Comment on mr-2021-6

Abstract

In this work we derive conditions under which a level crossing line in magnetic field effect curve for a recombining radical pair will be equivalent to ESR spectrum, and discuss three simple rules for qualitative prediction of the level crossing spectra. Introduction 10Spin-correlated nature of radical (ion) pairs arising as intermediates in many natural or induced chemical transformations gives rise to a host of "magnetic and spin effects" in chemical reactions. It all started with observing (Bargon, 1967;Ward and Lawler, 1967) and understanding (Closs, 1969;Kaptein and Oosterhoff, 1969) strange-looking "polarized" NMR spectra, and has evolved into a mature field in itself with a wide range of powerful experimental and theoretical techniques relying on magnetically manipulating spins in chemical processes (Salikhov et al., 1984; Steiner and Ulrich, 1989; Hayashi, 15 2004), culminating in the modern high-tech finesse of advanced hyperpolarized NMR (Ivanov et al., 2014). This paper deals with a curious bridge between the most humble magnetic field effect curves (MFE), i.e., dependence of reaction yield on applied static magnetic field, and hyperpolarized NMR: additional sharp resonance-like lines that may occur against the smooth background of MFE due to genuine level crossings in the spin system of the radical pair. The lines were first discovered in zero magnetic field (Anisimov et al. 1983;Fischer, 1983) and attributed to 20 interference of pair states in the higher, spherical, symmetry conditions of zero external field similar to Hanle effect in atomic spectroscopy (Hanle, 1924). The zero field line, or Low Field Effect, was then put to the front as the possible physical mechanism of magnetoreception, and the research that followed was plenty. However, this completely overshadowed the other, spectroscopic, aspect of the level crossing lines possible in field other than zero.