Sample records for double bonds
from WorldWideScience.org

Sample records 1 - 5 shown.



1

Reações de ozonólise de olefinas em fase gasosa/ Gas phase reactions of ozone with olefins

Nunes, Fabíola Maria Nobre; Pereira, Pedro Afonso de Paula; Andrade, Jailson B. de
2000-12-01

Resumo em inglês Biogenic emissions of volatile organic compounds play a fundamental role in the atmospheric chemistry, vegetation being one of their major sources. Amongst the VOCs emitted by plants, olefins and terpenoids are the most abundant. These compounds, due to the presence of two or more double bonds and other structural features, are very reactive in the atmosphere and act as precursors of the photochemical smog and aerosols. This article presents a review of the reactions of olefins and terpenoids with ozone, in the gas phase, with emphasis toward the mechanisms and kinetic aspects.

Scientific Electronic Library Online (Portuguese)

2

O modelo AM1 na previsão de frequências vibracionais/ The vibration frequencies predicted by the AM1 model

Ramos, João Carlos Silva; Hollauer, Eduardo; Cardoso, Sheila Pressentin
1999-09-01

Resumo em inglês We analyse vibrational frequencies of 168 compounds with the AM1 model concerning its experimentally observed gaseous frequencies. Stretching of CH, NH, OH and CO bonds, its related bending frequencies, and the CC frame movements are the studied vibrations. The results show problems with the AM1 vibrational splittings. Often symmetric stretching frequencies, like in CH3, CH2 and NH3, appear switched with the corresponding antisymmetrical ones. Among the studied vibrations (mais) many stretchings are overestimated, while bendings oscillate around experimental values. Fluorine stretchings, NN, OO, CH, double and triples CC bonds and cyclic hydrocarbon breathing modes are always overestimated while torsions, umbrella modes and OH/SH stretching are, in average, underestimated. Graphical analysis show that compounds with the lowest molecular masses are the ones with the largest difference to the experimental values. From our results it is not possible to fit confortably the calculated frequencies by a simple linear relationship of the type, n(obs)=a*n(AM1). Better aggreement is obtained when different curves are adjusted for the stretching and bending modes, and when a complete linear function is used. Among our studies the best obtained statistical results are for CH, NH and OH. The conclusions obtained in this work will improve the AM1 calculated frequencies leading to accurate results for these properties.

Scientific Electronic Library Online (Portuguese)

3

Influência do Sistema de Vulcanização nas Propriedades da Mistura NBR/EPDM/ The Influence of the Vulcanizing System on Curing, Mechanical and Ageing Properties of NBR/EPDM Blends

Oliveira, Marcia G.; Soares, Bluma G.
2002-01-01

Resumo em português A mistura NBR/EPDM apresenta diferentes velocidades de cura, o que resulta na vulcanização heterogênea das fases, isto é, a fase NBR encontra-se em grande parte reticulada enquanto que a fase EPDM encontra-se pouco reticulada ou não-reticulada em casos extremos, prejudicando as propriedades finais da mistura. Este trabalho avalia os efeitos de diferentes sistemas de vulcanização à base de enxofre no processo de cura, na dureza, na resistência à tração, ao rasg (mais) amento e ao envelhecimento da mistura NBR/EPDM. Foram estudados os sistemas S/MBTS, S/TMTD e S/MBTS/TMTD variando-se a quantidade de enxofre em 0,3 e 1,0 phr. O processo de cura é influenciado pelo tipo de acelerador (MBTS, TMTD e a combinação destes) e pela quantidade de enxofre (0,3 e 1,0 phr) presente nas formulações. Esta influência é percebida na resistência à pré-cura e na velocidade de cura da mistura NBR/EPDM. A reticulação da fase EPDM também varia com o sistema de vulcanização empregado e influi diretamente nas propriedades mecânicas e na resistência ao envelhecimento. Resumo em inglês NBR and EPDM differ in the number of double bonds and also in polarity, thus presenting different cure rates. Their blends frequently result in a maldistribution of crosslinks. In this work, sulfur-based systems varying in accelerator type and sulfur amount were investigated as vulcanizing systems for NBR/EPDM blends. The accelerator type and the sulfur concentration affected the curing characteristics of the blends. Some formulations were able to crosslink the EPDM phase to a larger extent and consequently displayed better mechanical and ageing properties.

Scientific Electronic Library Online (Portuguese)

4

Hidrogenação eletrocatalítica de substratos orgânicos utilizando eletrodos modificados poliméricos contendo partículas de Ni/Pd e Ni/Pt/ Electrocatalytic hydrogenation of organic substrates on polymer modified electrodes embedding Ni/Pd and Ni/Pt particles

Purgato, Fabiana L. S.; Romero, José R.; Abrantes, Luisa M.
2009-01-01

Resumo em inglês Novel modified electrodes bearing dispersed Pd and Pt particles have been prepared from poly (allyl ether of the p-benzenesulfonic acid) films with incorporated nickel particles making use of galvanic displacement reactions. The SEM analysis of the new modified electrodes revealed efficient deposition of Pd but weak up-take of Pt. Electrocatalytic hydrogenation of several classes of organic substrates were carried out using the MEs Ni, Ni/Pd and Ni/Pt. The Ni/Pd ME showed (mais) to be the best of them for the hydrogenation of double, triple and carbonyl bonds. The complete hydrogenation of the aromatic rings for the well-adsorbed substrates acetophenone and benzophenone is noteworthy.

Scientific Electronic Library Online (Portuguese)

5

Consumo e identidade no meio juvenil: considerações a partir de uma área popular do Distrito Federal/ Consumption and identity among youngsters: considerations from a popular area of the Brazilian Federal District

Nunes, Brasilmar Ferreira
2007-12-01

Resumo em português Este texto reflete sobre a natureza das relações entre os jovens e a cidade, com um recorte que privilegia a esfera do consumo, dimensão essencial na construção identitária nesta etapa da vida. As reflexões partem de uma pesquisa feita em uma área específica do Distrito Federal, a cidade da Estrutural, uma das que compõem o espaço urbano da capital do País. Em se tratando de uma área pobre para os padrões locais, os jovens que ali moram são marcados por est (mais) a condição que, por um lado, deixa-os socialmente vulneráveis e, por outro, produz resistências que se manifestam no estilo de se comportar na vida cotidiana. Tem-se, neste caso, uma dupla determinação da posição social destes jovens: o local de moradia e as suas faixas etárias, ou seja, estigmatizados por um lado e vulneráveis pelo outro, criam uma forma de existência social particular, que nos auxiliam na compreensão dos vínculos sociais na cidade. Resumo em inglês This text reflects on the nature of the relations between youngsters and the city, by privileging the sphere of the consumption; essential dimension in the personal identity construction in this stage of life. The reflections are made from a research conducted in a specific area of the Federal District, Estrutural city, which composes the urban space of the capital of the Country. When compared to local standards, this is considered a poor area; the youths who live there (mais) are marked by this condition, which, on one hand, leaves them socially vulnerable, and on the other, produces a resistance that is revealed through their behaviour in daily life. One observes, in this case, a double determination of the social status of these youngsters: the place where they live and their age, i. e., they are stigmatised, on one hand, and vulnerable on the other; they create a way of particular social existence, which assists us in understanding the social bonds in the city.

Scientific Electronic Library Online (Portuguese)