13C Nuclei Research Articles

The AMS technique as an important tool for the measurement of astrophysical cross sections

Accelerator Mass Spectrometry is a technique commonly used to approach low concentrations of certain long half-life radioisotopes. The most important contribution of the technique is the accurate measure of organic sample ages, by separating masses 12,13 and 14 in the case of carbon allocated in such samples. However, the reach of AMS could cover many other scientific scopes, since it can give us a precise measure of a very small concentration of a radioisotope. On this direction, AMS can be used to approach reactions of interest for astrophysics, if we spot an specific radioisotope which concentration can be measure with AMS. Starting with this, we have selected specific reactions involving 14C, 10Be and 26Al, produced with slow neutrons from a reactor and positive ions at an accelerator. The main idea is to produce a particular reaction and later to measure the radioisotopic concentration using AMS. In this study our first results for 14C and 10Be nuclei produced with neutrons, and the preliminary results for 26Al nuclei produced with deuterium are shown.

Механізми реакції 12С(15N,14N)13C при енергії 81 МеВ

The new experimental data of the 12С(15N,14N)13С reaction cross-sections at the energy Еlab(15N) = 81 MeV were measured for the ground and excited states of 14N and 13C nuclei. The experimental data were analyzed within the coupled-reaction-channels method (CRC) using channels-coupling scheme with the 15N + 12С elastic scattering and one- as well as two-step transfers of nucleons and clusters performing CRC-calculations with the Woods - Saxon potentials (WS) for the entrance and exit reaction channels. The WS potential parameters for the 12С + 15N nuclear interaction were taken from the previously studied elastic and inelastic scattering of 15N ions by 12С nuclei at the energy 81 MeV, and the WS potential parameters for the 14N + 13С reaction exit channel were deduced from fitting of the 12С(15N,14N)13C reaction data. The spectroscopic factors (amplitudes) of transferred in the reaction nucleons and clusters, used in the CRC-calculations, were computed within translational invariant shell model of 1p-shell. It was found that transfers of neutrons (n) and deuterons (d) dominate in this reaction. Multi-step transfers of nucleons and clusters give small contributions to the reaction data.

Carbon Isotopes in the PAMELA Experiment

An analysis of the isotopic composition of nuclei in galactic cosmic rays (GCR) in the orbital experiment of the PAMELA collaboration allows us to study the problems of the origin and propagation of cosmic rays in the Galaxy. Due to the high statistical and methodological accuracy, the data of the PAMELA magnetic spectrometer provided significant progress in studying the isotopic composition of light nuclei from H to Be in the GCR in the energy region of ∼ 0.1-1 GeV / nucleon and for the first time made it possible to estimate the contribution of local sources from close ones to the GCR (∼ 100 pc) of recent (∼ 106 yrs) supernova explosions. To date, the isotopic composition of carbon nuclei in the GCR has been measured only for the 13C / 12C ratio in the energy region ∼ 0.05-0.13 GeV / nucleon in the VOYAGER 1.2 space experiment and the upper limit for the 14C / 12C ratio was estimated in the ACE / CRIS experiment for energies 0.12-0.43 MeV / nucleon. In this work, using PAMELA flight data 2006-2014, on the rigidity of the detected nuclei and their speed (time-of-flight analysis and ionization losses in the multilayer calorimeter of the device), an attempt was made to determine the isotopic composition of carbon nuclei in the energy region of ∼ 0.1-1 GeV / nucleon. The half-life of 14C nuclei is 5730 years and can be detected in the case of a supernova explosion in the last ∼ 5 104 years at a distance of ∼ 100 200 pc. The results of isotope analysis of carbon nuclei in GCR (spectra 12C, 13C, 14C and 14C / 12C - ratio depending on the rigidity and energy of the nuclei) in comparison with the existing measurement data will be presented.

