Scielo RSS <![CDATA[Química Nova]]> vol. 38 num. 1 lang. pt <![CDATA[SciELO Logo]]> <![CDATA[A INFLUÊNCIA DA LIGAÇÃO DE HIDROGÊNIO EM REAÇÕES QUÍMICAS: REAÇÃO DE PRILESCHAJEW]]> Theoretical analysis at the BHandHLYP/6-311++G(d,p) level of theory with the support of QTAIM calculations was used to reinvestigate the structure of an intermolecular system formed between ethylene oxide (C2H4O) and formic acid (HCO2H) after the epoxidation known as the Prileschajew reaction. Geometric and infrared vibration results revealed that HO2CH forms a strong hydrogen bond with C2H4O followed by a larger red-shift of the H−O bond. NBO analysis was applied to justify this frequency shift. Finally, QTAIM calculations identified the formation of two hydrogen bonds, namely O···H−O and H···O=C. <![CDATA[THE APPLICATION OF AUTOMATED CORRELATION OPTIMIZED WARPING TO THE QUALITY EVALUATION OF <em>Radix Puerariae thomsonii</em>: CORRECTING RETENTION TIME SHIFT IN THE CHROMATOGRAPHIC FINGERPRINTS]]> The application of automated correlation optimized warping (ACOW) to the correction of retention time shift in the chromatographic fingerprints of Radix Puerariae thomsonii (RPT) was investigated. Twenty-seven samples were extracted from 9 batches of RPT products. The fingerprints of the 27 samples were established by the HPLC method. Because there is a retention time shift in the established fingerprints, the quality of these samples cannot be correctly evaluated by using similarity estimation and principal component analysis (PCA). Thus, the ACOW method was used to align these fingerprints. In the ACOW procedure, the warping parameters, which have a significant influence on the alignment result, were optimized by an automated algorithm. After correcting the retention time shift, the quality of these RPT samples was correctly evaluated by similarity estimation and PCA. It is demonstrated that ACOW is a practical method for aligning the chromatographic fingerprints of RPT. The combination of ACOW, similarity estimation, and PCA is shown to be a promising method for evaluating the quality of Traditional Chinese Medicine. <![CDATA[BIOFILMS BASED ON CANIHUA FLOUR (<em>Chenopodium Pallidicaule</em>): DESIGN AND CHARACTERIZATION]]> This work aims to (1) produce and characterize the flour obtained from two varieties of canihua, cupi and illpa-inia, and (2) evaluate the ability of these flours to form biofilms. The flours produced contain proteins, starches, lipids, organic substances containing phenol groups, and high percentages of unsaturated fatty acids. Films produced from the illpa variety presented lower water vapor permeability and larger Young’s modulus values than the films formed from the cupi variety. Both films were yellowish and displayed a high light blocking ability (as compared with polyethylene films), which can be attributed to the presence of phenolic compounds. Furthermore, they showed lesser solubility and water permeability than other polysaccharide films, which may be the result of the higher protein (12%–13.8%) and lipid (11%) contents in canihua flours, as well as the formation of a larger number of S–S bonds. On the other hand, these films presented a single vitreous transition temperature at low temperatures (&lt; 0 °C), crystallization of the A and Vh types, and an additional diffraction peak at 2 = 7.5º, ascribed to the presence of essential fatty acids in canihua flour. Canihua flour can form films with adequate properties and shows promise for potential applications in food packaging, because it acts as a good barrier to incident ultraviolet light. <![CDATA[EFEITO DA ESTRUTURA MOLECULAR DE LIGANTES DE SUPERFÍCIE EM PONTOS QUÂNTICOS DE CdTe DISPERSOS EM ÁGUA]]> Water-soluble CdTe quantum dots are synthesized to investigate how short-chain surface ligands bearing -SH, -COOH, and -NH2 groups interact with CdTe during nucleation/growth processes. Their optical properties and colloidal stability after the ligand exchange are also investigated. We then characterize the resulting CdTe by fluorescence, UV–Vis absorption, and infrared spectroscopies. The stability of the colloidal dispersions was determined by their Zeta potentials. The results show that in the synthesis of water-soluble CdTe, surface ligands with at least two functional groups are required and the hard/soft character of