Carbon Chemical Shift Tensors Research Articles

Carbon-13 Chemical Shift Tensors of Disaccharides: Measurement, Computation and Assignment

A recently developed chemical shift anisotropy amplification solid-state nuclear magnetic resonance (NMR) experiment is applied to the measurement of the chemical shift tensors in three disaccharides: sucrose, maltose, and trehalose. The measured tensor principal values are compared with those calculated from first principles using density functional theory within the planewave-pseudopotential approach. In addition, a method of assigning poorly dispersed NMR spectra, based on comparing experimental and calculated shift anisotropies as well as isotropic shifts, is demonstrated.

High nematic ordering in a bent-core smectogen showing a biaxial nematic phase: A 13C NMR study

Carbon-13 NMR spectroscopy has been used to probe the orientational ordering of a bent-core liquid crystal A131. Both the carbon chemical shift tensors in a rotating solid sample and chemical shifts from the aligned mesophases were determined. These results allow us to probe the local order parameters of the five phenyl rings in the liquid crystalline molecule. It is found that the conformation and ordering of the bent-shaped core significantly change between the uniaxial and biaxial nematic phases, with the highest local nematic order not associated to the central ring in this azo substituted bent-core mesogen.

Order parameters of α,ω-diphenylpolyenes in a nematic liquid crystal from an integrated computational and C13 NMR spectroscopic approach

The orientational order parameters and conformational behavior of five relatively large rodlike molecules, biphenyl, trans-stilbene, 1,3-diphenyl-butadiene, 1,3,5-diphenyl-hexatriene, and 1,3,5,7-diphenyl-octatetraene, dissolved in the thermotropic liquid crystal ZLI-1167, have been studied using an integrated approach combining (13)C NMR measurements and quantum mechanical computations of carbon chemical shift tensors. Besides biphenyl, the phenyl moiety of all structures has been found to have a high rotational mobility in the temperature range of the present experiments. The rank-two order parameter <P(2)> in the nematic phase is found to increase steadily from the shortest to the longest term of the series at any temperature within the nematic range. The molecular biaxiality order parameter <D(02) (2)> is found to be small and essentially constant with temperature, giving further support to the common assumption of effective uniaxiality for these probes.

Acetylenic Carbon-13 Chemical Shift Tensors for Diphenylacetylene and (η2-Diphenylacetylene)Pt(PPh3)2: A Solid-State NMR and Theoretical Study

The structure of (eta2-diphenylacetylene)Pt(PPh3) (2), as well as those of its dichloromethane and benzene solvates, is determined via X-ray crystallography. An investigation of the chemical shift (CS) tensors of the 13C-labeled carbons in Ph13C13CPh and (eta2-Ph13C13CPh)Pt(PPh3)2.(C6H6) is carried out via analysis of 13C NMR spectra from stationary solid samples. The principal components of the CS tensors as well as their orientations with respect to the 13C,13C internuclear vector are determined. DFT calculations of these CS tensors are in close agreement with the experimental values. For diphenylacetylene (tolane), the orientations and principal-component magnitudes of the alkynyl carbon CS tensors are comparable to those for other alkynyl carbons, although the CS tensor is not axially symmetric in this case. Coordination to platinum causes a change in the CS tensor orientation and a net increase in the isotropic chemical shift, resulting from a significant increase in two principal components (delta11 and delta33) while the third (delta22) decreases only slightly. The measured carbon CS tensors in the platinum complex bear a striking similarity to those of the alkenyl carbons in trans-Ph(H)C=C(H)Ph, and a short theoretical discussion of these observations is presented.

