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51.
Conformational problems often involve very small energy differences, even low as 0.5 kcal mol(-1). This accuracy can be achieved by theoretical methods in the gas phase with the appropriate accounting of electron correlation. The solution behavior, on the other hand, comprises a much greater challenge. In this study, we conduct and analysis for cis-2-fluoro-, cis-2-chloro-, and cis-2-bromocyclohexanol using low temperature NMR experiments and theoretical calculations (DFT, perturbation theory, and classical molecular dynamics simulations). In the experimental part, the conformers' populations were measured at 193 K in CD(2)Cl(2), acetone-d(6), and methanol-d(4) solutions; the preferred conformer has the hydroxyl group in the equatorial and the halogen in the axial position (ea), and its population stays at about 60-70%, no matter the solvent or the halogen. Theoretical calculations, on the other hand, put the ae conformer at a lower energy in the gas phase (MP2/6-311++G(3df,2p)). Moreover, the theoretical calculations predict a markedly increase in the conformational energy on going from fluorine to bromine, which is not observed experimentally. The solvation models IEF-PCM and C-PCM were tested with two different approaches for defining the atomic radii used to build the molecular cavity, from which it was found that only with explicit consideration of hydrogens can the conformational preference be properly described. Molecular dynamic simulations in combination with ab initio calculations showed that the ea conformer is slightly favored by hydrogen bonding. 相似文献
52.
Fernanda Vera Cruz de Vasconcelos Paulo Fernandes Barbosa de Souza Jr. Maria Fernanda Pimentel Márcio José Coelho Pontes Claudete Fernandes Pereira 《Analytica chimica acta》2012
This work evaluates the use of near-infrared (NIR) overtone regions to determine biodiesel content, as well potential adulteration with vegetable oil, in diesel/biodiesel blends. For this purpose, NIR spectra (12,000–6300 cm−1) were obtained using three different optical path lengths: 10 mm, 20 mm and 50 mm. Two strategies of regression with variable selection were evaluated: partial least squares (PLS) with significant regression coefficients selected by Jack-Knife algorithm (PLS/JK) and multiple linear regression (MLR) with wavenumber selection by successive projections algorithm (MLR/SPA). For comparison, the results obtained by using PLS full-spectrum models are also presented. In addition, the performance of models using NIR (1.0 mm optical path length, 9000–4000 cm−1) and MIR (UATR – universal attenuated total reflectance, 4000–650 cm−1) spectral regions was also investigated. The results demonstrated the potential of overtone regions with MLR/SPA regression strategy to determine biodiesel content in diesel/biodiesel blends, considering the possible presence of raw oil as a contaminant. This strategy is simple, fast and uses a fewer number of spectral variables. Considering this, the overtone regions can be useful to develop low cost instruments for quality control of diesel/biodiesel blends, considering the lower cost of optical components for this spectral region. 相似文献
53.
M. Pontes 《Microchemical Journal》2009,93(1):1-11
Dynamic headspace solid-phase microextraction (HS-SPME) followed by thermal desorption gas chromatography-quadrupole mass spectrometry analysis (GC-qMS), was used to investigate the aroma profile of different species of passion fruit samples. The performance of five commercially available SPME fibres: 65 μm polydimethylsiloxane/divinylbenzene, PDMS/DVB; 100 μm polydimethylsiloxane, PDMS; 85 μm polyacrylate, PA; 50/30 μm divinylbenzene/carboxen on polydimethylsiloxane, DVB/CAR/PDMS (StableFlex); and 75 μm carboxen/polydimethylsiloxane, CAR/PDMS; was evaluated and compared. Several extraction times and temperature conditions were also tested to achieve optimum recovery. The SPME fibre coated with 65 μm PDMS/DVB afforded the highest extraction efficiency, when the samples were extracted at 50 °C for 40 min with a constant stirring velocity of 750 rpm, after saturating the sample with NaCl (17%, w/v — 0.2 g). A comparison among different passion fruit species has been established in terms of qualitative and semi-quantitative differences in volatile composition. By using the optimal extraction conditions and GC-qMS it was possible to tentatively identify seventy one different compounds in Passiflora species: 51 volatiles in Passiflora edulis Sims (purple passion fruit), 24 in P. edulis Sims f. flavicarpa (yellow passion fruit) and 21 compounds in Passiflora mollissima (banana passion fruit). It was found that the ethyl esters comprise the largest class of the passion fruit volatiles, including 82.8% in P. edulis variety, 77.4% in P. edulis Sims f. flavicarpa variety and 39.9% in P. mollissima.The semi-quantitative results were then submitted to principal component analysis (PCA) in order to establish relationships between the compounds and the different passion fruit species under investigation. 相似文献
54.
