14N Atoms Research Articles

-=SUP=-15-=/SUP=-N isotope enrichment of the atomic nitrogen component in atomic-molecular exchange reactions in the post-discharge zone

A kinetic model is presented that explains the more than thirty-fold enrichment of the atomic nitrogen component with the 15N isotope in the post-discharge zone. Isotopic enrichment is due to the activation nature of the exchange reaction of atoms with vibrationally excited nitrogen molecules, the upper vibrational levels of which are enriched with molecules containing a heavier isotope during non-equilibrium vibrational-vibrational exchange. The dependences of the concentrations of 14N and 15N atoms on the time of nitrogen flow through a quartz tube emerging from the region of a pulsed electric discharge are obtained. In addition to exchange, the model took into account the dissociation of molecules, the recombination of atoms, and the loss of the vibrational energy of molecules on the surface of the tube. The obtained dependence of the increase in the ratio of the concentrations of 15N to 14N atoms on the time of nitrogen flow through the post-discharge zone satisfactorily describes the experimental data published earlier and obtained in this work. Keywords: Trinor distribution, gas excitation regime, inverse population.

Обогащение изотопом -=SUP=-15-=/SUP=-N атомарной компоненты азота в реакциях атомно-молекулярного обмена в послеразрядной зоне

A kinetic model is presented that explains the more than thirty-fold enrichment of the atomic nitrogen component with the 15N isotope in the post-discharge zone. Isotopic enrichment is due to the activation nature of the exchange reaction of atoms with vibrationally excited nitrogen molecules, the upper vibrational levels of which are enriched with molecules containing a heavier isotope during non-equilibrium vibrational-vibrational exchange. The dependences of the concentrations of 14N and 15N atoms on the time of nitrogen flow through a quartz tube emerging from the region of a pulsed electric discharge are obtained. In addition to exchange, the model took into account the dissociation of molecules, the recombination of atoms, and the loss of the vibrational energy of molecules on the surface of the tube. The obtained dependence of the increase in the ratio of the concentrations of 15N to 14N atoms on the time of nitrogen flow through the post-discharge zone satisfactorily describes the experimental data published earlier and obtained in this work.

Uric Acid as a Photosensitizer in the Reaction of Deoxyribonucleosides with UV Light of Wavelength Longer than 300 nm: Identification of Products from 2'-Deoxycytidine.

DNA reacts directly with UV light with a wavelength shorter than 300 nm. Although ground surface sunlight includes little of this short-wavelength UV light due to its almost complete absorption by the atmosphere, sunlight is the primary cause of skin cancer. Photosensitization by endogenous substances must therefore be involved in skin cancer development mechanisms. Uric acid is the final metabolic product of purines in humans, and is present at relatively high concentrations in cells and fluids. When a neutral mixed solution of 2'-deoxycytidine, 2'-deoxyguanosine, thymidine, and 2'-deoxyadenosine was irradiated with UV light with a wavelength longer than 300 nm in the presence of uric acid, all the nucleosides were consumed in a uric acid dose-dependent manner. These reactions were inhibited by the addition of radical scavengers, ethanol and sodium azide. Two products from 2'-deoxycytidine were isolated and identified as N4-hydroxy-2'-deoxycytidine and N4,5-cyclic amide-2'-deoxycytidine, formed by cycloaddition of an amide group from uric acid. A 15N-labeled uric acid, uric acid-1,3-15N2, having two 14N and two 15N atoms per molecule, produced N4,5-cyclic amide-2'-deoxycytidine containing both 14N and 15N atoms from uric acid-1,3-15N2. Singlet oxygen, hydroxyl radical, peroxynitrous acid, hypochlorous acid, and hypobromous acid generated neither N4-hydroxy-2'-deoxycytidine nor N4,5-cyclic amide-2'-deoxycytidine in the presence of uric acid. These results indicate that uric acid is a photosensitizer for the reaction of nucleosides by UV light with a wavelength longer than 300 nm, and that an unidentified radical derived from uric acid with a delocalized unpaired electron may be generated.

Selective 1H-14N Distance Measurements by 14N Overtone Solid-State NMR Spectroscopy at Fast MAS.