A solid-state NMR investigation of orexin-B

Some key aspects of the secondary structure of solid orexin-B, a 28 amino-acid peptide, have been investigated by solid-state NMR spectroscopy. The 13C–15N dipolar coupling between the carbonyl carbon of Leu11 and the nitrogen of Leu15, as determined by rotational echo double resonance (REDOR) experiments, is 35 Hz, indicating that these nuclei are separated by approximately 4.5 Å. This distance is consistent with the α-helical structure determined for this segment of orexin-B by solution NMR measurements. REDOR measurements of the dipolar coupling between the carbonyl carbon of Ala17 and the nitrogen of Ala22 support the contention in an earlier solution NMR study that a bend exists between the two α helices of orexin-B. However, in the solid state the internuclear distance (6.4 Å) is significantly greater than that observed for orexin-B in aqueous solution. In addition to the distance measurements, the principal components of the amide carbonyl carbon chemical shift (CS) tensors for Leu11 and Ala17 and of the amide nitrogen CS tensors for Leu15 and Ala22 are reported. There are only minor differences between the amide carbonyl carbon CS tensors for Leu11 and Ala17 and between the nitrogen CS tensors for Leu15 and Ala22.Key words: orexin-B, solid-state NMR, REDOR, chemical shift tensors.

13C NMR chemical shifts of the triclinic and monoclinic crystal forms of valinomycin.

Two different crystalline polymorphs of valinomycin, the triclinic and monoclinic forms, have been studied by high resolution, solid state (13)C CP-MAS NMR spectroscopy. Although the two polymorphs of the crystal are remarkably similar, there are distinct differences in the isotropic chemical shifts between the two spectra. For the triclinic form, the carbon chemical shift tensor components for the alpha carbons adjacent to oxygen in the lactic acid and hydroxyisovaleric acid residues and the ester carbonyls of the valine residue were obtained using the FIREMAT experiment. From the measured components, it was found that the behavior of the isotropic chemical shift, delta(iso), for valine residue ester carbonyl carbons is predominately influenced by the intermediate component, delta(22). Additionally it was found that the smallest shift component, delta(33), for the L -lactic acid ( L -Lac) and D -alpha-hydroxyisovaleric acid ( D -Hyi) C(alpha)-O carbon was significantly displaced depending upon the nature of individual amino acid residues, and it is the delta(33) component that governs the behavior of delta(iso) in these alpha carbons.

Mechanochemical preparation of hydrogen-bonded adducts between the diamine 1,4-diazabicyclo[2.2.2]octane and dicarboxylic acids of variable chain length: an X-ray diffraction and solid-state NMR study.

Mechanical mixing of solid dicarboxylic acids of variable chain length HOOC(CH(2))(n)COOH (n = 1-7) with solid 1,4-diazabicyclo[2.2.2]octane generates the corresponding salts or co-crystals of the formula [N(CH(2)CH(2))(3)N]-H-[OOC(CH(2))(n)COOH] (n=1-7). Preparation of the same systems from solution has been instrumental for a full characterization of the mechanochemical products by means of single-crystal and powder-diffraction X-ray analyses, as well as by solid-state NMR. The acid-base adducts, whether involving proton transfer from the COOH group to the N-acceptor, that is having ((-))O...H-N((+)) interactions, or the formation of neutral O-H...N hydrogen bonds, show a melting point alternation phenomenon analogous to that shown by the neutral carboxylic acids. The carbon chemical shift tensors of the COOH group obtained from the sideband intensity of low speed spinning NMR spectra provide a reliable criterion for assigning the protonation state of the adducts.

Probing Segmental Mobility in the Cyanogenic Glycoside Amygdalin by 13C Solid-State NMR

Local motions in the cyanogenic glycoside amygdalin have been characterized by variable-temperature 1 3 C solid-state NMR spectroscopy. Carbon-13 chemical shift tensors and 1 H- 1 3 C dipolar couplings were measured by advanced magic-angle spinning NMR techniques over the temperature range 0-90 °C. The main conclusions from our study are that the phenyl ring exhibits thermally activated dynamics about the para axis, whereas the glucopyranosyl residues remain immobile upon heating. It was also found that sample heating is accompanied by loss of water. The experimental results were complemented by quantum chemical calculations of the 1 3 C chemical shift tensors, which were utilized in the analysis of the experimental NMR parameters.

A solid-state nitrogen-15 NMR and ab initio study of nitrobenzenes.

