Quantitative <Superscript>2</Superscript>H NMR spectroscopy

The selectivity of deuterium distribution between the nonequivalent positions in 3-carene (1), 4-α-acetyl-2-carene (2), and 4-(1-hydroxyethyl)-2-carene (3) has been measured by ²H-{¹H} NMR spectroscopy at the natural abundance of deuterium. These “H/D-isotope portraits” were shown to be typical of terpenes and terpenoids produced in plants via the biosynthetic DXP pathway. The mechanism of acylation of 1 was studied by the density functional theory method (PBE functional, TZ2p basis set). The six-membered ring in compound 1 is planar. However, the endo attack of electrophiles on this ring is more favorable both kinetically and thermodynamically. It was shown both experimentally and theoretically that the elimination of a hydrogen atom in the second reaction step proceeds stereoselectively at the C(2) atom from the anti position with respect to the three-membered ring and occurs with pronounced nucleophilic assistance from the carbonyl group. For Part 2, see Ref. 1. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1657–1664, August, 2008.     

<Superscript>1</Superscript>H and <Superscript>7</Superscript>Li NMR in defect cobaltite Li<Subscript>0.6</Subscript>CoO<Subscript>2</Subscript>

Within a temperature range of 120–330 K, ⁷Li NMR spectra in Li_0.6CoO₂ are obtained. It is shown that as the temperature increases, both smooth and stepwise variation of ⁷Li NMR contact shifts occurs. The observed effects are explained by the occupation of the excited levels of cobalt ions. The stepwise change of the resonance line width depending on the temperature is revealed. It is driven by the features of the diffusive motion of lithium ions. The calculation of the ¹H NMR line shape provides the determination of the ratio of one-, two-, and three-spin proton clusters in Li_0.6CoO₂·xH₂O.     

Diffusion mobility of alkali metals in perfluorinated sulfocationic and carboxylic membranes as probed by <Superscript>1</Superscript>H, <Superscript>7</Superscript>Li, <Superscript>23</Superscript>Na,and <Superscript>133</Superscript>Cs NMR spectroscopy

Water self-diffusion and ion mobilities in various ionic forms (H⁺, Li⁺, Na⁺, Rb⁺, Cs⁺, and Ba²⁺) of perfluorinated sulfocationic membranes MF-4SK were studied by NMR and impedance spectroscopy. When degrees of hydration are low, the self-diffusion coefficients of water and ionic conductivities are considerably affected by the water content of the membrane. The self-diffusion coefficients decrease in the order H⁺ > Ba²⁺ > Cs⁺ > Rb⁺ > Na⁺ > Li⁺, whereas the ion mobility decreases in the order H⁺ > Li⁺ > Na⁺ > Cs⁺ > Ba²⁺.     

Structural studies of bacterial cellulose through the solid-phase nitration and acetylation by CP/MAS <Superscript>13</Superscript>C NMR spectroscopy

The solid-phase nitration and acetylation processes of bacterial cellulose have been investigated mainly by CP/MAS ¹³C NMR spectroscopy to clarify the features of these reactions in relation to the characterization of the disordered component included in the microfibrils. CP/MAS ¹³C NMR spectra of bacterial and Valonia cellulose samples are markedly changed as the nitration progresses, in a similar way to the case of cotton linters previously reported; and the relative reactivity of the OH groups in the glucose residues is found to decrease in the order of O(6)H>O(2)H>O(3)H. Moreover, the nitration rate and mode greatly depend on the concentration of nitric acid in the reaction media. At dilute and medium concentrations, the O(6)H groups in the crystalline and disordered components are subjected to nitration at nearly the same rate, indicating that these two components are distributed almost at random in the entire region of each microfibril. The preferential penetration of nitric acid into each microfibril also occurs prior to nitration at the medium concentration, resulting in an increase in the mole fraction of the disordered component. In contrast, all OH groups undergo nitration very rapidly at the higher concentration, although nitration levels off to a certain extent for O(3)H groups. In solid-phase acetylation, no regio-selective reactivity is observed among the three kinds of OH groups, which may be due to the characteristic reaction that proceeds in a very thin layer between the acetylated and nonacetylated regions in each microfibril. The almost random distribution of the disordered component in the entire region of the microfibrils is also confirmed in this solid-phase acetylation. On the basis of these results, the mechanism of the solid-phase reactions and the microfibril structure are discussed.     

