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Steric effects on carbon‐13 NMR shifts: carbon–hydrogen bond polarization contributions

The shielding observed in the chemical shifts of carbon atoms subjected to steric compression was originally attributed to carbon–hydrogen bond polarization. There is increasing evidence that this is not universally the case and theoretical studies reveal that changes in dihedral angles may be responsible for many steric effects. Hydrogen shifts, bond lengths and charge distributions were used as probes for these effects in methylnorbornanes and similar tetracyclic norbornyl hydrocarbons. Carbon–hydrogen bond polarization can make a significant contribution to shielding and can be distinguished from effects caused by changes in dihedral angles due to steric congestion.

Two known bis-indole alkaloids isolated from Tabernaemontana laeta: complete 1H and 13C chemical shift assignments

An NMR study of two known bis‐indole alkaloids is described. In addition to conventional 1D NMR methods, 2D shift‐correlated NMR experiments [1Hx1H‐COSY, 1Hx13C‐HMQC‐1J(C,H), 1Hx13C‐HMBC‐nJ(C,H) (n = 2 and 3)] and 2D 1Hx1H‐NOESY were used for 1H and 13C chemical shift assignments of these alkaloids. Copyright

Ab initio charge distributions in half-cage compounds

Abstract Charge distributions in half-cage inside and outside isomer alcohols were obtained by STO-3G ab initio calculations. Input optimized geometries were calculated by MM2 and MNDO methods. The MM2 geometry accompanied neutron-diffraction data on relative bond lengths more closely, but this fact had little influence on charge distributions. In C-H bonds under strong non-bonded pressure the electron density maximum of the hydrogen atom is shifted toward the bound carbon resulting in a significant increase in population and relative p character. In such cases, the influence of steric compression may be comparable to that of bond angles in determining hybridization.

NMR analysis of asphaltenes separated from vacuum residues by selected solvents

Abstract The characterization of petroleum asphaltenes from different sources has been the object of much current research. However, even employing standard separation methods to obtain representative samples, certain variations in properties are observed. In the present work, we report the results of combining solvent extraction with isolation procedures. We fractionated three asphaltenes obtained from vacuum residues by the IP‐143 method by ethyl acetate, acetone, methyl isobutyl ketone, acetyl acetone, and carbon tetrachloride as single solvents or in a given sucession. The extracts were isolated, weighed and analyzed by IR and 1H NMR. Our results indicate that the solubility of asphaltene constituents in these solvents is affected by the presence of certain aromatic ring systems and saturated chains or rings. The solubility of certain constituents does not depend exclusively on the polarity of the solvent.

Synthesis and biological evaluation of novel 6-hydroxy-benzo[d][1,3]oxathiol-2-one Schiff bases as potential anticancer agents.

With the aim of discovering new anticancer agents, we have designed and synthesized novel 6-hydroxy-benzo[d][1,3]oxathiol-2-one Schiff bases. The synthesis started with the selective nitration at 5-position of 6-hydroxybenzo[d][1,3]oxathiol-2-one (1) leading to the nitro derivative 2. The nitro group of 2 was reduced to give the amino intermediate 3. Schiff bases 4a–r were obtained from coupling reactions between 3 and various benzaldehydes and heteroaromatic aldehydes. All the new compounds were fully identified and characterized by NMR (1H and 13C) and specifically for 4q by X-ray crystallography. The in vitro cytotoxicity of the compounds was evaluated against cancer cell lines (ACP-03, SKMEL-19 and HCT-116) by using MTT assay. Schiff bases 4b and 4o exhibited promising cytotoxicity against ACP-03 and SKMEL-19, respectively, with IC50 values lower than 5 μM. This class of compounds can be considered as a good starting point for the development of new lead molecules in the fight against cancer.