Encapsulation of essential oils using cinnamic acid grafted chitosan nanogel: Preparation, characterization and antifungal activity

Chemical modifications in the chitosan structure may result in obtaining a new material with improved chemical properties, such as an ability to encapsulate lipophilic compounds. This study aimed to synthesize cinnamic acid grafted chitosan nanogel to encapsulate the essential oils of Syzygium aromaticum and Cinnamomum ssp., in order to develop a material to be applied in the control of dermatophytosis caused by the fungus Microsporum canis. The cinnamic acid graft in chitosan was verified by the Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), Solid State Nuclear Magnetic Resonance of the 13C Nucleus (13C SSNMR) and Thermal analysis coupled to mass spectrometry (TG-MS) techniques. The nanogel obtained showed affinity for the essential oils of S. aromaticum and Cinnamomum, with encapsulation efficiencies equal to 74% and 89%, respectively. When in an aqueous medium the nanogel with the encapsulated essential oils was able to form stable nanoparticles with average sizes of 176.0 ± 54.3 nm and 263.0 ± 81.4 nm. The cinnamic acid grafted chitosan nanogel showed antifungal activity in vitro against M. canis, inhibiting up to 53.96% of its mycelial growth. Complete inhibition of mycelial growth was achieved by the nanogel with encapsulated essential oils. The results found in this work demonstrated the development of a material with potential application in the control of dermatophytosis caused by the fungus M. canis.

Трансформация гидрофобных флавоноидов катехина, дигидрокверцетина и кверцетина в водорастворимые структуры

The paper describes methods for the transformation of poorly water-soluble flavonoids: dihydroquercetin, catechin and quercetin into water-soluble forms during the formation of supromolecular adducts with β-cyclodextrin and salt-type ionic complexes with the natural amino acid L-arginine, which contains a guanidine moiety properties necessary to create a cationic structure when interacting with phenolic groups. First, a methodology was developed for the synthesis of supromolecular structures, in which flavonoids were incorporated into a cyclodextrin matrix. As a result, the solubility in water at 20 °C of encapsulated flavonoids increased by more than two orders of magnitude. In the formation of complexes of cyclodextrin with flavonoids, the main role is played not by hydrogen bonds between the hydroxyl groups of flavonoids and β-cyclodextrin - in the case of dihydroquercetin and catechin, they are different and similar in the case of dihydroquercetin and quercetin, but the spatial orientation of the pyrocatechol cycle B (due to the flat conjugation of the entire molecule and due to sp2 is the structure of the second carbon atom of the pyran ring). Another implemented approach for obtaining water-soluble flavonoids at room temperature is the creation of their salts with the natural amino acid L-arginine, which is a part of proteins and is involved in several vital processes in the body. The structure of the obtained compounds was proved by the methods of NMR spectroscopy on 13C nuclei and X-ray structural analysis, the composition – by elemental analysis.

Multi-spectroscopic (FT-IR, FT-Raman, 1H NMR and 13C NMR) investigations on syringaldehyde

Spectroscopic properties of syringaldehyde have been investigated by FT-IR, FT-Raman and NMR analytical techniques with help of high level quantum chemical calculations. The vibration spectra of syringaldehyde were recorded in the mid IR region. The 1H and 13C NMR spectra were recorded in CDCl3 solvent at base frequencies of 400 MHz for 1H and 100 MHz for 13C nuclei. Besides, the Hirshfeld surface, mulliken charge distribution, MESP and thermodynamical parameters have been investigated to attain deep insight on molecular properties of syringaldehyde. The obtained experimental and theoretical findings show complete agreement, thereby to confirm the molecular structure of syringaldehyde.

Optimized selection of slow-relaxing 13C transitions in methyl groups of proteins: application to relaxation dispersion.

Optimized selection of the slow-relaxing components of single-quantum 13C magnetization in 13CH3 methyl groups of proteins using acute (< 90°) angle 1H radio-frequency pulses, is described. The optimal selection scheme is more relaxation-tolerant and provides sensitivity gains in comparison to the experiment where the undesired (fast-relaxing) components of 13C magnetization are simply 'filtered-out' and only 90° 1H pulses are employed for magnetization transfer to and from 13C nuclei. When applied to methyl 13C single-quantum Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion experiments for studies of chemical exchange, the selection of the slow-relaxing 13C transitions results in a significant decrease in intrinsic (exchange-free) transverse spin relaxation rates of all exchanging species. For exchanging systems involving high-molecular-weight species, the lower transverse relaxation rates translate into an increase in the information content of the resulting relaxation dispersion profiles.

