Magnetic shielding tensors of carbon nuclei in 3,5-dichlorophenylacetylene,a theoretical ab initio and experimental study

Ab initio GIAO-CHF and DFT(B3LYP) molecular geometry optimization and magnetic shielding tensor calculations of carbon nuclei of 3,5-dichlorophenylacetylene have been performed using 6–311G?? and 6–311+G(2d,p) basis sets. The isotropic ¹³C chemical shifts, needed for comparison, have been measured in C₆D₁₂ solution. The principal elements of the shielding tensor of the carbon nuclei in the investigated molecule in the solid state have been determined from an intensity analysis of the spinning sidebands in ¹H-¹³C CP/MAS NMR spectra. Shielding anisotropy parameters of the acetylenic carbons have been independently determined using the method based on proton-coupled ¹³C nematic phase spectra as well as from the interpretation of the ¹³C longitudinal relaxation rates. The latter data have been analysed assuming the molecular reorientation to be the rotational diffusion of an asymmetrical top, which has provided, apart from the diffusion coefficients, an additional check on the reliability of the theoretical calculation of the shielding tensors. In general, satisfactory agreement between the theoretical and experimental results has been achieved.     

Study of diamond film by dynamic nuclear polarization-enhanced13C nuclear magnetic resonance spectroscopy

Three chemical vapor deposited diamond films were studied by dynamic nuclear polarization (DNP)-enhanced high-resolution solid-state¹³C nuclear magnetic resonance (NMR) spectroscopy. Enhanced¹³C direct-polarization spectra of diamond films were obtained by irradiating the samples with microwaves at or near electron spin resonance Larmor frequency of carbon center free radicals. No NMR signal for sp² hybridized carbons could be observed. From the curve of the DNP enhancement as a function of frequency, it is found that the dominant DNP mechanism is the solid-state effect. The¹³C cross-polarization spectrum, which is an evidence for existence of the proton defect in the lattice of diamond films, is much broader than the¹³C single pulse spectrum. The reason is discussed shortly.     

X-ray photoelectron spectroscopic study of nitrogen incorporated amorphous carbon films embedded with nanoparticles

The effect of substrate bias on X-ray photoelectron spectroscopy (XPS) study of nitrogen incorporated amorphous carbon (a-C:N) films embedded with nanoparticles deposited by filtered cathodic jet carbon arc technique is discussed. High resolution transmission electron microscope exhibited initially the amorphous structure but on closer examination the film was constituted of amorphous phase with the nanoparticle embedded in the amorphous matrix. X-ray diffraction study reveals dominantly an amorphous nature of the film. A straight forward method of deconvolution of XPS spectra has been used to evaluate the sp³ and sp² contents present in these a-C:N films. The carbon (C 1s) peaks have been deconvoluted into four different peaks and nitrogen (N 1s) peaks have been deconvoluted into three different peaks which attribute to different bonding state between C, N and O. The full width at half maxima (FWHM) of C 1s peak, sp³ content and sp³/sp² ratio of a-C:N films increase up to −150 V substrate bias and beyond −150 V substrate bias these parameters are found to decrease. Thus, the parameters evaluated are found to be dependent on the substrate bias which peaks at −150 V substrate bias.     

Frequency-selective REDOR and spin-diffusion relays in uniformly labeled whole cells

Solid-state NMR is a powerful and non-perturbative method to measure and define chemical composition and architecture in bacterial cell walls, even in the context of whole cells. Most NMR studies on whole cells have used selectively labeled samples. Here, we introduce an NMR sequence relay using frequency-selective REDOR (fsREDOR) and spin diffusion elements to probe a unique amine contribution in uniformly ¹³C- and ¹⁵N-labeled Staphylococcus aureus whole cells that we attribute to the d-alanine of teichoic acid. In addition to the primary peptidoglycan structural scaffold, cell walls can contain significant amounts of teichoic acid that contribute to cell-wall function. When incorporated into teichoic acid, d-alanine is present as an ester, connected via its carbonyl to a ribitol carbon, and thus has a free amine. Teichoic acid d-Ala is removed during cell-wall isolations and can only be detected in the context of whole cells. The sequence presented here begins with fsREDOR and a chemical shift evolution period for 2D data acquisition, followed by DARR spin diffusion and then an additional fsREDOR period. fsREDOR elements were used for ¹³C observation to avoid complications from ¹³C–¹³C couplings due to uniform labeling and for ¹⁵N dephasing to achieve selectivity in the nitrogens serving as dephasers. The results show that the selected amine nitrogen of interest is near to teichoic acid ribitol carbons and also the methyl group carbon associated with alanine. In addition, its carbonyl is not significantly dephased by amide nitrogens, consistent with the expected microenvironment around teichoic acid.     

