Magnetic excitation and readout of methyl group tunnel coherence

Abstract

Methyl groups are ubiquitous in synthetic materials and biomolecules. At sufficiently low temperature, they behave as quantum rotors and populate only the rotational ground state. In a symmetric potential, the three localized substates are degenerate and become mixed by the tunnel overlap to delocalized states separated by the tunnel splitting νt. Although νt can be inferred by several techniques, coherent superposition of the tunnel-split states and direct measurement of νt have proven elusive. Here, we show that a nearby electron spin provides a handle on the tunnel transition, allowing for its excitation and readout. Unlike existing dynamical nuclear polarization techniques, our experiment transfers polarization from the electron spin to methyl proton spins with an efficiency that is independent of the magnetic field and does not rely on an unusually large tunnel splitting. Our results also demonstrate control of quantum states despite the lack of an associated transition dipole moment.

Electron paramagnetic resonance study of ferroelectric phase transition and dynamic effects in a Mn2+doped [NH4][Zn(HCOO)3] hybrid formate framework

“…At about 90 K, this process seems to slow down or change character causing much faster increase of Tm below this temperature. Note that a similar kink in the temperature behavior of Tm was also observed in the related Mn 2+ doped [(CH) 3 NH 2 ][Zn(HCOO) 3 ] perovskite framework 28 , where it is likely caused by the slowing down of the stochastic methyl group reorientation, which is followed by the tunneling dynamics 59 . The absence of methyl groups in AmZn requires a different relaxation mechanism.…”

Regularized dynamical decoupling noise spectroscopy – a decoherence descriptor for radicals in glassy matrices

“…Recently, the excitation and indirect detection of tunnel coherence has been demonstrated by three-pulse electron spin echo envelope modulation (ESEEM) experiments for two methyl groups of a dimethylammonium cation within a Mn(II)-doped perovskite framework. 28 In this case, the determined tunnel frequencies n t match the methyl proton HF couplings in size, 28 whereas methyl groups bound to a sp 3 carbon atom typically exhibit smaller n t of 100-300 kHz. 29 Density functional theory (DFT) calculations of the rotational barrier for one methyl group of the here investigated nitroxide moiety result in a tunnel frequency in this range, as the computed E rot = 13.7 kJ mol À1 converts into n t = 207 kHz via the hindered quantum rotor model.…”

“…DFT calculations involving simultaneous rotation of two methyl groups bound to the same carbon atom allow estimation of rotational coupling effects. Based on the free quantum rotor model 30 implemented in MATLAB as previously described, 28 we convert E rot into a tunnel frequency ν t .…”

“…29 Density functional theory (DFT) calculations of the rotational barrier for one methyl group of the here investigated nitroxide moiety result in a tunnel frequency in this range, as the computed E rot = 13.7 kJ mol À1 converts into n t = 207 kHz via the hindered quantum rotor model. 28,30 Compared to the HF couplings with the methyl protons, this tunnel frequency is small and only weakly perturbs the HF interactions. 28 We rely on density operator formalism to simulate the quantum rotor dynamics under the Hahn (also called two-pulse ESEEM) experiment in order to understand whether the tunnel effect explains the observed echo envelope on the T m 1 time scale.…”

“…28,30 Compared to the HF couplings with the methyl protons, this tunnel frequency is small and only weakly perturbs the HF interactions. 28 We rely on density operator formalism to simulate the quantum rotor dynamics under the Hahn (also called two-pulse ESEEM) experiment in order to understand whether the tunnel effect explains the observed echo envelope on the T m 1 time scale. In absence of rotational tunneling, the secular and pseudo-secular HF couplings to methyl protons result in an unmodulated trace at B 0 = 1.224 T, whereas n t = 225 kHz induces a signal with a sizable modulation depth of 0.25 and a period in the microsecond range (Fig.…”