Accurate distance measurements between proton and nitrogen can provide detailed information on the structures and dynamics of various molecules. The combination of broadband phase-modulated (PM) pulse and rotational-echo saturation-pulse double-resonance (RESPDOR) sequence at fast magic-angle spinning (MAS) has enabled the measurement of multiple 1H-14N distances with high accuracy. However, complications may arise when applying this sequence to systems with multiple inequivalent 14N nuclei, especially a single 1H sitting close to multiple 14N atoms. Due to its broadband characteristics, the PM pulse saturates all 14N atoms; hence, the single 1H simultaneously experiences the RESPDOR effect from multiple 1H-14N couplings. Consequently, no reliable H-N distances are obtained. To overcome the problem, selective 14N saturation is desired, but it is difficult because 14N is an integer quadrupolar nucleus. Alternatively, 14N overtone (OT) NMR spectroscopy can be employed owing to its narrow bandwidth for selectivity. Moreover, owing to the sole presence of two energy levels (m = ± 1), the 14N OT spin dynamics behaves similarly to that of spin-1/2. This allows the interchangeability between RESPDOR and rotational-echo double-resonance (REDOR) since their principles are the same except the degree of 14N OT population transfer; saturation for the former whereas inversion for the latter. As the ideal saturation/inversion is impractical due to the slow and orientation-dependent effective nutation of 14N OT, the working condition is usually an intermediate between REDOR and RESPDOR. The degree of 14N OT population transfer can be determined from the results of protons with short distances to 14N and then can be used to obtain long-distance determination of other protons to the same 14N site. Herein, we combine the 14N OT and REDOR/RESPDOR to explore the feasibility of selective 1H-14N distance measurements. Experimental demonstrations on simple biological compounds of L-tyrosine.HCl, N-acetyl-L-alanine, and L-alanyl-L-alanine were performed at 14.1 T and MAS frequency of 62.5 kHz. The former two consist of a single 14N site, whereas the latter consists of two 14N sites. The experimental optimizations and reliable fittings by the universal curves are described. The extracted 1H-14N distances by OT-REDOR are in good agreement with those determined by PM-RESPDOR and diffraction techniques.

Microwave measurements of rotational transitions and nitrogen quadrupole coupling for 2-aminopyridine

The high-resolution microwave rotational spectrum was measured for 2-aminopyridine in the 4.5–13.2 GHz range using a pulsed beam Fourier transform microwave spectrometer. The measured transitions for the parent isotopologue in the 0+ vibrational state of the amino group inversion vibration were used to determine accurate rotational constants and quadrupole coupling constants for the two 14N atoms in the molecule. The molecular parameters determined for the 2-aminopyridine in 0+ vibrational state have the following values: A = 5780.374(1) MHz, B = 2733.5017(3) MHz, C = 1857.6768(3) MHz, 1.5χaa (14N1) = 3.5789(45) MHz, 0.25(χbb – χcc) (14N1) = 1.5033(24)MHz, 1.5χaa (14N2) = -0.0958(70) MHz, and 0.25(χbb – χcc) (14N2) = −1.1615(21) MHz. The measured quadrupole coupling constants are in excellent agreement with those calculated using different computational methods and basis sets. It is noted that the calculation using the MP2 functional with cc-pvDz basis set closely reproduces the experimental values for quadrupole coupling constants for the two 14N atoms.

Plasma Scraping of 14C Surface Nanolayer Formed by Neutron Fluence of Graphite Reactor

In this article, we consider some mechanisms of the formation of a nanometer-thick surface layer enriched with 14C isotope. Main channel of 14C surface accumulation is believed to be the neutron capture reaction with 14N atoms of the cooling gas mixture during operation of uranium-graphite reactors, in particular, a high-power channel-type reactor (RBMK-type reactor). For the “dry” collection of thin layers enriched in 14C isotope from the irradiated graphite surface of graphite-type nuclear reactor and further compact burial, we propose argon plasma discharge technology based on the special type of a microdischarge at a high pressure, up to atmospheric pressure. In addition, there are some important technical applications, such as brachytherapy medicine or new beta-voltaic batteries, which require the deposition of thin layers of beta-active isotopes (e.g. 14C) with nanoscale precision to form a controlled output energy spectrum of secondary electrons. The proposed technological scheme can potentially be used both for graphite deactivation in the irradiated reactor during decommissioning of uranium-graphite reactors, and at the same time for fabrication of highly concentrated 14C nanosized coatings for use in radionuclide medicine and beta-voltaic batteries.

