Non-protonated Carbons Research Articles

A novel dipolar dephasing method for the slow magic angle turning experiment.

Complete suppression of the resonances from protonated carbons in a slow magic angle spinning experiment can be achieved using five dipolar dephasing (Five-DD) periods distributed in one rotor period. This produces a spectrum containing only the spinning sidebands (SSB) from the nonprotonated carbons. It is shown that the SSB patterns corresponding to the nonprotonated carbons are not distorted over a wide range of dipolar dephasing times. Hence, this method can be used to obtain reliable principal values of the chemical shift tensors for each nonprotonated carbon. The Five-DD method can be readily incorporated into isotropic-anisotropic 2D experiments such as FIREMAT and 2D-PASS to facilitate the measurement of the (13)C chemical shift tensors in complex systems.

Modification of Model Structures of Upper Freeport Coal Extracts Using 13C NMR Chemical Shift Calculations

The extract from carbon disulfide/N-methyl-2-pyrrolidinone mixed solvent extraction of Upper Freeport coal at room temperature was fractionated with acetone and pyridine into three fractions, and solid-state 13C NMR measurements of these fractions were made. Model structures for the three extract fractions constructed by Takanohashi et al. (Energy Fuels 1998, 12, 1168) were estimated using the NMR chemical shift calculation method. By comparing the spectra calculated for the model structures with the observed spectra, some corrections were made to the chemical structures of the original models to fit their spectra. Compared with the observed spectra, for the acetone-soluble fraction (AS), the model structure was richer in nonprotonated aromatic carbon and β methylene carbon, and poorer in protonated aromatic carbon and methyl carbon, and, for the acetone-insoluble−pyridine-soluble fraction (PS) and pyridine-insoluble−mixed-solvent-soluble fraction (PI), the spectra calculated for the model structures showed higher fractions for nonprotonated aromatic carbon and methylene carbon, and lower fractions for methyl carbon. We proposed modified model structures based on NMR chemical shift calculations for the three fractions.

Structural Determination in Carbonaceous Solids Using Advanced Solid State NMR Techniques

Detailed structural information in carbonaceous solids with complex molecular structures may be obtained with a combination of three advanced solid-state NMR techniques, i.e., dipolar dephasing with variable dephasing times, 13C CP/MAS spectral editing, and an isotropic−anisotropic correlation experiment called PHORMAT. It is shown that the ratio of protonated to nonprotonated aromatic carbons may be determined with high accuracy by the conventional dipolar dephasing experiment with variable dephasing times. The fractions of aliphatic CH, CH2, CH3 and nonprotonated (C) carbons may be obtained by employing 13C CP/MAS spectral editing techniques. The 2D PHORMAT experiment is used to obtain the structural information in the aromatic region of coal. Theoretical predictions of the principal values of chemical shift tensors in model compounds are used in the interpretation of the experimental chemical shift tensors. The potential application of these methods is illustrated using a sub-bituminous coal DIETZ (PSOC-1488).

Improvement of Spectral Editing in Solids: A Sequence for Obtaining 13CH + 13CH2-Only 13C Spectra

An improved spectral editing method for solids is described which allows one to obtain a set of subspectra in roughly two-thirds the amount of time as our original CPPI editing method for the same signal to noise. This improvement is afforded by a new pulse sequence that is used to acquire a 13CH + 13CH2 spectrum which has very little 13CH3 or nonprotonated carbon contamination. By using this new sequence the 13CH-only subspectrum is obtained much more efficiently. Criteria for optimizing the signal to noise in the edited subspectra are discussed.

