Separating chemical shift and quadrupolar anisotropies via multiple-quantum NMR spectroscopy

Chemical shift anisotropy (CSA) has been an invaluable probe of structure and dynamics for a variety of systems in NMR spectroscopy. Unfortunately, the presence of strong quadrupolar couplings has severely limited the ability to measure CSA in nuclei with spins I > 1/2. Here we show that these two interactions can be refocused at different times in a 2D multiple-quantum NMR experiment on polycrystalline samples. Combining this experiment with appropriate affine transformations allows these interactions to be cleanly separated into orthogonal dimensions. The 1D projection onto each axis can be fit to extract the respective principal tensor components. These components can then be used to fit the 2D spectrum for the relative orientation between the CSA and quadrupolar-coupling tensors. The necessary affine transformation parameters are given for all possible I values. Illustrative examples of spectra and analyses are given for 63Cu in K3[Cu(CN)4], 59Co in K3[Co(CN)6], and 87Rb in RbCrO4.     

13C and 119Sn NMR spectra of some tribenzyltin(IV) compounds

The ¹³C and ¹¹⁹Sn NMR spectra of some tribenzyltin(IV) compounds and their complexes in coordinating and non-coordinating solvents have been studied. The δ(¹¹⁹Sn) chemical shifts and coupling constants ¹J(¹¹⁹Sn, ¹³C) clearly depend on the coordination number of the central tin atom and the geometry of its coordination polyhedra. Approximate ranges of the characteristic values of both the NMR parameters were determined for various configurational types of tribenzyltin compound. The ¹³C and ¹¹⁹Sn NMR parameters found are indicative of a distinct interaction between the polarized σ(SnC) bond and adjacent π-electron system of the aromatic ring(s).     

13C and 119Sn NMR spectra of Di-n-butyltin(IV) compounds

The ¹³C and ¹¹⁹Sn NMR spectra of a set of di-n-butyltin(IV) compounds and their complexes in coordinating and non-coordinating solvents have been studied. The results have shown that it is possible to describe semiquantitatively the shape of coordination polyhedra of these compounds from analysis of their δ(¹¹⁹Sn) and ¹J(¹¹⁹Sn-¹³C) parameters. The values of δ(¹¹⁹Sn) define the regions with different coordination numbers of the central tin atom, so that four-coordinate compounds have δ(¹¹⁹Sn) ranging from about + 200 to −60 ppm, five-coordinate compounds, −90 to −190 ppm, and six-coordinate compounds, −210 to −400 ppm. The values of ¹J(¹¹⁹Sn-¹³C) were used for the calculation of the CSnC angle in the coordination polyhedron of individual compounds.     

119Sn NMR chemical shift tensors in anhydrous and hydrated Si8O20(SnMe3)8 crystals

(119)Sn chemical shift tensors of crystalline trialkyltin functionalized octameric spherosilicates, Si(8)O(20)(SnMe(3))(8), have been determined by fitting sideband intensities in solid-state magic angle spinning (MAS) NMR spectra. Tin chemical shift parameters are exquisitely sensitive to the presence of water of crystallization. Both hydrogen bonding and incipient oxygen-tin bonding from molecular water impact the local tin environment. Tin chemical shift tensors in the crystalline derivatives reflect the changes in geometry and coordination number at the tin centers. Chemical shift correlations on the crystalline derivatives, with known x-ray structures, are used to infer the tin coordination environment in an amorphous sample.     

Measurement of ribose carbon chemical shift tensors for A-form RNA by liquid crystal NMR spectroscopy

Incomplete motional averaging of chemical shift anisotropy upon weak alignment of nucleic acids and proteins in a magnetic field results in small changes in chemical shift. Knowledge of nucleus-specific chemical shift (CS) tensor magnitudes and orientations is necessary to take full advantage of these measurements in biomolecular structure determination. We report the determination by liquid crystal NMR of the CS tensors for all ribose carbons in A-form helical RNA, using a series of novel 3D NMR pulse sequences for accurate and resolved measurement of the ribose (13)C chemical shifts. The orientation of the riboses relative to the rhombic alignment tensor of the molecule studied, a stem-loop sequence corresponding to helix-35 of 23S rRNA, is known from an extensive set of residual dipolar couplings (RDC), previously used to refine its structure. Singular-value-decomposition fits of the chemical shift changes to this structure, or alternatively to a database of helical RNA X-ray structures, provide the CS tensor for each type of carbon. Quantum chemical calculations complement the experimental results and confirm that the most shielded tensor component lies approximately along the local carbon-oxygen bond axis in all cases and that shielding anisotropy for C3' and C4' is much larger than for C1' and C2', with C5' being intermediate.     

