13C NMR chemical shift of single-wall carbon nanotubes

We compute the magnetic shielding tensor within the London approximation and estimate the Knight shift of single-wall carbon nanotubes. Our results indicate that high resolution 13C NMR should be able to separate the metallic and insulator character of the nanotubes since a 11 ppm splitting is predicted from the respective resonances. As a model for disorder, bending, and defects in these structures, we investigate the magnetic response of nanotubes with finite size. We get a small line broadening coming from an intrinsic length dependent resonance effect. The nanotube packing is also studied and leads to a 20 ppm broadening which disappears under experimental high-resolution conditions.     

Homonuclear decoupled 13C chemical shift anisotropy in 13C doubly labeled peptides by selective-pulse solid-state NMR

We describe a new experiment for measuring homonuclear-decoupled anisotropic chemical shift patterns in doubly 13C-labeled compounds under magic-angle spinning. The experiment combines a pair of selective and non-selective 180 degrees pulses to suppress the 13C-13C scalar and dipolar interactions. This is combined with the recently developed SUPER technique to recouple the chemical shift anisotropy. Demonstrations on 13Calpha and 13CO-labeled amino acids and peptides show that accurate chemical shift powder patterns can be obtained. This permits the use of chemical shift anisotropy for conformational studies of suitably extensively 13C-labeled peptides and proteins.     

(13)C chemical shift and (13)C-(14)N dipolar coupling tensors determined by (13)C rotary resonance solid-state NMR

This work explores the utility of simple rotary resonance experiments for the determination of the magnitude and orientation of (13)C chemical shift tensors relative to one or more (13)C--(14)N internuclear axes from (13)C magic-angle-spinning NMR experiments. The experiment relies on simultaneous recoupling of the anisotropic (13)C chemical shift and (13)C--(14)N dipole--dipole coupling interactions using 2D rotary resonance NMR with RF irradiation on the (13)C spins only. The method is demonstrated by experiments and numerical simulations for the (13)C(alpha) spins in powder samples of L-alanine and glycine with (13)C in natural abundance. To investigate the potential of the experiment for determination of relative/absolute tensor orientations and backbone dihedral angles in peptides, the influence from long-range dipolar coupling to sequential (14)N spins in a peptide chain ((14)N(i)--(13)C(alpha)(i)--(14)N(i+1) and (14)N(i+1)--(13)C'(i)--(14)N(i) three-spin systems) as well as residual quadrupolar-dipolar coupling cross-terms is analyzed numerically.     

13C NMR spectral assignments for nematic liquid crystals by 2D chemical shift gamma-encoding NMR

A novel method to accomplish 13C NMR spectral assignments for nematic liquid crystals is proposed. The two-dimensional (2D) isotropic/anisotropic chemical shift correlation spectrum is observed in which the anisotropic shift parameters are represented as sharp lines by gamma-encoding. The 13C spectral assignments can be made from the 2D spectrum with the aid of the 13C isotropic shift assignments for the same compound in the isotropic liquid state. The experiments were performed on p-methoxybenzilidene-p-n-butylaniline.     

Formation of stable nanowires from ferromagnetic metals using 2-butyne-1,4-diol

We mechanically fabricated Fe, Co, and Ni nanowires showing conductance quantization under electrochemical potential control. The effects of 2-butyne-1,4-diol and saccharin on the stability of the nanowires were investigated by measuring the conductance and the stretched length of the nanowires. Nanowires of these ferromagnetic metals were stabilized by adding 2-butyne-1,4-diol. In Co, a monoatomic contact was stretched 0.4 nm in length, indicating the formation of a highly stable monoatomic contact of ferromagnetic metals. The improved stability of the nanowires could originate from specific adsorption of organic molecules on the contacts. Mechanical properties of these metals were considered to explain the origin of the metal-dependent stability of the nanowires.     