Probing the Structure and Binding Mode of EDTA on the Surface of Mn3O4 Nanoparticles for Water Oxidation by Advanced Electron Paramagnetic Resonance Spectroscopy.

Identification of the surface structure of nanoparticles is important for understanding the catalytic mechanism and improving the properties of the particles. Here, we provide a detailed description of the coordination modes of ethylenediaminetetraacetate (EDTA) on Mn3O4 nanoparticles at the atomic level, as obtained by advanced electron paramagnetic resonance (EPR) spectroscopy. Binding of EDTA to Mn3O4 leads to dramatic changes in the EPR spectrum, with a 5-fold increase in the axial zero-field splitting parameter of Mn(II). This indicates significant changes in the coordination environment of the Mn(II) site; hence, the binding of EDTA causes a profound change in the electronic structure of the manganese site. Furthermore, the electron spin echo envelope modulation results reveal that two 14N atoms of EDTA are directly coordinated to the Mn site and a water molecule is coordinated to the surface of the nanoparticles. An Fourier transform infrared spectroscopy study shows that the Ca(II) ion is coordinated to the carboxylic ligands via the pseudobridging mode. The EPR spectroscopic results provide an atomic picture of surface-modified Mn3O4 nanoparticles for the first time. These results can enhance our understanding of the rational design of catalysts, for example, for the water oxidation reaction.

Isotopic Nitrogen-15 Labeling of Mice Identified Long-lived Proteins of the Renal Basement Membranes

The kidney is comprised of highly complex structures that rely on self-maintenance for their functions, and tissue repair and regeneration in renal diseases. We devised a proteomics assay to measure the turnover of individual proteins in mouse kidney. Mice were metabolically labeled with a specially formulated chow containing nitrogen-15 (15N) with the absence of normal 14N atoms. Newly synthesized proteins with 15N contents were distinguished from their 14N counterparts by mass spectrometry. In total, we identified over 4,000 proteins from the renal cortex with a majority of them contained only 15N. About 100 proteins had both 14N- and 15N-contents. Notably, the long-lived proteins that had large 14N/15N ratios were mostly matrix proteins. These included proteins such as type IV and type VI collagen, laminin, nidogen and perlecan/HSPG2 that constitute the axial core of the glomerular basement membrane (GBM). In contrast, the surface lamina rara proteins such as agrin and integrin had much shorter longevity, suggesting their faster regeneration cycle. The data illustrated matrix proteins that constitute the basement membranes in the renal cortex are constantly renewed in an ordered fashion. In perspective, the global profile of protein turnover is usefully in understanding the protein-basis of GBM maintenance and repair.

In Situ Quantification of Biological N2 Production Using Naturally Occurring 15N15N.

We describe an approach for determining biological N2 production in soils based on the proportions of naturally occurring 15N15N in N2. Laboratory incubation experiments reveal that biological N2 production, whether by denitrification or anaerobic ammonia oxidation, yields proportions of 15N15N in N2 that are within 1‰ of that predicted for a random distribution of 15N and 14N atoms. This relatively invariant isotopic signature contrasts with that of the atmosphere, which has 15N15N proportions in excess of the random distribution by 19.1 ± 0.1‰. Depth profiles of gases in agricultural soils from the Kellogg Biological Station Long-Term Ecological Research site show biological N2 accumulation that accounts for up to 1.6% of the soil N2. One-dimensional reaction-diffusion modeling of these soil profiles suggests that subsurface N2 pulses leading to surface emission rates as low as 0.3 mmol N2 m–2 d–1 can be detected with current analytical precision, decoupled from N2O production.

