Chemical Shift Tensor Principal Values Research Articles

Establishing Accurate High-Resolution Crystal Structures in the Absence of Diffraction Data and Single Crystals—An NMR Approach

Predicting accurate crystal structures from theoretical consideration has proven to be remarkably challenging. Although significant progress has been made and numerous approaches have now been investigated, selection of the correct structure as the first choice in blind studies is still rarely achieved. Here a process is described that consistently identifies the correct structure from the myriad candidates created from typical crystal structure prediction software. This approach relies on 13C solid-state NMR data and a secondary refinement process that includes lattice fields. Four structures are considered, and in all cases the correct structure is selected as the first choice and the only statistically feasible candidate. Data from 13C chemical shift tensor principal values are found to provide better selectivity, but 13C isotropic shifts also consistently identify the correct structure. This process involves only experimental NMR data and computer-generated structures, yet the structures created appea...

15N Solid-State NMR as a Probe of Flavin H-Bonding

Flavins mediate a wide variety of chemical reactions in biology. To learn how one cofactor can be made to execute different reactions in different enzymes, we are developing solid-state NMR (SSNMR) to probe the flavin electronic structure, via the (15)N chemical shift tensor principal values (δ(ii)). We find that SSNMR has superior responsiveness to H-bonds, compared to solution NMR. H-bonding to a model of the flavodoxin active site produced an increase of 10 ppm in the δ(11) of N5, although none of the H-bonds directly engage N5, and solution NMR detected only a 4 ppm increase in the isotropic chemical shift (δ(iso)). Moreover SSNMR responded differently to different H-bonding environments, as H-bonding with water caused δ(11) to decrease by 6 ppm, whereas δ(iso) increased by less than 1 ppm. Our density functional theoretical (DFT) calculations reproduce the observations, validating the use of computed electronic structures to understand how H-bonds modulate the flavin's reactivity.

Solid‐state 13C NMR and quantum chemical investigation of metal diene complexes

This paper presents novel measurements and calculations of the olefinic (13)C chemical shift tensor principal values in several metal diene complexes. The experimental values and the calculations show shifts as large as 70 ppm with respect to the values in the parent olefinic compounds. These shifts are highly anisotropic, with the largest ones observed in the less shielded principal components and the smallest ones in the most shielded principal components of the tensor. The orientations of the principal components of the tensors remain, within 10 degrees , at their directions in ethylene and other olefinic compounds. The calculations, performed using the GIAO method and the LanDZ pseudopotential basis set, show good agreement with the experiments, and were used to establish definite evidence for the existence of a Cl-bridge structure in the bicyclo[2.2.1]hepta-2,5-diene (BCHD)dichlororuthenium(II) polymer.

15N NMR Chemical Shift Tensors of Substituted Hexaazaisowurtzitanes: The Intermediates in the Synthesis of CL-20

Several cage-substituted hexaazaisowurtzitanes [2,4,6,8,10,12-hexabenzyl-2,4,6,8,10,12-hexaazaisowurtzitane (HBIW); 2,6,8,12-tetraacetyl-4,10-dibenzyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADB); and 2,6,8,12-tetraacetyl-4,10-diformyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADF)] have been synthesized as intermediate steps in the synthesis of CL-20. The 15N NMR isotropic and chemical shift tensor principal values have been obtained for these intermediates using CP/MAS and FIREMAT methods. Solid-state 15N NMR offers unique advantages in the study of these materials, due to solubility issues and lack of high-quality X-ray crystallographic data. In addition, the caged structures possess unique electronic properties that can be useful in further refining theoretical methods. The effects of the different functional groups on the chemical shift tensor principal values of these caged amines have been evaluated and are shown to be reflected in the total anisotropy. The position of the amine within the cage is manifested in the δ22 values. Nitrogen-15 NMR shielding calculations using the embedded ion method (EIM) to account for long-range intermolecular interactions show a good correlation, rmsd = 4 ppm, with the observed chemical shift tensor values.

The 13C Chemical Shift Tensor Principal Values and Orientations in Dialkyl Carbonates and Trithiocarbonates

The 13C chemical shift tensor principal values for the trigonal carbonate and thiocarbonate carbon atoms in the dialkyl carbonates, dimethyl carbonate, ethylene carbonate, and diphenyl carbonate, and in the trithiocarbonates, ethylene trithiocarbonate and dimethyl trithiocarbonate, respectively, were measured in various solid-state one-dimensional and two-dimensional nuclear magnetic resonance experiments. Furthermore, the chemical shift tensor principal values and orientations were calculated for the corresponding isolated molecules with quantum mechanically fully optimized geometries. Proton-optimized X-ray geometries of ethylene carbonate, ethylene trithiocarbonate, and diphenyl carbonate were used in embedded ion method (EIM) calculations and in calculations on the isolated molecules to obtain the theoretical principal values and to assign the chemical shift tensor orientations in these three compounds. Considerable improvement in the correlation between the experimental and calculated principal values is obtained when the electrostatic crystal potentials are included in EIM calculations. The chemical shift tensor orientations and principal values obtained for the dialkyl compounds in this study complement the previous data on a series of ionic potassium carbonates and thiocarbonates.

Determination of 13C Chemical Shift Tensors in the Presence of Hydrogen Bonding and 14N Quadrupolar Coupling: p-Aminosalicylic Acid, Isoniazid, and Pyrazinamide

The 1 3 C chemical-shift tensor principal values were measured for p-aminosalicylic acid, pyrazinamide, and isoniazid. A moderate-strength hydrogen bond between adjacent p-aminosalicylic acid molecules in the crystal was simulated several different ways in attempts to find a reasonable model of the electrostatic interactions in the crystal. Crystal lattice effects on the calculated chemical-shift tensors are treated by application of the embedded ion method (EIM). Coupling between 1 4 N and several 1 3 C atoms was included in the data analysis to provide more accurate 1 3 C chemical-shift tensor principal values. The chemical-shift tensors are assigned with a permutative algorithm based upon the computed chemical shifts.

