Pulsed EPR characterization of encapsulated atomic hydrogen in octasilsesquioxane cages

Encapsulated Atomic Hydrogen in Octamethyl-POSS Cages: A Pulsed EPR Study

Encapsulated atomic hydrogen in polyhedral oligomeric silsesquioxane (POSS) cages is a promising candidate for spin-based quantum technologies. Key parameters such as spin relaxation times and magnetic interactions with surrounding electron and nuclear spins can be typically probed with advanced electron paramagnetic resonance (EPR) methods. Here we present a detailed pulsed EPR study of the species H@Si8O12R8 with R=CH3, namely encapsulated atomic hydrogen in the octamethyl POSS derivative. The temperature dependence of the spin-lattice relaxation rate 1/T 1 ${{T}_{1}}$ is analyzed in terms of a Raman process with a Debye temperature ofθ D = 145 ${{\theta }_{{\rm D}}=145}$ K and a thermally activated process withE a = 794 K ${{E}_{a}=794\hskip0.17em\hskip0.17em{\rm K}}$ (552 cm-1), whereas, the phase memory timeT M ${T_{\rm{M}} }$ shows the typical shortening behaviour atT < 150 ${T \char60 150}$ K observed for all methyl-containing derivatives. The hyperfine coupling of the cage29 ${^{29} }$ Si nuclei is measured by hyperfine sublevel correlation (HYSCORE) spectroscopy and is found to fulfil the so-called "matching condition" at the low-field EPR transition. The potential of this paramagnetic molecule to perform one-qubit quantum operations is probed by room-temperature Rabi oscillations.

Long Electron Spin Coherence Times of Atomic Hydrogen Trapped in Silsesquioxane Cages

Encapsulated atomic hydrogen in cube-shaped octasilsesquioxane (POSS) cages of the Si8O12R8 type (where R is an organic group) is one of the simplest alternative stable systems to paramagnetic endofullerenes that have been regarded as key elements of spin-based quantum technologies. Apart from common sources of decoherence such as nuclear spin and spectral diffusion, all H@POSS species studied so far suffer from additional shortening of T2 at low temperatures due to methyl group rotations. Here we eliminate this factor for the first time by studying the smallest methyl-free derivative with R = H, namely, H@T8H8. By applying dynamical decoupling methods, we measure electron spin coherence times T2 up to 280 ± 76 μs at T = 90 K and observe a linear dependence of the decoherence rate 1/T2 on trapped hydrogen concentrations, which we attribute to the spin dephasing mechanism of instantaneous diffusion and a nonuniform spatial distribution of encapsulated H atoms.

Pulsed Electron-Nuclear Double Resonance in the Fourier Regime

Nuclear magnetic resonance (NMR) spectroscopy provides atomic-level molecular structural information. However, in molecules containing unpaired electron spins, NMR signals are difficult to measure directly. In such cases, data is obtained using the electron-nuclear double resonance (ENDOR) method, where nuclei are detected through their interaction with nearby unpaired electron spins. Unfortunately, electron spins spread the ENDOR signals, which challenges current acquisition techniques, often resulting in low spectral resolution that provides limited structural details. Here, we show that by using miniature microwave resonators to detect a small number of electron spins, integrated with miniature NMR coils, one can excite and detect a wide bandwidth of ENDOR data in a single pulse. This facilitates the measurement of ENDOR spectra with narrow lines spread over a large frequency range at much better spectral resolution than conventional approaches, which helps reveal details of the paramagnetic molecules' chemical structure that were not accessible before.

