13C‐NMR data of daphnane diterpenoids

Daphnane diterpenoids are mainly distributed in Thymelaeaceae and Euphorbiaceae and have various bioactivities. About 100 daphnane diterpenoids have been isolated from natural plants. In this review, we systematically summarize the ¹³C‐NMR data of daphnane diterpenoids isolated from natural plants over the past several decades and briefly discussed their biological activities and basic structural–activity relationship. Copyright © 2013 John Wiley & Sons, Ltd.     

Use of glass capillaries to suppress thermal convection in NMR tubes in diffusion measurements

Diffusion ordered spectroscopy (DOSY) is used to determine the translational diffusion coefficients of molecules in solution. However, DOSY is highly susceptible to spurious spectral peaks resulting from thermal convection occurring in the NMR tube. Thermal convection therefore must be suppressed for accurate estimation of translational diffusion coefficients. In this study, we developed a new method to effectively suppress thermal convection using glass capillaries. A total of 6 to 18 capillaries (0.8‐mm outer diameter) were inserted into a regular 5‐mm NMR tube. The capillaries had minimal effect on magnetic field homogeneity and enabled us to obtain clean DOSY spectra of a mixture of small organic compounds. Moreover, the capillaries did not affect chemical shifts or signal intensities in two‐dimensional heteronuclear single quantum coherence spectra. Capillaries are a simple and inexpensive means of suppressing thermal convection and thus can be used in a wide variety of DOSY experiments. Copyright © 2016 John Wiley & Sons, Ltd.     

Deuterium Quantification through Deuterium‐Induced Remote 1H and 13C NMR Shifts

Partial labeling by deuterium may be quantified through simple integrations of those ¹H (200 or 400 MHz ) and ¹³C (100.6 MHz ) NMR resonances that are split into pairs by chemical shifts ⁿΔ=δ(deuterated)?δ(nondeuterated) as induced by deuterium across n>2 chemical bonds. The relative intensities of the two components of a pair are shown to be influenced to practically equal degrees by relaxation effects, so that a deuterium fraction may be determined from ¹H and ¹³C integral pairs at more remote molecular positions under the routine conditions of fast accumulative spectral acquisition.     

13C‐NMR detection of STD spectra

We have investigated the use of ¹³C for the detection of saturation transfer difference (STD) NMR spectra. By detecting the STD spectrum in the ¹³C channel it is possible to eliminate the residual water signal in the STD‐NMR spectrum. We have employed an INEPT transfer in order to shift the magnetization from the proton channel to ¹³C. As a sample system to check our method we have used human serum albumin and phenylalanine. We have shown that such a transfer can be accomplished and gives reasonable signal intensities. Copyright © 2009 John Wiley & Sons, Ltd.     

High resolution solid‐state 13C‐NMR study of as‐cured and irradiated epoxy resins

This article presents the effects of strong ionizing radiations on the physico‐chemical modifications of aliphatic or aromatic amine‐cured epoxy resins based on diglycidyl ether of bisphenol A (DGEBA). Such epoxy resins have a considerable number of applications in the nuclear industrial field and are known to be very stable under moderate irradiation conditions. Using extensively high resolution solid‐state ¹³C‐NMR spectroscopy we show that the aliphatic amine‐cured resin (DGEBA‐TETA) appears much more sensitive to gamma rays than the aromatic amine‐cured one (DGEBA‐DDM). On the one hand, qualitative analyses of the high resolution solid‐state ¹³C‐NMR spectra of both epoxy resins, irradiated under similar conditions (8.5 MGy), reveal almost no change in the aromatic amine‐cured resin whereas new resonances are observed for the aliphatic amine‐cured resin. These new peaks were interpreted as the formation of new functional groups such as amides, acids and/or esters and to alkene groups probably formed in the aliphatic amine skeleton. On the other hand, molecular dynamics of these polymers are investigated by measuring the relaxation times, T_CH, T_1ρH and T_1C , before and after irradiation. The study of relaxation data shows the formation, under irradiation, of a more rigid network, especially for the aliphatic amine‐cured system and confirms that aromatic amine‐cured resin [DGEBA‐4,4′‐diaminodiphenylmethane(DDM)] is much less affected by ionizing radiations than the aliphatic amine‐cured resin [DGEBA‐triethylenetetramine(TETA)]. Moreover, it has been shown that the molecular modifications generated by irradiation on the powder of the aliphatic‐amine‐cured resin appear to be homogeneously distributed inside the polymers as no phase separations can be deduced from the above analyses. Copyright © 2001 John Wiley & Sons, Ltd.     