Linear response of the indirect J‐coupling alternation to the energy gap of increasing π‐conjugated oligomers

AbstractWe have investigated the dependence of the electronic energy gap of organic conjugated oligomers on the J‐coupling alternation (JCA) parameter, derived from the nuclear magnetic resonance (NMR) spectroscopy. We exploit the systematic and concerted oscillations observed for the indirect spin‐spin coupling constants (SSCCs), between two adjacent 13C nuclei (1JCC), in the π‐conjugated systems. Hence, we define a scalar JCA parameter in terms of the differences of 1JCC, which is further analyzed as a function of the HOMO‐LUMO gaps (the energy difference between the frontier molecular orbitals), as well as other properties of the studied oligomers. Using an adequate exchange‐correlation density functional approach, we show that, for all systems considered here, that is, polyacetylene (PA), poly(para‐phenylene) (PPP), polypyrrole (PPy), and polythiophene (PTh), their calculated electronic energy gaps may exhibit a linear dependence on the JCA. Our results indicate that a single‐valued NMR‐based parameter properly renders the electronic features of highly conjugated oligomers, allowing us to compare the strengths of J‐couplings in different systems. The present description yields a way to capture both the bonding features and the electronic absorption of these systems provided by the SSCCs. The JCA gives evidence for the differences in the electronic structure of distinct classes of conjugated systems, such as PA, and those that can exhibit a quinoid character, such as PPP, PPy, and PTh.

An Internuclear J-Coupling of 3He Induced by Molecular Confinement.

The solution-state 13C NMR spectrum of the endofullerene 3He@C60 displays a doublet structure due to a J-coupling of magnitude 77.5 ± 0.2 mHz at 340 K between the 3He nucleus and a 13C nucleus of the enclosing carbon surface. The J-coupling increases in magnitude with increasing temperature. Quantum chemistry calculations successfully predict the approximate magnitude of the coupling. This observation shows that the mutual proximity of molecular or atomic species is sufficient to induce a finite scalar nuclear spin-spin coupling, providing that translational motion is restricted by confinement. The phenomenon may have applications to the study of surface interactions and to mechanically bound species.

Enantiospecific Response in Cross-Polarization Solid-State Nuclear Magnetic Resonance of Optically Active Metal Organic Frameworks.

We report herein on a NMR-based enantiospecific response for a family of optically active metal-organic frameworks. Cross-polarization of the 1H-13C couple was performed, and the intensities of the 13C nuclei NMR signals were measured to be different for the two enantiomers. In a direct-pulse experiment, which prevents cross-polarization, the intensity difference of the 13C NMR signals of the two nanostructured enantiomers vanished. This result is due to changes of the nuclear spin relaxation times due to the electron spin spatial asymmetry induced by chemical bond polarization involving a chiral center. These experiments put forward on firm ground that the chiral-induced spin selectivity effect, which induces chemical bond polarization in the J-coupling, is the mechanism responsible for the enantiospecific response. The implications of this finding for the theory of this molecular electron spin polarization effect and the development of quantum biosensing and quantum storage devices are discussed.

Enhanced Optical 13C Hyperpolarization in Diamond Treated by High‐Temperature Rapid Thermal Annealing