13C NMR investigation of carbon nanotubes and derivatives

We report on nuclear magnetic resonance on single wall carbon nanotubes. Depending on the chemical preparation the electronic and dynamical properties of carbon nanotubes are presented and discussed. From a room temperature study of the spin lattice relaxation of carbon nanotubes prepared with various catalysts we clearly identified two components. In agreement with previous NMR studies and theoretical predictions, one-third of the intensity of the signal is found with a short relaxation time (about 5 s) attributed to metallic nanobutes while the rest of the signal presents a relaxation time of about 90 s corresponding to semiconducting nanotubes. In the case of oxidized or cut nanotubes only one relaxation time is observed with characteristics similar to the slow component. The disappearance of the fast relaxing component is associated with the absence of metallic nanotubes damaged by the chemical or mechanical treatments. In this case, the T dependence of the spin lattice relaxation reveals the effect of thermally activated small amplitude motions (twistons) of the nanotube in ropes. If diffusion of twistons might induce movement of ¹³C sites and local magnetic field fluctuations, orientational order could appear below the transition temperature of 170 K. In the last part, we present the theoretical predictions of chemical shift tensor in carbon nanotubes.     

Characterization of 15N chemical shift tensors via 15N-13C REDOR and 1N-1H dipolar-shift CPMAS NMR spectroscopy

As part of our studies on the characterization of 15N chemical shift anisotropy (CSA) via magic angle spinning (MAS) NMR spectroscopy, we have investigated via numerical simulations the sensitivity of two different REDOR experimental protocols to the angles defining the orientation of the 15N-13C' bond vector in the principal axis system of the 15N CSA tensor of the amide nitrogen in a peptide bond. Additionally, employing polycrystalline samples of 15N and 13C', 15N-labeled acetanilide, we have obtained, in a first study of this type, the orientation of the 15N CSA tensor in the molecular frame by orienting the tensor with respect to the 15N-3C' and 15N-1H dipolar vectors via 15N-13C' REDOR and 15N-1H dipolar-shift MAS experiments, respectively.     

Three-dimensional 13C shift/1H-15N coupling/15N shift solid-state NMR correlation spectroscopy.

Triple-resonance experiments capable of correlating directly bonded and proximate carbon and nitrogen backbone sites of uniformly 13C- and 15N-labeled peptides in stationary oriented samples are described. The pulse sequences integrate cross-polarization from 1H to 13C and from 13C to 15N with flip-flop (phase and frequency switched) Lee-Goldburg irradiation for both 13C homonuclear decoupling and 1H-15N spin exchange at the magic angle. Because heteronuclear decoupling is applied throughout, the three-dimensional pulse sequence yields 13C shift/1H-15N coupling/15N shift correlation spectra with single-line resonances in all three frequency dimensions. Not only do the three-dimensional spectra correlate 13C and 15N resonances, they are well resolved due to the three independent frequency dimensions, and they can provide up to four orientationally dependent frequencies as input for structure determination. These experiments have the potential to make sequential backbone resonance assignments in uniformly 13C- and 15N-labeled proteins.     

Symmetry-based recoupling of proton chemical shift anisotropies in ultrahigh-field solid-state NMR

A two-dimensional NMR experiment for estimating proton chemical shift anisotropies (CSAs) in solid powders under magic-angle spinning conditions is demonstrated in which 1H CSAs are reintroduced with a symmetry-based recoupling sequence while the individual proton sites are resolved according to their isotropic chemical shifts by magic-angle spinning (MAS) or combined rotation and multiple pulse (CRAMPS) homonuclear decoupling. The experiments where carried out on an ultrahigh-field solid-state NMR instrument (900 MHz 1H frequency) which leads to increased resolution and reliability of the measured 1H CSAs. The experiment is expected to be important for investigating hydrogen bonding in solids.     