them is an important factor in the stability of CdTe. <![CDATA[DISTRIBUIÇÃO E FRACIONAMENTO DO Hg EM SEDIMENTOS DO RIO PARAÍBA DO SUL – RJ, BRASIL]]> Mercury distribution and fractionation were determined in sediments from the Paraíba do Sul River – RJ, Brazil. Total mercury concentration ranged from 1 to 158 ng g-1. Hg associated with the weakly bound fraction was dominant in the estuarine areas (main - 60% and secondary - 55%); followed by fluvial end member (48%) and mangrove (18%). These results reinforce the mercury availability to fluvial and estuarine areas and emphasize the key role played by mangroves as an efficient biogeochemical barrier. In conclusion, the continuous reduction of the mangrove ecosystem around the world can exacerbate the damage resulting from the mercury accumulation. <![CDATA[DI-HIDROCUCURBITACINA B: SEMI-SÍNTESE DE NOVOS DERIVADOS GLICOSILADOS]]> In the last ten years, the interest in natural and semi-synthetic cucurbitacin derivatives has increased, primarily due their cytotoxic and anti-tumoral activities. However, the isolation of glycosylated cucurbitacins has been difficult due the presence of β-glucosidase enzyme. With the aim of obtaining new glycosylated derivatives, the glycosylation of dihydrocucurbitacin B under Köenigs-Knorr and imidate reaction conditions was studied. Novel glycoside derivatives 16-(1,2-orthoacetate-3,4,6-tri-O-acetyl-α-D-glucopyranosyl)-dihydrocucurbitacin B (2), 2-O-β-D-2,3,4,6-tetra-O-acetyl-galactopyranosyl dihydrocucurbitacin B (3) and 2-O-β-D-galactopyranosyl dihydrocucurbitacin B (4) were synthesized for the first time in 17% (2 and 3) and 48% (4) yields. <![CDATA[CHEMICAL STUDY OF <em>Hortia superba</em> (Rutaceae) AND INVESTIGATION OF THE ANTIMYCOBACTERIAL ACTIVITY OF CRUDE EXTRACTS AND CONSTITUENTS ISOLATED FROM <em>Hortia</em> SPECIES]]> In this paper, the chemical study of Hortia superba and antimycobacterial potential of Hortia species were investigated. Crude extracts and limonoids, alkaloids, dihydrocinnamic acid derivatives and coumarins isolated from Hortia superba, Hortia oreadica and Hortia brasiliana were evaluated against Mycobacterium tuberculosis H37Rv, Mycobacterium kansasii and Mycobacterium avium. The results obtained demonstrated an inhibitory effect of the dichloromethane extract of leaves of H. oreadica (MIC 31.25 µg mL-1), indolequinazoline (15.62 µg mL-1) and furoquinoline (31.25 µg mL-1) alkaloids, and dihydrocinnamic acid derivatives (62.50 µg mL-1), on the growth of M. tuberculosis. These results are promising in relation to the search for biologically active natural products and could be useful in the development of effective new drugs against mycobacteria. <![CDATA[ESTERIFICAÇÃO E TRANSESTERIFICAÇÃO SIMULTÂNEAS DE ÓLEOS ÁCIDOS UTILIZANDO CARBOXILATOS LAMELARES DE ZINCO COMO CATALISADORES BIFUNCIONAIS]]> Palm oil containing 40% fatty acids was converted to methyl esters using zinc carboxylates as the esterification/transesterification catalyst. The reaction was optimized using a factorial design in which the effects of the alcohol:fatty acids molar ratio (MRAG) and the catalyst concentration (CAT) were assessed. The best conversion was achieved with CAT at 4 wt% and MRAG at 4:1. However, the solid catalyst presented significant structural changes after use. For instance, laurate anions were replaced by carboxylates of higher molecular mass, leading to the formation of a new catalytically active layered structure. Also, the glycerin obtained as a co-product contained 86 wt% glycerol. <![CDATA[INFLUÊNCIA DO TRATAMENTO TÉRMICO NO NANOCOMPÓSITO FOTOCATALISADOR ZnO/TiO<sub>2</sub>]]> ZnO/TiO2 nanocomposites were prepared by impregnating zinc acetate dihydrate on the surface of titanium dioxide P25, followed by thermal treatment at 350, 600, 750, and 900 °C, in order to investigate the TiO2 phase and titanate formation and the role of the latter in the photocatalytic activity of the nanocomposite. In the nanocomposites, the anatase-to-rutile transition is favored due to the presence of Zn2+, and the conversion is nearly complete at 750 °C. The presence of zinc metatitanate in the sample heated at 600 °C had no significant effect on the nanocomposite photocatalytic activity. <![CDATA[NOVOS FLAVONOIDES DE <em>Margaritopsis carrascoana</em> COM ATIVIDADE ANTIOXIDANTE]]> EtOH extracts of