Abstract Methods of optical dynamic nuclear polarization open the door to the replenishable hyperpolarization of nuclear spins, boosting their nuclear magnetic resonance/imaging signatures by orders of magnitude. Nanodiamond powder rich in negatively charged nitrogen vacancy defect centers has recently emerged as one such promising platform, wherein 13C nuclei can be hyperpolarized through the optically pumped defects completely at room temperature. Given the compelling possibility of relaying this 13C polarization to nuclei in external liquids, there is an urgent need for the engineered production of highly “hyperpolarizable” diamond particles. Here, a systematic study of various material dimensions affecting optical 13C hyperpolarization in diamond particles is reported on. It is discovered surprisingly that diamond annealing at elevated temperatures ∼1720 °C has remarkable effects on the hyperpolarization levels enhancing them by above an order of magnitude over materials annealed through conventional means. It is demonstrated these gains arise from a simultaneous improvement in NV− electron relaxation/coherence times, as well as the reduction of paramagnetic content, and an increase in 13C relaxation lifetimes. This work suggests methods for the guided materials production of fluorescent, 13C hyperpolarized, nanodiamonds and pathways for their use as multimodal (optical and magnetic resonance) imaging and hyperpolarization agents.

Sensitive and simplified: a combinatorial acquisition of five distinct 2D constant-time 13C-1H NMR protein correlation spectra.

A procedure is presented for the substantial simplification of 2D constant-time 13C-1H heteronuclear single-quantum correlation (HSQC) spectra of 13C-enriched proteins. In this approach, a single pulse sequence simultaneously records eight sub-spectra wherein the phases of the NMR signals depend on spin topology. Signals from different chemical groups are then stratified into different sub-spectra through linear combination based on Hadamard encoding of 13CHn multiplicity (n = 1, 2, and 3) and the chemical nature of neighboring 13C nuclei (aliphatic, carbonyl/carboxyl, aromatic). This results in five sets of 2D NMR spectra containing mutually exclusive signals from: (i) 13Cβ-1Hβ correlations of asparagine and aspartic acid, 13Cγ-1Hγ correlations of glutamine and glutamic acid, and 13Cα-1Hα correlations of glycine, (ii) 13Cα-1Hα correlations of all residues but glycine, and (iii) 13Cβ-1Hβ correlations of phenylalanine, tyrosine, histidine, and tryptophan, and the remaining (iv) aliphatic 13CH2 and (v) aliphatic 13CH/13CH3 resonances. As HSQC is a common element of many NMR experiments, the spectral simplification proposed in this article can be straightforwardly implemented in experiments for resonance assignment and structure determination and should be of widespread utility.

Combining fast magic angle spinning dynamic nuclear polarization with indirect detection to further enhance the sensitivity of solid-state NMR spectroscopy

Dynamic nuclear polarization (DNP) and indirect detection are two commonly applied approaches for enhancing the sensitivity of solid-state NMR spectroscopy. However, their use in tandem has not yet been investigated. With the advent of low-temperature fast magic angle spinning (MAS) probes with 1.3-mm diameter rotors capable of MAS at 40 ​kHz it becomes feasible to combine these two techniques. In this study, we performed DNP-enhanced 2D indirectly detected heteronuclear correlation (idHETCOR) experiments on 13C, 15N, 113Cd and 89Y nuclei in functionalized mesoporous silica, CdS nanoparticles, and Y2O3 nanoparticles. The sensitivity of the 2D idHETCOR experiments was compared with those of DNP-enhanced directly-detected 1D cross polarization (CP) and 2D HETCOR experiments performed with a standard 3.2-mm rotor. Due to low CP polarization transfer efficiencies and large proton linewidth, the sensitivity gains achieved by indirect detection alone were lower than in conventional (non-DNP) experiments. Nevertheless, despite the smaller sample volume the 2D idHETCOR experiments showed better absolute sensitivities than 2D HETCOR experiments for nuclei with the lowest gyromagnetic ratios. For 89Y, 2D idHETCOR provided 8.2 times better sensitivity than the 1 D89Y-detected CP experiment performed with a 3.2-mm rotor.

