摘要:

The time-resolved discharge patterns generated in a gaseous dielectric breakdown (DB) are imaged using a fast optical imaging system ( approx 40 ns exposure time). The discharge patterns from various gases (He, air, and SF6) show general features consistent with their known dielectric strength. A new stochastic pattern-forming model that explicitly includes time is presented and is shown to produce theoretical DB patterns in qualitative agreement with the present experimental discharge patterns for air (N2). The stochastic DB model (with time) also provides an approximation to the discharge current, which is in good agreement with published current wave forms.Key words: gas discharge, electron scattering, fractal.

ISSN:0008-4042    

DOI:10.1139/v97-207

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

A series of cation-exchange membranes were obtained by photoinitiated grafting of acrylic acid onto a polypropylene microporous support having 0.2 µm diameter pores. The poly(acrylic acid) was shown to be contained within the pores of the membrane. The ion-exchange capacities of these "pore-filled" membranes ranged from 65 to 80% of the theoretical values calculated on the basis of their measured graft yields, with water contents ranging from 72 to 77%. The membranes exhibited a chemical valve effect of flux as a function of pH. Treatment of a poly(acrylic acid) grafted membrane with a solution containing ferrous ions resulted in a large uptake of iron (>24% by weight of iron with a membrane containing 152% graft yield of poly(acrylic acid)). Subsequent alkaline (pH 14) oxidation at 70°C with either oxygen or hydrogen peroxide led to the formation of a superparamagnetic nanocrystalline form of ferrihydrite, 5Fe2O3·9H2O. The oxide was identified by temperature-dependent Mössbauer spectroscopy recorded from 300 K to 4.2 K. TEM micrographs of sectioned membranes that were treated with H2O2revealed the presence of two iron oxide morphologies, disk-shaped particles 3-4 nm in diameter and needle-like particles about 4 nm wide and 50 nm long. Only disk-shaped particles were observed in membranes treated with O2. Magnetization measurements indicate that the membranes are superparamagnetic above 50 K with a room temperature magnetization of 3.3 emu g-1at 20 kOe.Key words: membrane, poly(acrylic acid); iron oxide, magnetic, Mössbauer spectroscopy; magnetization, photografting

ISSN:0008-4042    

DOI:10.1139/v97-210

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The present work is concerned with a thermodynamic study of the three aminopyridine (AP) isomers (general formula: C5H6N2). It was achieved using three techniques: combustion calorimetry of small amounts of substance (a few milligrams), sublimation calorimetry, and differential thermal analysis. From this study, it was possible to determine the enthalpies of combustion, sublimation, and fusion of these compounds and their triple point temperatures; to discuss the relative stability of the three molecules; to determine the experimental resonance energies and compare them with the theoretical values; to determine the atomization enthalpies and compare them with the values calculated from the energetical contributions previously determined in our laboratory; to determine the enthalpy of the intermolecular bonds.Key words: aminopyridine isomers; enthalpy of combustion, of sublimation, of fusion, of inter- and intramolecular bonds.

ISSN:0008-4042    

DOI:10.1139/v97-206

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

A1H NMR chemical shift ( delta ) study of a homologous series of hydrocarbon (hc) (CxH2x+ 1CO2Na,x= 5, 7, 9, 11, 13) surfactants (S) has been carried out in water and in binary solvent (D2O + cyclodextrin (CD)) systems at 22°C. Complementary1H NMR chemical shift ( delta ) data of the cyclodextrins in binary (D2O + S) systems containing hc surfactants have also been obtained. Complex induced shift (CIS) values for selected host or guest protons were found to increase as the alkyl chain (Cx) length of the surfactant increased. The CIS values are found to depend on the following factors: (i) the magnitude of the binding constant (Ki,i= 1:1, 2:1), (ii) the chain length of the surfactant, (iii) the mole ratio of the host to guest species, (iv) the host-guest stoichiometry, and (v) the host-guest inclusion geometry. The CIS values of the CD-S systems have been analyzed using equilibrium models in which 1:1 complexes, 1:1 plus 2:1 complexes, and uncomplexed species are present. Differences in the binding affinity, stoichiometry, and inclusion geometry of the complexes formed between a given hc surfactant and the various cyclodextrins were observed.Key words: cyclodextrin, surfactant, NMR, chemical shift, complex, binding constant.