Chemical nitrogen fractionation in dense molecular clouds

Nitrogen-bearing molecules display variable isotopic fractionation levels in different astronomical environments such as in the interstellar medium or in the Solar system. Models of interstellar chemistry are unable to induce nitrogen fraction in cold molecular clouds as exchange reactions for 15N are mostly inefficient. Here, we developed a new gas–grain model for nitrogen fractionation including a thorough search for new nitrogen fractionation reactions and a realistic description of atom depletion on to interstellar dust particles. We show that, while dense molecular cloud gas-phase chemistry alone leads to very low fractionation, 14N atoms are preferentially depleted from the gas phase due to a mass-dependent grain surface sticking rate for atomic nitrogen. However, assuming an elementary 14N/15N ratio of 441 (equal to the solar wind value), our model leads to only low 15N enrichment for all N-containing species synthesized in the gas phase with predicted 14N/15N ratios in the range 360–400. Higher enrichment levels can neither be explained by this mechanism, nor through chemistry, with two possible explanations: (i) The elementary 14N/15N ratio in the local ISM is smaller, as suggested by the recent work of Romano et al., with a hypothetic 15NNH+ and 15NNH+ depletion due to variation of the electronic recombination rate constant variation with the isotopes and (ii) N2 photodissociation leads to variable nitrogen fractionation in diffuse molecular clouds where photons play an important role, which is conserved during dense molecular cloud formation as suggested by the work of Furuya & Aikawa.

The Structure and Internal Dynamics of R6-p-C6H4-R6 Biradical: EPR, X-ray Crystallography and DFT Calculations

A purposefully synthesized nitroxide biradical R6-p-C6H4-R6 (B1), where R6 is the 1-oxyl-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine group with a relatively short distance between the two radical sites, has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Hyperfine splitting (hfs) constants on the 14N atoms, electron spin exchange integral |J|, and the distance between the two N–O fragments rNO–NO were experimentally measured. Density functional theory, DFT, calculations were performed using the ORCA 4.0.1.2 program package. The optimized geometry was compared with X-ray crystallographic data and theoretical hfs constants were compared with the respective experimental EPR values. It is concluded that the current quantum chemical approaches provide good results in calculating hfs constants as well as some other EPR parameters. It is confirmed that the intramolecular electron spin exchange in biradicals analogous to B1 is realized by the indirect mechanism rather than direct collision of the N–O· groups. It is also shown that one can calculate and predict values of |J| in other similar biradicals based on the principle of “attenuation coefficients.

Ultra-Narrow Low-Field Nuclear Spin Resonance in NV Centers in a Bulk Diamond Crystal

Negatively charged nitrogen vacancy (NV) color centers in diamond crystals have been intensively studied in the last few decades with the use of the optically detected magnetic resonance method which usually implies resonance excitation using a DC or pulsed drive field in the gigahertz range (MW field). Various techniques for multi-frequency MW excitation of magnetic resonance in the nuclear structure of 14N atoms in NV centers were developed. Most of them use MW excitation in combination with RF modulation or direct RF excitation of nuclear transition. Here, we report on the possibility to detect ultra-narrow resonances in the nuclear structure of 14N using the ODMR methods with single-frequency DC RF excitation of the nuclear transition at 4.95 MHz. The resonances detected in a bulk sample in weak (0–10 mT) magnetic fields show approximately 7 kHz width HWHM, corresponding to T 2 * = 23 μs, and they seem to be insensitive to the magnetic field direction. These resonances may be of interest for solid-state quantum information processing, as well as for quantum magnetometry, especially in biological and medical applications where a strong MW drive field may be undesirable. We demonstrate that optical pumping can be used to create and detect not only the electron orientation, but also the nuclear one, even in the state with a zero electron spin projection. This allows sensitive non-pulsed detection methods like coherent population trapping to be applied to nuclear spins.

Isotope Shifts of Nitrogen around 800 nm

The Doppler-limited absorption spectra of 14N and 15N atoms were measured around 800 nm using concentration modulation spectroscopy to study their isotope shifts. The nitrogen atoms were generated by discharging molecular nitrogen buffered with helium in a home-made discharge tube. The isotope shifts of four multiplets (3s4PJ→3p4DoJ, 3s4PJ→3p4PoJ, 3s2DJ→5p2DoJ, and 3p2PoJ→5s2PJ) were measured and their J-dependent specific mass shifts were observed and discussed.