Ionic mobility and <Superscript>19</Superscript>F MAS NMR spectra of lithium octafluorozirconate Li<Subscript>4</Subscript>ZrF<Subscript>8</Subscript>

A new 1D cadmium coordination polymer [Cd(PhCOO)₂(bbbm)] _n (1) (bbbm = 1,1′-(1,4-butanediyl)bis-1H-benzimidazole ) is synthesized under hydrothermal conditions and characterized by elemental analysis, IR, TG, and X-ray single-crystal diffraction. Compound 1 crystallizes in the triclinic system, space group P-1 with a = 10.4289(17) Å, b = 12.5198(9) Å, c = 12.6130(9) Å, α = 118.4260(10)°, β = 95.1990(10)°, γ = 94.3820(10)°, V = 1428.8(3) ų, Z = 2. In the structure of 1, each cadmium center is six-coordinated in a strongly distorted octahedron by two N and four O atoms; an infinite one-dimensional linear chain was built by the flexible bbbm ligand that adopts a bis-monodentate bridging mode linking Cd^II atoms.     

Charge distribution and mobility of lithium ions in Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript> from <Superscript>6,7</Superscript>Li NMR data

A comparative analysis of ^6,7Li NMR spectra is performed for the samples of monoclinic lithium titanate obtained at different synthesis temperatures. In the ⁷Li NMR spectra three lines are found, which differ in quadrupole splitting frequencies v _Q and according to ab initio EFG calculations are assigned to three crystallographic sites of lithium: Li1 (v _Q ～ 27 kHz); Li2 (v _Q ～ 59 kHz); Li3 (v _Q ～ 6 kHz). The dynamics of lithium ions is studied in a wide temperature range from 300 K to 900 K. It is found that the narrowing of ⁷Li NMR spectra as a result of thermally activated diffusion of lithium ions in the low-temperature Li₂TiO₃ sample is observed at a higher temperature in comparison with a sample of high-temperature lithium titanate. Based on the analysis of ⁶Li NMR spectra it is assumed that there is mixed occupancy of lithium and titanium sites in the corresponding layers of the crystal structure of low-temperature lithium titanate, which hinders lithium ion transfer over regular crystallographic sites.     

<Superscript>129</Superscript>Xe NMR spectroscopy of adsorbed xenon: Possibilites for exploration of microporous carbon materials

Despite the extensive use of ¹²⁹Xe NMR for characterization of high surface-to-volume porous solids, particularly zeolites, this method has not been widely used to explore the properties of microporous carbon materials. In this study, commercial amorphous carbons of different origin (produced from different precursors) and a series of activated carbons obtained by successive cyclic air oxidation/pyrolysis treatments of a single precursor were examined. Models of ¹²⁹Xe chemical shift as a function of local Xe density, mean pore size, and temperature are discussed. The virial coefficient arising from binary xenon collisions, σ_Xe-Xe, varied linearly with the mean pore size given by N₂ adsorption analysis; σ_Xe-Xe appeared to be a better probe of the mean pore size than the chemical shift extrapolated to zero pressure, σ_S.     

Crystalline cadmium dialkyl phosphorodithioate complexes: Synthesis and structural organization as probed by multinuclear <Superscript>13</Superscript>C, <Superscript>31</Superscript>P,and <Superscript>113</Superscript>Cd CP/MAS NMR and single-crystal X-ray diffraction

The cadmium O,O′-dethyl (I) and O,O′-di-sec-butyl phosphorodithioate (II) complexes have been synthesized and characterized in detail by ¹³C, ³¹P, and ¹¹³Cd CP/MAS NMR. X-ray crystallography shows that complex II has a binuclear molecular structure [Cd₂{S₂P(O-s-C₄H₉)₂}₄]. For ³¹P and ¹¹³Cd NMR signals, the chemical shift anisotropy δ_aniso and the asymmetry parameter η have been calculated. The ³¹P NMR signals are assigned to the terminal and bridging ligands in the complexes.     

Accurate assignments in PMR and <Superscript>13</Superscript>C NMR spectra of anhydrolycoctonine using 2D spectroscopy

Resonances in PMR and ¹³C NMR spectra of anhydrolycoctonine were fully assigned based on a series of 1D and 2D NMR experiments. The conformation of ring A was concluded to be a distorted boat with H-1β from a comprehensive analysis of chemical shifts, SSCC, and the NOE for two possible conformations. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 267–269, May–June, 2008.     