Insight into the unexpectedly small range of isotropic nitrogen chemical shifts in nitrobenzene derivatives is gained through measurements of the chemical shift (CS) tensor by solid-state NMR experiments and ab initio molecular orbital (MO) and density functional theory (DFT) calculations. The principal components, delta(ii), of the (15)N CS tensors have been measured for nitrobenzene, 4-nitroaniline, 4-nitrotoluene, 4-nitroanisole, 4-nitroacetophenone, nitromesitylene, and 2,4,6-tri-tert-butylnitrobenzene. No obvious correlations of the delta(ii) values with traditional reactivity parameters were observed. The CS tensor components change significantly for the para-substituted nitrobenzenes, but these variations nearly cancel to yield isotropic shifts that fall in a range of only 3 ppm. Ab initio calculations of the delta(ii) values at the HF level are in poor agreement with the experimental values, whereas MP2 calculations and DFT calculations employing the B3LYP functional are in better agreement with experiment. The calculated (B3LYP/6-311G) delta(ii) values follow a trend in which delta(11) and delta(33) increase while delta(22) decreases with the accepted electron withdrawing ability of the para substituent. These changes tend to cancel yielding a variation in delta(iso) of only 4 ppm. These calculations indicate that the CS tensor has the same orientation as the carbon CS tensor in the isoelectronic benzoate anion: delta(11) bisects the O-N-O angle, delta(33) is perpendicular to the NO(2) plane, and delta(22) is in the NO(2) plane and perpendicular to delta(11).

A 95Mo and 13C solid-state NMR and relativistic DFT investigation of mesitylenetricarbonylmolybdenum(0) – a typical transition metal piano-stool complex

The chemical shift (CS) and electric field gradient (EFG) tensors in the piano-stool compound mesitylenetricarbonylmolybdenum(0), 1, have been investigated via95Mo and 13C solid-state magic-angle spinning (MAS) NMR as well as relativistic zeroth-order regular approximation density functional theory (ZORA-DFT) calculations. Molybdenum-95 (I = 5/2) MAS NMR spectra acquired at 18.8 T are dominated by the anisotropic chemical shift interaction (Ω = 775 ± 30 ppm) rather than the 2nd-order quadrupolar interaction (CQ = −0.96 ± 0.15 MHz), an unusual situation for a quadrupolar nucleus. ZORA-DFT calculations of the 95Mo EFG and CS tensors are in agreement with the experimental data. Mixing of appropriate occupied and virtual d-orbital dominated MOs in the region of the HOMO-LUMO gap are shown to be responsible for the large chemical shift anisotropy. The small, but non-negligible, 95Mo quadrupolar interaction is discussed in terms of the geometry about Mo. Carbon-13 CPMAS spectra acquired at 4.7 T demonstrate the crystallographic and magnetic nonequivalence of the twelve 13C nuclei in 1, despite the chemical equivalence of some of these nuclei in isotropic solutions. The principal components of the carbon CS tensors are determined via a Herzfeld–Berger analysis, and indicate that motion of the mesitylene ring is slow compared to a rate which would influence the carbon CS tensors (i.e. tens of μs). ZORA-DFT calculations reproduce the experimental carbon CS tensors accurately. Oxygen-17 EFG and CS tensors for 1 are also calculated and discussed in terms of existing experimental data for related molybdenum carbonyl compounds. This work provides an example of the information available from combined multi-field solid-state multinuclear magnetic resonance and computational investigations of transition metal compounds, in particular the direct study of quadrupolar transition metal nuclei with relatively small magnetic moments.

