DFT calculations of quadrupolar solid-state NMR properties: Some examples in solid-state inorganic chemistry

Revealing impacts of electrolyte speciation on ionic charge storage in aluminum-quinone batteries by NMR spectroscopy

Rechargeable aluminum-organic batteries are composed of earth-abundant, sustainable electrode materials while the molecular structures of the organic molecules can be controlled to tune their electrochemical properties. Aluminum metal batteries typically use electrolytes based on chloroaluminate ionic liquids or deep eutectic solvents that are comprised of polyatomic aluminum-containing species. Quinone-based organic electrodes store charge when chloroaluminous cations (AlCl₂⁺) charge compensate their electrochemically reduced carbonyl groups, even when such cations are not natively present in the electrolyte. However, how ion speciation in the electrolyte affects the ion charge storage mechanism, and resultant battery performance, is not well understood. Here, we couple solid-state NMR spectroscopy with electrochemical and computational methods to show for the first time that electrolyte-dependent ion speciation significantly alters the molecular-level environments of the charge-compensating cations, which in turn influences battery properties. Using 1,5-dichloroanthraquinone (DCQ) for the first time as an organic electrode material, we utilize solid-state dipolar-mediated and multiple-quantum NMR experiments to elucidate distinct aluminum coordination environments upon discharge that depend significantly on electrolyte speciation. We relate DFT-calculated NMR parameters to experimentally determined quantities, revealing insights into their origins. The results establish that electrolyte ion speciation impacts the local environments of charge-compensating chloroaluminous cations and is a crucial design parameter for rechargeable aluminum-quinone batteries.

GIAO versus GIPAW: Comparison of Methods To Calculate 11B NMR Shifts of Icosahedral Closo-Heteroboranes toward Boron-Rich Borides

In this work, we perform first-principle density functional theory calculations with the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional to compare the results of the gauge-including atomic orbital (GIAO) method with the gauge-including projector-augmented wave (GIPAW) approach for isotropic ¹¹B nuclear magnetic resonance shifts. GIPAW had been used successfully for the theoretical calculation of nuclear magnetic parameters of ¹¹B species in strong ionic solid-phase compounds such as borates but had been applied very rarely to structures where boron is mainly involved in complex covalent bonding situations, for example, in icosahedra of boron-rich borides. Thus, we investigate the accuracy of both well-known methods and reliability of the effective treatment of core electrons on a test set containing 16 experimentally known closo-(hetero)dodecaboranes. In general, we find very good agreement between GIAO and GIPAW when compared to experimental observations. However, accidental degeneracies of the shift values are better predicted by GIPAW. The optimized molecular geometries on the PBE level agree well with gaseous electron diffraction data and lead to theoretical isotropic chemical ¹¹B shifts with root-mean-square errors of 2.1 and 1.0 ppm depending on the used model of converting absolute shieldings to chemical shifts. The comparison with results from hybrid functionals (B3LYP, B3LYP-D2, and PBE0) shows a minor improvement in accuracy, which is in agreement with ¹³C shifts of sp³-hybridized species. In order to prove the reliability of the conversion parameters obtained by PBE, we report the calculated ¹¹B shifts of 1,2-, 1,7-, and 1,12-PCB₁₀H₁₁ with GIAO and GIPAW to our knowledge for the first time. Additionally, Bader's analysis is carried out on the converged electron density for all boron species within the molecular test set, yielding no simple direct relation between charge and isotropic shifts.

Local Electronic Structure in AlN Studied by Single-Crystal 27Al and 14N NMR and DFT Calculations

Both the chemical shift and quadrupole coupling tensors for 14N and 27Al in the wurtzite structure of aluminum nitride have been determined to high precision by single-crystal NMR spectroscopy. A homoepitaxially grown AlN single crystal with known morphology was used, which allowed for optical alignment of the crystal on the goniometer axis. From the analysis of the rotation patterns of 14N (I=1) and 27Al (I=5/2), the quadrupolar coupling constants were determined to χ(14N)=(8.19±0.02) kHz, and χ(27Al)=(1.914±0.001) MHz. The chemical shift parameters obtained from the data fit were δiso=−(292.6±0.6) ppm and δΔ=−(1.9±1.1) ppm for 14N, and (after correcting for the second-order quadrupolar shift) δiso=(113.6±0.3) ppm and δΔ=(12.7±0.6) ppm for 27Al. DFT calculations of the NMR parameters for non-optimized crystal geometries of AlN generally did not match the experimental values, whereas optimized geometries came close for 27Al with χ¯calc=(1.791±0.003) MHz, but not for 14N with χ¯calc=−(19.5±3.3) kHz.