ISSN:0008-4042    

DOI:10.1139/v97-208

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The diphenylmethylenecyclohexane,2, and the substituted 2-methyleneadamantanes,3-7, have been prepared. The radical cation of each was generated by photochemical oxidation using 1,4-dicyanobenzene as the sensitizer, and their reactivity was examined in methanol-acetonitrile mixtures with added tetraethylammonium cyanide. Although compound2reacted only by tautomerization, the other compounds all gave addition products. For the monoaryl alkenes3,4,and5, comparable yields of both methanol and HCN addition products were obtained, resulting from nucleophilic attack at either end of the alkene radical cation. For the diphenylalkenes6and7, the regioselectivity favored nucleophilic attack at the adamantyl carbon, and the addition products were predominantly those of HCN addition. For the 5-methoxy compound7, HCN addition was facially selective with asyn:antiratio of 58:42.Key words: methyleneadamantanes, facial selectivity, radical cations.

ISSN:0008-4042    

DOI:10.1139/v97-201

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

A survey of crystal structures containing the xanthine ring shows that caffeine-like molecules tend to stack in parallel in a variety of stacking geometries, suggesting that a stacked pair of caffeine molecules has many conformations of comparable energy. Molecular modelling suggests that the caffeine dimer, the major associated species in aqueous solution, may occur in nine distinct conformations. In agreement with the crystal survey and the modelling results, NMR spectra of caffeine solutions exhibit intermolecular nuclear Overhauser enhancements (NOEs), consistent with a dynamic equilibrium of several conformations of the dimer of comparable energies.Key words: caffeine, dimer, self-association, molecular modelling, intermolecular nuclear Overhauser enhancements (NOEs).

ISSN:0008-4042    

DOI:10.1139/v97-211

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Structural parameters, harmonic frequencies, and force constants of B2F4were obtained in internal coordinates from ab initio RHF calculations with the 6-311G* basis set. The results obtained are used to calculate the Coriolis coupling constants and the inertial defect for10B2F4and11B2F4and to compare these values with those obtained by using the autoconsistency method. Both the ab initio and the iterative force fields, despite some significant differences, reproduce the vibrational wave numbers equally well and yield nearly the same Coriolis coupling constants and inertial defects. The calculated inertial defects for10B2F4and11B2F4are -0.942 and -0.919 amu Å2, respectively.Key words: Coriolis coupling constants, inertial defect, B2F4, harmonic frequencies, force constants.

ISSN:0008-4042    

DOI:10.1139/v97-209

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Kinetics for the reactions of [1] [PtIV2Me8(&mgr; - SMe2)2] + 2 Me2Skf<-->kd2[PtIVMe4(SMe2)2] [2] [PtII2Me4(&mgr; - SMe2)2] + 2 Me2Skf--> 2[PtIIMe2(SMe2)2]and [3] [PtIV2Me8(&mgr; - SMe2)2] + 2NN --> 2 [PtIVMe4(NN)]+2Me2Swhere NN = bipy or 4,4'-Me2-bipy, have been studied at various temperatures and pressures. Reaction [3] was shown to consist of the rapid conversion of the dimer to a monomer and the much slower subsequent substitution of the dimethyl sulfide with bidentate ligand NN: [PtIV2Me8(&mgr; - SMe2)2] + 2 Me2S <--> 2 [PtIVMe4(SMe2)2] and [PtIVMe4(SMe2)2] + NN --> [PtIVMe4(NN)] + 2 Me2S The rate constants and activation parameters for the reactions are as follows:kf= 3.16 ± 0.06 M-1s-1(25°C), &Dgr;Hdoubledaggerf= 51.8 ± 1.7 kJ mol-1, &Dgr;Sdoubledaggerf= -61.0 ± 5.8 J mol-1K-1,kd= 1.18 ± 0.22 M-1s-1(25°C), &Dgr;Hdoubledaggerd= 65 ± 22 kJ mol-1, &Dgr;Sdoubledaggerd= -26 ± 73 J mol-1K-1for reaction [1] inn-hexane;kf= 6.68 ± 0.06 M-1s-1(25°C), &Dgr;Hdoubledaggerf= 58.0 ± 3.1 kJ mol-1, &Dgr;Sdoubledaggerf= -34.5 ± 10.5 J mol-1K-1, &Dgr;Vdoubledaggerf= -10.7 ± 1.3 cm3mol-1for reaction [2] in dichloromethane;k2= (7.09 ± 1.89) x 10-4M-1s-1, &Dgr;Hdoubledagger2= 95 ± 21 kJ mol-1, and &Dgr;Sdoubledagger2= 18 ± 70 J mol-1K-1, &Dgr;Vdoubledagger2= 9 ± 9 cm3mol-1for reaction [3] with bipy, andk1= (1.10 ± 0.10) x 10-2s-1,k3/k-1= (4.33 ± 0.30) x 10-2, andk2= (6.09 ± 1.35) x 10-4M-1s-1for reaction [3] with 4,4'-Me2-bipy. It was shown that reactions [1] and [2] proceed nucleophilically without any intermediates, and reaction [3] proceeds through a mainlyk2path for NN = bipy and through bothk1andk2paths for NN = 4,4'-Me2-bipy, without appreciable participation of the direct reaction between the dimer and NN as shown by the following reaction scheme.Key words: octamethylplatinum(IV) dimer, tetramethylplatinum(II) dimer, reaction mechanism, high-pressure kinetics.