Structural dynamics of CH3NH3+ and PbBr3\u2212 in tetragonal and cubic phases of CH3NH3PbBr3 hybrid perovskite by nuclear magnetic resonance

Understanding the structural dynamics of lead-halide perovskites is essential for their advanced use as photovoltaics.Here, the structural dynamics of the CH3NH3 cation and PbBr6 octahedra in the perovskite CH3NH3PbBr3 were studied via nuclear magnetic resonance (NMR) to determine the mechanism of the transition from the tetragonal to cubic phase. The chemical shifts were obtained by 1H, 13C, and 207Pb magic angle spinning NMR and 14N static NMR. The chemical shifts of the 1H nuclei in CH3 and NH3 remained constant with increasing temperature, whereas those of the 13C and 207Pb nuclei varied near the phase transition temperature (TC = 236 K), indicating that the structural environments of 13C and 207Pb change near TC. The spin–lattice relaxation time T1ρ values for 1H, 13C, and 207Pb nuclei increased with increasing temperature and did not exhibit an abrupt change near TC. In addition, the two lines in the 14N NMR spectra superposed into one line near TC, indicating the occurrence of a phase transition to a cubic phase with higher symmetry than tetragonal. Consequently, the main factor causing the phase transition from the tetragonal to cubic phase near TC is a change in the surroundings of the 207Pb nuclei in the PbBr6 octahedra and of the C–N groups in the CH3NH3 cations.

Searching for 9He upon the Absorption of Stopped Pions by 14C Nuclei

An experimental search is conducted for the formation of the heavy 9He isotope of helium upon the absorption of stopped pions by 14C nuclei. The structure of 9He levels in the missing mass spectra in reactions 14C(π–, p4He)Х and 14C(π–, d3He)Х is studied. A state with a high energy of excitation of 12.5 MeV and a state width of 1.5 MeV is detected for the first time. Suppression of the formation of ground state 9He in the considered reactions is established.

Application of INADEQUATE NMR techniques for directly tracing out the carbon skeleton of a natural product

Nuclear magnetic resonance (NMR) measurement of 1 JCC coupling by two-dimensional (2D) INADEQUATE (incredible natural abundance double quantum transfer experiment), which is a special case of double-quantum (DQ) spectroscopy that offers unambiguous determination of 13 C-13 C spin-spin connectivities through the DQ transitions of the spin system, is especially suited to solving structures rich in quaternary carbons and poor in hydrogen content (Crews rule). To review published literature on the application of NMR methods to determine structure in the liquid-state, which specifically considers the interaction of a pair of carbon-13 (13 C) nuclei adjacent to one another, to allow direct tracing out of contiguous carbon connectivity using 2D INADEQUATE. A comprehensive literature search was implemented with various databases: Web of Knowledge, PubMed and SciFinder, and other relevant published materials including published monographs. The keywords used, in various combinations, with INADEQUATE being present in all combinations, in the search were 2D NMR, 1 JCC coupling, natural product, structure elucidation, 13 C-13 C connectivity, cryoprobe and CASE (computer-assisted structure elucidation)/PANACEA (protons and nitrogen and carbon et alia). The 2D INADEQUATE continues to solve "intractable" problems in natural product chemistry, and using milligram quantities with cryoprobe techniques combined with CASE/PANACEA experiments can increase machine time efficiency. The 13 C-13 C-based structural elucidation by dissolution single-scan dynamic nuclear polarisation NMR can overcome disadvantages of 13 C insensitivity at natural abundance. Selected examples have demonstrated the trajectory of INADEQUATE spectroscopy from structural determination to clarification of metabolomics analysis and use of DFT (density functional theory) and coupling constants to clarify the connectivity, hybridisation and stereochemistry within natural products. Somewhat neglected over the years because of perceived lack of sensitivity, the 2D INADEQUATE NMR technique has re-emerged as a useful tool for solving natural products structures, which are rich in quaternary carbons and poor in hydrogen content.

Structural Evolution of Diisopropylammonium Chloride (DIPAC) Molecular Ferroelectric

Room-temperature changes in the structure of a new diisopropylammonium chloride (C6H16ClN, DIPAC) molecular ferroelectric have been investigated by high-resolution cross polarization/magic-angle spinning nuclear magnetic resonance (CP/MAS NMR) and X-ray diffraction (XRD) methods. The NMR measurements are carried out using 13C nuclei. Depending on time and heat treatment, monoclinic ferroelectric or orthorhombic nonpolar phases (as well as their coexistence) are observed. It is shown that the polar DIPAC modification at room temperature is transformed in the course of time into the nonpolar orthorhombic structure.