NMR shifts in aluminosilicate glasses via machine learning

Machine learning (ML) approaches are investigated for the prediction of nuclear magnetic resonance (NMR) shifts in aluminosilicate glasses, for which NMR has proven to be a cutting-edge method over the last decade.

Multinuclear NMR as a tool for studying local order and dynamics in CH3NH3PbX3 (X = Cl, Br, I) hybrid perovskites

We report on ²⁰⁷Pb, ⁷⁹Br, ¹⁴N, ¹H, ¹³C and ²H NMR experiments for studying the local order and dynamics in hybrid perovskite lattices. ²⁰⁷Pb NMR experiments conducted at room temperature on a series of MAPbX₃ compounds (MA = CH₃NH₃⁺; X = Cl, Br and I) showed that the isotropic ²⁰⁷Pb NMR shift is strongly dependent on the nature of the halogen ions. Therefore ²⁰⁷Pb NMR appears to be a very promising tool for the characterisation of local order in mixed halogen hybrid perovskites. ²⁰⁷Pb NMR on MAPbBr₂I served as a proof of concept. Proton, ¹³C and ¹⁴N NMR experiments confirmed the results previously reported in the literature. Low temperature deuterium NMR measurements, down to 25 K, were carried out to investigate the structural phase transitions of MAPbBr₃. Spectral lineshapes allow following the successive phase transitions of MAPbBr₃. Finally, quadrupolar NMR lineshapes recorded in the orthorhombic phase were compared with simulated spectra, using DFT calculated electric field gradients (EFG). Computed data do not take into account any temperature effect. Thus, the discrepancy between the calculated and experimental EFG evidences the fact that MA cations are still subject to significant dynamics, even at 25 K.

Solid-state NMR and computational insights into the crystal structure of silicocarnotite-based bioceramic materials synthesized mechanochemically

In this work, we report the results of a detailed structural study of a promising bioceramic material silicocarnotite Ca₅(PO₄)₂SiO₄ (SC) synthesized from mechanochemically treated nanosized silicon-substituted hydroxyapatite by annealing at 1000°C. This novel synthetic approach represents an attractive and efficient route towards large-scale manufacturing of the silicocarnotite-based bioceramics. A combination of solid-state nuclear magnetic resonance (NMR), powder X-ray crystallography and density function theory (DFT) calculations has been implemented to characterize the phase composition of the prepared composite materials and to gain insight into the crystal structure of silicocarnotite. The phase composition analysis based on the multinuclear solid-state NMR has been found in agreement with X-ray powder diffraction indicating the minority phases of CaO (5-6wt%) and residual silicon-apatite (7-8wt%), while the rest of the material being a fairly crystalline silicocarnotite phase (86-88wt%). A combination of computational (CASTEP) and experimental methods was used to address the anionic site disorder in the silicocarnotite crystal structure. Distorted [OPO₃] pyramids have appeared as an important structural motif in the SC crystal structure. The ratio between regular [PO₄] and distorted [OPO₃] tetrahedra is found between 2:1 and 3:1 based on XRD experiments and CASTEP calculations. The natural abundance ⁴³Ca magic angle spinning NMR spectra of silicocarnotite are reported for the first time.

