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1. ³¹ P spin–lattice and singlet order relaxation mechanisms in pyrophosphate studied by isotopic substitution, field shuttling NMR, and molecular dynamics simulation

   https://doi.org/10.1039/D2CP03801C  

   Korenchan, David E.; Lu, Jiaqi; Sabba, Mohamed; Dagys, Laurynas; Brown, Lynda J.; Levitt, Malcolm H.; Jerschow, Alexej (October 2022, Physical Chemistry Chemical Physics)

   Nuclear spin relaxation mechanisms are often difficult to isolate and identify, especially in molecules with internal flexibility. Here we combine experimental work with computation in order to determine the major mechanisms responsible for 31 P spin–lattice and singlet order (SO) relaxation in pyrophosphate, a physiologically relevant molecule. Using field-shuttling relaxation measurements (from 2 μT to 9.4 T) and rates calculated from molecular dynamics (MD) trajectories, we identified chemical shift anisotropy (CSA) and spin–rotation as the major mechanisms, with minor contributions from intra- and intermolecular coupling. The significant spin–rotation interaction is a consequence of the relatively rapid rotation of the –PO 3 2− entities around the bridging P–O bonds, and is treated by a combination of MD simulations and quantum chemistry calculations. Spin–lattice relaxation was predicted well without adjustable parameters, and for SO relaxation one parameter was extracted from the comparison between experiment and computation (a correlation coefficient between the rotational motion of the groups). 

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2. Singlet excitation in the intermediate magnetic equivalence regime and field-dependent study of singlet–triplet leakage

   https://doi.org/10.1039/C8CP06883F  

   Kharkov, Boris; Duan, Xueyou; Tovar, Emily S.; Canary, James W.; Jerschow, Alexej (January 2019, Physical Chemistry Chemical Physics)

   The examination and optimized preparation of nuclear spin singlet order has enabled the development of new types of applications that rely on potentially long-term polarization storage. Lifetimes several orders of magnitude longer than T 1 have been observed. The efficient creation of such states relies on special pulse sequences. The extreme cases of very large and very small magnetic equivalence received primary attention, while relatively little effort has been directed towards studying singlet relaxation in the intermediate regime. The intermediate case is of interest as it is relevant for many spin systems, and would also apply to heteronuclear systems in very low magnetic fields. Experimental evidence for singlet–triplet leakage in the intermediate regime is sparse. Here we describe a pulse sequence for efficiently creating singlets in the intermediate regime in a broad-band fashion. Singlet lifetimes are studied with a specially synthesized molecule over a wide range of magnetic fields using a home-built sample-lift apparatus. The experimental results are supplemented with spin simulations using parameters obtained from ab initio calculations. This work indicates that the chemical shift anisotropy (CSA) mechanism is relatively weak compared to singlet–triplet leakage for the proton system observed over a large magnetic field range. These experiments provide a mechanism for expanding the scope of singlet NMR to a larger class of molecules, and provide new insights into singlet lifetime limiting factors. 

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3. Entangled spin-polarized excitons from singlet fission in a rigid dimer

   https://doi.org/10.1038/s41467-023-36529-6  

   Dill, Ryan_D; Smyser, Kori_E; Rugg, Brandon_K; Damrauer, Niels_H; Eaves, Joel_D (March 2023, Nature Communications)

   Abstract Singlet fission, a process that splits a singlet exciton into a biexciton, has promise in quantum information. We report time-resolved electron paramagnetic resonance measurements on a conformationally well-defined acene dimer molecule, TIPS-BP1', designed to exhibit strongly state-selective relaxation to specific magnetic spin sublevels. The resulting optically pumped spin polarization is a nearly pure initial state from the ensemble. The long-lived spin coherences modulate the signal intrinsically, allowing a measurement scheme that substantially removes noise and uncertainty in the magnetic resonance spectra. A nonadiabatic transition theory with a minimal number of spectroscopic parameters allows the quantitative assignment and interpretation of the spectra. In this work, we show that the rigid dimer TIPS-BP1' supports persistent spin coherences at temperatures far higher than those used in conventional superconducting quantum hardware. 

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4. Origin of Magnetic Anisotropy in Nickelocene Molecular Magnet and Resilience of Its Magnetic Behavior

   Alessio, Maristella; Kotaru, Saikiran; Giudetti, Goran; Krylov, Anna I (February 2023, Journal of physical chemistry C)

   The robustness of nickelocene’s (NiCp2, Cp = cyclopentadienyl) magnetic anisotropy and addressability of its spin states make this molecular magnet attractive as a spin sensor. However, microscopic understanding of its magnetic anisotropy is still lacking, especially when NiCp2 is deposited on a surface to make quantum sensing devices. Quantum chemical calculations of such molecule/solid-state systems are limited to density functional theory (DFT) or DFT+U (Hubbard correction to DFT). We investigate the magnetic behavior of NiCp2 using the spin-flip variant of the equation-of-motion coupled-cluster (EOM-SF-CC) method and use the EOM-SF-CC results to benchmark SF-TD-DFT. Our first-principle calculations agree well with experimentally derived magnetic anisotropy and susceptibility values. The calculations show that magnetic anisotropy in NiCp2 originates from a large spin–orbit coupling (SOC) between the triplet ground state and the third singlet state, whereas the coupling with lower singlet excited states is negligible. We also considered a set of six ring-substituted NiCp2 derivatives and a model system of the NiCp2/MgO(001) adsorption complex, for which we used SF-TD-DFT method. To gain insight into the electronic structure of these systems, we analyze spinless transition density matrices and their natural transition orbitals (NTOs). The NTO analysis of SOCs explains how spin states and magnetic properties are retained upon modification of the NiCp2 coordination environment and upon its adsorption on a surface. Such resilience of the NiCp2 magnetic behavior supports using NiCp2 as a spin-probe molecule by functionalization of the tip of a scanning tunneling microscope. 

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5. J -Driven dynamic nuclear polarization for sensitizing high field solution state NMR

   https://doi.org/10.1039/d1cp04186j  

   Concilio, Maria Grazia; Kuprov, Ilya; Frydman, Lucio (January 2022, Physical Chemistry Chemical Physics)

   Dynamic nuclear polarization (DNP) is widely used to enhance solid state nuclear magnetic resonance (NMR) sensitivity. Its efficiency as a generic signal-enhancing approach for liquid state NMR, however, decays rapidly with magnetic field B 0 , unless mediated by scalar interactions arising only in exceptional cases. This has prevented a more widespread use of DNP in structural and dynamical solution NMR analyses. This study introduces a potential solution to this problem, relying on biradicals with exchange couplings J ex of the order of the electron Larmor frequency ω E . Numerical and analytical calculations show that in such J ex ≈ ± ω E cases a phenomenon akin to that occurring in chemically induced DNP (CIDNP) happens, leading to different relaxation rates for the biradical singlet and triplet states which are hyperfine-coupled to the nuclear α or β states. Microwave irradiation can then generate a transient nuclear polarization build-up with high efficiency, at all magnetic fields that are relevant in contemporary NMR, and for all rotational diffusion correlation times that occur in small- and medium-sized molecules in conventional solvents. 

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