The effect of CuII ions in l-asparagine single crystals

We report the synthesis, crystal growth, and spectroscopic characterization of l-asparagine monohydrate (LAM) single crystals doped with CuII. The crystals were successfully grown by slow cooling from a supersaturated aqueous solution up to size of 16×12×2mm3;the effect of copper impurities in the crystals morphology was discussed. Electron Paramagnetic Resonance (EPR) was used to calculate the g and hyperfine coupling (A) tensors of the CuII ions (g1=2.044, g2=2.105, g3=2.383and A1≈0, A2=35, A3=108Gauss). The EPR spectra for certain orientations of the magnetic field suggest that CuII ions are coordinated to two 14N atoms. Correlating the EPR and optical absorption results, the crystal field and the CuII orbital bond parameters were calculated. The results indicate that the paramagnetic center occupies interstitial rhombic distorted site and the ground orbital state for the unpaired electron is the d(x2-y2).

High-Resolution Electronic Structure of the Primary Electron Acceptor A0 of Photosystem I

The Type I photosynthetic reaction center, Photosystem I, is an exquisitely tuned protein complex comprised of multiple polypeptide subunits and protein-bound electron-transfer (ET) cofactors. Upon illumination, the special chlorophylls, P700, are photoexcited which results in the rapid formation of the charge-separated state, P700+A0-. In order to prevent charge recombination, the electron is transferred to the phylloquinone cofactor, A1, and subsequently to three four iron-four sulfur [4Fe4S] clusters, FA, FB, and FX, respectively. Through the ET reactions of PSI, the reducing equivalents that are required for the carbon fixation reactions are generated and stored as NADPH or ‘biohydrogen’. The PSI reaction center displays pseudo-C2 symmetry, such that the ET cofactors, A0 and A1, are duplicated in what is termed the A- and B-branch. Although ET is bidirectional along the A- and B-branch, it has been demonstrated that ET in the A-branch is highly preferred. This is thought to be due to the tuning of the redox-potential of the ET cofactors by smart matrix effects from the surrounding protein environment. This research explores the electronic structure of the primary electron acceptor A0 in the A-branch (A0A) through the use of two-dimensional (2D) hyperfine sublevel correlation (HYSCORE) spectroscopy in tandem with density functional theory calculations. The application of 2D HYSCORE spectroscopy allows for the identification of the 14N atoms and protons that are magnetically interacting with the paramagnetic center of A0A-. The 14N and 1H hyperfine parameters obtained here provide a direct measure of the electron spin density distribution of A0- in the A-branch of PSI. These are then be compared with simulations of the electronic structure of the A-branch of Photosystem I in order to better understand the mechanism of electron transfer from P700 to the primary electron acceptor, A0A.

Adsorption and reactivity of nitrogen atoms on silica surface under plasma exposure

The kinetics of adsorption, desorption and recombination of nitrogen atoms on a silica surface is investigated. Stable nitrogen atoms are grafted to the inner surface of a fused silica discharge tube by a discharge in N2 at 0.53 mbar. After the pre-treatment, the surface is analysed using x-ray photoelectron spectroscopy and an isotopic exchange technique. The latter consists of the exposure of the pre-treated surface with a discharge in the heavy nitrogen isotope 30N2. Nitrogen isotopologues 29N2 and 28N2 produced on the surface are detected using a mass spectrometer and provide information about the coverage and reactivity of adsorbed 14N atoms. It is found that during the pre-treatment, a silicon oxynitride (SiOxNy) layer is formed on the initially clean SiO2 surface. The coverage of N on the surface increases from 5 × 1013 to 5 × 1015 cm−2 for a pre-treatment duration in the range of 10−2 − 104 s. Atoms on the surface demonstrate a distribution of reactivity, which is attributed to a distribution of their binding energies and configurations on the surface. We demonstrate that stable chemisorbed Nads are not the main recombination sites for N atoms on the surface contrary to previous studies. We conclude that recombination takes place mainly on weakly bonding active sites with the binding energy smaller than 1 eV.