Local Electronic Structure in γ-LiAlO2 Studied by Single-Crystal 27Al NMR and DFT Calculations

From single-crystal ²⁷Al NMR experiments, the full tensors for both the electrical field gradient (EFG) and the chemical shift (CS) for the aluminum atoms in γ-LiAlO₂ have been determined. A simultaneous fit of the quadrupolar splittings observed for the four ²⁷Al in the unit cell gave the EFG tensor in the crystal frame, from which a quadrupolar coupling constant of χ = C_Q = 3.330 ± 0.005 MHz and an asymmetry parameter of η_Q = 0.656 ± 0.002 were derived. The experimentally determined quadrupolar splittings were sufficiently sensitive to quantify small deviations of both rotation axis direction and starting direction by the data fitting routine. For determination of the CS tensor, the evolution of the outer satellite centers over the crystal rotation was tracked, and the contribution of the quadrupolar shift was subtracted according to the previously determined EFG tensor. The resulting CS tensor of ²⁷Al yields an isotropic chemical shift of δ_iso = 81.8 ± 0.25 ppm and an asymmetry parameter of η_CS = 0.532 ± 0.004, in good agreement with the fit of a MAS NMR spectrum acquired at B₀ = 21.1 T. From both experiments and DFT calculations using the Castep code, we find the eigenvectors of the EFG and CS tensors to be practically colinear.

Combining solid-state NMR spectroscopy with first-principles calculations - a guide to NMR crystallography

Recent advances in the application of first-principles calculations of NMR parameters to periodic systems have resulted in widespread interest in their use to support experimental measurement. Such calculations often play an important role in the emerging field of "NMR crystallography", where NMR spectroscopy is combined with techniques such as diffraction, to aid structure determination. Here, we discuss the current state-of-the-art for combining experiment and calculation in NMR spectroscopy, considering the basic theory behind the computational approaches and their practical application. We consider the issues associated with geometry optimisation and how the effects of temperature may be included in the calculation. The automated prediction of structural candidates and the treatment of disordered and dynamic solids are discussed. Finally, we consider the areas where further development is needed in this field and its potential future impact.

The coloring problem in the solid-state metal boride carbide ScB2C2: a theoretical analysis

The electronic properties of the layered ternary metal boride carbide ScB2C2, the structure of which consists of B/C layers made of fused five- and seven-membered rings alternating with scandium sheets, are analyzed. In particular, the respective positions of the B and C atoms (the so-called coloring problem) are tackled using density functional theory, quantum theory of atoms in molecules, and electron localizability indicator calculations. Results reveal that (i) the most stable coloring minimizes the number of B–B and C–C contacts and maximizes the number of boron atoms in the heptagons, (ii) the compound is metallic in character, and (iii) rather important covalent bonding occurs between the metallic sheets and the boron–carbon network.

Recent developments in solid-state NMR spectroscopy of crystalline microporous materials

Microporous materials, having pores and channels on the same size scale as small to medium molecules, have found many important applications in current technologies, including catalysis, gas separation and drug storage and delivery. Many of their properties and functions are related to their detailed local structure, such as the type and distribution of active sites within the pores, and the specific structures of these active sites. Solid-state NMR spectroscopy has a strong track record of providing the requisite detailed atomic-level insight into the structures of microporous materials, in addition to being able to probe dynamic processes occurring on timescales spanning many orders of magnitude (i.e., from s to ps). In this Perspective, we provide a brief review of some of the basic experimental approaches used in solid-state NMR spectroscopy of microporous materials, and then discuss some more recent advances in this field, particularly those applied to the study of crystalline materials such as zeolites and metal-organic frameworks. These advances include improved software for aiding spectral interpretation, the development of the NMR-crystallography approach to structure determination, new routes for the synthesis of isotopically-labelled materials, methods for the characterisation of host-guest interactions, and methodologies suitable for observing NMR spectra of paramagnetic microporous materials. Finally, we discuss possible future directions, which we believe will have the greatest impact on the field over the coming years.

An NMR crystallographic approach to monitoring cation substitution in the aluminophosphate STA-2