A 2D MAS solid-state NMR method to recover the amplified heteronuclear dipolar and chemical shift anisotropic interactions

A two-dimensional solid-state NMR method for the measurement of chemical shift anisotropy tensors of X nuclei (15N or 13C) from multiple sites of a polypeptide powder sample is presented. This method employs rotor-synchronized pi pulses to amplify the magnitude of the inhomogeneous X-CSA and 1H-X dipolar coupling interactions. A combination of on-resonance and magic angle rf irradiation of protons is used to vary the ratio of the magnitudes of the 1H-X dipolar and X-CSA interactions which are recovered under MAS, in addition to suppressing the 1H-1H dipolar interactions. The increased number of spinning sidebands in the recovered anisotropic interactions is useful to determine the CSA tensors accurately. The performance of this method is examined for powder samples of N-acetyl-(15)N-L-valine (NAV), N-acetyl-15N-L-valyl-15N-L-leucine (NAVL), and alpha-13C-L-leucine. The sources of experimental errors in the measurement of CSA tensors and the application of the pulse sequences under high-field fast MAS operations are discussed.     

Modeling NMR chemical shifts: a comparison of charge models for solid state effects on N chemical shift tensors

This paper presents results from applying different point charge models to take into account intermolecular interactions to model the solid state effects on the ¹⁵N NMR chemical shifts tensors. The DFT approach with the BLYP gradient corrected exchange correlation functional has been used because it can include electron correlation effects at a reasonable cost and is able to reproduce ¹⁵N NMR chemical shifts with reasonable accuracy. The results obtained with the point charge models are compared with the experimental data and with results obtained using the cluster model, which includes explicitly neighboring molecular fragments. The results show that the point charge models can take into account solid state effects at a cost much lower than the cluster methods.     

1H and 13C solution- and solid-state NMR investigation into wax products from the Fischer-Tropsch process

(1)H and (13)C solid- and solution-state NMR have been used to characterise waxes produced in the Fischer-Tropsch reaction, using Co-based catalysts either unpromoted or promoted with approximately 1 wt% of either cerium or rhenium. The aim was to measure average structural information at the submolecular level of the hydrocarbon waxes produced, along with identification of the minor products, such as oxygenates and olefins, which are typically observed in these waxes. A parameter of key interest is the average number of carbon atoms within the hydrocarbon chain (N(C)). A wax prepared using an unpromoted Co/Al(2)O(3) catalyst had N(C)～20, whilst waxes made using rhenium- or cerium-promoted Co/Al(2)O(3) catalysts were found to have N(C)～21. All three samples contained small amounts of oxygenates and alkenes. The subtle differences found in the waxes, in particular the minor species produced, demonstrate that the different promoters have different effects during the reaction, with the Re-promoted catalyst producing the fewest by-products. It is shown in (13)C solid-state NMR spectra that for that for longer chain (compared to the lengths of chain in previous studies) waxes that the lack of resolution and the complexities added by the differential cross-polarisation (CP) dynamics mean that it is difficult to accurately determine N(C) from this approach. However the N(C) determined by (13)C CP magic angle spinning NMR is broadly consistent with the more accurate solution approaches used and suggest that the wax characteristics do not change in solution. On this basis an alternative approach for determining N(C) is suggested based on (1)H solution state NMR that provides a higher degree of accuracy of the chain length as well as information on the minor constituents.     

Calculation of 13C chemical shifts in rna nucleosides: structure-13C chemical shift relationships.

Isotropic 13C chemical shifts of the ribose sugar in model RNA nucleosides are calculated using SCF and DFT-GIAO ab initio methods for different combinations of ribose sugar pucker, exocyclic torsion angle, and glycosidic torsion angle. Idealized conformations were obtained using structures that were fully optimized by ab initio DFT methods starting with averaged parameters from a collection of crystallographic data. Solid-state coordinates of accurate crystal or neutron diffraction structures were also examined directly without optimization. The resulting 13C chemical shifts for the two sets of calculations are then compared. The GIAO-DFT method overestimates the shifts by an average of 5 ppm while the GIAO-SCF underestimates the shifts by the same amount. However, in the majority of cases the errors appear to be systematic, as the slope of a plot of calculated vs experimental shifts is very close to unity, with minimal scatter. The values of the 13C NMR shifts of the ribose sugar are therefore sufficiently precise to allow for statistical separation of sugar puckering modes and exocyclic torsion angle conformers, based on the canonical equation model formulated in a previous paper.     

