High resolution studies of the effects of magnetic fields on chemical reactions

Abstract

A simple and inexpensive experiment is described which detects magnetic field effects on chemical reactions with high signal-to-noise ratio and high resolution. It consists in applying a small modulation field to the sample, whilst the main field it experiences is varied, with optical detection at the modulation frequency. It consequently measures the derivative of the normal MARY spectrum. It is shown by theoretical analysis that when using this method it is better to monitor reaction intermediates than products. The method is demonstrated by application to known systems in which additional features are sometimes observed, in particular a low-field feature which is shown to vary as electron hopping occurs. MARY spectra obtained using modulated light excitation of the sample and pulsed excitation with laser-induced fluorescence detection are provided for comparison. The method represents a general technique for studying field effects in systems containing low stationary state concentrations of radicals, produced by any method.

Invited article

“…The rate of change of fluorescence intensity I from a bioluminescent reaction with the strength B of an applied magnetic field, showing the reaction yield to be influenced by small magnetic fields. This trace was obtained using a field-modulation technique, developed to have high sensitivity [26], and shows that the effect reaches a plateau value by the time the field reaches about 12mT, when the derivative no longer changes as the field is increased. The derivative trace exhibits inversion symmetry about the zero-field position as the field is reversed, and the maxima occur at the B~l 2 values.…”

Magnetic field modulation of the delayed fluorescenceyield in the photoionization reaction of N, N, N′, N′‐tetramethyl‐p‐phenylenediamine in water

“…In contrast to the previous reports [5,7], the MFE on the delayed fluorescence [8] can only be explained if the singlet RIP is more reactive than the triplet one. In the present study we report on the MFE on the delayed fluorescence from TMPPD in water and water/DMSO mixture using modulation technique [9][10]. This technique enables the characterization of the RIP observed and allows the registration of its lifetime.…”

“…In some cases however the RIP's lifetime is long enough for effective spin mixings but the detection methods are too slow to register or/and insensitive to very small variations accompanying these spin mixings. Introduction of the modulation technique to the methods of detection of MFEs introduced much better signal-to-noise ratio and higher time resolution to the detection of radical pairs in solution [9,10,12]. MARY (MAgnetic field modulation of the Reaction Yield) spectra, or more precisely the MFEs on the recombination fluorescence yield, exhibit relatively narrow lines.…”

“…MFE on fluorescence yield was monitored by a fieldmodulation setup similar to that built by McLauchlan and Steiner [9]. Light from a highly stabilized 150 watt high pressure xenon arc lamp is collimated and passed through an appropriate collection of solution filters, UV band pass filters, diaphragms, and a Lumatec Series 300 liquid light guide (300 nm< λ trans < 650 nm) to control excitation wavelength.…”

Typical applications of MARY spectroscopy: Radical ions of substituted benzenes

“…Other possibilities that have been realized elsewhere include photoionization in nonpolar solutions [5] and photoinduced electron transfer in moderately polar solutions [6,7]. The flux of X-ray quanta hitting the solution leads to ionization of the more abundant solvent molecule, knocking the electron as much as about I0 nm from its parent motecule and producing the primary radical ion pair of the solvent radical cation and the thermalized electron.…”