroxy-4 0 -(2-hydroxyethoxy)-2-methylpropiophenone in cyclohexanol. 95 The RP has large HFCs in one radical and negligible ones in the other giving rise to a substantial geminate LFE. As the reaction progresses, the size of the LFE decreases while the MFE increases.…”