31P and 13C chemical shielding tensors in the phosphoenolpyruvate moiety from rotary resonance recoupling 13C and 31P MAS and single crystal 31P NMR

A ³¹P and ¹³C NMR study of powder and single crystal samples of two phosphoenolpyruvate (PEP) compounds, the tris-ammonium salt monohydrate (NH₄)₃(PEP)·H₂O (1), and the mono-ammonium-salt (NH₄)(H₂PEP) (2) is presented. The P chemical shielding tensors in 1 are measured by ³¹P single crystal NMR on four minuscule samples and assigned without ambiguity by exploiting the orientation-dependent ³¹P-³¹p dipolar splittings of the resonance lines. The orientation of the ³¹P chemical shielding tensor is discussed in terms of the C_2v — and C₃-type distortions of the phosphate PO₄-coordination sphere. From ¹³C MAS NMR experiments with ³¹P rotary resonance recoupling on polycrystalline powder samples the orientations of the ³¹P chemical shielding tensors in 1 and 2 are obtained, for 1 in very good agreement with the ³¹P single crystal NMR results. Only some of the orientational parameters of the three ¹³C chemical shielding tensors in the PEP moiety of 1 could be derived from ¹³C MAS NMR experiments with ³¹P rotary resonance recoupling.     

A structure refinement strategy for NMR crystallography: an improved crystal structure of silica-ZSM-12 zeolite from 29Si chemical shift tensors

A strategy for performing crystal structure refinements with NMR chemical shift tensors is described in detail and implemented for the zeolite silica-ZSM-12 (framework type code MTW). The 29Si chemical shift tensors were determined from a slow magic-angle spinning spectrum obtained at an ultrahigh magnetic field of 21.1T. The Si and O atomic coordinate parameters were optimized to give the best agreement between experimentally measured and ab initio calculated principal components of the 29Si chemical shift tensors, with the closest Si-O, O-O, and Si-Si distances restrained to correspond with the distributions of the distances found in a set of single-crystal X-ray diffraction (XRD) structures of high-silica zeolites. An improved structure for the silica-ZSM-12 zeolite, compared to a prior structure derived from powder XRD data, is obtained in which the agreement between the experimental and calculated 29Si chemical shift tensors is dramatically improved, the Si-O, O-O, and Si-Si distances correspond to the expected distributions, while the calculated powder XRD pattern remains in good agreement with the experimental powder XRD data. It is anticipated that this "NMR crystallography" structure refinement strategy will be an important tool for the accurate structure determination of materials that are difficult to fully characterize by traditional diffraction methods.     

The Effect of pH on solid-state 13C NMR spectra of histidine

High-resolution solid-state ¹³C NMR spectra of histidine powder samples prepared from solutions at several pH values near the pK_a of the imidazole group are presented. These spectra demonstrate several effects due to the titration of the imidazole group. The chemical shifts for all of the carbon sites change upon titration. They are in “slow exchange” in the solid state in contrast to the “fast exchange” behavior seen in solution. Changes in the quadrupole interactions in the ¹⁴N sites occur upon titration and are observed by their effects on the resonance lineshapes of the ¹³C sites to which they are bonded.     

Double-quantum solid-state NMR of 13C spin pairs coupled to 14N

We examine the double-quantum magic angle spinning NMR spectra of pairs of 13C nuclei coupled to one or more 14N nuclei. The experimental spectra of 13C(2)-glycine and glycyl-[13C(2)]-glycyl-glycine are used to demonstrate the sensitivity of the spectra to the orientation of 14N quadrupole interaction tensors and to the molecular torsional angles.     

Chemisorption of cis-2-butene-1,4-diol on Si(100): A theoretical investigation

The adsorption structures of cis-2-butene-1,4-diol (C₄H₈O₂) on a Si(100) surface were investigated using density functional theory (DFT) calculations. The most stable configuration involves the adsorbed cis-2-butene-1,4-diol molecule with dissociated H forming a bridged structure between two surface Si atoms through dual O–Si bonding. The corresponding simulated images were able to explain previously reported experimental observations. The two stable bridged structures, either on-top or end-bridged, produced STM images consistent with the experimentally identified features.