the stems and leaves of Margaritopsis carrascoana were found to contain new flavonoids luteolin 7-O-{β-D-apiofuranosil-(1→6)-[β-Lrhamnopyranosyl-( 1→2)]-β-D-glucopyranosyl} (5) and luteolin 7-O-{α-L-rhamnopyranosyl-(1→6)-[β-L-rhamnopyranosyl- (1→2)]-β-D-glucopyranosyl} (6), in addition to the known dihydrodehydrodiconiferyl alcohol 4-O-b-D-glucopyranoside (1), luteolin 7-O-b-L-rhamnopyranosyl-(1→2)-β-D-glucopyranosyl (2), luteolin 7-O-[b-D-apiofuranosyl-(1→6)]-β-D-glucopyranoside (3), and chrysoeriol 7-O-[b-D-apiofuranosyl-(1→6)]-β-D-glucopyranoside (4). All isolated compounds presented higher antioxidant activities than the controls, BHT and quercetin, while the extract of the stems showed strong AChE inhibition. <![CDATA[VOLATILE COMPONENTS FROM GALLS INDUCED BY <em>Baccharopelma dracunculifoliae</em> (Hemiptera: Psyllidae) ON LEAVES OF <em>Baccharis dracunculifolia</em> (Asteraceae)]]> The volatile components of the galls induced by the insect Baccharopelma dracunculifoliae (Hemiptera: Psyllidae) on leaves of Baccharis dracunculifolia (Asteraceae) were analyzed by gas chromatography-mass spectrometry (GC-MS) and gas chromatographyflame- ionisation detection (GC-FID), and then comparison with volatile oil samples from healthy leaves collected in the vicinity. The galls produced around 3.5% of the total organic volatiles whereas healthy leaves rendered an average yield of 0.6%. The observed higher proportions of germacrene D, bicyclogermacrene, limonene, and β-pinene in the galls suggest that all these compounds are important targets in the search for natural enemies of this Psyllid. Moreover, higher relative percentages of (E)-nerolidol and spathulenol were found in healthy leaves. <![CDATA[GLOBAL AND LOCAL REACTIVITY DESCRIPTORS FOR PICLORAM HERBICIDE: A THEORETICAL QUANTUM STUDY]]> In this work, we studied the reactivity of picloram in the aqueous phase at the B3LYP/6-311++G(2d,2p) and MP2/6-311++G(2d,2p) levels of theory through global and local reactivity descriptors. The results obtained at the MP2 level indicate that the cationic form of picloram exhibits the highest hardness while the anionic form is the most nucleophilic. From the Fukui function values, the most reactive site for electrophilic and free radical attacks are on the nitrogen in the pyridine ring. The more reactive sites for nucleophilic attacks are located on the nitrogen atom of the amide group and on the carbon atoms located at positions 2 and 3 in the pyridine ring. <![CDATA[AVALIAÇÃO DE MÉTODOS DE SOLUBILIZAÇÃO PARA DETERMINAÇÃO DE METAIS EM GLICERINA PROVENIENTE DA PRODUÇÃO DE BIODIESEL POR TÉCNICAS DE ESPECTROMETRIA ATÔMICA]]> This paper describes the evaluation of simple and fast solubilization methods for the determination of Ca, Mg, and K in glycerin samples from biodiesel production by atomic spectrometry. The solubilization in water was compared with two other methods: solubilization in formic acid and solubilization in ethanol. Using solubilization in water, determination of the three analytes was possible; the values of limits of detection for Ca, K, Mg were 0.31, 0.06, and 0.16 mg kg−1, respectively. Because no adequate reference material was available, the accuracy was evaluated by assessing the recoveries tests with both solubilization methods; the evaluation ranged from 90% to 115%, with values of relative standard deviation &gt;8%, indicating good accuracy of the measure. Four crude glycerin samples obtained from biodiesel plants of Rio Grande do Sul were analyzed after treatment with the different methods of solubilization, and the obtained results of Ca, Mg, and K concentration were in agreement with the values obtained from both solubilization methods. Therefore, solubilization in water is concluded to be a simpler, faster, and viable method for sample preparation of glycerin. <![CDATA[INVESTIGAÇÃO DO USO DE ELETRODO DE CARBONO VÍTREO MODIFICADO COM HEXACIANOFERRATO DE RUTÊNIO PARA DETECÇÃO DE PROCAÍNA]]> A glassy carbon electrode modified with ruthenium hexacyanoferrate (RuOHCF) was investigated as an electrocatalyst for the detection of procaine with the aim of quantification in pharmaceutical and forensic samples. The RuOHCF films were prepared by electrochemical deposition, and the parameters used in this process (concentration of RuCl3, K3Fe(CN)6, temperature, and number