Chemical Shifts In Solids Research Articles

Modeling Small Structural and Environmental Differences in Solids with 15 N NMR Chemical Shift Tensors.

The ability to theoretically predict accurate NMR chemical shifts in solids is increasingly important due to the role such shifts play in selecting among proposed model structures. Herein, two theoretical methods are evaluated for their ability to assign 15 N shifts from guanosine dihydrate to one of the two independent molecules present in the lattice. The NMR data consist of 15 N shift tensors from 10 resonances. Analysis using periodic boundary or fragment methods consider a benchmark dataset to estimate errors and predict uncertainties of 5.6 and 6.2 ppm, respectively. Despite this high accuracy, only one of the five sites were confidently assigned to a specific molecule of the asymmetric unit. This limitation is not due to negligible differences in experimental data, as most sites exhibit differences of >6.0 ppm between pairs of resonances representing a given position. Instead, the theoretical methods are insufficiently accurate to make assignments at most positions.

Chemical shifts in molecular solids by machine learning

Due to their strong dependence on local atonic environments, NMR chemical shifts are among the most powerful tools for strucutre elucidation of powdered solids or amorphous materials. Unfortunately, using them for structure determination depends on the ability to calculate them, which comes at the cost of high accuracy first-principles calculations. Machine learning has recently emerged as a way to overcome the need for quantum chemical calculations, but for chemical shifts in solids it is hindered by the chemical and combinatorial space spanned by molecular solids, the strong dependency of chemical shifts on their environment, and the lack of an experimental database of shifts. We propose a machine learning method based on local environments to accurately predict chemical shifts of molecular solids and their polymorphs to within DFT accuracy. We also demonstrate that the trained model is able to determine, based on the match between experimentally measured and ML-predicted shifts, the structures of cocaine and the drug 4-[4-(2-adamantylcarbamoyl)-5-tert-butylpyrazol-1-yl]benzoic acid.

Visualising crystal packing interactions in solid-state NMR: Concepts and applications.

In this article, we introduce and apply a methodology, based on density functional theory and the gauge-including projector augmented wave approach, to explore the effects of packing interactions on solid-state nuclear magnetic resonance (NMR) parameters. A visual map derived from a so-termed "magnetic shielding contribution field" can be made of the contributions to the magnetic shielding of a specific site-partitioning the chemical shift to specific interactions. The relation to the established approaches of examining the molecule to crystal change in the chemical shift and the nuclear independent chemical shift is established. The results are applied to a large sample of 71 molecular crystals and three further specific examples from supermolecular chemistry and pharmaceuticals. This approach extends the NMR crystallography toolkit and provides insight into the development of both cluster based approaches to the predictions of chemical shifts and for empirical predictions of chemical shifts in solids.

Natural-Abundance Solid-State 2H NMR Spectroscopy at High Magnetic Field

High-resolution solid-state (2)H NMR spectroscopy provides a method for measuring (1)H NMR chemical shifts in solids and is advantageous over the direct measurement of high-resolution solid-state (1)H NMR spectra, as it requires only the application of routine magic angle sample spinning (MAS) and routine (1)H decoupling methods, in contrast to the requirement for complex pulse sequences for homonuclear (1)H decoupling and ultrafast MAS in the case of high-resolution solid-state (1)H NMR. However, a significant obstacle to the routine application of high-resolution solid-state (2)H NMR is the very low natural abundance of (2)H, with the consequent problem of inherently low sensitivity. Here, we explore the feasibility of measuring (2)H MAS NMR spectra of various solids with natural isotopic abundances at high magnetic field (850 MHz), focusing on samples of amino acids, peptides, collagen, and various organic solids. The results show that high-resolution solid-state (2)H NMR can be used successfully to measure isotropic (1)H chemical shifts in favorable cases, particularly for mobile functional groups, such as methyl and -N(+)H(3) groups, and in some cases phenyl groups. Furthermore, we demonstrate that routine (2)H MAS NMR measurements can be exploited for assessing the relative dynamics of different functional groups in a molecule and for assessing whole-molecule motions in the solid state. The magnitude and field-dependence of second-order shifts due to the (2)H quadrupole interaction are also investigated, on the basis of analysis of simulated and experimental (1)H and (2)H MAS NMR spectra of fully deuterated and selectively deuterated samples of the α polymorph of glycine at two different magnetic field strengths.