Role of scandium atom in ScxY3−xN@C80(CF3)n (n = 0, 2) on nuclear quadrupole interaction, electric field gradient and asymmetric parameters

Electronic structures and magnetic properties of ScxY3−xN@C80(CF3)n (n=0, 2) were studied by theoretical calculation. Metallofullerene of Sc2YN@C80(CF3)2 anion radical had magnetic parameters of chemical shift with a large extent of A-tensor in hyperfine coupling constant (hfc) of 30MHz and nuclear quadrupole interaction based on Q-tensor of 20MHz. Especially, the magnetic parameters of isotropic chemical shift, principle g-tensor, A-tensor in hfc and Q-tensor of 14N, 45Sc and 89Y atoms in ScxY3−xN@C80(CF3)n were originated in distorted molecular structure of electron density distribution with a large extent of charge distribution on the fullerene cage. Spin oscillation of nuclear spin depends on quite extent of nuclear quadrupole interaction based on multiplication of nuclear quadrupole moment, electronic field gradient and asymmetrical parameters in spin–lattice relaxation time of 0.3s, which will be appropriate to control spin gate multi-entanglement in parallel process.

Electronic structures and magnetic properties of Sc2YN@C80(CF3)2 dimer

Electronic structures and magnetic properties of a Sc2YN@C80(CF3)2 dimer, a Sc3N@C80(CF3)2 dimer, and Sc2YN@C80(CF3)2 were investigated by quantum calculation using the density functional theory. The effect of the perturbation of dimerization on the electronic structure was based on the distorted molecular structure of the electron density distribution with an s,p,d-hybrid orbital at decreasing energy levels near the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Magnetic parameters of the isotropic chemical shifts of 14N, 45Sc, and 13C-NMR in 14N, 45Sc, and 13C atoms on the fullerene cage were calculated. The chemical shifts of 45Sc and 14N atoms in a dimer were due to the dipole–dipole and nuclear quadrupole interactions generated by an electrostatic potential with a slight modified electron density distribution and an atomic charge around a nucleus.

Theoretical study of substituents effects on characteristics of resonance-assisted hydrogen bond in (Z)-(thionitrosomethylene)hydrazine and its derivatives in ground and electronic excited state

The molecular geometry, intramolecular hydrogen bond strength, vibrational frequencies, 1H NMR chemical shift, and nuclear quadrupole resonance parameters of 14N, 35S and 2H atoms and several well-established indices of aromaticity in (Z)-(thionitrosomethylene)hydrazine molecule and its derivatives were studied by density functional theory method. The results of calculations were obtained at B3LYP/6-311++G** level of approximation on model species, with the resonance-assisted hydrogen bonds. A set of simple and mostly common substituents having different properties in resonance effect according to values of substituents constants were chosen to simulate the influence of substitution in R position of title molecule on the quasi-delocalization and H-bonding. The following substituents have been taken into consideration: F, Cl, NO2, OCH3, OCF3, SCH3, SH, and OH. The excited-state properties of intramolecular hydrogen bonding in substituted systems have been investigated theoretically using the time-dependent density functional theory method. Also, the possible charge transfer and the topological properties of investigated molecule and its derivatives were studied by means of natural bond orbital and atoms in molecules (AIM) theory. The energy of the N–H···S interactions studied here was found medium in strength ( $$ E_{\text{HB}}^{*} $$ = −36.5 to −45.3 kJ mol−1). The electron density (ρ), Laplacian (∇2 ρ) properties and the total electron energy density (HC), estimated by AIM calculations, indicate that H···S bond possesses low ρ, positive ∇2 ρ and HC < 0 which are in agreement with partially covalent character of HB.

Electronic structure and magnetic properties of endohedral metallofullerenes based on mixed metal cluster within fullerene cage with trifluoromethyl groups

Endohedral metallofullerenes based on encapsulated nitride scandium cluster within C80 fullerene cage with trifluoromethyl group of CF3 were characterized as a candidate model for controlling nuclear spin gate in NMR quantum computers. Effects of yttrium in nitride scandium cluster of Sc3N within the fullerene cage with trifluoromethyl group on electronic structures and magnetic properties were investigated. Electronic structure and chemical shifts in 45Sc, 89Y, 19F, 13C and 14N atoms were calculated by quantum calculation using density functional theory. Addition of yttrium in nitride scandium cluster of Sc3N within the fullerene cage provided a different position of chemical shift in 45Sc, 89Y, 19F, 13C and 14N atoms. This result was originated in magnetic interaction with spin-local interaction, dipole-dipole interaction and nuclear quadrupole interaction based on nuclear quadrupole moment and asymmetry of electron field gradient with a bias value of charge and spin density distribution of spd-hybrid orbital by inductive effect.