Conformations and 13C NMR chemical shifts of some polyamides in the solid state as studied by high-resolution 13C NMR spectroscopy

High-resolution ¹³Carbon nuclear magnetic resonance (NMR) spectra of Nylons 4, 6, and 66 in the solid state were measured over a wide range of temperature. From the results, it was found that resonance lines of crystalline and noncrystalline components were separable and their chemical shifts were determined. The ¹³C chemical shift behavior is closely related to their conformation. The origin of the conformational effects on the chemical shifts is discussed.     

Fourier transform NMR investigations of organotin compounds : VIII. Propenyl- and isopropenyl-tin compounds; detection and identification of isomers using 119Sn and 13c NMR

Starting from the equilibrium mixture of cis- and trans-1-bromo-1-propene, isomeric mixtures of compounds Me_n Sn(CH=GHMe)_4-n (n = 0–3) have been prepared and studied. While proton NMR only allows distinction between the methyltin signals of the various isomers (except where n = 3), the ¹³C spectra show separate signals for almost all isomeric carbons even when n = 0. In the ¹¹⁹Sn spectra the signals due to the various isomers are separated by ca. 20 ppm for a given value of n; the peak areas can be used to estimate the proportions of cis- and trans-propenyl residues present in the mixtures. Addition of 2-bromo-propene to the starting 1-bromo-1-propenes leads to the formation of further isomers, which can in all cases be observed and identified in the ¹¹⁹Sn spectra; ¹¹⁹Sn shifts can be calculated using the shifts for the Me₃SnC₃H₅ isomers as increments.     

Heteronuclear chemical shift correlation and J-resolved MAS NMR spectroscopy of lipid membranes

Direct observation of J-couplings remains a challenge in high-resolution solid-state NMR. In some cases, it is possible to use Lee-Goldburg (LG) homonuclear decoupling during rare spin observation in MAS NMR correlation spectroscopy of lipid membranes to obtain J-resolved spectra in the direct dimension. In one simple implementation, a wide line separation-type (13)C-(1)H HETCOR can provide high-resolution (1)H/(13)C spectra, which are J-resolved in both dimensions. Coupling constants, (1)J(HC), obtained from (1)H doublets, can be compared with scaled (1)J(θ)(CH)-values obtained from the (13)C multiplets to assess the LG efficiency and scaling factor. The use of homonuclear decoupling during proton evolution, LG-HETCOR-LG, can provide J-values, at least in the rare spin dimension, and allows measurements in less mobile membrane environments. The LG-decoupled spectroscopic approach is demonstrated on pure dioleoylphosphatidylcholine (DOPC) membranes and used to investigate lipid mixtures of DOPC/cholesterol and DOPC/cholesterol/sphingomyelin.     

A 31P and 119Sn NMR study of trichlorostannate derivatives of some chloride-bridged palladium(II) dimers

The chloride-bridged dimers [Pd(μ-Cl)(SnCl₃)L]₂ (L = P(p-tolyl)₃, P(p-C₆H₄OMe)₃, As(p-tolyl)₃, AsEt₃, PEt₃, PPrⁿ₃) have been synthesized and their ¹¹⁹Sn and, where relevant, ³¹P NMR spectra recorded. For the aryl phosphine and both arsine complexes the dimers exist as both the sym-trans and sym-cis isomers; however, for the PEt₃ and PPrⁿ₃ compounds, there is only one form, which we assign to the sym-trans structure. There is a geometric dependence of both ⁴J(¹¹⁹Sn, ¹¹⁷Sn) and ⁴J(¹¹⁹Sn, ³¹P). The dimeric complex with P(p-tolyl)₃ carbonylates 1-heptyne in the presence of excess SnCl₂, but is inactive in its absence.     