13C and (15)N chemical shift tensors in adenosine, guanosine dihydrate, 2'-deoxythymidine, and cytidine.

The (13)C and (15)N chemical shift tensor principal values for adenosine, guanosine dihydrate, 2'-deoxythymidine, and cytidine are measured on natural abundance samples. Additionally, the (13)C and (15)N chemical shielding tensor principal values in these four nucleosides are calculated utilizing various theoretical approaches. Embedded ion method (EIM) calculations improve significantly the precision with which the experimental principal values are reproduced over calculations on the corresponding isolated molecules with proton-optimized geometries. The (13)C and (15)N chemical shift tensor orientations are reliably assigned in the molecular frames of the nucleosides based upon chemical shielding tensor calculations employing the EIM. The differences between principal values obtained in EIM calculations and in calculations on isolated molecules with proton positions optimized inside a point charge array are used to estimate the contributions to chemical shielding arising from intermolecular interactions. Moreover, the (13)C and (15)N chemical shift tensor orientations and principal values correlate with the molecular structure and the crystallographic environment for the nucleosides and agree with data obtained previously for related compounds. The effects of variations in certain EIM parameters on the accuracy of the shielding tensor calculations are investigated.

207Pb NMR of PbZrO3 and PbZr1-xTixO3 Solid Solutions

ABSTRACTPbZrO3 undergoes a phase transition to a paraelectric phase at 230°C. During this phase transition the unit cell changes from orthorhombic to cubic. Structural changes of PbZrO3 have been monitored using solid state NMR by measuring the variation in the 207Pb chemical shielding tensor as a function of temperature. The two distinct lead sites show rather different behavior as a function of temperature. The less shielded lead maintains an almost constant asymmetry parameter η = 0.2 from 0°C to 200°C while the more shielded lead resonance becomes more axially symmetric as the temperature is raised going from η = 0.24 at 0°C to [.eta] = 0.08 at 200°C. Powder pattern singularities become less distinct near the phase transitions, but the temperature dependence of the chemical shift tensor principal values remains continuous, and there is no evidence of an intervening higher-symmetry phase. At the phase transition, both resonances collapse into a single narrow line characteristic of a nucleus at a high-symmetry site. Results of a preliminary study of the PbZr1−xTixO3 solid solutions by 207Pb solid state NMR are also presented.

A sensitive, high resolution magic angle turning experiment for measuring chemical shift tensor principal values

A sensitive, high-resolution ‘FIREMAT’ two-dimensional (2D) magic-angle-turning experiment is described that measures chemical shift tensor principal values in powdered solids. The spectra display spinning-sideband patterns separated by their isotropic shifts. The new method's sensitivity and high resolution in the isotropic-shift dimension result from combining the 5π magic-angle-turning pulse sequence, an extension of the pseudo-2D sideband-suppression data rearrangement, and the TIGER protocol for processing 2D data. TPPM decoupling is used to enhance resolution. The method requires precise synchronization of the pulses and sampling to the rotor position. It is shown that the technique obtains 35 natural-abundance 13C tensors from erythromycin in 19 hours, and high quality natural-abundance 15N tensors from eight sites in potassium penicillin V in three days on a 400 MHz spectrometer.

A sensitive, high resolution magic angle turning experiment for measuring chemical shift tensor principal values

A sensitive, high-resolution ‘FIREMAT’ two-dimensional (2D) magic-angle-turning experiment is described that measures chemical shift tensor principal values in powdered solids. The spectra display spinning-sideband patterns separated by their isotropic shifts. The new method's sensitivity and high resolution in the isotropic-shift dimension result from combining the 5π magic-angle-turning pulse sequence, an extension of the pseudo-2D sideband-suppression data rearrangement, and the TIGER protocol for processing 2D data. TPPM decoupling is used to enhance resolution. The method requires precise synchronization of the pulses and sampling to the rotor position. It is shown that the technique obtains 35 natural-abundance 13C tensors from erythromycin in 19 hours, and high quality natural-abundance 15N tensors from eight sites in potassium penicillin V in three days on a 400 MHz spectrometer.

15N Chemical Shift Principal Values in Nitrogen Heterocycles

This paper presents data on the 15N chemical shift tensor principal values in a series of 15N-enriched heterocycles. Compounds that are liquids at room temperature were frozen, and the principal values of all compounds studied were measured from static powder patterns. Four different types of nitrogen tensors are described, consisting of protonated and nonprotonated nitrogens in both five- and six-membered rings. The principal values were oriented on the molecular frame using the DFT quantum mechanical calculations of the 15N chemical shielding tensors. The agreement between the calculated and experimental principal values is adequate to make these assignments, but the relative scatters are greater than those observed in similar 13C chemical shift calculations. The largest shift component, δ11, is always oriented in the radial direction to the ring for substituted nitrogens but is tangential to the ring for the nonsubstituted nitrogens. The large variations observed in the nitrogen chemical shift tensors upon changing the nitrogen hybridization can be explained using qualitative arguments on the localization of the smallest bonding-antibonding or HOMO−LUMO gap in the molecule. The orientation of the largest shift component is always found in the plane of the molecule and is approximately perpendicular to the plane containing the bonding−antibonding or HOMO−LUMO pair of orbitals with the smallest energy gap.