Curing chemistry of phenylethynyl‐terminated imide oligomers: Synthesis of 13C‐labeled oligomers and solid‐state NMR studies

4‐(Phenylethynyl‐α,β‐¹³C)phthalic anhydride (PEPA) and ¹³C‐labeled phenylethynyl‐terminated imide (PETI) oligomers were synthesized, and solid‐state ¹³C nuclear magnetic resonance (NMR) spectroscopy was used to determine the structure of cured oligomers. Solid‐state ¹³C NMR spectra were collected before and after thermal curing. Using solid‐state ¹³C NMR difference spectroscopy, several cure products were identified. The observed ¹³C NMR resonances were assigned to four different classes of cure products: aromatics, products from backbone addition (substituted stilbenes and tetraphenylethanes), polyenes, and cyclobutadiene cyclodimers. The effects of postcuring and oligomer chain length on the structure of the cured resins were examined. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3486–3497, 2000     

Quinuclidine complex with alpha-cyclodextrin: a diffusion and 13C NMR relaxation study

The stability of an inclusion complex of quinuclidine with alpha-cyclodextrin in solution was investigated by NMR measurements of the translational diffusion coefficient. A 1:1 stoichiometry model yielded an association constant of 35 +/- 3 M(-1). The guest molecules exchange rapidly between the host cavity and the bulk solution. The reorientational dynamics of the guest and host molecules was studied using carbon-13 NMR relaxation at two magnetic fields. The relaxation of the host nuclei showed very little dependence on the guest-host concentration ratio, while the 13C spins in quinuclidine were sensitive to the solution composition. Using mole-fraction data, it was possible to extract the relaxation parameters for the bound and free form of quinuclidine. Relaxation rates of the guest molecule, free in solution, were best described by an axially symmetric model, while the data of the complex species were analyzed using the Lipari-Szabo method. Applying the axially symmetric model to the complexed quinuclidine indicated that the anisotropy of its reorientation in the bound form was increased.     

Self‐diffusion coefficients obtained from proton‐decoupled carbon‐13 spectra for analyzing a mixture of terpenes

In the limit of sufficient sensitivity, natural abundance ¹³C offers a much better spectral resolution than proton NMR. This is due to an important chemical shift range and to proton‐decoupling conditions that yield one peak per carbon with practically no overlap. However, pulsed gradient spin echo experiments, which lead to the diffusion coefficient associated with each peak, have scarcely been employed. In this article, we present and compare different ways to access this quantity and we have effectively verified that, without any precaution, diffusion coefficients cannot be properly determined from standard procedures. The cause of such a failure is decoupling during the gradient pulses. We have used a very simple remedy that proved to be very successful on a model mixture of three monoterpenes and that appears as being of general applicability. Copyright © 2009 John Wiley & Sons, Ltd.     

13C isotope effects on 1H chemical shifts: NMR spectral analysis of 13C‐labelled D‐glucose and some 13C‐labelled amino acids

The one‐ and two‐bond ¹³C isotope shifts, typically ?1.5 to ?2.5 ppb and ?0.7 ppb respectively, in non‐cyclic aliphatic systems and up to ?4.4 ppb and ?1.0 ppb in glucose cause effects that need to be taken into account in the adaptive NMR spectral library‐based quantification of the isotopomer mixtures. In this work, NMR spectral analyses of some ¹³C‐labelled amino acids, D ‐glucose and other small compounds were performed in order to obtain rules for prediction of the ¹³C isotope effects on ¹H chemical shifts. It is proposed that using the additivity rules, the isotope effects can be predicted with a sufficient accuracy for amino acid isotopomer applications. For glucose the effects were found strongly non‐additive. The complete spectral analysis of fully ¹³C‐labelled D ‐glucose made it also possible to assign the exocyclic proton signals of the glucose. Copyright © 2009 John Wiley & Sons, Ltd.     

