1,1-Organoboration of tri-1-alkynyltin compounds: novel triorganotin cations, stannoles, 3-stannolenes and 1-stanna-4-bora-2,5-cyclohexadienes

Tri-1-alkynyltin compounds [R²Sn(CCR¹)₃ (1), R² = Me, R¹ = Me (a), ⁿBu (b), ^tBu (c), Me₃Si (d), 1-(1-cyclohexenyl) (e); R² = Et, R¹ = Me (a(Et)), ⁿBu (b(Et)), ^tBu (c(Et)), SiMe₃ (d(Et)); R² = ⁿBu, R¹ = Me (a(Bu)), ⁿBu (b(Bu))] were prepared, and their reactivity towards trialkylboranes Et₃B (2) and ⁱPr₃B (3) in 1,1-organoboration reactions was studied. The first step in each reaction is an intermolecular 1,1-alkytoboration. Afterwards, intramolecular 1,1-vinyloboration or 1,1-alkyloboration compete with further intermolecular 1,1-alkyloboration. Various triorganotin cations (4-7), stabilized by intramolecular side-on coordination to the CC bond of an alkynylborate moiety, were detected as highly fluxional intermediates prior to rearrangement into heterocyclic systems such as stannoles (9-11), 1-stanna-4-bora-2, 5-cyclohexadienes (8, 12). The reactions between 1a or 1a(Bu) and an excess of Et₃B (2) afford the tris(alkenyl)tin compounds 13 via threefold intermolecular 1, 1-ethyloboration. 13 rearrange to the 3-stannolenes (14a or 14a(Bu)). The intermediates and final products were characterized by multinuclear one- and two-dimensional ¹H, ¹¹B, ¹³C, ²⁹Si and ¹¹⁹Sn NMR.     

Sesquiterpene polyol esters from Celastrus rosthornianus

Two new sesquiterpene polyol esters with β-dihydroagarofuran skeleton were isolated from the root bark of Celastrus rosthornianus. Their structures were elucidated, mainly on the basis of spectral analyses, as 1 β-acetoxy-8β,9-dibenzoyloxy-6-hydroxy-2β(-methylbutanoyloxy)-β-dihydroagarofuran and 1β-acetoxy-9-benzoyloxy-8β-(β-furanocarbonyloxy)-6-hydroxy-2β(-methylbutanoyloxy)-β-dihydroagarofuran. The complete assignments of ¹³C NMR chemical shifts for both compounds on the basis of ¹H-¹³C chemical-shift correlation spectrum were also carried out.     

Variation in δC values for the seagrass Thalassia testudinum and its relations to mangrove carbon

Carbon isotope ratios (¹³C/¹²C) were measured for the leaves of the seagrass Thalassia testudinum Banks ex König and carbonates of shells collected at the seagrass beds from seven sites along the coast of southern Florida, U.S.A. The δ¹³C values of seagrass leaves ranged from −7.3 to −16.3‰ among different study sites, with a significantly lower mean value for seagrass leaves from those sites near mangrove forests (−12.8 ± 1.1‰) than those far from mangrove forests (−8.3 ± 0.9‰; P < 0.05). Furthermore, seagrass leaves from a shallow water area had significantly lower δ¹³C values than those found in a deep water area (P < 0.01). There was no significant variation in δ¹³C values between young and mature leaves (P = 0.59) or between the tip and base of a leaf blade (P = 0.46). Carbonates of shells also showed a significantly lower mean δ¹³C value in the mangrove areas (−2.3 ± 0.6‰) than in the non-mangrove areas (0.6 ± 0.3‰; P <0.025). In addition, the δ¹³C values of seagrass leaves were significantly correlated with those of shell carbonates (δ¹³C seagrass leaf = −9.1 + 1.3δ¹³C shell carbonate (R² = 0.83, P < 0.01)). These results indicated that the input of carbon dioxide from the mineralization of mangrove detritus caused the variation in carbon isotope ratios of seagrass leaves among different sites in this study.     