Can proton-proton recoupling in fully protonated solids provide quantitative, selective and efficient polarization transfer?

Dipolar recoupling sequences have been used to probe spatial proximity of nuclear spins and were traditionally designed to probe rare spins such as 13C and/or 15N nuclei. The multi-spin dipolar-coupling network of the rare spins is weak due to smaller couplings and large chemical shift dispersion. Therefore, the recoupling approaches were tailored to design offset compensated or broadband sequences. In contrast, protons have a substantially stronger dipolar-coupling network and much narrower chemical shift range. Broadband recoupling sequences such as radio-frequency driven recoupling (RFDR), back-to-back (BABA), and lab frame proton-proton spin diffusion have been routinely used to characterize the structures of protein/macromolecules and small molecules. Recently selective 1H-1H recoupling sequences have been proposed that combine chemical shift offset of the resolved proton spectrum (at fast MAS) with first- and second-order dipolar recoupling Hamiltonians to obtain quantitative and qualitative proton distances, respectively. Herein, we evaluate the performances of broadband and selective proton recoupling sequences such as finite pulse RFDR (fp-RFDR), band-selective spectral spin diffusion (BASS-SD), second-order cross-polarization (SOCP), and selective recoupling of proton (SERP) in terms of the selectivity and efficiency of 1H-1H polarization transfers in a dense network of proton spins and explore the possibility of measuring 1H-1H distances. We use theoretical considerations, numerical simulations, and experiments to support the distinct advantages and disadvantages of each recoupling sequence. Experiments were performed on L-histidine.HCl.H2O at a MAS frequency of 71.43 kHz. This study rationalizes the proper selection of 1H-1H recoupling sequences when working with fully protonated solids.

Experimental and Theoretical Evidence of Spin-Orbit Heavy Atom on the Light Atom 1 H NMR Chemical Shifts Induced through H⋅⋅⋅I- Hydrogen Bond.

Spin-orbit (SO) heavy-atom on the light-atom (SO-HALA) effect is the largest relativistic effect caused by a heavy atom on its light-atom neighbors, leading, for example, to unexpected NMR chemical shifts of 1 H, 13 C, and 15 N nuclei. In this study, a combined experimental and theoretical evidence for the SO-HALA effect transmitted through hydrogen bond is presented. Solid-state NMR data for a series of 4-dimethylaminopyridine salts containing I- , Br- and Cl- counter ions were obtained experimentally and by theoretical calculations. A comparison of the experimental chemical shifts with those calculated by a standard DFT methodology without the SO contribution to the chemical shifts revealed a remarkable error of the calculated proton chemical shift of a hydrogen atom that is in close contact with the iodide anion. The addition of the relativistic SO correction in the calculations significantly improves overall agreement with the experiment and confirms the propagation of the SO-HALA effect through hydrogen bonds.

The Alkali Metal Salts of Methyl Xanthic Acid

Methyl xanthates of the type M(SSC‐OMe) (M = Li–Cs) are readily formed when carbon disulfide is reacted with the corresponding alkali metal hydroxides in methanol exposed to air, or with the alkali metal methoxides in dry methanol or THF under exclusion of air. The reactions are easily monitored by 13C NMR spectroscopy. The Na, K, Rb, and Cs salt could be isolated in high yields, while the Li salt decomposed upon attempted isolation. All compounds are readily complexed by crown ethers and form isolable 1:1 adducts, including the elusive Li salt. All products were studied by NMR (1H, 13C, and alkali metal nuclei) and IR spectroscopy, and most of them where structurally characterized by single‐crystal X‐ray diffraction. Li(SSC‐OMe)(12c4) (12c4 = [12]crown‐4) and Cs(SSC‐OMe)(18c6) (18c6 = [18]crown‐6) represent the first structurally characterized lithium and caesium xanthate complexes, respectively.