13C‐TmDOTA as versatile thermometer compound for solid‐state NMR of hydrated lipid bilayer membranes

Recent advances in solid‐state nuclear magnetic resonance (NMR) techniques, such as magic angle spinning and high‐power decoupling, have dramatically increased the sensitivity and resolution of NMR. However, these NMR techniques generate extra heat, causing a temperature difference between the sample in the rotor and the variable temperature gas. This extra heating is a particularly crucial problem for hydrated lipid membrane samples. Thus, to develop an NMR thermometer that is suitable for hydrated lipid samples, thulium‐1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetate (TmDOTA) was synthesized and labeled with ¹³C (i.e., ¹³C‐TmDOTA) to increase the NMR sensitivity. The complex was mixed with a hydrated lipid membrane, and the system was subjected to solid‐state NMR and differential scanning calorimetric analyses. The physical properties of the lipid bilayer and the quality of the NMR spectra of the membrane were negligibly affected by the presence of ¹³C‐TmDOTA, and the ¹³C chemical shift of the complex exhibited a large‐temperature dependence. The results demonstrated that ¹³C‐TmDOTA could be successfully used as a thermometer to accurately monitor temperature changes induced by ¹H decoupling pulses and/or by magic angle spinning and the temperature distribution of the sample inside the rotor. Thus, ¹³C‐TmDOTA was shown to be a versatile thermometer for hydrated lipid assemblies. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterization of 4,5‐Dihydro‐1H‐Pyrazole Derivatives by 13C NMR Spectroscopy

The ¹³ C NMR resonances of 19 1‐acyl‐3‐(2‐nitro‐5‐substitutedphenyl)‐4,5‐dihydro‐1H‐pyrazoles, and 19 1‐acyl‐3‐(2‐amino‐5‐substituted)‐4,5‐dihydro‐1H‐pyrazoles, were completely assigned using the concerted application of one‐ and two‐dimensional NMR experiments (DEPT, gs‐HSQC and gs‐HMBC). Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of 2‐aryl‐1,3,4‐oxadiazoles by 15N and 13C NMR spectroscopy

¹⁵N NMR chemical shifts of 2‐aryl‐1,3,4‐oxadiazoles were assigned on the basis of the ¹H–¹⁵N HMBC experiment. Chemical shifts of the nitrogen and carbon atoms in the oxadiazole ring correlate with the Hammett σ‐constants of substituents in the aryl ring (r² ≥ 0.966 for N atoms). ¹⁵N NMR data are a suitable and sensitive means for characterizing long‐range electronic substituent effects. Additionally, ¹³C NMR data for these compounds are presented. Copyright © 2003 John Wiley & Sons, Ltd.     

NMR artifacts caused by decoupling of multiple‐spin coherences: improved SLAP experiment

Contrary to common expectations, multiple‐spin coherences containing products of proton and heteronucleus operators (e.g. H_uC_x, u = x, y, z) can produce not only sidebands but also noticeable centerband NMR signals of the heteronucleus during acquisition under ¹H broadband decoupling. Such centerband signals of low abundant heteronuclei can be sources of relatively strong unexpected artifacts in NMR experiments that aim to detect very weak signals from much less‐abundant isotopomers, e.g. ¹³C–¹³C ones. These findings lead to a new design of Sign Labeled Polarization Transfer (SLAP) pulse sequence (MSS‐SLAP) with improved suppression of centerband peaks that are because of singly, e.g. ¹³C, labeled molecules (parent peaks). The MSS‐SLAP experiment and its MSS‐BIRD‐SLAP variant are compared with a few older SLAP versions. Copyright © 2015 John Wiley & Sons, Ltd.     

13C cross‐polarization magic angle spinning NMR of ferrocene derivatives

The 13C chemical shifts of the CP/MAS NMR for ferrocene derivatives have been measured. Copyright © 2002 John Wiley & Sons, Ltd.     