Incorporation of Petroleum Carbon Into Soil Organic Matter During Biodegradation

A procedure, based on measurement of the stable carbon isotope ¹³C, has been developed for determining the extent to which petroleum carbon is incorporated into soil organic matter (SOM) by humification of biomass produced during biodegradation of the petroleum in soil. We have shown that a crude oil having a δ¹³C of-27.4%, when biodegraded in a soil containing SOM with a δ¹³C of-15.7%, resulted in a change of the δ¹³C of the bound SOM reflecting that of petroleum carbon. Comparison of five soil biodegradation tests using different amounts and types of fertilizer to stimulate biodegradation of the oil in this soil showed that the extent of the δ¹³C change in the bound SOM varied with the extent of oil biodegradation observed. To obtain ¹³C data on the SOM, the residual petroleum was first removed by rigorous extraction with dichloromethane using a Soxhlet apparatus. The extracted soil was then combusted to release bound carbon as CO₂, which was analyzed for ¹³C. Where the SOM has a δ¹³C similar to that of petroleum, ¹⁴C measurements of SOM would give similar results. This type of data, referred to as the petroleum “footprint” in the SOM, could be useful in identifying or confirming intrinsic biodegradation of petroleum in contaminated soil.     

Synthesis and NMR structural analysis of O-succinyl derivative of low-molecular-weight κ-carrageenan

Low-molecular-weight (LMW) κ-carrageenan was achieved through mild hydrochloric acid hydrolysis of κ-carrageenan. The acylation of LMW κ-carrageenan was performed by use of tetrabutylammonium (TBA) salt of the anionic polysaccharide fragments, succinic anhydride, 4-dimethylaminopyridine and tributylamine under homogeneous conditions in N,N-dimethylformamide at 80 °C. Investigation of FT-IR spectrum of the succinylated LMW κ-carrageenan showed that a monoester derivative with succinyl group was formed when LMW κ-carrageenan reacted with succinic anhydride. The ¹H and ¹³C NMR spectroscopy has been used to characterize the fine structure of O-succinyl derivative of the LMW κ-carrageenan. The ¹³C and ¹H NMR chemical shifts of disaccharide unit of O-succinyl LMW κ-carrageenan have been fully assigned using 2D NMR spectroscopic techniques.     

Changes in C NMR chemical shifts of DNA as a tool for monitoring drug interactions

The antibiotic drug, netropsin, was complexed with the DNA oligonucleotide duplex [d(GGTATACC)]₂ to explore the effects of ligand binding on the ¹³C NMR chemical shifts of the DNA base and sugar carbons. The binding mode of netrospin to TA-rich tracts of DNA has been well documented and served as an attractive model system. For the base carbons, four large changes in resonance chemical shifts were observed upon complex formation: −0.64 ppm for carbon 4 of either Ado4 or Ado6, 1.36 ppm for carbon 2 of Thd5, 1.33 ppm for carbon 5 of Thd5 and 0.94 for carbon 6 of Thd5. AdoC4 is covalently bonded to a heteroatom that is hydrogen bonded to netropsin; this relatively large deshielding is consistent with the known hydrogen bond formed at AdoN3. The three large shielding increases are consistent with hydrogen bonds to water in the minor groove being disrupted upon netropsin binding. For the DNA sugar resonances, large changes in chemical shifts were observed upon netropsin complexation. The 2′, 3′ and 5′ ¹³C resonances of Thd3 and Thd5 were shielded whereas those of Ado4 and Ado6 were deshielded; the ¹³C resonances of 1′ and 4′ could not be assigned. These changes are consistent with alteration of the dynamic pseudorotational states occupied by the DNA sugars. A significant alteration in the pseudorotational states of Ado4 or Ado6 must occur as suggested by the large change in chemical shift of −1.65 ppm of the C3′ carbon. In conclusion, ¹³C NMR may serve as a practical tool for analyzing structural changes in DNA-ligand complexes.     

Sources and accumulation rates of organic carbon in an equatorial peat bog (Burundi, East Africa) during the Holocene: carbon isotope constraints

The relationship between organic carbon accumulation rates and ¹³C/¹²C ratios of total organic carbon (TOC) was investigated in an highland peat bog core (Ru-3) from Equatorial Africa. This core yielded a sequence spanning the last 14 kyr and was analysed with a 100–300 yr resolution for TOC-δ¹³C values. The Holocene section shows contrasted TOC accumulation regimes and TOC δ¹³C varying between −28.5 and −19.5‰ with a few very short ‘isotopic excursions' (dated at ca. 9.3, 7.5, 4.2 ka B.P.). The organic carbon accumulation rates range from 2 to 20 mg C cm⁻² yr⁻¹. They increase when TOC becomes more depleted in ¹³C, notably between 12 and 9.8 ka B.P., 8.5 and 7.8 ka B.P. and after 1.6 ka B.P. Periods of restricted carbon storage correspond to heavier TOC accumulation at 9.3, and between 7.5 and 1.6 ka B.P. At the study site, the δ-variations can be related to variable C4-plant inputs, and possibly, to changes in the fractionation between CO₂ and the organic carbon in C3 vascular plants. The Ru-3 record indicates restricted carbon storage during the periods of increased contribution from C4 plants and/or of decreased fractionation between CO₂ and organic carbon in C3 plants. Changes in TOC-δ¹³C values in core Ru-3 seem to match fluctuations of East Equatorial African lakes. High lake stands correspond to low δ¹³C intervals and vice versa. This points to indirect climatic forcing of δ¹³C changes in intertropical peats.     