Calculation of NMR chemical shifts in organic solids: Accounting for motional effects

NMR chemical shifts were calculated from first principles for well defined crystalline organic solids. These density functional theory calculations were carried out within the plane-wave pseudopotential framework, in which truly extended systems are implicitly considered. The influence of motional effects was assessed by averaging over vibrational modes or over snapshots taken from ab initio molecular dynamics simulations. It is observed that the zero-point correction to chemical shifts can be significant, and that thermal effects are particularly noticeable for shielding anisotropies and for a temperature-dependent chemical shift. This study provides insight into the development of highly accurate first principles calculations of chemical shifts in solids, highlighting the role of motional effects on well defined systems.

Further Conventions for NMR Shielding and Chemical Shifts (IUPAC Recommendations 2008)

IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem. 73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the (1)H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a part per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS). This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating (13)C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids are reviewed in some detail, and recommendations are given for best practice.

Further conventions for NMR shielding and chemical shifts IUPAC recommendations 2008

IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem. 73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the 1H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a part per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS). This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating 13C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids are reviewed in some detail, and recommendations are given for best practice.

Further conventions for NMR shielding and chemical shifts (IUPAC Recommendations 2008)

Abstract IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem. 73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the 1H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a parts per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS). This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating 13C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids is reviewed in some detail, and recommendations are given for best practice.

Applications of solid-state NMR to pharmaceutical polymorphism and related matters

Magic-angle spinning NMR is now making a significant contribution to our understanding of the structure of polymorphs and solvates of pharmaceutical significance. This personal review article discusses a range of applications, with particular emphasis on information about crystallography, for which NMR can address problems that cannot be readily solved by diffraction techniques (such as dynamic disorder and non-stoichiometric hydration). Unlike diffractograms, NMR spectra yield immediate chemical information. Moreover, heterogeneous samples can be investigated and amorphous content provides no significant barrier to studies. Furthermore, NMR can be an effective technique for quantitation (down to the level of ca. 1%). Additional strength is being derived from computation of chemical shifts in solids, using a code that takes account of the spatial repetition inherent in crystalline materials.

2H Natural-Abundance MAS NMR Spectroscopy: An Alternative Approach to Obtain 1H Chemical Shifts in Solids

(2)H NMR was examined as an approach to determine (1)H chemical shifts in solids. For high-resolution observation, the line width due to (2)H quadrupole interaction and chemical-shift anisotropy was removed by magic-angle spinning and that due to (1)H-(2)H dipolar interactions by (1)H decoupling. Further, we showed that the sensitivity can be enhanced by applying (1)H to (2)H cross polarization and by adding spinning-sideband spectra. These make it possible to obtain (2)H natural-abundance MAS spectra revealing highly resolved (2)H signals. The second-order quadrupole effects of (2)H are also examined.

The 15N chemical shifts in mixed NB 2Si and NBSi 2 environments of Si 3B 3N 7—A theoretical investigation

Nuclear magnetic resonance (NMR) chemical shifts in solids may be calculated by ab initio methods approximating the solid state by molecular clusters. We employed this technique to obtain estimates of 15N chemical shifts in NB 2Si and NBSi 2 environments in the solid state. Such nitrogen environments are found in amorphous (Si/B/N-)ceramics which exhibit very interesting features such as high thermal and mechanical stability. We based our calculations on cutouts of hypothetical Si 3B 3N 7 crystals suggested by Kroll and Hoffmann [Silicon boron nitrides: hypothetical polymorphs of Si 3B 3N 7, Angew. Chem. Int. Ed. 37 (1998) 2527]. Taking the systematic errors of our calculations into account we expect the chemical shifts in NBSi 2 environments around - 293 ± 5 ppm . Chemical shifts in NB 2Si environments are expected at - 272 ± 6 ppm . The range of the calculated chemical shifts in NBSi 2 environments coincides with experimental chemical shifts in molecular compounds. Experimental chemical shifts of NB 2Si nitrogen in molecules appear at lower field than our calculated chemical shifts in the solid state.