The substitution of the divalent cations Mg(2+) and Zn(2+) into the aluminophosphate (AlPO) framework of STA-2 has been studied using an "NMR crystallographic" approach, combining multinuclear solid-state NMR spectroscopy, X-ray diffraction and first-principles calculations. Although the AlPO framework itself is inherently neutral, the positive charge of the organocation template in an as-made material is usually balanced either by the coordination to the framework of anions from the synthesis solution, such as OH(-) or F(-), and/or by the substitution of aliovalent cations. However, the exact position and distribution of the substituted cations can be difficult to determine, but can have a significant impact upon the catalytic properties a material exhibits once calcined. For as-made Mg substituted STA-2, the positive charge of the organocation template is balanced by the substitution of Mg(2+) for Al(3+) and, where required, by hydroxide anions coordinated to the framework [27] Al MAS NMR spectra show that Al is present in both tetrahedral and five-fold coordination, with the latter dependent on the amount of substituted cations, and confirms the bridging nature of the hydroxyl groups, while high-resolution MQMAS spectra are able to show that Mg appears to preferentially substitute on the Al1 site. This conclusion is also supported by first-principles calculations. The calculations also show that (31)P chemical shifts depend not only on the topologically-distinct site in the SAT framework, but also on the number of next-nearest-neighbour Mg species, and the exact nature of the coordinated hydroxyls (whether the P atom forms part of a six-membered ring, P(OAl)2OH, where OH bridges between two Al atoms). The calculations demonstrate a strong correlation between the (31)P isotropic chemical shift and the average 〈P-O-M〉 bond angle. In contrast, for Zn substituted STA-2, both X-ray diffraction and NMR spectroscopy show less preference for substitution onto Al1 or Al2, with both appearing to be present, although that into Al1 appears slightly more favoured.

New methods and applications in solid-state NMR spectroscopy of quadrupolar nuclei

Solid-state nuclear magnetic resonance (NMR) spectroscopy has long been established as offering unique atomic-scale and element-specific insight into the structure, disorder, and dynamics of materials. NMR spectra of quadrupolar nuclei (I > (1)/2) are often perceived as being challenging to acquire and to interpret because of the presence of anisotropic broadening arising from the interaction of the electric field gradient and the nuclear electric quadrupole moment, which broadens the spectral lines, often over several megahertz. Despite the vast amount of information contained in the spectral line shapes, the problems with sensitivity and resolution have, until very recently, limited the application of NMR spectroscopy of quadrupolar nuclei in the solid state. In this Perspective, we provide a brief overview of the quadrupolar interaction, describe some of the basic experimental approaches used for acquiring high-resolution NMR spectra, and discuss the information that these spectra can provide. We then describe some interesting recent examples to showcase some of the more exciting and challenging new applications of NMR spectra of quadrupolar nuclei in the fields of energy materials, microporous materials, Earth sciences, and biomaterials. Finally, we consider the possible directions that this highly informative technique may take in the future.

Conformational and NMR study of some furan derivatives by DFT methods

4'-substituted neutral/protonated furfurylidenanilines and trans-styrylfurans are able to exist in two different conformations related to the rotation around the furan ring-bridge double bond. In this work, the equilibrium geometry and the corresponding rotational barrier of the benzene ring for each furan derivative conformation were calculated by DFT methods. The trend and shape of the rotational barrier are rationalized within natural bond orbitals as well as atoms-in-molecules approach. For the corresponding equilibrium geometries, (1)H and (13)C substituent induced shifts (SIS) were calculated and compared with experimental values. Calculated shielding constants are shown to be sensitive to the substituent effect through a linear fit with substituent's Hammett constants. An alternative approach was followed for assessing the effect of substituents over SIS through comparing the differences in isotropic shielding constants with NBO charges as well as with (1)H and (13)C experimental chemical shifts.

First-principles calculations of NMR parameters for phosphate materials

In this short review, we discuss the ability to reproduce NMR parameters in the case of phosphates materials through electronic structure calculation within density functional theory linear response. Indeed, the gauge-including projector-augmented wave is today largely used by the solid-state NMR community as a tool for structural determination and it has been applied to a large variety of materials. We emphasise on the crucial points that should be taken into account to perform such calculations. In particular, we discuss the influence of the electronic structure and of the geometry on the calculation of NMR parameters. To illustrate the review, we present experimental and theoretical comparison of (31)P, (1)H and (23)Na NMR data on a series of sodium phosphate systems.

Electric field gradient calculations in paramagnetic compounds using the PAW approach. Application to ²³Na NMR in layered vanadium phosphates

This article presents ab initio calculations of electric field gradient (EFG) parameters as a tool for the structural characterization of paramagnetic crystalline compounds. Previously reported ²³Na NMR parameters of vanadium + IV containing vanado-phosphate compounds were computed within density functional theory using both cluster and fully periodic approaches. Quadrupolar parameter values measured by ²³Na NMR experiments were reproduced with a level of accuracy comparable to that achievable in diamagnetic compounds and allowed the assignment of observed ²³Na NMR signals. This work demonstrates the utility of the periodic planewave pseudopotential + PAW approach for the calculation of EFG parameters in paramagnetic compounds.