Nuclear magnetic resonance spectroscopy of 3 beta,7 beta-dihydroxy-5-cholen-24-oic acid multi-conjugates: unusual bile acid metabolites in human urine

Complete ¹H and ¹³C resonance assignments were made for a new type of 3β,7β-dihydroxy-5-cholen-24-oic acid doubly conjugated with sulfuric acid at C-3 and N-acetylglucosamine at C-7 and its glycine- and taurine-amidated triple-conjugates by the combined use of several homonuclear and heteronuclear shift-correlated 2D NMR techniques. The effects of sulfation at C-3, N-acetylglucosaminidation at C-7, and aminoacyl amidation at C-24 on the ¹H and ¹³C chemical shifts and signal multiplicity were clarified. The shielding data serving to characterize each of the bile acid multi-conjugates are also discussed.     

Pleistocene paleoenvironmental evolution at continental middle latitude inferred from carbon and oxygen stable isotope analysis of ostracodes from the Guadix-Baza Basin (Granada, SE Spain)

A representative paleoenvironmental reconstruction of continental middle latitude from ca. 2 my to the upper part of Middle Pleistocene (279 ± 77 ky) was obtained from the carbon and oxygen stable isotopes analyzed in ostracode shells (Cyprideis torosa) recovered in the Guadix-Baza Basin (SE Spain), an intramontaneous closed depression filled by alluvial and lacustrine sediments. This study was performed along a 356-m-thick composite section, dated previously by paleomagnetism and the amino acid racemization method. δ¹³C and δ¹⁸O profiles reflected changes in temperature, the evaporation/infill ratio in the water bodies and the amount of rain. δ¹³C is also affected by changes in plant biomass: periods with high δ¹³C and δ¹⁸O values are associated with warm and dry regimes, and with less vegetation, which, in some cases, coincide with the development of displacive gypsum crystals, whereas low δ¹³C and δ¹⁸O values correlate with cold and humid episodes, which cause more vegetation biomass and, therefore, increasing the input of isotopically light carbon. Intermediate δ¹⁸O values are linked to temperate dry or humid episodes when they coincide with high or low δ¹³C values, respectively. 86 paleoclimatic events were distinguished in the Pleistocene record from the δ¹³C and δ¹⁸O profiles. From both the statistical analysis of the geochemical data and the geological observations, four Cold and Humid Long Periods (low δ¹⁸O) and four Warm and Dry Long Periods (high δ¹⁸O) were defined. This differs with respect to the paleoclimatological behavior established for the Northern Hemisphere where during cold periods (glacial), no water was available while permafrost conditions persisted, whereas in warm episodes (interglacial), higher precipitation rates occurred. Good correspondences between the Guadix-Baza Basin paleoclimatic record and a marine oxygen-isotope sequence, two continental cores and other long Mediterranean paleoenvironmental records (pollen sequences from Israel) were found, which suggested that climate changes in the Guadix-Baza Basin were in tune with global climatic changes.     

Monitoring Anaerobic Natural Attenuation of Petroleum Using a Novel In Situ Respiration Method in Low-Permeability Sediment

Assessing petroleum biodegradation rates is an important part of predicting natural attenuation in subsurface sediments. Monitoring carbon dioxide (CO₂) and methane (CH₄) produced in situ, and their radiocarbon ¹⁴C), stable carbon (¹³C) and deuterium (D). signature provide a novel method to assess anaerobic microbial processes. Our objectives were to: (1) estimate the rate of anaerobic petroleum hydrocarbon (PH) mineralization by monitoring the production of soil gas CH₄ and CO₂ in the vadose zone of low-permeability sediment, (2) evaluate the dominant microbial processes using δ¹³C and δD, and (3) determine the proportion of CH₄ and CO₂ attributable to anaerobic mineralization of PH using ¹⁴C analysis. Argon was sparged into the subsurface to dilute existing CO₂ and CH₄ concentrations. Vadose zone CO₂, CH₄, oxygen, total combustible hydrocarbons, and argon concentrations were measured for 75 days. CO₂ and CH₄ samples were collected on day 86 and analyzed for ¹⁴C, δ¹³C, and δD. Based on CH₄ soil gas production, the anaerobic biodegradation rate was estimated between 0.017 to 0.055 mg/kg soil-d. CH₄ ¹⁴C (2.6 pMC), δ¹³C (-45.64‰), and δD (-316‰) values indicated that fermentation of PH was the sale source of CH₄ in the vadose zone. CO₂ ¹⁴C (62 pMC) indicated that approximately 47% of the total CO₂ was from PH mineralization and 53% from plant root respiration. Although low-permeability sediment increases the difficulty of completely replacing in situ soil gas and assuring anaerobic conditions, this novel respiration method distinguished between anaerobic processes responsible for PH degradation.     

