13C Chemical Shielding Tensors Research Articles

13C Chemical Shielding Tensors: A Combined Solid-State NMR and DFT Study of the Role of Small-Amplitude Motions

Nuclear chemical shielding tensors (CSTs) provide valuable information on structural and dynamic properties of molecular fragments in the solid state, and they are often exploited to obtain a detailed characterization of reorientational or interconformational motions in the kilohertz regime. In this work, we introduce a new combined computational DFT and experimental solid-state NMR approach to investigate the effects of small-amplitude vibrational motions on 13C CSTs. This approach was tested on ibuprofen, for which the influence of reorientational motions on CSTs could be ruled out from a previous detailed investigation of its dynamic properties. The data relative to the carbons belonging to the phenyl moiety were analyzed and discussed in terms of phenyl ring librations and in-plane and out-of-plane C–H bendings.

Probing 13C chemical shielding tensors in cryptolepine and two bromo-substituted analogs for antiplasmodial activity

Density functional theory calculations were applied to investigate (13)C chemical shielding tensors in cryptolepine and its bromo-substituted analogs, 2-bromocryptolepine and 2,7-dibromocryptolepine. The fact that bromo-substituted cryptolepine shows higher antiplasmodial activity than cryptolepine raises the question of whether this effect can be related to the electronic properties around carbon atoms. The results show that changes to the principal components of the shielding tensors upon substitution are significant. In particular, σ (33) is the most affected tensor for carbons in the substituted ring, which could be related to the increased antiplasmodial activity of bromosubstituted cryptolepine. The analyses were also focused on atomic charges and dipole moment.

Influence of N–H…O and O–H…O hydrogen bonds on the 17O, 15N and 13C chemical shielding tensors in crystalline acetaminophen: A density functional theory study

A computational investigation was carried out to characterize the 17O, 15N and 13C chemical shielding tensors in crystalline acetaminophen. We found that N–H…O and O–H…O hydrogen bonds around the acetaminophen molecule in the crystal lattice have different influences on the calculated 17O, 15N and 13C chemical shielding eigenvalues and their orientations in the molecular frame of axes. The calculations were performed with the B3LYP method and 6–311++G(d, p) and 6–311+G(d) standard basis sets using the Gaussian 98 suite of programs. Calculated chemical shielding tensors were used to evaluate the 17O, 15N, and 13C NMR chemical shift tensors in crystalline acetaminophen, which are in reasonable agreement with available experimental data. The difference between the calculated NMR parameters of the monomer and molecular clusters shows how much hydrogen-bonding interactions affect the chemical shielding tensors of each nucleus. The computed 17O chemical shielding tensor on O(1), which is involved in two intermolecular hydrogen bonds, shows remarkable sensitivity toward the choice of the cluster model, whereas the 17O chemical shielding tensor on O(2) involved in one N–H…O hydrogen bond, shows smaller improvement toward the hydrogen-bonding interactions. Also, a reasonably good agreement between the experimentally obtained solid-state 15N and 13C NMR chemical shifts and B3LYP/6–311++G(d, p) calculations is achievable only in molecular cluster model where a complete hydrogen-bonding network is considered. Moreover, at the B3LYP/6–311++G(d, p) level of theory, the calculated 17O, 15N and 13C chemical shielding tensor orientations are able to reproduce the experimental values to a reasonably good degree of accuracy.

13C chemical shielding tensor orientations in a phosphoenolpyruvate moiety from 13C rotational-resonance MAS NMR lineshapes

The absolute orientations of the three 13C chemical shielding tensors in the phosphoenolpyruvate (PEP) moiety in a PEP-model compound with known crystal structure are reported. The study uses a fully 13C-enriched polycrystalline sample of triammonium phosphoenolpyruvate monohydrate, (NH 4) 3(PEP)⋅H 2O, and 13C MAS NMR experiments fulfilling various different 13C rotational-resonance conditions. The absolute 13C chemical shielding tensor orientations are derived by iterative fitting, employing numerically exact simulations, of various rotational-resonance 13C MAS NMR lineshapes of the three- 13C-spin system in fully 13C-enriched (NH 4) 3(PEP)⋅H 2O. The implications of the results of this study for future, biochemically oriented solid-state NMR studies on the PEP moiety are outlined.

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 C2v — 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.