13C CP MAS NMR, FTIR, X-ray diffraction and PM3 studies of some N-(ω-carboxyalkyl)morpholine hydrohalides

N-(ω-carboxyalkyl)morpholine hydrochlorides, OC4H8N(CH2)nCOOH·HCl, n=1–5, were obtained and analyzed by 13C cross polarization (CP) magic angle spinning (MAS) NMR, FTIR and PM3 calculations. The structure of N-(3-carboxypropyl)morpholine hydrochloride (n=3) has been solved by X-ray diffraction method at 100K and refined to the R=0.031. The crystals are monoclinic, space group P21/c, a=14.307(3), b=9.879(2), c=7.166(1)Å,β=93.20(3)°, V=1011.3(3)Å3,Z=4. In this compound the nitrogen atom is protonated and two molecules form a centrosymmetric dimer, connected by two N+–H⋯Cl− (3.095(1)Å) and two O–H⋯Cl− (3.003(1)Å) hydrogen bonds. 13C CP MAS NMR spectra, contrary to the solution, showed non-equivalence of the ring carbon atoms. The PM3 calculations predict a molecular dimer without proton transfer for an HCl complex, while for an HBr complex an ion pairs with proton transfer, and reproduces correctly the conformation of both dimers but overestimates H-bond distances. Shielding constants calculated from the PM3 geometry of ion pairs gave a linear correlation with the 13C chemical shifts in solids.

Isotropic second-order dipolar shifts in the rotating frame

An experiment is described that utilizes the truncation of the Hamiltonian in the rotating frame by a radio-frequency field designed to yield an isotropic shift for the dipolar coupling. This approach allows the measurement of a normally orientation-dependent coupling constant by a single isotropic value. The dipolar isotropic shift is closely related to the field-dependent chemical shift in solids due to the second-order dipolar perturbation observed in magic-angle spinning experiments. In the rotating frame, larger shifts of up to 1000 Hz can be observed for the case of a one-bond C–H coupling compared to a shift of a few Hertz in the laboratory-frame experiment. In addition to the isotropic shift, a line broadening due to the P4(cos β) terms is observed when the experiment is carried out under magic-angle sample spinning (MAS) conditions, leading to the requirement of higher-order averaging such as double rotation (DOR) for obtaining narrow lines. As an application of this new experiment the separation of CH, CH2, and CH3 groups in a 2D spectrum under MAS is demonstrated. Implemented under DOR it could be used as a technique to select carbon atoms according to the number of directly attached protons.

Correlation of Isotropic and Anisotropic Chemical Shifts in Solids by Two‐Dimensional Variable‐Angle‐Spinning NMR

AbstractWe describe a new solid‐state nuclear magnetic resonance (NMR) technique for correlating anisotropic and isotropic chemical shifts in powdered samples. Two‐dimensional (2D) NMR spectra are obtained by processing signals acquired in independent experiments for different angles between the sample spinning axis and the Zeeman magnetic field. This 2D NMR approach can therefore resolve individual static anisotropic lineshapes according to their isotropic chemical shift frequencies, without use of sudden mechanical motions or multiple‐pulse irradiation schemes. Applications of the technique are illustrated with an analysis of the chemical shift anisotropy for the eight distinct 13C sites in tyrosine.

Field-dependent C-13 chemical shifts in solids: A second-order dipolar perturbation

The observation of field-dependent C-13 chemical shifts in the presence of high-power proton decoupling and magic angle sample spinning (MAS) is documented. While the principal data were taken at fields of 1.4 and 4.7 T, the difference in chemical shift, in ppm, between the crystalline resonance of polyethylene and a reference resonance of solid adamantane varied as (a+bB−20) in measurements taken at six different fields in as many laboratories. At a given field there is no dependence of this shift difference on proton resonance offset, proton rf field strength, or sample spinning speed. In rigid solids, the ‘‘b’’ term in the foregoing relationship is twice as large for a methylene as for a methine carbon. Results of chemical shift measurements at two fields are reported for polyethylene, polypropylene, and three molecular solids including the normal alkane, nonadecane, which exhibits fast well-defined molecular rotation in the solid rotator phase. The observed shift differences for several kinds of carbons at 1.4 and 4.7 T agree very well with the explanation that the b term in the above expression results from a second-order energy perturbation involving the nonsecular ‘‘C’’ and ‘‘D’’ terms of the dipolar Hamiltonian. A simple theory is presented for a C-13-proton spin pair, and the role of MAS is examined. A line broadening equal to about one-half of the mean shift is also associated with this phenomenon. The behavior of this second-order shift is also considered for cases where fast molecular motion is present or where multiple protons are bonded to a carbon. Even at 1.4 T the shifts are rather small, i.e., 0.72 and 0.36 ppm, respectively, for a methylene and a methine carbon. Corrections which will account for this second-order shift are easily made in powder samples of rigid molecules; thus, true chemical shifts can be recovered. Above an operating field of, say, 3 T, the degradation of linewidth due to the spread of these second-order shifts is negligible (<3 Hz) even for a methylene group. Finally, a challenge is offered to consider whether similar second-order shifts operate in homonuclear coupled systems in the presence of homonuclear decoupling and MAS.