Reaction of sulfur diimides with organoboranes — studied by multinuclear magnetic resonance in solution

Reactions between sulfur diimides R(NSN)R′ (R=R′=^tBu (1a), SiMe₃ (1b), SnMe₃ (1c); R=^tBu, R′=SnMe₃ (1d); R=SiMez₃, R′=SnMe₃ (1e)) and various organoboranes were studied, and the products were characterized by multinuclear magnetic resonance data (¹H, ¹¹B, ¹³C, ¹⁵N, ²⁹Si and ¹¹⁹Sn NMR). Tetraalkyldiboranes(6) (Et₂BH₂BEt₂ (2), dimeric 9-borobicyclo[3,3.1]nonane (3)) react with 1a and 1b by 1,3-hydroboration to give the N-sulfanyl-dialkylaminoboranes 4 and 5 which are instable with respect to eliminatio of short-lived [R---NS]. Trialkylboranes (Et₃B (8)) react only sluggishly with 1a, but more readily with 1b mainly via S-ethylation, formally a 1,2-ethyloboration, to give the diethylborylamido-imino-ethanesulfinic acid 9b decomposes slowly at room temperature via ethene elimination to give 4b, followed by further decomposition via [R---NS] elimination. The compounds 9 can be prepared independently from the reaction between the N-lithio-imino-ethanesulfinic acid amide 10 and diorganoboron halides. The molecular structure of the lithium amide 10a (R=R′=tBu) was determined by X-ray analysis as a dimer in which the four nitrogen, two sulfur and two lithium atoms adopt a boat conformation, in contrast with other known derivatives of this type. If the sulfur diimides bear at least one trimethylstannyl group (1c-e), their reactions with Et₃B (8), iPr₃B (12) or 9-iso-butyl-9-borabicyclo[3,3,1]nonane (13) lead to the novel aminoboranes 14–16. These are products of a 1,1,-organoboration, since the Me₃Sn group moves from one nitrogen atom to the other, and both the boryl and an alkyl group end up at the same nitrogen atom.     

Synthesis and solution behavior of the trinuclear palladium(II) unsaturated carboxylate complexes triangle-Pd3[μ-O2CC(R′) = CHMe]6 (R′ = Me, H): X-ray structure of palladium(II) tiglate (R′ = Me)

The first examples of binary palladium(II) derivatives of unsaturated carboxylic acids are reported. It was found that the interaction of Pd₃(μ-OAc)₆ with the ,β-unsaturated 1-methylcrotonic (tiglic) and crotonic acids leads to the corresponding carboxylates of composition Pd₃[μ-O₂CC(R′) = CHMe]₆, where R′ = Me (1) or H (2). The new compounds have been characterized by elemental analysis, solid and solution IR, ¹H and ¹³C NMR, and ESI mass spectrometry. The crystal structure of 1 has been determined. This molecule displays a central Pd₃ cyclic core with Pd–Pd distances of 3.093–3.171 Å. Each Pd–Pd bond is bridged by a pair of carboxylate ligands, one above and the other below the Pd₃ plane, providing a square planar coordination for each Pd atom in an approximate D_3h overall symmetry arrangement. Solution spectroscopic data show that the bridging η¹:η¹:μ₂ interaction of the carboxylates of 1 and 2 is readily displaced, with a change of the ligand to the terminal (η¹) coordination mode.     

1H and 13C NMR characteristics of synthetic derivatives of steroid sapogenins. Part III. 16Beta,23:23,26-diepoxy side chains

The full assignments of the ¹H and ¹³C NMR signals of steroids bearing the 16β,23:23,26-diepoxy side chain are provided. Differentiation of the diasterotopic H-26 pair was achieved with the aid of NOESY experiments. The main substituent and steric effects associated with this moiety and their influence on the chemical shifts of the neighboring atoms are discussed.     

