The Radical Pair Mechanism is a canonical model for the magnetosensitivity of chemical reaction processes. The key ingredient of this model is the hyperfine interaction that induces a coherent mixing of singlet and triplet electron spin states in pairs of radicals, thereby facilitating magnetic field effects (MFEs) on reaction yields through spin-selective reaction channels. We show that the hyperfine interaction is not a categorical requirement to realize the sensitivity of radical reactions to weak magnetic fields. We propose that, in systems comprising three instead of two radicals, dipolar interactions provide an alternative pathway for MFEs. By considering the role of symmetries and energy level crossings, we present a model that demonstrates a directional sensitivity to fields weaker than the geomagnetic field and remarkable spikes in the reaction yield as a function of the magnetic field intensity; these effects can moreover be tuned by the exchange interaction. Our results further the current understanding of the effects of weak magnetic fields on chemical reactions, could pave the way to a clearer understanding of the mysteries of magnetoreception and other biological MFEs and motivate the design of quantum sensors. Further still, this phenomenon will affect spin systems used in quantum information processing in the solid state and may also be applicable to spintronics.There is growing excitement about the possibility of quantum coherence and entanglement underpinning the optimal functioning of biological processes [1]. A notable example is the avian inclination compass [2][3][4][5][6][7][8][9][10][11][12], which has recently been realised as a truly quantum-biological process [3]. The leading explanation of this phenomenon utilizes the Radical Pair Mechanism (RPM), which describes the unitary evolution of singlet-triplet (S-T) coherences in systems comprising two radicals, i.e. two electron spins [2,13,14]. The RPM has also been suggested to underpin controversial health-related implications of exposure to weak electromagnetic fields [15][16][17][18]. For these phenomena, the so-called low-field effect (LFE) is crucial to foster sensitivity to magnetic fields of intensity comparable to the geomagnetic field (≈ 50 µT) [6,[19][20][21][22][23][24]. The electron-electron dipolar interaction is often neglected when addressing MFEs within the RPM framework, but preliminary explorations have been conducted: electron-electron dipolar coupling is expected to resemble the exchange coupling, which, as the dominant interaction, suppresses S-T conversion by lifting the neardegeneracy of triplet and singlet states, reducing their susceptibility to mixing by weak hyperfine interactions [25], and quenching the LFE [26]. Efimova et al. proposed that the dipolar interaction could be partly compensated by the exchange interaction, thereby allowing high sensitivity to the geomagnetic field despite sizeable electron-electron dipolar coupling interactions [27].In contrast to the two-spin systems of the classical RPM, spin tr...

The electron–electron dipolar coupling of more than two radicals provides a new mechanism for the magnetosensitivity of lipid autoxidation.

We adapt the Monte-Carlo wavefunction (MCWF) approach to treat the open-system spin dynamics of radical pairs subject to spin-selective recombination reactions. For these systems, non-Lindbladian master equations are widely employed, which account for recombination via the non trace-preserving Haberkorn superoperator in combination with reaction-dependent exchange and singlet–triplet dephasing terms. We show that this type of master equation can be accommodated in the MCWF approach, by introducing a second type of quantum jump that accounts for the reaction simply by suitably terminating the propagation. In this way, we are able to evaluate approximate solutions to the time-dependent radical pair survival probability for systems that have been considered untreatable with the master equation approach until now. We explicate the suggested approach with calculations for radical pair reactions that have been suggested to be relevant for the quantum compass of birds and related phenomena.

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