A comparison of the chemical bonding and reactivity of Si8H8O12 and Ge8H8O12: A theoretical study

We have analyzed the chemical bonding and reactivity in the cubic molecule octahydridosilsesquioxane, Si₈H₈O₁₂, and its counterpart Ge₈H₈O₁₂ by means of ab initio quantum chemical methods and group theory. Density functional theory and MP2 methods combined with the basis sets 6-311+G(d) and 6-311++G(2d,p) were used for geometry optimization and vibrational frequency analysis. The geometries of Si₈H₈O₁₂ and Ge₈H₈O₁₂ are unstable under O_h symmetry and distort to the rare T_h molecular symmetry. The energy gained from this pseudo-Jahn-Teller distortion ranges from 0.78 to 6.14 kcal mol^-1 depending on methodological treatment. The Fukui functions and the molecular electrostatic potential were both used as DFT-based reactivity descriptors. Our study shows that Si₈H₈O₁₂ and Ge₈H₈O₁₂ are both hard amphoteric molecules. The cavity within each cage is acidic and able to encapsulate hard small bases such as F^-. The exterior of the cages is basic and can form stable exohedral complexes with hard acids, as in the case of H⁺. The insertion of F^- in Si₈H₈O₁₂ and Ge₈H₈O₁₂ cages gives the most stable endohedral complexes of the series studied, characterized by formation energies of -3.50 and -3.45 eV at CAM-B3LYP/6-311+G(d) and -3.61 and -3.68 eV at the MP2/6-311++G(d,p) level, respectively. The calculated formation energies of the exohedral and endohedral complexes align with the DFT reactivity descriptor analysis.

Electron spin relaxation properties of atomic hydrogen encapsulated in octavinyl POSS cages

The electron spin relaxation times of encapsulated atomic hydrogen in the vinyl derivative of silsesquioxane (R₈Si₈O₁₂) cages (R = CH[double bond, length as m-dash]CH₂) are studied in detail by pulse electron paramagnetic resonance (EPR) methods in the temperature range between 10 and 300 K. The temperature dependence of the spin-lattice relaxation time, T₁, shows similar behaviour with previously studied derivatives that typically involve Raman and thermally activated processes. The room-temperature phase-memory time T_M = 9 μs is comparable to those reported for different alkyl derivatives and exhibits a characteristic temperature dependence with a considerable reduction below 200 K as a result of dynamic effects like methyl group rotation. However, this reduction is modest for the vinyl derivative since the minimum observed T_M = 5 μs is much longer than the value of 1 μs reported for methyl-containing derivatives like R = C₂H₅, C₃H₇ (n-propyl), or OSi(CH₃)₂H. This discrepancy is attributed to the different rotation dynamics of the vinyl group, as evidenced by the determined activation energy and rotation frequency.

Modulation depth enhancement of ESEEM experiments using pulse trains

We present a new way to increase the modulation amplitude of electron spin echo envelope modulation (ESEEM) experiments that are based on electron spin coherence. The method uses a train of N refocusing π-pulses where each one of them redistributes the electron spin coherence among allowed and forbidden EPR transitions. This in turn leads to a significant enhancement of the ESEEM effect, depending on the strength of the hyperfine interaction and the number of applied pulses, N. We derive analytical expressions for a general two-dimensional (2D) scheme which is based on the refocused primary echo and we explore the expected modulation enhancement of various correlation peaks as a function of k (modulation depth parameter) and N. In addition, we inspect two different one-dimensional (1D) versions of the method, namely the Carr-Purcell-Meiboom-Gill (CPMG) sequence occurring for t1=t2, and an extension of the primary echo sequence occurring for t2=0. Our study shows that these methods are particularly useful for detecting weak hyperfine couplings of magnetic nuclei having small gn factors and low natural abundances like (13)C and (29)Si. The theoretically predicted features are confirmed by experiments in disordered spin systems.

Extending the electron spin coherence time of atomic hydrogen by dynamical decoupling

We study the electron spin decoherence of encapsulated atomic hydrogen in octasilsesquioxane cages induced by the (1)H and (29)Si nuclear spin bath. By applying the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence we significantly suppress the low-frequency noise due to nuclear spin flip-flops up to the point where a maximum T2 = 56 μs is observed. Moreover, dynamical decoupling with the CPMG sequence reveals the existence of two other sources of decoherence: first, a classical magnetic field noise imposed by the (1)H nuclear spins of the cage organic substituents, which can be described by a virtual fluctuating magnetic field with the proton Larmor frequency, and second, decoherence due to anisotropic hyperfine coupling between the electron and the inner (29)Si spins of the cage.