Sonochemical and soft-chemical intercalation of lithium ions into MnO<Subscript>2</Subscript> polymorphs

We explored routes for the synthesis of LiMn₂O₄ spinel from five different polymorphs of MnO₂ as the manganese source. These included and -MnO₂ and three types of MnO₂ (electrochemically produced EMD, and two types of chemically produced CMD). The synthesis included a lithiation step by mild reduction of the MnO₂ with glucose in a LiOH solution, followed by calcination of the lithiated product. This route was shown in a previous study to produce highly pure, nanocrystalline LiMn₂O₄. The effect of the application of ultrasound radiation in the lithiation step on the quality of the products was also explored. It was found that the degree of lithiation, the purity of the Li_xMn₂O₄ spinel phase obtained and its electrochemical behavior as a Li insertion electrode material depend strongly on the nature of the MnO₂ material in terms of crystal structure and morphology. The effect of ultrasound radiation was found to be detrimental. A very good electrochemical performance (capacity, stability) in repeated Li intercalation–deintercalation cycling was obtained with LiMn₂O₄ originating from nanometric CMD. The tools for this study included XRD, TEM, surface area measurements (BET method), atomic absorption and standard electrochemical techniques (voltammetry, chronopotentiometry).     

Adducts of zinc and copper(II) dialkyldithiocarbamate complexes with dialkylamines: Synthesis,EPR, and <Superscript>13</Superscript>C and <Superscript>15</Superscript>N CP/MAS NMR

Crystalline adducts of zinc and copper(II) dithiocarbamate (Dtc) complexes with dialkylamines [M(NHR′₂)(S₂CNR₂)₂] (M = Zn, ⁶³Cu, ⁶⁵Cu; R = CH₃, C₂H₅, or R₂ = (CH₂)₄O; R′ = C₂H₅, C₃H₇) have been preparatively isolated. The structures and spectral properties of the adducts have been studied by EPR and ¹³C and ¹⁵N MAS NMR. Chemisorption of bases on powders of dinuclear dithiocarbamates leads to their dissociation into monomeric adducts. Computer simulation demonstrates that the experimental EPR spectra of isotope-substituted copper(II) adducts have an individual character. The geometry of the copper polyhedra is intermediate between a trigonal bipyramid (TBP) and a tetragonal pyramid (TP). The TBP and TP contributions have been quantified based on EPR data. ¹³C and ¹⁵N MAS NMR data show that the Dtc ligands incorporated into the zinc adduct molecule are structurally nonequivalent. The dependence of the isotropic ¹⁵N chemical shifts of the Dtc groups on the alkyl substituents at the nitrogen atom is interpreted based on the concept of joint manifestation of the (+)inductive effect of the alkyl substituents and the mesomeric effect of the Dtc groups.     

Optical energy storage properties of Sr<Subscript>2</Subscript>MgSi<Subscript>2</Subscript>O<Subscript>7</Subscript>:Eu<Superscript>2+</Superscript>,R<Superscript>3+</Superscript> persistent luminescence materials

The details of the mechanism of persistent luminescence were probed by investigating the trap level structure of Sr₂MgSi₂O₇:Eu²⁺,R³⁺ materials (R: Y, La-Lu, excluding Pm and Eu) with thermoluminescence (TL) measurements and Density Functional Theory (DFT) calculations. The TL results indicated that the shallowest traps for each Sr₂MgSi₂O₇:Eu²⁺,R³⁺ material above room temperature were always ca. 0.7 eV corresponding to a strong TL maximum at ca. 90 °C. This main trap energy was only slightly modified by the different co-dopants, which, in contrast, had a significant effect on the depths of the deeper traps. The combined results of the trap level energies obtained from the experimental data and DFT calculations suggest that the main trap responsible for the persistent luminescence of the Sr₂MgSi₂O₇:Eu²⁺,R³⁺ materials is created by charge compensation lattice defects, identified tentatively as oxygen vacancies, induced by the R³⁺ co-dopants.     