13C NMR study of carbon distribution in Fe−C,Fe−Ni−C and Fe−Mn−C martensite

The distribution of carbon atoms in martensite have been studied by NMR. It was established that in virgin martensite metalloid was located only in octahedral intersticies. The analysis of hyperfine fields on¹³C nuclei for various types of martensite local arrangement was carried out. Comparison of analysis results with experimental data let to determine the possible models of carbon clustering in martensite at steel aging.     

The influence of ion irradiation on the structure and properties of amorphous carbon films

Presented in this work are the results of investigation of the structure and electrophysical properties of amorphous carbon films. The films were produced by sputtering of graphite by ion beam and usin ion irradiation (E=0–200 eV) during condensation process. The structure of i-C films has been studied by means of transmission electron microscope. The electron diffraction data have been interpretated by employing the calculated interference function of carbon clusters. The structure of V-band was obtained from AES by deconvolution method. Experimental data shows that under ion irradiation the transformation of short range order and electron bonds is an oscillating function of ion energy E. This paper presents a theoretical calculation of tunneling neutralization cross-section of Ar⁺ ions on carbon surface. The process also has an oscillating dependence on ion energy. A significant importance of inelastic processes in carbon phase transformation has been revealed.     

The carbon isotopic effect in photolysis of an aqueous solution of cyclopentanol measured by the NMR method

Using the¹³C NMR method, the manifestation of the carbon isotopic effect on the products of the photolysis of an aqueous solution of cyclopentanone is investigated. A relative enrichment by the isotope¹³C of the carbonyl (13%) and α-carbon (6%) atoms of initial cyclopentanone and of the carbonyl atom of pent-4-en-1-ale (11%), as well as the depletion of the carbonyl (15%) and methyl (7%) carbon atoms of valeric acid are revealed. The isotopic effect observed is shown to have a magnetic nature. Institute of Physical-Organic Chemistry, Academy of Sciences of Belarus, 13, Surganov St., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 260–262, March–April, 1997.     

Proton and Carbon - 13 NMR Assignments for Fluorinated and Nonfluorinated Aromatic Ketimines

The ¹H and ¹³C NMR spectra of three aromatic ketimines with varying degrees of fluoro substitution have been extensively studied using one and two dimensional techniques. COSY experiments were conducted to identify the protons on each of the aromatic groups, HETCORR experiments were then utilized to identify the corresponding carbon atoms, and then the order of the carbon atoms was established by long-range HETCORR (HRTCORRLR) results. These studies have allowed rigorous assignments to be made for most of the carbon and hydrogen atoms present in these compounds. Fluorine splittings were very helpful in the analyses. In the course of this study, the NMR absorbances (¹H and ¹³C) of related aldimines have also been assigned. This constitutes the first report on the assignments of ¹H and ¹³C absorbances for ketimines. The observed spectral properties suggest that the structure of aromatic ketimines is one in which the aromatic rings are in three different planes. Two of the aromatic rings, the N-substituted ring and the C-substituted ring in the cis configuration, are twisted substantially out of the plane with respect to the -C=N- bond. The remaining C-substituted aromatic ring, trans to the N-substituted ring, lies in the deshielding zone of the imine bond.     