of cyclic voltammograms recorded in the modification step) were carefully optimized. Based on the optimal conditions achieved, the RuOHCF modified electrode allows the determination of procaine at 0.0 V with a detection limit of 11 nmol L-1 using square wave voltammetry. <![CDATA[NANOPARTÍCULAS DE PROTEÍNA ISOLADA DE SOJA EM ÁGUA: EFEITO DA FORÇA IÔNICA E DAS CONCENTRAÇÕES DE PROTEÍNA E SURFACTANTE]]> Soft nanoparticles of size 200-400 nm were obtained from soybean protein isolate (SPI). The particles were formed and suspended in water by the coacervation of aqueous suspensions of SPI in hostile buffered aqueous solutions in the presence of surfactants. We investigate the effect of storage, ionic strength, and concentrations of SPI and surfactant on nanoparticle size and zeta potential. Transmission electron microscopy images and scattering techniques (SLS/ DLS) revealed that the particles are spherical, with hydrophilic chains at the surface. <![CDATA[A VERSATILIDADE DOS COMPOSTOS DE COORDENAÇÃO NA PRODUÇÃO DE POLIETILENOS: UMA REVISÃO DOS SISTEMAS CATALÍTICOS]]> Taking into account the relevance of polyethylene for modern society and the role of catalysts for the production of this material, in the present work, we carried out a review of the main catalytic systems used in industry and academia. Most systems consist of coordination compounds, whose structural versatility allows the tuning of the characteristics of polyethylene for different applications. The structural aspects and chemical reactivity of such systems are discussed based on the existing literature and experimental data. <![CDATA[ÓXIDOS DE NIÓBIO: UMA VISÃO SOBRE A SÍNTESE DO Nb2O5 E SUA APLICAÇÃO EM FOTOCATÁLISE HETEROGÊNEA]]> Despite the fact that Brazil is the world’s largest niobium mineral producer, governmental interest in exploration of the mineral leading to more valuable derived materials is scarce, which has reduced the country’s knowledge about a wider range of technological applications for this metal. Niobium pentoxide stands out due its remarkable electronic, structural, and textural properties. Therefore, this review aims to highlight its main properties, synthetic methods, and applications, with a particular focus on photocatalysts based on Nb2O5. This review will highlight the potential of Nb2O5 and encourage the study of niobium and its compounds in technological and environmental applications. <![CDATA[ARSÊNIO E ARROZ: TOXICIDADE, METABOLISMO E SEGURANÇA ALIMENTAR]]> Arsenic is considered a semimetal, and its wide distribution in the Earth's crust in different chemical forms, including organic and inorganic species, has a great deal of influence on the mechanisms of toxicity. Exposure to arsenic can be either through occupational practice (use of pesticides) or by the consumption of water and food containing the element. Rice is considered a fundamental constituent of the basic diet of Brazilians and is usually cultivated in flooded conditions. Such a plantation system results in an increased amount of As in the soil and hence a greater accumulation of As in the plant, which is highlighted by the inorganic species' classification as highly toxic. Besides the use of mitigation techniques to reduce the toxicological risk, monitoring the concentration of As and its chemical species in rice and rice products is required through the establishment of legislation in the area. Thus, some world organizations are conducting improved research to determine and establish acceptable concentrations of As and its chemical species in rice, e. g., in 2012, FDA researchers described a chemical speciation methodology for As in rice and rice products. Hence, the application of existing chemical speciation methods and the establishment of parameters for ensuring food security and exposure risk assessment deserve particular consideration. <![CDATA[CONSTRUÇÃO DE UM REATOR DE PLASMA DESCARGA CORONA PARA ELIMINAÇÃO DE COMPOSTOS ORGÂNICOS VOLÁTEIS]]> The impacts derived from the emission of volatile organic compounds (VOC) into the atmosphere can have harmful consequences for human health and the environment. In this regard, the present paper proposes the construction of a low-cost cold plasma reactor for the treatment of these compounds. Tests with the prototype were performed to confirm the efficiency for BTEX (benzene, toluene and xylene) samples. Degradation efficiency was confirmed by the gas chromatography method. <![CDATA[CONSTRUÇÃO DE UM DISPOSITIVO DE PRESSURIZAÇÃO A VÁCUO PARA A PREPARAÇÃO DE FASES ESTACIONÁRIAS MONOLÍTICAS VIA PROCESSO SOL-GEL]]> The filling of capillaries via the sol-gel process is growing. Therefore, this technical note focuses on disseminating knowledge acquired in the Group of Analytical Chemistry and Chemometrics over seven years working with monolithic stationary phase preparation in fused silica capillaries. We believe that the detailed information presented in this technical note concerning the construction of an alternative high pressurization device, used to fill capillary columns via the sol-gel process, which has promising potential for applications involving capillary electrochromatography and liquid chromatography in nano scale, may be enlightening and motivating for groups interested in developing research activities within this theme. <![CDATA[APARATO ELETROQUÍMICO PARA SISTEMA DE AQUISIÇÃO DE DADOS MULTICANAL COM RESOLUÇÃO ESPAÇO-TEMPORAL]]> Spatiotemporal pattern formation in reaction-transport systems takes place spontaneously when the system is kept far from thermodynamic equilibrium. Targets, reaction fronts, waves, spirals, spots and stripes are some typical examples of selforganized structuring. In electrochemical systems, monitoring spatiotemporal patterns of potential in the solid/liquid interface can be done by the use of equally distributed microprobes located close to the working electrode. However, the physical size of each probe can limit the spatial resolution and alter mass transport properties. In contrast, the direct measurement of discrete electrodes does not suffer from this limitation and allows the accurate manipulation of the spatial coupling through changes in resistors connected to the electric circuit. In this paper, the development of an electrochemical setup for multichannel data acquisition with spatiotemporal resolution is described, especially to monitor low levels of currents usually observed in the electro-oxidation of small organic molecules. <![CDATA[A FORMAÇÃO DE PROFESSORES EM COMUNIDADES DE PRÁTICA: O CASO DE UM GRUPO DE PROFESSORES DE QUÍMICA EM FORMAÇÃO INICIAL]]> This work deals with communities of practice and their contribution to pre-service teacher training. A group of eight pre-service chemistry teachers was accompanied during their participation in the PIBID program. Based on their interaction in planning teaching activities, the group was characterized as a community of practice. For this characterization the three dimensions of communities of practice were observed: mutual engagement, joint enterprise and shared repertoire. The results showed that the community of practice was essential for the training of pre-service chemistry teachers. Through community practice, pre-service teachers were able to learn more about their future practice as chemistry teachers. <![CDATA[AVOIDING FIRST-YEAR UNIVERSITY CHEMISTRY TEXTBOOKS’ MISREPRESENTATIONS IN THE TEACHING OF SPONTANEOUS REACTIONS]]> This paper summarizes the misrepresentations related to Gibbs energy in general chemistry textbooks. These misrepresentations arise from a problem in the terminology textbooks use. Thus, after reviewing the proper definition of each of the terms analyzed, we present two problems to exemplify the correct treatment of the quantities involved, which may help in the discussion and clarification of the misleading conventions and assumptions reported in this study. <![CDATA[MÉTODO DE SÍNTESE DE AZIDAS AROMÁTICAS USANDO VINAGRE]]> The present paper describes a simple and low-cost procedure for preparation of aryl azides from anilines using vinegar as an unusual solvent/reagent. We observed the sequence of diazotation followed by diazo displacement with sodium azide can be carried out in aqueous solution of acetic acid <![CDATA[COMENTÁRIO SOBRE O ARTIGO: “MODIFICAÇÃO DO POLÍMERO CONDUTOR POLIANILINA PARA USO COMO TROCADOR CATIÔNICO”]]> The present paper describes a simple and low-cost procedure for preparation of aryl azides from anilines using vinegar as an unusual solvent/reagent. We observed the sequence of diazotation followed by diazo displacement with sodium azide can be carried out in aqueous solution of acetic acid