Solid‐state 13C NMR study on thermal dehydration process of poly(methacrylic acid)(PMAA)

Thermal dehydration process of PMAA was investigated by solid‐state ¹³C NMR. For heat‐treated PMAA at 150°C, at which the dehydration goes very slowly, we observed three ¹³C peaks at 172, 178, and 187 ppm in the carboxyl group region. The peak at 172 ppm is due to the intramolecular cyclic anhydrides by comparing the reported value of ¹³C chemical shift. The peaks at 178 and 187 ppm were assigned to regularly aligned free carboxylic acids and intermolecular acid dimers, respectively, from the 2D‐exchange ¹³C NMR spectra, ¹³C chemical shift values and IR spectra. We concluded that by heat‐treatment the rearrangement of intermolecular hydrogen bonding of the carboxylic acids in PMAA occurs firstly to form the regularly aligned acid dimers, and the dimers dissociated to be the regularly aligned free carboxylic acids at high temperatures. The adjacent free carboxyl acids dehydrate with each other, resulting in the formation of intramolecular anhydrides. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2007–2012, 1999     

1H‐19F REDOR‐filtered NMR spin diffusion measurements of domain size in heterogeneous polymers

Solid state NMR spectroscopy is inherently sensitive to chemical structure and composition and thus makes an ideal method to probe the heterogeneity of multicomponent polymers. Specifically, NMR spin diffusion experiments can be used to extract reliable information about spatial domain sizes on multiple length scales, provided that magnetization selection of one domain can be achieved. In this paper, we demonstrate the preferential filtering of protons in fluorinated domains during NMR spin diffusion experiments using ¹H‐¹⁹F heteronuclear dipolar dephasing based on rotational echo double resonance (REDOR) MAS NMR techniques. Three pulse sequence variations are demonstrated based on the different nuclei detected: direct ¹H detection, plus both ¹H?¹³C cross polarization and ¹H?¹⁹F cross polarization detection schemes. This ¹H‐¹⁹F REDOR‐filtered spin diffusion method was used to measure fluorinated domain sizes for a complex polymer blend. The efficacy of the REDOR‐based spin filter does not rely on spin relaxation behavior or chemical shift differences and thus is applicable for performing NMR spin diffusion experiments in samples where traditional magnetization filters may prove unsuccessful. This REDOR‐filtered NMR spin diffusion method can also be extended to other samples where a heteronuclear spin pair exists that is unique to the domain of interest.     

Production of Pure Aqueous 13C‐Hyperpolarized Acetate by Heterogeneous Parahydrogen‐Induced Polarization

A supported metal catalyst was designed, characterized, and tested for aqueous phase heterogeneous hydrogenation of vinyl acetate with parahydrogen to produce ¹³C‐hyperpolarized ethyl acetate for potential biomedical applications. The Rh/TiO₂ catalyst with a metal loading of 23.2 wt % produced strongly hyperpolarized ¹³C‐enriched ethyl acetate‐1‐¹³C detected at 9.4 T. An approximately 14‐fold ¹³C signal enhancement was detected using circa 50 % parahydrogen gas without taking into account relaxation losses before and after polarization transfer by magnetic field cycling from nascent parahydrogen‐derived protons to ¹³C nuclei. This first observation of ¹³C PHIP‐hyperpolarized products over a supported metal catalyst in an aqueous medium opens up new possibilities for production of catalyst‐free aqueous solutions of nontoxic hyperpolarized contrast agents for a wide range of biomolecules amenable to the parahydrogen induced polarization by side arm hydrogenation (PHIP‐SAH) approach.     

Study of phenothiazine and N‐methyl phenothiazine by infrared,raman, 1H‐, and 13C‐NMR spectroscopies

A complete vibrational analysis of the Fourier transform (FT) infrared (IR) and FT‐Raman spectra of both molecules was carried out using quantum chemical calculations. The structure of phenothiazine (PTZ) and N‐methylphenothiazine (N‐MePTZ) were studied by semiempirical, and ab initio methods. Different basis sets and two new procedures for scaling the frequencies of the ring modes were used. Vibrational data of the methyl group in N‐MePTZ were interpreted in terms of the different molecular conformations in the solid state. The ¹H‐ and ¹³C–nuclear magnetic resonance (NMR) data were interpreted in terms of the electron densities on the atoms and the stacking solute–solute association in dimethyl sulfoxide solution. Chemical shifts were related to the Merz‐Kollman atomic charges. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002