Correlation of sp and sp fraction of carbon with electrical, optical and nano-mechanical properties of argon-diluted diamond-like carbon films

In the present work the correlation of electrical, optical and nano-mechanical properties of argon-diluted diamond-like carbon (Ar-DLC) thin films with sp³ and sp² fractions of carbon have been explored. These Ar-DLC thin films have been deposited, under varying C₂H₂ gas pressures from 25 to 75 mTorr, by radio frequency-plasma enhanced chemical vapor deposition technique. X-ray photoelectron spectroscopy studies are performed to estimate the sp³ and sp² fractions of carbon by deconvoluting C 1s core level spectra. Various electrical, optical and nano-mechanical parameters such as conductivity, I-V characteristics, optical band gap, stress, hardness, elastic modulus, plastic resistance parameter, elastic recovery and plastic deformation energy have been estimated and then correlated with calculated sp³ and sp² fractions of carbon and sp³/sp² ratios. Observed tremendous electrical, optical and nano-mechanical properties in Ar-DLC films deposited under high base pressure conditions made it a cost effective material for not only hard and protective coating applications but also for electronic and optoelectronic applications.     

A C solid-state NMR analysis of vitamin D compounds

¹³C cross-polarization/magic-angle spinning (CP/MAS) solid-state NMR spectroscopy has been employed to analyze four vitamin D compounds, namely vitamin D3 (D3), vitamin D2 (D2), and the precursors ergosterol (Erg) and 7-dehydrocholesterol (7DHC). The ¹³C NMR spectrum of D3 displays a doublet pattern for each of the carbon atoms, while that of Erg contains both singlet and doublet patterns. In the cases of 7DHC and D2, the ¹³C spectra display various multiplet patterns, viz. singlets, doublets, triplets, and quartets. To overcome the signal overlap between the ¹³C resonances of protonated and unprotonated carbons, we have subjected these vitamin D compounds to 1D ¹H-filtered ¹³C CP/MAS and {¹H}/¹³C heteronuclear correlation (Hetcor) NMR experiments. As a result, assisted by solution NMR data, all of the ¹³C resonances have been successfully assigned to the respective carbon atoms of these vitamin D compounds. The ¹³C multiplets are interpreted due to the presence of s-cis and s-trans configurations in the α- and β-molecular conformers, consistent with computer molecular modeling determined by molecular dynamics and energy minimization calculations. To further characterize the ring conformations in D3, we have successfully extracted chemical shift tensor elements for the ¹³C doublets. It is demonstrated that ¹³C solid-state NMR spectroscopy provides a robust and high sensitive means of characterizing molecular conformations in vitamin D compounds.     

Bonding configurations in amorphous carbon and nitrogenated carbon films synthesised by femtosecond laser deposition

The effect of nitrogen addition and laser fluence on the atomic structure of amorphous carbon films (a-C) synthesized by femtosecond pulsed laser deposition has been studied. The chemical bonding in the films was investigated by means of X-ray photoelectron (XPS) and Raman spectroscopies. XPS studies revealed a decrease in the sp³ bonded carbon sites and an associated increase in the N-sp²C bonding sites with increasing nitrogen content in the CN_x films. An increase in laser fluence from 0.36 to 1.7 J/cm² led to a rise in sp³C sites. These results were further confirmed by Raman spectroscopy. The I_D/I_G ratio increased monotonically and G line-width decreased with the increase of nitrogen content in the films indicating a rise in either the number or the size of the sp² clusters. Furthermore a visible excitation wavelength dependence study established the resonant Raman process in a-C and CN_x films. PACS 81.05.Uw; 81.15.Fg; 82.80     

The electronic structure of different forms of solid carbon studied by electron momentum spectroscopy

The purpose of this paper is to first give an overview of the information obtained by electron momentum spectroscopy (EMS) on the electronic structure of different forms of solid carbon (single crystal graphite, polycrystalline carbon, different forms of amorphous carbon, and fullerenes). We show for sp² bonded carbon that, with decreasing order, the structures observed change but certain basic features remain. sp³-bonded carbon films and fullerene films both have electronic structures different from these graphitic films. The results of EMS are compared with the results of other spectroscopic and scattering techniques. It is argued that the combination of energy and momentum resolution is crucial in order to get a clear picture of the changes in the electronic structure due to the changes in atomic arangements. Only EMS resolves both energy and momentum for all these solids.     