Solid-state nitrogen-15 NMR and quantum chemical study of N,N-dimethylaniline derivatives

In this study the components of the nitrogen chemical shift (CS) tensor are examined for a series of para substituted N,N-dimethylaniline derivatives. This is done through measurement of the 15N NMR spectra of powder samples and through quantum chemical calculations on the isolated molecules. Experiments and calculations show that the isotropic CS, delta(iso), decreases with increasing electron donating ability of the para substituent, in agreement with previous solution studies. More importantly, this study shows that this decrease in the isotropic (solution) CS is due to decreasing values of the CS tensor component delta(11) and component delta(33). The component delta(22) is essentially invariant to the electron donating/withdrawing ability of the para substituent. Through Ramsey's theory of nuclear magnetic shielding, it can be seen that the variation in delta(11) and delta(33), and hence delta(iso), is due to changes in the n-pi* and the sigma-pi* energy gaps in N,N-dimethylaniline. This, in turn, is a result of the change in the energy of the pi* molecular orbital with change in the pi-electron donating ability of the para substituent. The effects of nitrogen inversion on the components of the nitrogen CS tensor components are also discussed. This study also shows the feasibility of performing 15N cross-polarization experiments on nonspinning powder samples at natural isotopic abundance.     

Modes of coordination of 1,2-aminothiols in organotin(IV) complexes,as demonstrated by 119Sn, 15N and 13C NMR spectroscopy

Three series of organotin(IV) cysteamine complexes have been prepared in which the 1,2-aminothiol may be monodentate, chelating or bridging, on the ¹¹⁹Sn, ¹⁵N or ¹³C NMR spectroscopic and other physical evidence. Triorganotin(IV) complexes [SnR₃(SCH₂ CH₂NR′₂] (R = Me, Buⁿ or Ph; R′ = H, Me or Et) are monomeric and the aminothiol monodentate in non-coordinating solvents. Dialkyltin(IV) chloro complexes [SnR₂Cl(SCH₂CH₂NR′₂)] (R = Me, Et, Buⁿ or Octⁿ; R′ = H, Me or Et), however, are monomeric with chelation of the aminothiol, like the SnR₂Cl complex of ethyl cysteinate, although nitrogen ligation is hindered by N,N-dialkylation. These solution properties contrast with the polymeric solid structures that are likely for [SnR₂Cl(SCH₂CH₂NH₂)] complexes with R = Me or Et, though not for R = Buⁿ or Octⁿ. Dialkyltin bis-cysteamine complexes [SnR₂(SCH₂CH₂NR′₂)₂] (R = Me, Et, Buⁿ or Octⁿ; R′ = H or Et) show an intermolecular association as neat liquids and in non-coordinating solvents, increasingly with concentration, but again, nitrogen ligation is hindered by tertiary amino groups. The ¹⁵N results usefully complement those from ¹¹⁹Sn and ¹³C NMR spectroscopy.     

Conformational studies of some 2-phenylpropyl derivatives by NMR spectroscopy and use of lanthanide shift reagents

The conformational distribution of CH₃CH(Ph)CH₂X (X = OH, OCH₃, NH₂ Cl) has been studied by NMR and IR spectroscopy. The results are interpreted in favour of the conformers with methoxy- or chloro-groups anti to the phenyl group, but the amino group anti to the methyl group. For the alcohol both forms are about equally populated. It is suggested that intra-molecular hydrogen bonding might be affecting the conformational equilibria when X = OH, NH₂.     

Simultaneous enhancement of chemical shift dispersion and diffusion resolution in mixture analysis by diffusion-ordered NMR spectroscopy

Mixture analysis by high resolution diffusion-ordered NMR spectroscopy (HR-DOSY) requires differences in both chemical shift and diffusion coefficient; resolution can be greatly enhanced by exploiting the chemical specificity of lanthanide shift reagent binding to increase chemical shift and diffusion dispersion simultaneously.     

Phosphorus chemical shift tensors of phosphido ligands in ruthenium carbonyl compounds: (31)P NMR spectroscopy of single-crystal and powder samples and ab initio calculations