Chain Packing in Linear Phenol−Polycarbonate by 13C{2H} REDOR

Interchain packing in [carbonyl-13C, phenol-2H]phenol-substituted bisphenol A polycarbonate (a totally amorphous polymer by X-ray powder diffraction) has been characterized by 13C{2H} rotational-echo double-resonance (REDOR) NMR measurements on a 10 A distance scale. Differences in the REDOR dephasing rates of the centerband and spinning sidebands of the carbonyl carbon resonance prove the presence of local orientational order. Approximately 70% of the repeat units are locally ordered. For the most tightly packed pairs of chains, the phenol 2H of one chain is 4−5 A from the carbonate 13C of the other chain, and the interchain 2H−13C dipolar vector has an orientation of approximately 30° (azimuthal) and 70° (polar) in the carbonyl carbon chemical shift tensor reference frame. The REDOR determination of these packing parameters is model-independent. Both random and chain-pair models were generated to describe the chain packing, but only the chain-pair model was consistent with the REDOR distance and orienta...

A 13C Solid-State NMR Investigation of the Alkynyl Carbon Chemical Shift Tensors for 2-Butyne-1,4-diol

The alkynyl carbon chemical shift (CS) tensors for 2-butyne-1,4-diol are reported, based on analyses of the carbon-13 NMR spectra of stationary-powder and slow magic-angle spinning (MAS) samples for which the alkynyl carbon nuclei are enriched in 13C. NMR spectra of slow MAS samples exhibit spinning-frequency-dependent fine structure typical of crystallographically equivalent but magnetically distinct nuclei. Simulated spectra of slow MAS samples of this two-spin system are particularly sensitive to the relative orientations of the CS tensors. In addition, the value of 1J(13C, 13C),+175±10 Hz, is determined by examination of the total NMR lineshape of slow MAS samples. The CS tensors are almost axially symmetric, δ11=158.9±1.0 ppm and δ22=155.7±1.0 ppm; the direction of greatest shielding is approximately along the alkynyl CC bond, δ33=−57.8±2.0 ppm. Both the magnitudes of the principal components of the CS tensors and their orientations are in agreement with those predicted from first-principles calculations at the HF and MP2 levels of theory. This study demonstrates the importance of examining the NMR spectra of homonuclear two-spin systems with and without MAS under a variety of conditions (e.g., two or more applied magnetic fields and slow MAS).

A Carbon-13 and Deuterium NMR Investigation of Solid Platinum−Ethylene Complexes: Zeise's Salt and Pt(η2-C2H4)(PPh3)2

The carbon chemical shift (CS) tensors for two platinum−ethylene complexes, ethylene-13C2-bis(triphenylphosphine)platinum(0) and potassium trichloro(ethylene-13C2)platinate(II), have been characterized by the dipolar-chemical shift method and with 2D spin−echo NMR experiments. The carbon CS tensors of the ethylene ligand are significantly modified upon coordination with platinum, particularly for the Pt(0) complex, to which ethylene is strongly coordinated. The most shielded principal component, δ33, perpendicular to the molecular plane in ethylene, is relatively unaffected by coordination; the changes to the CS tensors arise mainly from the increased shielding in the directions corresponding to δ11 and δ22. Hence, the span of the chemical shift tensor decreases from 210 ppm for ethylene (Zilm, K. W.; Conlin, R. T.; Grant, D. M.; Michl, J. J. Am. Chem. Soc. 1980, 102, 6672) to 150 ppm for the Pt(II) complex, and to 48 and 55 ppm, respectively, for the two nonequivalent carbon nuclei of the Pt(0) complex. The orientations of the carbon CS tensor components relative to the 13C,13C dipolar vector are also determined from this analysis. Orientations of the carbon CS tensors in the molecular framework are proposed on the basis of a combination of the experimental results and ab initio calculations using the GIAO method. Deuterium NMR studies of the ethylene-2H4 derivatives of the title compounds are characterized by long 2H T1s and by quadrupolar coupling constants which are comparable in magnitude to that observed for rigid olefins, demonstrating that the ethylene ligand is not subject to significant motion. This conclusion is supported by ab initio calculations which indicate barriers to internal rotation for the ethylene ligand in excess of 80 kJ mol-1 in both complexes.