Gasification and removal of carbon materials and redeposited boron–carbon layers exposed in an oxygen–ozone mixture

The results from experiments on measuring the rate of gasification for carbon and boron–carbon films and carbon fiber composite (CFC) exposed in oxygen–ozone mixtures are presented. The rate of gasification is 0.4–0.6 μm h^–1 (at temperatures of 220–250°C, a pressure of 0.3 atm, and an ozone concentration of 0.6 at %) for carbon films; plane CFC samples; gaps 1 and 2 mm wide with walls of stainless steel; and gaps 1 mm wide with walls of CFC. It is 15 μm h^–1 for plane CFC at a temperature of 250°C, a pressure of 1 atm, and an ozone concentration of 10 at %. The rate of gasification for boron–carbon films is from 3 to 30 nm h^–1 for B/C ratios of 2.1 to 0.8 (at 250°C, 1 atm, and ozone concentration of 10 at %).     

Isotopic fractionation between gaseous and dissolved carbon dioxide

The isotopic fractionation between gaseous carbon dioxide and an equilibrated aqueous solution of the gas has been measured at temperatures between 0° and 60 °C for the carbon isotopes and close to 0 °C for the oxygen isotopes. ¹³C¹⁶O₂ is slightly less, and¹²C¹⁶O¹⁸O slightly more soluble than¹²C¹⁶O₂, the actual values for the fractionation,?, being ?(1.18?0.0041 ·t)‰ (temperature,t, in °C) for the carbon isotopes and about 0.8‰ at 0°C for the oxygen isotopes. A theoretical expression is derived for the vapour-solute equilibrium system by treating it in a similar manner as the pure vapour-liquid system. The calculated isotopic fractionation for carbon dioxide, using data on the pure vapour-liquid system, is in satisfactory agreement with the experimental results.     

Opto-electrical properties of amorphous carbon thin film deposited from natural precursor camphor

A simple thermal chemical vapor deposition technique is employed for the pyrolysis of a natural precursor “camphor” and deposition of carbon films on alumina substrate at higher temperatures (600-900 °C). X-ray diffraction measurement reveals the amorphous structure of these films. The carbon films properties are found to significantly vary with the deposition temperatures. At higher deposition temperature, films have shown predominately sp²-bonded carbon and therefore, higher conductivity and lower optical band gap (Tauc gap). These amorphous carbon (a-C) films are also characterized with Raman and X-ray photoelectron spectroscopy. In addition, electrical and optical properties are measured. The thermoelectric measurement shows these as-grown a-C films are p-type in nature.     

Characterization of doped diamond-like carbon films deposited by hot wire plasma sputtering of graphite

Diamond-like carbon (DLC) films doped with nitrogen and oxygen were deposited on silicon(100) and polytetrafluoroethylene (PTFE) substrates by hot wire plasma sputtering of graphite. The morphology and chemical composition of deposited films has been characterized by scanning electron microscopy, XPS, Auger, FTIR spectroscopy and micro-Raman scattering. Plasmon loss structure accompanying the XPS C 1s peak and electron energy loss spectroscopy (EELS) in reflection mode was used to study the fraction of sp³ bonded C atoms and the density of valence electrons. Raman spectra show two basic C–C bands around 1575 cm^-1 (G line) and 1360 cm^-1 (D line) . Auger depth profiling spectroscopy was used to measure the spatial distributions of C, N and O atoms in the surface layer of DLC films. The fraction of sp³ bonded atoms of about 40% was detected in DLC films by XPS plasmon loss and EELS techniques. Nitrile and iso-nitrile groups observed in FTIR spectra demonstrated the existence of sp bonded carbon in doped DLC films. The typical for DLC films specific density 1.7–1.8 g/cm³ was obtained from EELS and XPS data. PACS 52.77.Dq; 81.65.-b; 82.80.Pv     