ISSN:0008-4042    

DOI:10.1139/v97-203

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The effect of NaF, NaCl, NaBr, NaI, NaNO3, CH3COONH4, NaH2PO4, and (NH4)2SO4onmer-(+)-tris(L-alaninato)cobalt(III) (mer-(+)-[Co(L-ala)3]) was examined by determining the solubility of the chelate. While the solubility ofmer-(+)-[Co(L-ala)3] (salting-in effect) increased with the addition of NaCl, NaBr, NaI, and NaNO3, the solubility (salting-out effect) decreased with the addition of NaF, CH3COONH4, and NaH2PO4. In low salt concentration (NH4)2SO4caused slight salting-in, but in salt concentration >0.5 mol dm-3(NH4)2SO4brought about salting-out. The magnitude of the salting-in effect increased in the order of NaCl < NaNO3< NaBr < NaI. The standard enthalpy (&Dgr;Hos) and entropy (&Dgr;Sos) of dissolution former-(+)-[Co(L-ala)3] were estimated from the temperature dependence of the solubility. When &Dgr;Hoswas plotted againstT&Dgr;Sos, a single-correlation straight line was obtained for the salts that brought about the salting-in effect. On the basis of these findings, the origin of the salting-in effect was discussed. Furthermore, while NaF, NaCl, and NaBr showed larger salting-in effect onfac-tris(glycinato)cobalt(III) (fac-[Co(gly)3]) than onmer-(+)-[Co(L-ala)3], NaI greatly decreased the solubility offac-[Co(gly)3]. At the present stage, an explanation for this anomalous salting-out effect of NaI cannot be presented.Key words: salt effect, solubility, thermodynamic parameters of dissolution,mer-tris(L-alaninato)cobalt(III),fac-tris(glycinato)cobalt(III).

ISSN:0008-4042    

DOI:10.1139/v97-200

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

4-Azido-X'-substituted biphenyls (X' = 4'-MeO, 4'-Me, 4'-F, 3'-Me, 4'-Cl, H, 3'-MeO, 3'-Cl, 4'-CF3) have been prepared and subjected to 248 nm flash photolysis irradiation in 20:80 acetonitrile:water. Transient X'-substituted 4-biphenylylnitrenium ions10(Ar C6H4-N+H) are observed, with lifetimes ranging from 0.6 ms (4'-MeO) to 26 ns (4'-CF3). These cations are quenched by azide ion, with values ofkazranging from 6 to 10 x 109M-1s-1, with the majority in the range (9-10) x 109. This near constantkazprovides further evidence that arylnitrenium ions are quenched by azide ion at the diffusion limit. The solvent reactivities, plotted in a single-parameter Hammett plot against &sgr; sup + (X), exhibit a poor correlation, with the points for thepara&pgr;-electron donors deviating from the correlation line based on the other substituents in the direction of requiring a more negative substituent parameter. The data are more satisfactorily fit to the two-parameter Yukawa-Tsuno equation; the parameterr+obtained in this fit is 2.8. Thus the resonance interaction of thepara&pgr; -donor X'-substituents with the positive charge of the cation is underestimated by &sgr; sup + , a situation that has previously been observed with benzylic-type carbenium ions. The conclusion is made that, in their reaction with water, 4-biphenylylnitrenium ions behave like benzyl cations bearing two additional stabilizing vinyl groups, i.e., as if they had the structure Ar-C+(C = C)2. The inherent reactivity and the pattern of the aryl substituent effects are in fact similar to those in the carbocation series Ar-C+(Ph)2.Key words: nitrenium, nitrene, aryl azide, photochemistry, lifetime.

ISSN:0008-4042    

DOI:10.1139/v97-204

出版商:NRC Research Press    

年代:1998

数据来源: NRC