Modified Spectral Editing Methods for 13C CP/MAS Experiments in Solids

The spectral editing approach of Zilm and coworkers utilizes polarization, polarization inversion, and spin depolarization methods for enhancing or suppressing NMR spectral lines in solids. The proposed pulse sequences allow nonprotonated C, CH, CH2, and CH3 types of carbon resonances to be separated from one another and identified accordingly. The former method tentatively separates the nonprotonated C and CH3 peaks with a cutoff shift of 35 ppm. This shift is a reasonable demarcation shift for a preponderance of organic molecules, but exceptions do exist that could constitute a serious drawback in a few instances. The new approach separates the nonprotonated C and CH3 carbon peaks unequivocally using modified pulse sequences similar to those of Zilm. Further, both the CH only and CH2 only spectra, respectively, can be acquired directly from combining so called (+) and (−) sequences using different spectral delay periods and pulse parameters. The (+) and the (−) pulse sequences produce signals for the nonprotonated and methyl carbons that have essentially the same amplitude but opposite phases. These spectra, combined with the previously reported CH3 and nonprontonated C only spectra, offer a complete spectral editing technique for solid samples. Examples of these spectral editing methods are provided for 3-methylglutaric acid, fumaric acid monoethyl ester, and two complex natural products: methyl o-methylpodocarpate and 10-deacetylbaccatin III.

Determination of the sign of long-range1H-13C scalar coupling constants to non-protonated carbons: application to assignment

The sign of the scalar coupling between a carbonyl carbon and α-proton J(C′,Hα), for the cyclic dipeptide cyclo(Leu–Gly) was determined relative to that between an amide proton and α-proton 3J(HN,Hα) by an E.COSY-type HSQC experiment. The pulse sequence used in this experiment gives a cross peak between the amide proton and carbonyl carbon which shows an E.COSY-type tilt caused by the α-proton. The sign information on J(C′,Hα) was used to distinguish between two-bond (intra-residue) and three-bond (inter-residue) J(C′,Hα)s, leading to the assignment of the carbonyl carbons of the cyclic dipeptide cyclo(Leu–Gly). Copyright © 1999 John Wiley & Sons, Ltd.

SPECTROSCOPIC EVIDENCE FOR CONDENSED DOMAINS IN SOIL ORGANIC MATTER

Soil organic matter (SOM) has previously been proposed as a dual-mode sorbent for hydrophobic organic compounds (HOC), i.e., partitioning takes place in both expanded (rubbery) and condensed (glassy) domains, whereas competitive sorption takes place only in condensed domains. The objective of this paper was to provide some spectroscopic evidence of condensed domains in SOM. Six humic acids (HA) extracted from different depths of a single soil profile were analyzed using X-ray diffraction (XRD) and solid-state [sup 13]C nuclear magnetic resonance (NMR) with cross-polarization (CP) and magic angle spinning (MAS) techniques. The XRD results showed a strong peak at 0.35 nm for the condensed aromatic structure of the HA from mineral horizons and a dominant peak at 0.43 nm for less tightly packed side chains of the HA from surface organic horizons. The [sup 13]C CP/MAS NMR results indicated further that HA becomes more aromatic from surface organic to mineral horizons. This increase of aromatic structure in HA was confirmed by the increase of nonprotonated carbon signals at 130 ppm using a dipolar-dephasing technique and by the increase of atomic C/H and C/O ratios. These spectroscopic and elemental data corroborate further the dual-mode model for the sorption of HOC in SOM.

Solid-state carbon-13 n.m.r. studies of molecular motion in an aromatic copolyester

Carbon-13 n.m.r. studies have been carried out on an oriented copolyester material comprising 73 mol% hydroxybenzoic acid (HBA) and 27 mol% hydroxynaphthoic acid (HNA). The T1ρ values for the carbonyl carbon, and non-protonated aromatic carbons of HBA and HNA rings have been measured over a range of temperature, and show minima at temperatures corresponding to the mechanical and dielectric relaxation transitions. The T1ρ-temperature plots have been fitted using the Cole-Davidson distribution, and indicate a broad distribution of correlation times. The activation energies obtained from the fits provide support for the association of the γ relaxation with the HBA ring, and the β relaxation with the HNA ring. Studies of the variation in chemical shift anisotropy indicate an increase in oscillations of both rings about the chain axis, with increasing temperature, and these motions appear not to be coupled with the motion of the carbonyl group.

