Unexpected NMR shieldings of sp- and sp²-hybridized carbon atoms in graphyne systems

Abstract Graphynes (GYs) are two-dimensional alloptropic forms of carbon consisting of periodically arranged sp- and sp2-hybridized carbon atoms in a planar structure. Graphynes can be formally created from graphene by inserting sp-hybridized carbon links into selected points of the graphene lattice. Depending on where the links are introduced, several forms of graphynes have been proposed with properties that make them potential candidates for new generation electronics or for applications in chemical processes. Since the applications of each form of GY depend on its structure, it is of interest to experimentally distinguish different forms of graphynes. In this paper we propose nuclear magnetic resonance (NMR) as a potential method of choice for such distinction. We computationally investigate on the DFT level the ¹³C-NMR chemical shifts for α-, β-, γ-, rhombic, and 6,6,12-graphynes, and α- and γ-graphdiynes. We perform the calculations both in periodic systems and with approximate finite models. The results show that NMR chemical shifts in graphynes are dependent on the structure and reflect the local bonding around the carbon nucleus. Interestingly, NMR shifts of several graphynes show anomalous values, differing significantly from shifts found in typical sp²-hybridized systems. We analyze these results in terms of local structural parameters and qualitatively investigate the possible origins of these anomalous NMR shifts. The results show that NMR is a viable method for determining the structure of graphynes and their finite precursor molecules

Orienting spins in dually doped monolayer MoS₂:from one-sided to double-sided doping

Abstract Electron spins of the doped monolayer MoS₂ were aligned by placing two magnetic impurities at sulfur vacancies, both on the same side and different sides of the slab. Origins of the calculated magnetisms are beyond most conventional physical models, yet interactions of single-molecule magnets are tentatively proposed

Ratcheting rotation or speedy spinning: EPR and dynamics of Sc₃C₂@C₈₀

Abstract Besides their technological applications, endohedral fullerenes provide ideal conditions for investigating molecular dynamics in restricted geometries. A representative of this class of systems, Sc₃C₂@C₈₀ displays complex intramolecular dynamics. The motion of the ⁴⁵Sc trimer has a remarkable effect on its electron paramagnetic resonance (EPR) spectrum, which changes from a symmetric 22-peak pattern at high temperature to a single broad lineshape at low temperature. The scandium trimer consists of two equivalent and one inequivalent metal atom, due to the carbon dimer rocking through the Sc₃ triangle. We demonstrate through first-principles molecular dynamics (MD), EPR parameter tensor averaging, and spectral modelling that, at high temperatures, three-dimensional movement of the enclosed Sc₃C₂ moiety takes place, which renders the metal centers equivalent and their magnetic parameters effectively isotropic. In contrast, at low temperatures the dynamics becomes restricted to two dimensions within the equatorial belt of the Ih symmetric C₈₀ host fullerene. This restores the inequivalence of the scandium centers and causes their anisotropic hyperfine couplings to broaden the experimental spectrum

Direct magnetic-field dependence of NMR chemical shift

Abstract Nuclear shielding and chemical shift are considered independent of the magnetic-field strength. Ramsey proposed on theoretical grounds in 1970 that this may not be valid for heavy nuclei. Here we present experimental evidence for the direct field dependence of shielding, using 59Co shielding in Co(acac)3 [tris(acetylacetonate)cobalt(III)] as an example. We carry out NMR experiments in four field strengths for this low-spin diamagnetic Co(III) complex, which features a very large and negative nuclear shielding constant of the central Co nucleus. This is due to a magnetically accessible, low-energy eg ← t2g orbital excitation of the d6 system. The experiments result in temperature-dependent magnetic-field dependence of −5.7 to −5.2 ppb T−2 of the 59Co shielding constant, arising from the direct modification of the electron cloud of the complex by the field. First-principles multiconfigurational non-linear response theory calculations verify the sign and order of magnitude of the experimental results

Electron and nuclear spin polarization in Rb-Xe spin-exchange optical hyperpolarization

Abstract Spin-exchange optical hyperpolarization of 129Xe gas enhances the signal-to-noise ratio in nuclear magnetic resonance experiments. The governing parameter of the Rb-Xe spin-exchange process, the so-called enhancement factor, was recently reevaluated experimentally. However, the underlying hyperfine coupling and atomic interaction potential as functions of the internuclear distance of the open-shell Rb-Xe dimer have not been accurately determined to date. We present a piecewise approximation based on first-principles calculations of these parameters contributing to the NMR and EPR frequency shifts in the low-density Rb-Xe gas mixture of relevance to hyperpolarization experiments. Both Rb electron and 129Xe nuclear spin polarizations are estimated based on a combination of electronic-structure calculations, observed frequency shifts, and an estimate of the Rb number density. Finally, an expression for the enhancement factor in terms of modern electronic-structure theory is obtained

Bi(III) complexes containing dithiocarbamate ligands:synthesis, structure elucidation by X‐ray diffraction, solid‐state ¹³C/¹⁵N NMR, and DFT calculations