Stable isotope-enhanced two- and three-dimensional diffusion ordered C NMR spectroscopy (SIE-DOSY C NMR)

The feasibility of obtaining high quality homonuclear or heteronuclear diffusion-ordered ¹³C NMR data is shown to be greatly improved by using ¹³C isotopically-enriched samples. Stable isotope-enhanced diffusion ordered (SIE-DOSY) ¹³C NMR has been applied to ¹³C-enriched carbohydrates, and has been used to determine diffusion coefficients for pentose and hexose monosaccharides, and a disaccharide and trisaccharide. These 2D spectra were obtained with as little as 8 min of acquisition time. Fully resolved 3D DOSY-HMQC NMR spectra of [U-¹³C]xylose, [U-¹³C]glucose, and [1-¹³C^gal]lactose were obtained in 5 h. Sample derivatization with [carbonyl-¹³C]acetate (peracetylation) extends the usefulness of the technique to included non-labeled sugars; the ¹³C-carbonyl – carbohydrate ring proton ¹H–¹³C correlations also provide additional structural information, as shown for the 3-D DOSY-HMQC analysis of a mixture of maltotriose and lactose per-[carbonyl-¹³C]acetates.     

Electrical conduction in nanostructured carbon and carbon-metal films grown by supersonic cluster beam deposition

We have studied the electrical conduction in nanostructured carbon (ns-C) films produced by deposition of a supersonic beam of neutral carbon clusters. The d.c. conduction properties of these films have been measured in situ during the deposition process and ex situ as a function of the temperature in vacuum and in ambient of different gases (H₂, N₂, CH₄, He). The ns-C films exhibit an ohmic behavior with a room temperature resistivity in the range of 10⁷-10⁹ W{\rm\Omega} cm depending on the growth and storage conditions. Conductivity vs. temperature measured in vacuum in the range 290-400 K is characterized by activation energies in the range of 0.3-1.7 eV, the current response does not differ significantly in gas atmosphere. Nanocomposite carbon-metal films have been obtained by adding small amounts of metallorganic precursors containing molybdenum and cobalt during the formation of carbon clusters prior to deposition. The films are characterized by porous carbon networks containing small metal and metal carbide clusters. Electrical transport properties of these films have been studied as a function of temperature, gas pressure and relative humidity. In particular, the electrical conductivity of the sample produced with molybdenum showed to be much sensitive to changes in gas pressure and relative humidity, being characterised by fast and reversible responses.     

Heavy carbon doping of AlAs by elemental-source molecular beam epitaxy

Heavily carbon-doped AlAs films with free-hole concentrations in excess of 10¹⁹ cm^–3 have been grown by conventional molecular beam epitaxy using elemental sources. The hole concentration in AlAs:C saturates at 6×10¹⁹ cm^–3 without any detectable deterioration of the smooth surface morphology and of the structural properties. At very high carbon concentrations the lattice contraction due to the smaller covalent radius of carbon leads to an in-plane lattice constant of the AlAs:C films which is even smaller than that of the GaAs substrate. The high freehole concentration and the tunability of the lattice constant are important for application in p-type GaAs/AlAs Bragg reflectors in surface emitting lasers having a low series resistance and a significantly reduced lattice mismatch.     

Synchronized 13C-15N-NMR Spectroscopy to Follow Kinetics of Glycine Metabolization in the Rat Liver Non-Destructively

A method is proposed for reiterated measurements by turns of ¹³C and ¹⁵N NMR signals at intact rat liver tissue. Eight minutes after Wistar rats had been injected 1,2-¹³C; ¹⁵Nglycine, the liver was excised and stored in the triple resonance probe-head of a 2.1 Tesla NMR spectrometer. Alternatively ¹³C and ¹⁵N spectra were taken over some hours. Continued enzymatic activities of the liver tissue were seen in terms of metabolic changes of each of the carbon and nitrogen groups of glycine. Different half lives were found for the disappearance of the groups. The measurements gave insights into the kinetics and metabolism of an amino acid as an example to follow the vitality of an isolated organ non-destructively.