Quantitative 13C NMR study of structural variations within the vitrinite and inertinite maceral groups for a semifusinite-rich bituminous coal

To determine the structural variation within vitrinite and inertinite maceral groups, fractions with purities over 90% in vitrinite and semifusinite were obtained by density gradient centrifugation from a medium volatile Australian bituminous coal and the bulk structural compositions of the maceral concentrates were determined by the quantitatively reliable single pulse excitation (SPE) solid state 13C NMR technique. As previously reported for coals and chars, the aromaticities determined by cross polarisation are often lower than those by SPE, due to the unfavourable spin dynamics. As expected, the aromaticities of the vitrinite fractions are significantly lower than those of the semifusinite ones, but the aromaticity, the fraction of non-protonated aromatic carbon and the number of rings per cluster all increase with density within both the maceral groups. The vitrinite and semifusinite fractions contain 3–6 and 9 to over 15 aromatic rings, respectively. Methyl groups account for greater proportions of the aliphatic carbon with increasing density. These structural trends are consistent with the variations evident in random reflectance.

Solid-State NMR Study of Poly(phenylacetylene) Synthesized with a Rhodium Complex Initiator

The polymerization mechanism and the structure of poly(phenylacetylene) synthesized using the Rh(C≡CC 6 H 5 )(2,5-norbornadiene)[P(C 6 H 5 ) 3 ] 2 initiator were investigated by solid-state NMR. The 13 C Pake doublet of the poly(phenylacetylene) containing 5% [1,2- 13 C]phenylacetylene was observed by effectively removing the chemical shift anisotropies using a composite inversion pulse sequence. The 13 C- 13 C bond distance was determined to be 1.386 A from the spectrum, which corresponds to a double bond length. This result clearly shows that the polymerization by the Rh initiator progresses by the cis-insertion mechanism. The 13 C isotropic chemical shifts of the protonated and nonprotonated olefin carbons were deduced to be 131.8 and 141.4 ppm, respectively, from differences between some overlapped CPMAS spectra. Two-dimensional 13 C CPMAS exchange experiments revealed conformational inhomogeneity of the main chain and the presence of slow π-flip motion of the phenyl ring.

Structural Uniformity of Toluene-Insolubles from Heat-Treated Coal Tar Pitch As Determined by Solid State13C NMR Spectroscopy

A suite of coal tar pitches (CTPs) thermally treated to varying degrees and their toluene-insoluble (TI) fractions have been characterized by solid state 13C nuclear magnetic resonance (NMR). The nonprotonated aromatic carbon concentrations estimated from dipolar dephasing experiments using cross polarization with a relatively long contact time (5 ms) and the quantitatively reliable single-pulse excitation (SPE) technique were in good agreement and these concentrations were used to deduce the average ring sizes. Complementary information has been obtained from X-ray diffraction and thermogravimetry analysis. An increase in the degree of condensation for the aromatic structure was evident for the whole pitches upon heat treatment, consistent with the increasing concentrations of TIs and published liquid chromatographic results on the toluene-soluble fractions. In contrast, the proportions of nonprotonated and bridgehead/internal aromatic carbons remained constant over the heat treatment range for the TIs, ...

Scopolamine and atropineN-methyl diastereomer stability.Ab initio calculations onO-formylscopine andO-formyltropine model compounds

Ab initio geometry optimizations at the RHF-21G basis set level were calculated forequatorial andaxial N-methyl diastereomers ofO-formyltropine andO-formylscopine esters and other model compounds. These optimized geometries were then utilized as input for single-point energy calculations using the higher level RHF/6-31G* basis set to afford a more precise estimation of the total energies and atomic charges. Ethano bridge “pinching” of theN-protonated tropanyl piperidine moiety pushes the smalleraxial N-proton closer toward the neighboring twoaxial C-H bonds compared with the analogous case for a bulkyaxial N-methyl. Increasedcis 1,3-diaxial interactions in theaxial N-methyl diastereomer destabilize this epimer in favor of theequatorial N-methyl counterpart [e.g., 2.121 kcal/mol lower energy for theequatorial N-methylO-formyltropineN-protonated diastereomer (12) than for theaxial epimer (13)]. Lower pyramidality at nitrogen in the free base maintains the relative stability of theequatorial N-methyl free base diastereomer (14) (1.120 kcal/mol more stable than theaxial free base15). A nonprotonated carbon atom at the apex of a three-membered ring fused to the 6,7-positions of theO-formyltropine skeleton results in severe transannular nonbonding steric interactions involving the neighboringequatorial N-methyl group inN-protonated16 (3.335 kcal/mol less stable than theaxial N-methyl epimer17, where these transannular interactions are reduced due to the smallerequatorial N-H proton). Oxygen atom occupation of the apex of a similar fused three-membered ring retains the same severe transannular nonbonding steric interactions involving the neighboringequatorial N-methyl group inN-protonated18. These transannular interactions now become electrostatically attractive in theN-protonatedaxial N-methyl epimer19 (2.031 kcal/mol more stable than theequatorial epimer). Reduced pyramidality at nitrogen in theO-formylscopine free base reduces the repulsive transannular interaction with the neighboringequatorial N-methyl group compared to that in theN-protonated form. Lowered pyramidality also reduces thecis-1,3-diaxial interactions in theaxial N-methyl epimer, but the nitrogen lone pair is pushed close to the transannular oxygen lone pair as a result (theequatorial N-methyl free base20 is 3.870 kcal/mol more stable than theaxial epimer21). Theseab initio-calculated models ofO-formyltropines andO-formylscopineN-methyl diastereomeric protonated cations and free bases provide stereochemical insight into the relative stabilities of solution-state atropine and scopolamineN-methyl species previously observed by NMR spectroscopic methods.