Abstract We report on syntheses, characterisation by nuclear magnetic resonance (NMR) spectroscopy, X‐ray diffraction (XRD) measurements, and density functional theory (DFT) calculations of electronic/molecular structure and NMR chemical shifts of complexes of Bi(III), having the molecular formulae: [Bi{S₂CN(C₂H₅)₂)}₃] (1), [Bi{S₂CN(C₂H₅)₂)}₂(C₁₂H₈N₂)NO₃)] (2), and [Bi₂{S₂CN(CH₂)₅}₆ • H₂O] (3). The powder XRD patterns of complexes (1) and (2) resembled the corresponding calculated powder XRD patterns for previously reported single crystal structures. Single crystal XRD structure of complex (3), reported in this work, adopted an orthorhombic system with a space group Pbca with a=10.9956(3) Å, b=27.7733(8) Å, c=35.1229(10) Å and α=β=γ=90°. The experimental solid‐state ¹³C/¹⁵N NMR data of the complexes (1)‐(3) were in accord with their X‐ray single crystal structures. The unit cell of the complex (3) shows a weak supramolecular Bi···S interaction leading to the formation of a non‐centrosymmetric binuclear molecule [Bi₂{S₂CN(CH₂)₅}₆ • H₂O], which displays structural inequivalence in both ¹³C/¹⁵N NMR, and XRD data. Assignments of resonance lines in solid‐state ¹³C/¹⁵N NMR spectra of complexes (1)‐(3) were assisted by chemical shift calculations using periodic DFT methods. The findings of the present multidisciplinary approach will contribute in designing molecular models and further understanding of the structures and properties of (diamagnetic) metal complexes, including heavy metal ones

Local structures of rare earth phosphate minerals by NMR

Abstract 31P solid state NMR studies combined with DFT calculations were conducted over a chosen series of rare earth element phosphates (REEPO4s), selected on the basis of the size and magnetic properties of REEs (La, Sm, Lu and Yb). PXRD analysis revealed the presence of rhabdophane (La, Sm), monazite (La) and xenotime (Lu, Yb) phases of these phosphate compounds. The direct excitation and cross-polarization 31P NMR studies together with calculations confirmed the PXRD results for the abovementioned bulk structures, but also revealed presence of several local phosphorus environments on surfaces. NMR is sensitive to the atomic level local interactions, and we were able to show that the combination of experimental and theoretical NMR methods can provide information unavailable with other methods. Due to the distinct coordination of the water molecules to crystal surfaces with different Miller plane cleavages, we were able to identify from the NMR spectra the surface structures of the studied minerals. This adds to the knowledge of the bulk structures of REE phosphates and provides preliminary data for studies on coordination of various ligands on REE phosphate surfaces. This combination of experimental and computational methods can further be used for studies on surface chemistry, important for applications in catalysis and extraction of REEs from the minerals

NMR relaxation and modelling study of the dynamics of SF6 and Xe in porous organic cages

Abstract The porous solid formed from organic CC3 cage molecules has exceptional performance for rare gas separation. NMR spectroscopy provides a way to reveal the dynamical details by using experimental relaxation and diffusion measurements. Here, we investigated T₁ and T₂ relaxation as well as diffusion of ¹²⁹Xe and SF₆ gases in the CC3-R molecular crystal at various temperatures and magnetic field strengths. Advanced relaxation modelling made it possible to extract various important dynamical parameters for gases in CC3-R, such as exchange rates, activation energies and mobility rates of xenon, occupancies of the cavities, rotational correlational times, effective relaxation rates, and diffusion coefficients of SF₆

Clathrate structure determination by combining crystal structure prediction with computational and experimental ¹²⁹Xe NMR spectroscopy

Abstract We present an approach for the structure determination of clathrates using NMR spectroscopy of enclathrated xenon to select froma set of predicted crystal structures. Crystal structure prediction methods have been used to generate an ensemble of putative structures of o- and m-fluorophenol, whose previously unknown clathrate structures have been studied by ¹²⁹Xe NMR spectroscopy. The high sensitivity of the ¹²⁹Xe chemical shift tensor to the chemical environment and shape of the crystalline cavity makes it ideal as a probe for porous materials. The experimental powder NMR spectra can be used to directly confirm or reject hypothetical crystal structures generated by computational prediction, whose chemical shift tensors have been simulated using density functional theory. For each fluorophenol isomer we find one predicted crystal structure whose measured and computed chemical shift tensors agree within experimental and computational error margins and these are thus proposed as the true fluorophenol xenon clathrate structures

Structural insights into the polymorphism of bismuth(III) di-𝑛-butyldithiocarbamate by X-ray diffraction, solid-state (¹³C/¹⁵N) CP-MAS NMR and DFT calculations

Abstract Two crystalline polymorphs of a binuclear tris(di-𝑛-butyldithiocarbamato)bismuth(III) complex, I and II, with an empirical formula of [Bi{S₂CN(𝑛-C₄H₉)₂}₃] were synthesised and characterised by X-ray diffraction (XRD), solid-state NMR and density functional theory (DFT) calculations. At the supramolecular level, these mononuclear molecular units interact in pairs via secondary Bi⋯S bonds, yielding binuclear formations of [Bi₂{S₂CN(𝑛-C₄H₉)₂}₆]. The polymorph I (𝑃1̅) contains two isomeric non-centrosymmetric binuclear molecules of [Bi₂{S₂CN(𝑛-C₄H₉)₂}₆], which are related to each other as conformers, therefore having four structurally inequivalent bismuth atoms and twelve inequivalent dithiocarbamate ligands. In contrast, the structurally simpler polymorph II (𝑃2₁/𝑛) exists as a single molecular form of the corresponding centrosymmetric binuclear formation, comprising two structurally equivalent bismuth atoms and three structurally different dithiocarbamate groups. The polymorphs I and II were found to be interconvertible by altering the solvent system during the recrystallisation process. Sun et al. (2012) has reported a crystalline form of the title compound which resembles, but is not identical with, polymorph II. Experimental solid-state ¹³C and ¹⁵N cross-polarisation (CP) magic-angle-spinning (MAS) NMR spectra of both polymorphs I and II were in accord with the direct structural data on these complexes. Assignments of the resonance lines in the solid-state ¹³C and ¹⁵N NMR spectra were assisted by chemical shift calculations of the crystals using periodic DFT