An Experimental and Theoretical Investigation of the Olefinic Carbon Chemical Shift Tensors intrans-Stilbene and Pt(η2-trans-stilbene)(PPh3)2

The olefinic carbon chemical shift tensors of trans-stilbene-α,β-13C2 (1) and (trans-stilbene-α,β-13C2)[bis(triphenylphosphine)]platinum(0) (2) have been characterized by solid-state 13C NMR spectroscopy. Analyses of the 13C NMR spectra obtained for stationary powder samples of 1 and 2 at 4.7 and 9.4 T yield the principal components of the carbon chemical shift tensors. The presence of a homonuclear spin pair in these compounds provides information about the orientation of the chemical shift tensors in the dipolar frame of reference. The span, Ω, and skew, κ, of the olefinic carbon chemical shift tensor of 1 are 166 ppm and −0.145, respectively, comparable to those of other known olefinic carbons. The carbon chemical shift is largest when the applied magnetic field is in the plane of the vinyl group, perpendicular to the olefinic C,C bond (δ11 = 215 ppm) and smallest when it is perpendicular to the vinyl group (δ33 = 49 ppm). The intermediate principal component of the chemical shift tensor, δ22 = 120 ppm, is oriented along the olefinic C,C bond. Upon coordination to platinum(0), the isotropic carbon chemical shift changes from 128 to 68 ppm, and the span of the chemical shift tensor decreases dramatically, Ω = 51 ppm. The experimental results are compared with those from ab initio shielding calculations performed with a combination of density functional theory (DFT) and the gauge-including atomic orbitals (GIAO) method. Overall, agreement with experiment is good. The combined experimental and theoretical approach allows one to suggest the most likely orientation of the carbon chemical shift tensor for 2 in the molecular axis system.

Carbon-13 Chemical Shift Tensors in Pentaerythritol

The carbon-13 chemical shift tensors of pentaerythritol (C(CH2OH)4, tetrakis(hydroxymethyl)methane) are measured with use of the two-dimensional chemical shift correlation technique. The results are interpreted with quantum-chemical ab initio gauge invariant atomic orbital computations of the nuclear shielding tensors based on X-ray and neutron diffraction structures, and on corresponding structures modified by optimizing the CH2 proton positions with the Gaussian 94 program. The X-ray based computations correlate with the experimental tensors with a root-mean-square deviation of 4.45 ppm, while the neutron structure gives a root-mean-square deviation of 0.86 ppm. After optimizing the CH2 proton positions for the X-ray and neutron structures, root-mean-square deviations of 1.19 and 0.84 ppm, respectively, are calculated. Tensor computations for the central carbon were done with hypothetical structures obtained by altering the C−C−C angle. The difference between the two computed principal values of this axially symmetric tensor varies at a rate of 2.7 ppm/deg as the angle changes. The experimentally measured chemical shift difference of 14.6 ppm corresponds to a C−C−C bond angle of 106.6°, in good agreement with the X-ray diffraction value of 107.3° and the neutron diffraction value of 107.1°.

13C MAS NMR Spectra ofcis,cis-Mucononitrile in Liquid-Crystalline Media and in the Solid State. Orientation of the Carbon Chemical-Shift Tensors

The spinning sidebands observed in the $^{13}C$ MAS NMR spectra ofcis,cis-mucononitrile oriented in liquid-crystalline media and of the neat sample in the solid state are studied. There are differences in the sideband intensity patterns in the two cases. These differences arise because the order parameters which characterize the orientation of the solute in the liquid-crystalline media differ for different axes. It is shown that, in general, the relative intensities of the sidebands contain information on the sign and magnitude of an effective chemical-shift parameter which is a function of the sum of the products of the principal components of the chemical-shift tensor and the corresponding order parameters with respect to the director. A method for obtaining the orientation of the carbon chemical-shift tensor is proposed. The carbon chemical-shift tensors obtained from gauge-including atomic orbital calculations are also presented for comparison.

Order parameters and carbon shielding tensors of bis-MSB from 13C NMR measurements in a nematic liquid crystal

Abstract The order and biaxiality of the fluorescent molecule p -bis(0-methylstyryl)benzene (bis-MSB) in the nematic phase of the liquid crystal ZLI-1167 have been determined as a function of temperature by means of a recently developed 13 C NMR approach. The method can also provide information on the carbon chemical shift tensors and we demonstrate that it can be readily applied to molecules containing a fairly large number of chemically different carbons, even if the assignment of the shifts in the isotropic phase is not available.