Marcos Reinaldo da Silva Marcos Carlos de Mattos Maria da Conceição Ferreira de Oliveira Telma Leda Gomes de Lemos Nágila Maria Pontes Silva Ricardo Gonzalo de Gonzalo Iván Lavandera Vicente Gotor-Fernández Vicente Gotor 《Tetrahedron》2014
Several phenylalanine analogs have been synthesized through a four-step route starting from easily available ethyl acetamidocyanoacetate. In a first reaction, and making use of phase transfer catalysts, this compound reacted with several alkyl halides, being benzyltributylammonium chloride identified as the best one for the production of a series of quaternary amino acids in moderate to excellent yields (52–95%). Then, the corresponding N-acetyl-phenylalanine methyl and allyl ester derivatives were obtained through acidic hydrolysis, esterification, and N-acetylation. Rhizomucor miehei lipase was found as a versatile enzyme for the resolution of these amino esters, finding the best results through interesterification reactions with butyl butyrate in acetonitrile. A great influence in the stereoselectivity was found depending on the chemical structure of the compound, achieving for the non- or para-substituted in the phenyl ring excellent stereoselectivities, being moderate for the meta-nitro derivative, while the ortho-nitro amino ester did not react. 相似文献
55.
56.
Cleverson C. Bocca Rodrigo M. Pontes Ernani A. Basso 《Journal of Molecular Structure》2004,710(1-3):105-110
The influence of hyperconjugative interactions on bond lengths of some allylic compounds (H2CCH–CH2–M(CH3)3; M=C, Si, Ge) has been investigated through NBO calculations using ab initio and density functional methods. The optimized structural parameters, at the B3LYP/6-31+G(d,p) and HF/6-31+G(d,p) levels, showed a good agreement with the resonance theory. Partial geometry optimization with orbital interactions removed confirmed the observations and revealed that σ→σ* interactions, together with the more common σ→π* ones, play an important role in determining the variations in bond lengths on going from C to Ge. 相似文献
57.
H. Polli L. A. M. Pontes M. J. B. Souza V. J. Fernandes Jr A. S. Araujo 《Journal of Thermal Analysis and Calorimetry》2006,86(2):469-473
The degradation kinetics
of polycarbonate with flame retardant additive was investigated by means of
thermogravimetric analysis. The samples were heated from 30 to 900°C in
nitrogen atmosphere, with three different heating rates: 5, 10 and 20°C
min–1. The Vyazovkin model-free kinetics
method was applied to calculate the activation energy (E
a)
of the degradation process as a function of conversion and temperature. The
results indicated that the polycarbonate without flame retardant additive
starts to loose mass slightly over 380°C and the polycarbonate with flame
retardant additive, slightly over 390°C (with heating rate of 5°C
min–1). The activation energy for flame retardant
polycarbonate and normal polycarbonate were 190 and 165 kJ mol–1,
respectively. 相似文献
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60.
D. S. L. Pontes E. Longo F. M Pontes Marcelo A. Pereira-da-Silva J. H. D. da Silva A. J. Chiquito P. S. Pizani 《Journal of Sol-Gel Science and Technology》2010,55(2):151-157
Dielectric and Raman scattering experiments were performed on polycrystalline Pb1−x−yCaxSryTiO3 thin films as a function of temperature. Temperature-dependent dielectric measurements revealed a decreasing ferroelectric-to-paraelectric
phase transition temperature and peak dielectric permittivity showed a broad phase transition near room temperature with increasing
levels of CaO12 and SrO12 clusters. Therefore, for higher levels of substitution, the possible random position of the CaO12 and SrO12 clusters leads to a diffuse state. At 100 kHz, the ferroelectric-to-paraelectric phase transition temperatures were 633,
495 and 206 K for PCST90 (Pb0.90Ca0.05Sr0.05TiO3), PCST70 (Pb0.70Ca0.15Sr0.15TiO3) and PCST30 (Pb0.30Ca0.35Sr0.35TiO3) thin films, respectively. The evolution of the Raman spectra was also studied as a function of temperature. The temperature
dependence of the E(1TO) soft mode frequencies was used to characterize the phase transition. Raman peaks were observed above
the ferroelectric-to-paraelectric phase transition temperature, although all optical modes should be inactive in Raman scattering.
The origin of these modes was interpreted as a breakdown of the local cubic symmetry by the random distribution of CaO12 and SrO12 clusters. 相似文献