The phosphorus chemical shift (CS) tensors of several ruthenium carbonyl compounds containing a phosphido ligand, micro), bridging a Ru [bond] Ru bond were characterized by solid-state (31)P NMR spectroscopy. As well, an analogous osmium compound was examined. The structures of most of the clusters investigated have approximate local C(2v) symmetry about the phosphorus atom. Compared to the "isolated" PH(2)(-) anion, the phosphorus nucleus of a bridging phosphido ligand exhibits considerable deshielding. The phosphorus CS tensors of most of the compounds have spans ranging from 230 to 350 ppm and skews of approximately zero. Single-crystal NMR was used to investigate the orientation of the phosphorus CS tensors for two of the compounds, Ru(2)(CO)(6)(mu(2)-C [triple bond] C [bond] Ph)(mu(2)-PPh(2)) and Ru(3)(CO)(9)(mu(2)-H)(mu(2)-PPh(2)). The intermediate component of the phosphorus CS tensor, delta(22), lies along the local C(2) axis in both compounds. The least shielded component, delta(11), lies perpendicular to the Ru [bond] P [bond] Ru plane while the most shielded component, delta(33), lies perpendicular to the C [bond]P [bond] C plane. The orientation of the phosphorus CS tensor for a third compound, Ru(2)(CO)(6)(mu(2)-PPh(2))(2), was investigated by the dipolar-chemical shift NMR technique and was found to be analogous, suggesting it to be the same in all compounds. Ab initio calculations of phosphorus magnetic shielding tensors have been carried out and reproduce the orientations found experimentally. The orientation of the CS tensor has been rationalized using simple frontier MO theory. Splittings due to (99,101)Ru [bond] (31)P spin-spin coupling have been observed for several of the complexes. A rare example of (189)Os [bond] (31)P spin-spin splittings is observed in the (31)P MAS NMR spectrum of the osmium cluster, where (1)J((189)Os, (31)P) is 367 Hz. For this complex, the (189)Os nuclear quadrupolar coupling constant is on the order of several hundred megahertz.     

Four-dimensional NMR spectroscopy of a 723-residue protein: chemical shift assignments and secondary structure of malate synthase g

A four-dimensional (4-D) NMR study of Escherichia coli malate synthase G (MSG), a 723-residue monomeric enzyme (81.4 kDa), is described. Virtually complete backbone (1)HN, (15)N, (13)C, and (13)C(beta) chemical shift assignments of this largely alpha-helical protein are reported. The assignment strategy follows from our previously described approach based on TROSY triple resonance 4-D NMR spectroscopy [Yang, D.; Kay, L. E. J. Am. Chem. Soc. 1999, 121, 2571-2575. Konrat, R; Yang, D; Kay, L. E. J. Biomol. NMR 1999, 15, 309-313] with a number of modifications necessitated by the large size of the protein. A protocol for refolding deuterated MSG in vitro was developed to protonate the amides deeply buried in the protein core. Of interest, during the course of the assignment, an isoaspartyl linkage in the protein sequence was unambiguously identified. Chemical shift assignments of this system are a first step in the study of how the domains of the protein change in response to ligand binding and for characterizing the dynamical properties of the enzyme that are likely important for function.     

A simple method for determining anisotropies of diamagnetic susceptibilities of liquid crystals: use of 1H NMR spectroscopy

In mixtures of oriented liquid crystals with opposite signs of diamagnetic susceptibility anisotropies, the point of macroscopic zero anisotropy can be detected by ¹H NMR from the degree of order of a dissolved molecule. On the basis of additivity and of the liquid-crystal molar concentrations relative anisotropies can be determined.     

Benzoxazine oligomers: evidence for a helical structure from solid-state NMR spectroscopy and DFT-based dynamics and chemical shift calculations

A combination of molecular modeling, DFT calculations, and advanced solid-state NMR experiments is used to elucidate the supramolecular structure of a series of benzoxazine oligomers. Intramolecular hydrogen bonds are characterized and identified as the driving forces for ring-shape and helical conformations of trimeric and tetrameric units. In fast MAS (1)H NMR spectra, the resonances of the protons forming the hydrogen bonds can be assigned and used for validating and refining the structure by means of DFT-based geometry optimizations and (1)H chemical-shift calculations. Also supporting these proposed structures are homonuclear (1)H[bond](1)H double-quantum NMR spectra, which identify the local proton-proton proximities in each material. Additionally, quantitative (15)N[bond](1)H distance measurements obtained by analysis of dipolar spinning sideband patterns confirm the optimized geometry of the tetramer. These results clearly support the predicted helical geometry of the benzoxazine polymer. This geometry, in which the N...H...O and O...H...O hydrogen bonds are protected on the inside of the helix, can account for many of the exemplary chemical properties of the polybenzoxazine materials. The combination of advanced experimental solid-state NMR spectroscopy with computational geometry optimizations, total energy, and NMR spectra calculations is a powerful tool for structural analysis. Its results provide significantly more confidence than the individual measurements or calculations alone, in particular, because the microscopic structure of many disordered systems cannot be elucidated by means of conventional methods due to lack of long-range order.