The Structure of Solid Copper(I) Cyanide: A Multinuclear Magnetic and Quadrupole Resonance Study

Carbon-13 and nitrogen-15 nuclear magnetic resonance (NMR) spectra of solid copper(I) cyanide present a clear picture of its molecular structure as comprising linear, polymeric chains: [−Cu−N−C−]n. Copper-63/65 nuclear quadrupole resonance reveals that the cyanide ligands are subject to “head-tail” disorder. Magic-angle spinning NMR spectra are analyzed by accounting for the quadrupolar effects of neighboring copper nuclei to yield one-bond indirect spin−spin coupling constants, 1J(63Cu,13C) ≈ +725 Hz and 1J(63Cu,15N) ≈ −250 Hz, and effective dipolar coupling constants, Reff(63Cu,13C) ≈ +1200 Hz and Reff(63Cu,15N) ≈ −460 Hz. The dipolar coupling data correspond to internuclear separations of rCu,C ≈ 1.88 A and rCu,N ≈ 1.92 A. The observation of axially symmetric 15N and 13C chemical shielding tensors in NMR spectra of nonspinning samples provides compelling evidence for the structural linearity of CuCN chains. Conclusions based on experimental data are supported by ab initio calculations of chemical shie...

Application of Optical Nuclear Polarization Enhanced 13C NMR

A gain in detection sensitivity of more than 3 orders of magnitude has been achieved in high-resolution solid-state 13C NMR of monocrystalline fluorene doped with acridine by applying optical nuclear polarization (ONP) via excited triplet states to protons and transferring this proton polarization to the 13C nuclei. This sensitivity gain was utilized to measure the angular dependence (rotation pattern) of the 13C NMR lines. In this way the principal values and orientations of all 13C chemical shielding tensors were determined. While the 13C shielding tensor of the bridging methylene group exhibits only a small anisotropy, at the aromatic carbon positions the typical strong anisotropy is observed. All tensors belonging to the same molecule have one principal axis, perpendicular to the molecular plane, in common, showing that in the crystal lattice the fluorene molecule is in a planar configuration. The experimental data are compared to ab initio calculations employing optimized geometries and gauge include...

Ab initio calculation and experimental determination of the 13C chemical shielding tensors of 4,4′-dimethoxybiphenyl

An ab initio calculation of the principal components of the 13C chemical shielding tensors of 4,4′-dimethoxybiphenyl are reported by employing the ab initio GIAO approach. The geometric parameters were optimized by using the Gaussian-92 program. The observed values of the 13C NMR chemical shifts of a powdered sample were determined by using the recently introduced magic angle turning (MAT) experiment. Theoretical calculations successfully reproduced the MAT experimental resonance peaks. The results presented in this paper demonstrate the importance of using tensor information in assigning the experimental resonances.

Determination of enantiomeric purity from solid‐state 31P MAS NMR of organophosphorus compounds

AbstractPhosphorus‐31 Magic‐Angle Spinning (MAS) NMR is shown to be an extremely useful and accurate method for the determination of the enantiomeric purity for chiral organophosphorus compounds. 31P and 13C chemical shielding tensors in racemic and enantiomeric crystals of organophosphorus compounds vary because of different crystal symmetries. This implies that solid‐state 31P (13C) MAS spectra of an enantiomerically impure sample reveal separate signals from the racemic and enantiomer crystallites. Simple integration (or deconvolution) of the MAS NMR signals can thus be used to determine the enantiomeric purity of the sample. For organophosphorus compounds the application of this technique to 31P MAS NMR is of particular interest due to the high sensitivity and large shielding anisotropy of the 31P nucleus.

Carbon‐13 chemical shielding tensors in alkanedicarboxylic acids. Influence of molecular geometry on the carboxyl carbon tensors in alkanedicarboxylic acids and related compounds

AbstractThe principal elements of 13C chemical shift tensors in some dicarboxylic acids were determined from powder samples by two methods: (i) from the theoretical fit of the experimental data and (ii) from the side‐band intensities of the CP‐MAS spectra using the graphical procedure of Herzfeld and Berger. Neither the methylene nor the carboxyl carbons are axially symmetrical. The methylene tensors corresponding to higher dicarboxylic acids were not analysed owing to severe overlap of the different carbon tensors. The influence of local molecular geometry on the carbon 13C chemical shielding tensors in dicarboxylic acids and other related organic molecules are also reported. The σ22 component of the tensor shows a linear correlation as a function of the structural parameter, the OCO bond angle, around this carboxyl carbon atom. The significance of the symmetry factor calculated for the carboxyl carbons is also presented.