COMPARISON BETWEEN ASPHALTENES (SUB)FRACTIONS EXTRACTED FROM TWO DIFFERENT ASPHALTIC RESIDUES: CHEMICAL CHARACTERIZATION AND PHASE BEHAVIOR

Asphaltenes are blamed for various problems in the petroleum industry, especially formation of solid deposits and stabilization of water-in-oil emulsions. Many studies have been conducted to characterize chemical structures of asphaltenes and assess their phase behavior in crude oil or in model-systems of asphaltenes extracted from oil or asphaltic residues from refineries. However, due to the diversity and complexity of these structures, there is still much to be investigated. In this study, asphaltene (sub)fractions were extracted from an asphaltic residue (AR02), characterized by NMR, elemental analysis, X-ray fluorescence and MS-TOF, and compared to asphaltene subfractions obtained from another asphaltic residue (AR01) described in a previous article. The (sub)fractions obtained from the two residues were used to prepare model-systems containing 1 wt% of asphaltenes in toluene and their phase behavior was evaluated by measuring asphaltene precipitation onset using optical microscopy. The results obtained indicated minor differences between the asphaltene fractions obtained from the asphaltic residues of distinct origins, with respect to aromaticity, elemental composition (CHN), presence and content of heteroelements and average molar mass. Regarding stability, minor differences in molecule polarity appear to promote major differences in the phase behavior of each of the asphaltene fractions isolated.

Conformational analysis of phloroglucinols from Hypericum brasiliense by using X-ray diffraction and molecular modeling.

In this work we intend to verify the applicability of a computational methodology to predict structural features of organic compounds with biological activity. We selected three phloroglucinols and compared their calculated conformational data with their X-ray crystallographic structure. The results showed that conformations obtained by conformational analysis with the AM1 method followed by geometry optimization by using the DFT B3LYP/6-31 G(d,p) basis set are in very good agreement with X-ray data, indicating that the methodology employed here seems to be a very useful tool in order to predict the conformational preference for this class of compounds.

Efficient removal of mercury from aqueous solutions and industrial effluent.

The objective of this study was to examine the ability of a solid waste produced during beneficiation of ornamental rocks to remove mercury (Hg) from an industrial effluent and aqueous solutions under various conditions. Batch studies have been carried out by observing the effects of pH, concentration of the adsorbate, contact time, and so on. Various sorption isotherm models such as Langmuir, Freundlich, and Tóth have been applied for the adsorbent. Film and intraparticle diffusion were both found to be rate-limiting steps. Adsorption was properly described by the Freundlich model (capacity constant of 0.3090 (mg g−1)(mg L−1)−1/n and adsorption intensity indicator of 2.2939), which indicated a favorable sorption and encouraged subsequent studies for treatment of Hg-containing industrial effluent. Industrial effluent treatment efficiency reached Hg removals greater than 90% by using ornamental rock solid waste (ORSW). Besides, desorption studies indicated that the maximum recovery of mercury was 100 ± 2% for 1 mol L−1 HNO3 and 74 ± 8% for 0.1 mol L−1 HNO3. The ORSW could be reused thrice without significant difference on the Hg removal rate from industrial effluent. These findings place ORSW as a promising efficient and low-cost adsorbent for the removal of Hg from aqueous solutions and industrial effluent.

Cânfora: um bom modelo para ilustrar técnicas de RMN

CAMPHOR: A GOOD MODEL FOR ILLUSTRATING NMR TECHNIQUES. The use of Nuclear Magnetic Resonance spectroscopyto establish the three-dimensional structures of molecules is an important component of modern Chemistry courses. The combinationof techniques that can be used for this purpose is conveniently illustrated by their application to the camphor molecule. This paperpresents applications of several techniques used in NMR spectral interpretation in an increasing order of complexity. The result ofindividual experiments is illustrated in order to familiarize the user with the way connectivity through bonds and through space isestablished from 1D/2D-NMR spectra and molecular stereochemistry is determined from different NMR experiments.Keywords: organic chemistry; NMR spectroscopy; camphor.

Combining Nuclear Magnetic Resonance Spectroscopy and Density Functional Theory Calculations to Characterize Carvedilol Polymorphs

The experiments of carvedilol form II, form III, and hydrate by (13)C and (15)N cross-polarization magic-angle spinning (CP MAS) are reported. The GIPAW (gauge-including projector-augmented wave) method from DFT (density functional theory) calculations was used to simulate (13)C and (15)N chemical shifts. A very good agreement was found for the comparison between the global results of experimental and calculated nuclear magnetic resonance (NMR) chemical shifts for carvedilol polymorphs. This work aims a comprehensive understanding of carvedilol crystalline forms employing solution and solid-state NMR as well as DFT calculations.