Accurate Determination of13C Enrichments in Nonprotonated Carbon Atoms of Isotopically Enriched Amino Acids by1H Nuclear Magnetic Resonance

A method for the accurate determination of13C enrichments in nonprotonated carbon atoms of organic compounds that makes use of unresolved13C satellites of proton(s) bonded to the vicinal carbon atom was developed. Using glutamate as a model molecule, this1H nuclear magnetic resonance (NMR) inverse spin-echo difference spectroscopy method was calibrated for inversion efficiency and relaxation effects which were then shown to cause only a minor loss of the measured13C satellite amplitude (2% for glutamate C-1 and 7% for glutamate C-5). The determination of13C enrichments in nonprotonated glutamate carbon atoms by this method was shown to be more precise than13C NMR. As a first application, a [5-13C]glucose labeling experiment withCorynebacterium glutamicumASK1 was performed. The labeling patterns of glutamate and arginine extracted from cellular protein were determined using the newly developed method and standard1H NMR with and without broadband13C decoupling. Determination of the13C enrichment in C-5 of glutamate and arginine, respectively, by the two methods showed good agreement. From the deduced labeling pattern of 2-oxoglutarate, anin vivocarbon flux distribution within the central metabolism ofC. glutamicumASK1 was calculated. Thus, the relative flux toward oxaloacetate via the tricarboxylic acid cycle enzyme malate dehydrogenase was determined as 45%, whereas that via anaplerotic C3carboxylation was determined as 55%.

Quantitative solid-state 13C n.m.r. measurements on cokes, chars and coal tar pitch fractions

Bloch decay or single pulse excitation (SPE) 13C n.m.r., generally recognized as the best approach to obtain quantitatively reliable aromaticity values and other skeletal parameters for coals, was applied to partly carbonized coal samples, a biomass char and the toluene-insolubles from a coal tar and a corresponding pitch. As found previously for coals, the aromaticities and non-protonated carbon concentrations were generally higher than those estimated by cross-polarization (CP). Furthermore, in terms of accumulation times, the shorter 13C T 1's of low-temperature chars makes SPE a more efficient technique than for coals. The higher concentrations of paramagnetic centres responsible for the shorter 13C T 1's still result in observability of 75% of the carbon in the chars by the SPE technique. The H C ratios derived from the SPE measurements agree well with those obtained from elemental analysis.

Determination of the 13C magnetic shielding tensor in partially oriented polymer systems

It is shown that, for an oriented sample, the 13C magnetic shielding tensor of a specific nucleus can be determined from the spinning sideband intensities in a 2D spectrum obtained from a 13C rotor-synchronized 2D CPMAS experiment, originally designed by Harbison and co-workers (G.S. Harbison, V. Vogt and H.W. Spiess, J. Chem. Phys., 86(3) (1987) 1206; G.S. Harbison, H.W. Spiess, Chem. Phys. Lett., 124 (1986) 128). In our fitting procedure, we combined their multilinear regression analysis for the determination of the order parameters with a variation both of the principal values of the magnetic shielding tensor and of the direction of the principal axes relative to the molecule. The magnetic shielding principal elements of the nonprotonated aromatic carbon in poly(ethylene terephthalate) (PET) fibres and of the aromatic carbons in the central ring of an oriented liquid crystalline polymer network are reported. We estimate that the accuracy of the principal values is within 5 ppm.