Analyzing and Assigning Carbon-13 Chemical-Shift Tensors in Fructose, Sorbose, and Xylose Single Crystals

Abstract Complete carbon-13 chemical-shift tensors are measured in single crystals of the monosaccharides β- D -fructopyranose [57-48-7], α- L -sorbopyranose [87-79-6], and α- D -xylopyranose [58-86-6] using the two-dimensional chemical-shift correlation technique with a multiple-axis sample-reorientation mechanism. Symmetry-constrained fitting of the experimental data extracts the tensors for the four equivalent molecules in theP212121unit cells and the directions of the three twofold axes in the orthorhombic crystals. The standard deviations of the symmetry fits to the data are 0.30, 0.40, and 0.29 ppm for the three monosaccharide molecules, respectively. Gauge-invariant atomic orbital (GIAO) computations using the D-95 double-zeta basis set are then used to assign the experimental tensors to the carbons in the molecules and to the molecules in the unit cell. All assignment permutations and combinations are explored, and that which minimizes the RMS deviation between the experimental and computed tensors is accepted as the best assignment. The GIAO results also identify the experimental twofold axes with the crystallographic axes. Because the hydroxymethyl hydroxyl group in sorbose jumps between two positions, it is necessary to compute the tensors for both molecular configurations and then to take a weighted average of the results for comparison with the experimental data. The D-95 GIAO results have RMS deviations from the experimental tensors of 1.90 ppm for fructose, 2.61 ppm for sorbose, and 2.66 ppm for xylose. Similar computations confirm a previous assignment of the carbon-13 tensors in sucrose and yield a 2.85 ppm RMS deviation.

Analyzing and Assigning Carbon-13 Chemical-Shift Tensors in α-L-Rhamnose Monohydrate Single Crystals

Abstract The carbon-13 chemical-shift tensors of α- L -rhamnose (6-deoxy- L -mannose) monohydrate (C6H12O5·H2O [10030-85-0]) are measured using two-dimensional chemical-shift correlation spectroscopy. The tensors and the orientation of the twofold axis along thebcrystallographic axis in the sample frame are obtained by symmetry-constrained fitting of the data.Ab initiogauge invariant atomic orbital computations of the chemical shielding are then used to assign the tensors to the carbons in the rhamnose molecule and to the two molecules in the unit cell. These computations are also used to find the direction of thebcrystallographic axis along the twofold axis and to make initial estimates of theaandccrystallographic axes in the sample. The finalaandccrystallographic axes are then determined with a 2D dipolar spectrum. A second independent method, 1D spectra taken with the applied field along directions perpendicular to the (011), the (110), and the (100) crystallographic planes, is used to confirm the result.

Carbon-13 Chemical Shift Tensors in Methyl Glycosides, Comparing Diffraction and Optimized Structures with Single-Crystal NMR

Complete carbon-13 chemical shift tensors are measured in single crystals of methyl α-d-galactopyranoside monohydrate, methyl α-d-glucopyranoside, methyl α-d-mannopyranoside, methyl β-d-galactopyranoside, methyl β-d-glucopyranoside hemihydrate, and methyl β-d-xylopyranoside. The fits of the experimental data to the second-rank form of shift tensors reflect the accuracy of the measured tensors and yield standard deviations that range between 0.27 and 0.75 ppm. Ab initio gauge-invariant atomic orbital (GIAO) computations using the D-95 double-ζ basis set are used to assign the experimental tensors to the carbons in the unit cell. The root-mean-square (rms) deviation of the diffraction-structure-based GIAO shieldings fitted to all of the experimental shifts is 4.99 ppm. By optimizing the ring and methyl proton positions with the Gaussian-92 program and repeating the GIAO computations, the root-mean-square deviation is reduced to 2.40 ppm. These results illustrate that complete 13C chemical shift tensors meas...