Hydrogen bonds and local symmetry in the crystal structure of gibbsite

First-principles quantum mechanical calculations of NMR chemical shifts and quadrupolar parameters have been carried out to assign the (27)Al MAS NMR resonances in gibbsite. The (27)Al NMR spectrum shows two signals for octahedral aluminum revealing two aluminum sites coordinated by six hydroxyl groups each, although the crystallographic positions of the two Al sites show little difference. The presence of two distinguished (27)Al NMR resonances characterized by rather similar chemical shifts but quadrupolar coupling constants differing by roughly a factor of two is explained by different character of the hydrogen bonds, in which the hydroxyls forming the corresponding octahedron around each aluminum site, are involved. The Al-I site characterized by a C(Q) = 4.6 MHz is surrounded by OH-groups participating in four intralayer and two interlayer hydrogen bonds, while the Al-II site with the smaller quadrupolar constant (2.2 MHz) is coordinated by hydroxides, of which two point toward the intralayer cavities and four OH-bonds are aligned toward the interlayer gallery. In high-resolution solid-state (1)H CRAMPS (combination of rotation and multiple-pulse spectroscopy) four signals with an intensity ratio of 1:2:2:1 are resolved which allow to distinguish six nonequivalent hydrogen sites reported in the gibbsite crystal structure and to ascribe them to two types of structural OH groups associated with intralayer and interlayer hydrogen bonds. This study can be applied to characterize the gibbsite-like layer-intergallery interactions associated with hydrogen bonding in the more complex systems, such as synthetic aluminum layered double hydroxides.

Improving sensitivity and resolution of MQMAS spectra: a 45Sc-NMR case study of scandium sulphate pentahydrate

To efficiently obtain multiple-quantum magic-angle spinning (MQMAS) spectra of the nuclide 45Sc (I=7/2), we have combined several previously suggested techniques to enhance the signal-to-noise ratio and to improve spectral resolution for the test sample, scandium sulphate pentahydrate (ScSPH). Whereas the 45Sc-3QMAS spectrum of ScSPH does not offer sufficient resolution to clearly distinguish between the 3 scandium sites present in the crystal structure, these sites are well-resolved in the 5QMAS spectrum. The loss of sensitivity incurred by using MQMAS with 5Q coherence order is partly compensated for by using fast-amplitude modulated (FAM) sequences to improve the efficiency of both 5Q coherence excitation and conversion. Also, heteronuclear decoupling is employed to minimise dephasing of the 45Sc signal during the 5Q evolution period due to dipolar couplings with the water protons in the ScSPH sample. Application of multi-pulse decoupling schemes such as TPPM and SPINAL results in improved sensitivity and resolution in the F(1) (isotropic) dimension of the 5QMAS spectrum, the best results being achieved with the recently suggested SW(f)-TPPM sequence. By numerical fitting of the 45Sc-NMR spectra of ScSPH from 3QMAS, 5QMAS and single-quantum MAS at magnetic fields B(0)=9.4 T and 17.6 T, the isotropic chemical shift delta(iso), the quadrupolar coupling constant chi, and the asymmetry parameter eta were obtained. Averaging over all experiments, the NMR parameters determined for the 3 scandium sites, designated (a), (b) and (c) are: delta(iso)(a)=-15.5+/-0.5 ppm, chi(a)=5.60+/-0.10 MHz, eta(a)=0.06+/-0.05; delta(iso)(b)=-12.9+/-0.5 ppm, chi(b)=4.50+/-0.10 MHz, eta(b)=1.00+/-0.00; and delta(iso)(c)=-4.7+/-0.2 ppm, chi(c)=4.55+/-0.05 MHz, eta(c)=0.50+/-0.02. The NMR scandium species were assigned to the independent crystallographic sites by evaluating their experimental response to proton decoupling, and by density functional theory (DFT) calculations using the PAW and GIPAW approaches, in the following way: Sc(1) to (c), Sc(2) to (a), and Sc(3) to (b). The need to compute NMR parameters using an energy-optimised crystal structure is once again demonstrated.