Use of Paramagnetic Relaxation Agents in the Characterization of Acrylic Polymers: Application of Chromium (III) Acetylacetonate and Iron (III) Acetylacetonate

Abstract The quantitation of acrylate polymers by high-resolution 13C-NMR requires a knowledge of the spin-lattice relaxation times. For signals from non-protonated carbons, such as carbonyls, these relaxation times are frequently on the order of 1.5–2.0 s. Through the use of paramagnetic relaxation agents, it is possible to decrease the relaxation times. For poly(methyl methacrylate), poly(butyl methacrylate), and poly(hydroxyethyl methacrylate), it has been shown that Cr(acac)3 and Fe(acac)3 can be used to decrease the T1 of all 13C signals. Guidelines for the use of these relaxation agents in acrylics are given.

Spectral editing in 13C CP/MAS experiments at high magnetic field

We show that the spinning side-bands of protonated and non-protonated carbon atoms can be well separated by means of the standard SCP and LCPD experiments at a relatively slow sample spinning rate or at high magnetic field. These experiments offer a promising way of measuring the principal values of chemical shift anisotropies via spinning side-band analysis in a moderately complex system. General spectral editing in 13C cross-polarization magic-angle spinning (CP/MAS) experiments at high field is achieved by incorporating the total side-band suppression (TOSS) pulse sequence into the standard series of spectral editing pulse sequences. It is confirmed that the relative signal intensity for a certain kind of functional group obtained at different polarization, polarization-in-version and depolarization times is about the same as that obtained at low magnetic field, and that the signal intensity distortion introduced by the TOSS sequence for resonances having different chemical shift anisotropies does not interfere with the spectral editing process. However, quantitative results can only be expected in those cases where full restoration of the intensity of the central band can be achieved by the TOSS sequence. This new strategy at high field is demonstrated by using fumaric acid monoethyl ester as a model compound. A typical application to a Chinese resin is presented, where the relative ratio of each functional group in the aliphatic portion to the total number of aliphatic carbon atoms is determined from only three experimental spectra.

96/02213 13C NMR determination of protonated and nonprotonated carbons in model compounds, mixtures, and coalderived liquid samples

96/02213 13C NMR determination of protonated and nonprotonated carbons in model compounds, mixtures, and coalderived liquid samples

Model Calculations of the 13C NMR Shieldings in the Crystalline Cyclic Pentamer of Poly(p-phenylene sulfide)

The cyclic pentamer of poly(p-phenylene sulfide) [c-(PS)5] cystallizes in an interesting manner. Though only a single c-(PS)5 molecule constitutes the asymmetric unit of its crystalline unit cell, the conformation about and the geometry of the C−S bonds vary widely around the cycle. The high-resolution, solid-state 13C NMR spectrum of c-(PS)5 reflects this heterogeneity of structural environments, where multiple resonances (at least six) are observed for both protonated (CP) and nonprotonated (CQ) carbons spread over 18 and 8 ppm chemical shift ranges, respectively. Crystalline c-(PS)5 provides us with a unique model system with which we may test our ability to calculate NMR nuclear shieldings with ab initio quantum mechanical methods. For this purpose, a gauge including atomic orbital (GIAO) approach was implemented with the use of locally dense basis sets. Diphenyl sulfide was adopted as a fragment model for c-(PS)5, with the S, CQ, and two CP atoms on a single adjacent phenyl ring treated with the loca...

Accurate Measurement of Heteronuclear Long-Range Coupling Constants from 1D Subspectra in Crowded Spectral Regions

A general review of novel NMR methods to measure heteronuclear long-range proton–carbon coupling constants (nJCH; n > 1) in small molecules is made. NMR experiments are classified in terms of NMR pulse scheme and cross-peak nature. A discussion about simplicity, general applicability and accuracy for each particular NMR experiment is presented and exemplified. Important aspects such as the sign determination and measurement of very small coupling values involving protonated and non-protonated carbons as well as the complementarity between different experiments are also discussed. Finally, a compilation of applications in structural and conformational analysis of different types of molecules since 2000 is surveyed.