Late Quaternary paleoproductivity changes off the Congo deduced from stable carbon isotopes of planktonic foraminifera

A detailed record (≈2-kyr intervals) of the difference in stable carbon isotopes (Δδ¹³C) between Globigerina bulloides and Globigerinoides ruber (pink) is used to reconstruct changes in upwelling intensity off the Congo River for the last 190,000 yr. Comparisons of the oxygen and carbon isotope data from this core with records from the Niger Fan and from pelagic cores in the eastern equatorial South Atlantic indicate that the Congo Fan isotope records do not contain a strong freshwater signal as is described off other major rivers. The temporal pattern of the Δδ¹³C signal correlates with the marine organic carbon record from the Congo Fan. Thus the planktonic Δδ¹³C record, reflecting past changes in upwelling intensity and nutrient content, corroborates the signal provided by sedimentary organic carbon, which is presumed to indicate changes in the amount of biological productivity and export flux to the seafloor.

The planktonic Δδ¹³C signal is characterized by a dominant 23-kyr periodicity which provides evidence for a strong response of upwelling fluctuations off the Congo to precessional forcing. Minima in the Δδ¹³C record are aligned with periods of minimum boreal summer insolation over Central Africa reflecting an increase of upwelling and biological productivity off the Congo at periods of enhanced zonal intensity of southeast trades and corresponding weak southeast monsoon over the eastern South Atlantic. A strong response to changes in river discharge probably did not occur, indicating that fertilization by river-derived nutrients has played only a minor role with respect to Late Quaternary changes in the total amount of primary productivity off the Congo.     

Synthesis, NMR study, and X-ray structure of 2-(N-methyl-N′-cyano-N″-ethylguanidino) thiotriophenylphosphinegold(I)

The reaction between cimetidine in a methanolic solution of KOH and a dichloromethane solution of PPh₃AuCl affords a new compound with formula [L-Au-PPh₃] (I) (L = 2-(N-methyl-N′-cyano-N″-ethylguanidino)thiolate), the thiolato ligand resulting from cleavage of one of the thioether bonds of cimetidine. (I) has been characterized by elemental analysis, infrared, and ¹H and ¹³C NMR spectroscopy. Single crystal x-ray structure determination shows that the gold atom is linearly coordinated by a phosphine ligand (Au-P 2.258(1) Å) and by an S atom (Au-S 2.282(1) Å) of the thiolato ligand. Crystal data: triclinic, space group P with a = 8.848(1), b = 11.343(3), c = 12.107(3)Å, = 87.63(1), β = 85.24(1), γ = 79.89(1)°, R = 0.024 for 3673 reflections with I > 3 δ (I).     

NMR studies of [U-C]cyclosporin A bound to human cyclophilin B

NMR data (¹H and ¹³C chemical shifts, NOEs) on [[U-¹³C]cyclosporin A bound to cyclophilin B were compared to previously published data on the [U-¹³C]CsA/CyPA complex [Fesik et al., (1991) Biochemistry 30, 6574–6583]. Despite only 64% sequence identity between CyPA and CyPB, the conformation and active site environment of CsA when bound to CyPA and CyPB are nearly identical as judged by the similarity of the NMR data.     

Methylenedioxy- and methoxyflavones from Melicope coodeana syn. Euodia simplex

Three new natural products, 3,8-dimethoxy-5,7-dihydroxy-3′,4′-methylenedioxyflavone, 3,6,8-trimethoxy-5,7-dihydroxy-3′,4′-methylenedioxyflavone and 3,6,8,3′,4′-pentamethoxy-5,7-dihydroxyflavone were isolated from Melicope coodeana syn. Euodia simplex (Rutaceae) along with 3,6,3′-trimethoxy-5,7,4′-trihydroxyflavone and 3,3′-dimethoxy-5,7,4′-trihydroxyflavone. The structural assignments are based on ¹H and ¹³C NMR data, including discussion of the chemical shifts of C-2 in 3,5-dihydroxy- and 3-methoxy-5-hydroxyflavones. The presence of highly methoxylated and methylenedioxyflavones is characteristic of the genus Melicope, and the present findings support the recent transfer of Euodia simplex to Melicope.     