13C and 31P chemical shielding tensors of a single crystal of dipotassium α- d-glucose-1-phosphate dihydrate. An application of a 13C-{ 1H, 31P} triple-resonance technique

The 13C NMR spectra of a single crystal of dipotassium α- d-glucose-l-phosphate dehydrate for different orientations in the external magnetic field, were recorded by using 1H and 31P double nuclear decoupling. To overcome difficulties encountered because of the high 13C RF power required to achieve the Hartmann-Hahn condition, a new cross-polarization method (K. Takegoshi and C. A. McDowell, J. Magn. Reson. 67, 356 (1986)) was used. The directions of the most shielded principal value of the 13C chemical shielding tensors for the C2–C6 carbon nuclei in the glucose group were along the CO bond, and that for the CI carbon nucleus made an angle of 42† with the C1-O5 bond direction in the O1-C1-O5 plane. The 31P chemical shielding tensors are axially symmetric and the direction of the least shielded principal value is almost parallel to the P-O1(R) bond, which is the longest among the four PO bonds in the phosphate moiety.

Correlation between a structural parameter and the carboxyl 13C chemical shielding tensors in compounds containing carboxylic carbon as a functional group

The influence of local molecular geometry on the carboxyl 13C chemical shielding tensors in some organic molecules is investigated. The σ 22 component of the tensor shows a linear variation as a function of the OCO bond angle around the carboxyl carbon. The significance of the symmetry factors calculated for these compounds is discussed.

Determination of the 14N quadrupole coupling tensors and the 13C chemical shielding tensors in a single crystal of L-asparagine monohydrate

The 14N quadrupole coupling tensors for the Nα and Nδ nitrogen nuclei in a single crystal of l-asparagine monohydrate were completely determined using proton enhanced 14N NMR. The quadrupole coupling constants and asymmetry parameters were evaluated to be e2Qq/h=1.152 MHz and η=0.220 for the Nα nitrogen nucleus, and e2Qq/h=−2.677 MHz and η=0.337 for the Nδ nitrogen nucleus. It was found that the direction of the largest absolute principal value of the quadrupole coupling tensor for the Nδ nitrogen nucleus was perpendicular to the NH2 plane (sp2 plane) and that for the Nα nitrogen nucleus was parallel to the Cα–Nα bond. The 13C chemical shielding tensors for all four chemically different nuclei (Cα, Cβ, Cγ, and COO−) were determined and assigned to the molecule in the unit cell on the basis of the 13C–14N dipolar splittings. It turned out that the directions of the most shielded value of the chemical shielding tensors for the Cγ and COO− carbon nuclei were perpendicular to their sp2 plane, and that of the least shielded value of the chemical shielding tensor for the COO− carbon nucleus was parallel to the Cα–COO− bond. The direction of the intermediate value of the Cγ chemical shielding tensor makes an angle of 7.7° with the Cγ–Oδ bond. The lengths of the Cα–Nα and Cγ–Nδ bonds were calculated to be 1.513 and 1.364 Å, respectively, by analyzing the 13C–14N dipolar coupling tenors. These values agree well with those determined by neutron diffraction studies.

1 1 3Cd and 13C chemical shielding tensors in a single crystal of cadmium (II) acetate dihydrate

Both the carbon-13 and cadmium-113 chemical shielding tensors in a single crystal of cadmium (II) acetate dihydrate have been determined by solid state proton-enhanced NMR at 1.32 T. The 13C shielding tensors of a carboxyl group bound to a metal ion is determined quantitatively and unambiguously. The most shielded direction lies perpendicular to the O–C–O plane and the least shielded principal value is along C⋅⋅⋅Cd direction. The influence of carbon–cadmium dipolar interaction on the observed spectra is discussed and used to make unique assignments of the carboxyl carbon tensors. The CP-MAS spectrum exhibits resolved resonances for the two chemically distinct carboxyl carbons and these are assigned based on single crystal analysis. The methyl carbon tensors exhibit significant deviation from axial symmetry. The most shielded direction is along the C3 symmetry axis and the least shielded direction is perpendicular to the C(H3)–COO plane. It is demonstrated that the orientation dependence of 13C–1H dipolar interaction is useful in assigning 13C resonances of the two chemically similar methyl groups in the asymmetric unit. The 113Cd results show that the cadmium shielding tensor is not axially symmetric and the most shielded direction lies approximately perpendicular to the plane formed by cadmium and the two water oxygens.