Adducts of zinc and copper(II) dialkyldithiocarbamate complexes with dibutyl-and diisobutylamines: Synthesis,structures, EPR and solid-state natural abundance <Superscript>13</Superscript>C and <Superscript>15</Superscript>N CP/MAS NMR spectra

Crystalline adducts of zinc and copper(II) dithiocarbamate complexes with dibutyl-and diisobutylamines of the general formula [M(NHR′₂)(S₂CNR₂)₂] (M = Zn, ⁶³Cu, and ⁶⁵Cu; R = CH₃ and C₂H₅; R₂ = (CH₂)₄O; R′ = C₄H₉ and i-C₄H₉) were synthesized. Their structures and spectroscopic properties were studied by EPR and solid-state natural abundance ¹³C and ¹⁵N CP/MAS NMR spectroscopy. Experimental EPR data and computer-assisted modeling confirmed the individual character of copper(II) adducts. The geometries of the copper coordination polyhedra were found to be intermediate between a tetragonal pyramid and a trigonal bipyramid (TBP). The contributions from TBP to the geometries of the adducts obtained were calculated from the EPR data. According to the X-ray diffraction data, the adduct of zinc diethyldithiocarbamate with diisobutylamine exists in two isomeric forms. The ¹³C and ¹⁵N CP/MAS NMR signals were assigned to the atomic positions in two crystallographically independent conformer molecules.     

Ion pairing in H<Subscript>2</Subscript>O and D<Subscript>2</Subscript>O solutions of lead nitrate,as determined with <Superscript>207</Superscript>Pb NMR spectroscopy

The temperature and concentration dependences of ²⁰⁷Pb NMR chemical shifts of Pb(NO₃)₂ in D₂O are reported. The results are analyzed in terms of exchange between a solvated lead ion and the Pb(NO₃)⁺ contact-ion pair. Predictions of the chemical shift difference between the aquated ion and contact-ion pair are carried out for the gas-phase entities and for the solvated species with a DFT calculation. Previously reported data on ²⁰⁷Pb NMR chemical shifts of Pb(NO₃)₂ in H₂O are reevaluated. From the analysis, the enthalpy of dissociation of the contact-ion pair is found to be–42.3±1.0 kJ/mol.     

Sol-gel polycondensation of methyltrimethoxysilane in ethanol studied by <Superscript>29</Superscript>Si NMR spectroscopy using a two-step acid/base procedure

Sol-gel polymerization of methyltrimethoxysilane (MTMS) in ethanol using a two-step acid/base catalyzed procedure (B2) is followed by ²⁹Si NMR spectroscopy. Analysis of the structural evolution of the B2 system shows that esterification of monomeric and end silicon species is rate-limited while that of linear and cyclic species is able to reach pseudoequilibrium in the second basic step. Condensation reactivity is reduced with increasing network connectivity, however, to a much less degree under B2 conditions than MTMS polymerization under acidic conditions. Steric effects as well as many other factors are attributed to this trend. The concentration of cyclic and polycyclic species of the B2 system is nearly 3 times lower compared to the acid-catalyzed system. The empirical degree of condensation at the gel point is determined to be 0.88. The effects of cyclization and phase separation on MTMS gelation are discussed for both B2 and acid-catalyzed systems. Based on these results it is believed that MTMS-based gels form for B2 and not acid-catalyzed conditions due to reduced cyclization, rapid hydrolysis and condensation, effective use of functional groups, and effective contribution of branched and polycyclic species as crosslinking points to connect polymeric chains in the B2 system.     

Excellent electrocatalytic performance of a Ni<Superscript>2+</Superscript>-loaded multiwalled carbon nanotube composite in glucose oxidation

A new type of Ni²⁺-loaded MWCNT composite was prepared by mixing carboxylated multiwalled carbon nanotubes (MWCNTs) and Ni²⁺ ions and allowing them to interact electrostatically. The resulting composite was subsequently used as an electrocatalyst for glucose (Glu) oxidation. Compared with electrodes modified through the addition of free Ni²⁺ ions or MWCNTs, the Ni²⁺/MWCNT composite electrode showed greatly improved properties such as hydrophilicity. Investigations of the Ni²⁺/MWCNT composite electrode via inductively coupled plasma atomic emission spectroscopy and nitrogen adsorption–desorption isotherms verified that Ni²⁺ ions had been adsorbed onto the surfaces of the MWCNTs in the composite. As expected, a Ni²⁺/MWCNT composite-based sensor showed extraordinary electrocatalytic performance in Glu oxidation. In the concentration range 0–4.3 mM, a good linear relationship between the Glu added and the current generated was observed, with a correlation coefficient (R ²) of 0.9988. The detection limit and sensitivity were calculated to be 0.081 μM and 2285 μA mM^?1 cm^?2, respectively. Finally, the new method was successfully applied to determine the Glu in a human blood sample. Recoveries of >100%, indicative of high reliability, accuracy, and precision, were obtained.