Relativistic effects on the chemical shift in solids: important points of a new contribution

The authors derive a theory of the chemical shift ( sigma ) in solids and show that sigma contains, in addition to the usual diamagnetic and paramagnetic shielding terms, a more important new term due to the spin-orbit interaction. The theory presented in this Letter is, they believe, the most general and thorough treatment which has yet been made of this problem.

A simple spin-echo experiment for accurate measurement of chemical shifts in solids: Application to 19F in metal difluorides

A simple NMR experiment requiring only a single channel pulse spectrometer is described for the accurate measurement of chemical shifts in solids. It is based upon the frequency dependence of the two pulse solid echo response. Its applicability is limited to materials in which all resonant spins are chemically and crystallographically equivalent. The method is demonstrated by application to measurement of the 19F chemical shifts in a variety of metal difluorides.

Detection of imogolite in soils using solid state 29Si NMR

High resolution solid state 29Si-NMR has recently been used to examine silicates, and synthetic and natural zeolites1–4. This has been possible through the use of dipolar decoupling, magicangle spinning and cross-polarization techniques which have enabled line narrowing and also considerable signal enhancement for 29Si nuclei in close proximity to protons. It has been found that isotropic 29Si chemical shifts in solids and solutions are generally similar and depend mainly on the degree of condensation of silicon–oxygen tetrahedra1 For silicates, increasing condensation from single (Q0, −66 to −72 p.p.m.) to double tetrahedra (Q1, −78 to −82 p.p.m.), to chains (Q2, −86 to −88 p.p.m.) and cyclic layered structures (Q3, −107 to −109 p.p.m.) leads to increasing diamagnetic shielding. In solid aluminosilicates, the 29Si chemical shift is sensitive to the substitution of aluminium. A further important point was that the 29Si linewidth in aluminosilicates is sensitive to the regularity of aluminium distribution in the lattice. Thus 29Si-NMR has obvious potential for the elucidation of the coordination of silicon in soils and mineral components. We report here the first 29Si-NMR study of the clay mineral imogolite, and show that 29Si-NMR is an excellent analytical technique for the identification of imogolite (or imogolite like materials, that is proto-imogolite) in clay fractions. The observed chemical shift of imogolite (−78 p.p.m.) is consistent with the proposal that imogolite is a tubular hydrated aluminosilicate in which silicon tetrahedra are isolated by coordination through oxygen with three aluminium atoms and one proton.

Measurement of 13C chemical shifts in solids

A pulse sequence and sample geometry which allows the measurement of 13C chemical shifts of solid materials relative to liquid tetramethylsilane (TMS) are described. Using this technique, the chemical shifts of a series of common engineering plastics were measured and reported. A small number of candidate secondary shift reference materials were considered and their chemical shifts measured. Most of these materials proved to be unsuitable for general 13C shift references for differing reasons. The most promising standard investigated was polydimethylsilane. The measurement of chemical shifts in solid materials is slightly complicated by anisotropic magnetic properties and sensitivity to magic-angle missetting when the material exhibits macroscopic orientation. These complications are discussed in detail and examples of misleading spectra are shown.

An experimental study of resolution of proton chemical shifts in solids: Combined multiple pulse NMR and magic-angle spinning

High-resolution nuclear magnetic resonance spectra of protons in rigid, randomly oriented solids have been measured using combined homonuclear dipolar decoupling (via multiple pulse techniques) and attenuation of chemical shift anisotropies (via magic-angle sample spinning). Under those conditions, isotropic proton chemical shifts were recorded for a variety of chemical species, with individual linewidths varying from about 55 to 110 Hz (1–2 ppm). Residual line broadening was due predominately to (i) magnetic-field instability and inhomogeneity, (ii) unresolved proton–proton spin couplings, (iii) chemical shift dispersion, (iv) residual dipolar broadening, and (v) lifetime broadening under the multiple pulse sequences used. The magnitudes of those effects and the current limits of resolution for this experiment in our spectrometer have been investigated. The compounds studied included organic solids (4, 4′-dimethylbenzophenone, 2, 6-dimethylbenzoic acid, and aspirin), polymers (polystyrene and polymethylmethacrylate), and the vitrain portion of a bituminous coal.