鼎湖山森林演替序列植物-土壤碳氮同位素特征

植物群落对水分利用和养分利用的优化策略, 土壤碳周转和氮循环过程对演替变化如何响应, 森林土壤有机碳积累机制等都是森林生态学需要解决的关键问题。然而, 这些生态学过程的变化在短时间内通过传统的研究手段难以被精确观测, 碳氮同位素(¹³C、¹⁵N)技术的应用或许能提供更多有价值的信息。该文通过对鼎湖山森林演替序列代表性群落——马尾松(Pinus massoniana)针叶林(PF)、针阔叶混交林(MF)和季风常绿阔叶林(BF)植物-土壤碳氮同位素自然丰度的测定, 分析了叶片稳定碳同位素比率(δ¹³C)和稳定氮同位素比率(δ¹⁵N)与其叶片元素含量的关系, 以及叶片-凋落物-土壤δ¹³C、δ¹⁵N在演替水平和垂直方向上的变化特征。结果显示: 1)主要优势树种叶片δ¹³C与其C:N极显著正相关(p < 0.01), 凋落物和各层土壤δ¹³C均表现为PF > MF > BF, 沿演替方向逐渐降低; 2)叶片δ¹⁵N与叶片N含量正相关(p = 0.05), 凋落物和表层土壤(0-10 cm) δ¹⁵N沿演替方向逐渐增大; 3)不同演替阶段土壤δ¹³C、δ¹⁵N均沿垂直剖面呈现增大的趋势。结果表明: 南亚热带地区植物群落的发展并不一定受水分利用和氮素利用的补偿制约; δ¹³C自然丰度法的应用有助于森林土壤有机碳积累机制, 尤其有助于成熟森林土壤“碳汇”机制的阐释; 植物-土壤δ¹⁵N值可作为评估土壤氮素有效性和生态系统“氮饱和”状态的潜在指标。     

13C and 15N isotopic signatures of plant-soil continuum along a successional gradient in Dinghushan Biosphere Reserve

Aims The optimal patterns of plant community for water use and nutrient utilization, the responses of soil carbon and nitrogen turnover processes to forest succession, and the mechanisms of soil organic carbon accumulation, are three critical issues in forest ecosystem study. It is difficult to accurately detect these ecological processes with conventional methodologies in the short term, yet the application of ¹³C and ¹⁵N natural abundance technique may yield important information about these processes.Methods This study was conducted in Dinghushan Biosphere Reserve. We investigated the natural isotopic abundance of both ¹³C and ¹⁵N of plant-soil continuum along a successional gradient from Pinus massoniana forest (PF) to coniferous and broad-leaved mixed forest (MF), and monsoon evergreen broad-leaved forest (BF). We also analyzed the correlations of foliar stable carbon isotope ratio (δ¹³C) and stable nitrogen isotope ratio (δ¹⁵N) with foliar elemental contents and the variations of soil δ¹³C and δ¹⁵N along soil profiles at different successional stages.Important findings A significant positive correlation between foliar δ¹³C and foliar C:N was observed. In both litter and soil, the δ¹³C values tended to decrease along the forest succession, with the order as PF > MF > BF. Foliar δ¹⁵N was positively correlated with foliar N content. The δ¹⁵N values of litter and upper soil (0-10 cm) increased with successional status. Both soil δ¹³C and δ¹⁵N values increased with increasing soil depth at all three forests. Our results imply that 1) trade-off between water use efficiency and nitrogen use efficiency did not necessarily exist in subtropical forests of China; 2) the application of isotopic technique could assist understanding of the mechanisms of soil carbon accumulation in subtropical forests, especially in old-grow forests; 3) the ¹⁵N natural abundance of plant-soil continuum could be a potential indicator of soil nitrogen availability and ecosystem nitrogen saturation status.     

Studies of mercurated derivatives of acetylacetone and ethyl acetoacetate by solid state nuclear magnetic resonance and vibrational spectroscopies

Seven new mono- and/or dimercurated compounds involving acetylacetone (2,4-pentanedione) or ethyl acetoacetate (3-ethyl ketobutanoate) were synthesized in aqueous medium. In all cases, mercuration occurred at methylene carbon atoms. All compounds were carefully analyzed by solid state carbon-13 nuclear magnetic resonance. Assignments were confirmed by using selective sequences, which allowed a total editing of the spectra. It was shown that deshielding of ¹³C mercurated sites occurred when the rate of mercuration increased. It was also possible to measure direct ¹J(¹⁹⁹Hg¹³C) coupling constants. The main bands of the vibrational spectra (infrared and Raman) were assigned. It was proved that v(C = O) and δ(C(γ)-H) could be directly related to the mercuration rate of molecules.