Determination of the 14N quadrupole coupling tensor and the 13C chemical shielding tensors in a single crystal of l-serine monohydrate

The 14N quadrupole coupling tensor for the NH3+ nitrogen nucleus in a single crystal of l-serine monohydrate was completely determined. The quadrupole coupling constant, and the asymmetry parameter, were evaluated to be e2Qq/h=1.069 MHz and η=0.214 at room temperature by measuring the angular variation of the proton-enhanced 14N NMR spectra about the three experimental axes. It was found that the direction of the largest principal value of the 14N quadrupole coupling tensor is at an angle of 3.5° to the direction of the C–N bond. The sign of the 14N quadrupole coupling constant was determined to be positive by analyzing the 13C–14N dipolar splitting pattern in the 13C NMR signal of the Cα carbon. The 13C chemical shielding tensors for three chemically different carbon nuclei, namely COO−, Cα, and Cβ were determined and assigned to the molecule in the unit cell by inspecting the 13C–14N dipolar interaction rather than relying only on the local symmetry. The most shielded direction of the COO− carbon is perpendicular to the O–C–O plane, and the least shielded direction is parallel to the Cα–C bond direction. It was found that the most shielded direction for the Cβ carbon is almost parallel to the Cβ–O bond direction.

Single-crystal study of 13C chemical shielding tensors in dimethyl maleic anhydride and acrylamide

Carbon chemical shielding tensors of dimethyl maleic anhydride (DMMA) and acrylamide were determined by single-crystal studies using proton-enhanced NMR. The double-bond carbon tensors in DMMA were found to have their most shielded component approximately perpendicular to the sp 2 plane and the component of intermediate shielding approximately in the bond direction. The most shielded and the intermediate shielding components of the anhydride carbon in DMMA have close values, the intermediate shielding component being approximately perpendicular to the sp 2 plane. For the acrylamide there are two possible assignments for each shielding tensor. One of the two possible assignments of each of the double-bond carbon tensors has the principal directions oriented similarly to those in the DMMA case. One of the two possible assignments of the amide carbon tensor has the orientation of the principal directions quite similar to those encountered in the studies of other tensors of carbons double bonded to oxygen (allowing for deviations caused by molecular deformation). A new kind of rf pulse power amplifier possessing good stability was used in the present work for H decoupling and is described.

NMR study of molecular motion of methanol in β-quinol clathrate

The 13C chemical shielding tensors in a single crystal of β-quinol clathrate with trapped methanol molecules have been obtained by high resolution double resonance. A single shielding tensor instead of three for the trapped methanol molecule results from time-averaging. It is axially symmetric about the crystallographic c-axis and the anisotropy is about 0.29 times as large as that of polycrystalline methanol. These results suggest a new type of motion of the trapped molecules.

Carbon-13 chemical shielding tensors in acetophenone

The 13C chemical shielding tensors in a single crystal of acetophenone have been determined with high-resolution solid-state NMR techniques. The principal directions appear to be in close agreement with the local symmetry of the carbon nuclei in the molecule. A comparison is made with the tensors in analogous compounds.

Carbon-13 chemical shielding tensors in p a r a-xylene

The 13C chemical shielding tensors in a single crystal of para-xylene have been determined with high resolution double resonance NMR techniques. The eigenvectors of the tensors are found to coincide with the local symmetry of the various nuclei. A comparison is made with the tensors of similar compounds that have been reported earlier.

13C chemical shielding in oxalic acid, oxalic acid dihydrate, and diammonium oxalate

13C chemical shielding tensors in three oxalates were investigated in order to determine the effects of protonation and conformation changes on the tensor elements and their orientations. The experimental results, together with those in the accompanying paper [R. G. Griffin and D. J. Ruben, J. Chem. Phys. elding) of σ22 and in one case a concurrent rotation of the tensor about σ33, the element which is perpendicular to the COO plane. On the other hand, the simple conformational change of the twisting of carboxyl groups relative to one another, which occurs in diammonium oxalate monohydrate, results in small rotation of the tensor around σ11 and also may contribute to a decrease in the shielding of the σ22 component in this molecule. The orientation of the 13C tensor in oxalic acid dihydrate is different from that observed in other oxalate ions and carboxyl groups. The influence of a change in hydrogen bonding strength on tensor elements is discussed.