摘要:

Reactions in mesoscopic, molecular clusters may proceed by mechanisms and with rates that differ from those in bulk solvents. Two examples of reactions in large, liquid-state, molecular clusters are described to illustrate the distinctive features of these reactions: acid dissociation and proton transfer in aprotic, polar solvents. Both of these reactions involve proton dynamics so methods for dealing with mixed quantum–classical systems must be utilized to investigate the reaction dynamics. Surface versus bulk solvation effects play an important role in determining the reaction mechanisms as do the strong cluster fluctuations. Mechanisms for proton transfer within clusters that have no bulk analogs will be described.Keywords: proton reactions, mesoscopic clusters.

ISSN:0008-4042    

DOI:10.1139/v97-001

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The mechanism of the oxidation of 3-mercaptopropionic acid (3-MPA) by hydrogen peroxide was studied in the acidic pH range. The nucleophilic attack by sulphur on the peroxide bond controls the rate. Extrapolation of the pH dependency suggests that the rate of attack by the deprotonated dianion is highest. Traces of Fe(III), at levels below 10−7 mol dm−3, do not catalyze efficiently the process through one-electron mechanisms; at higher concentrations, or on the surface of iron(III) oxides, this type of catalysis becomes important. The electrochemical oxidation of 3-MPA was also studied, using differential pulse polarography and cyclic voltammetry techniques. The mechanism is of the EC2E type, the second electron transfer step corresponding to the oxidation of the disulphide RS-SR. The rate constant for the dimerization of the RS•radicals was 1.8 × 103 mol−1dm3s−1; the slowness of this step agrees with the mechanisms observed in the course of one-electron oxidations by metal ions.Keywords: electrochemistry, kinetics, 3-mercaptopropionic acid, autooxidation.

ISSN:0008-4042    

DOI:10.1139/v97-002

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Humic substances (HS) were extracted from the sediments of four Sudbury area lakes, namely, Tilton, Clearwater, Silver, and Ramsey Lakes, with the aid of 0.1 M Na4P2O7and 0.5 M NaOH solutions. The HS (humic and fulvic acids) were purified and characterized using the methods of elemental analysis, visible spectroscopy (E4/E6ratio), FTIR, and solid-state13C CPMAS NMR. A substantial amount of information with regard to the composition and chemical nature of lake sediment HS was obtained. The results obtained for the Sudbury area lake sediments were compared with one another and with HS from other sources, such as soils. The elemental composition, atomic ratios,E4/E6ratios, and FTIR and NMR features of the samples from the above lakes were found to be nearly identical, suggesting that HS formed in the Sudbury area have similar chemical properties. Compared with soil HS, the Sudbury lake sediments HS have undergone a low degree of aromatic condensation and are considerably more aliphatic in nature.Keywords: humic substances, characterization, lake sediments, extraction,13C NMR.

ISSN:0008-4042    

DOI:10.1139/v97-003

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Preparation of bisannulated salts of 2,2′-biimidazole, 2-(2′-imidazolyl)benzimidazole, 2,2′-bibenzimidazole, and annulated salts of 1,1′-dimethyl-2,2′-biimidazole, 1,1′-dimethyl-2-(2′-imidazolyl)benzimidazole, and 1,1′-dimethyl-2,2′-bibenzimidazole is accomplished by direct alkylation of the parent azaheterocycle with excess 1,n-dihaloalkane. Discussions of the product conformations use electronic absorption spectra and NMR. The redox potentials of these salts are measured in DMF and CH3CN, and become increasingly more negative and less reversible as the systems become less planar.Keywords: electrochemistry, bridged azabiaryl salts.

ISSN:0008-4042    

DOI:10.1139/v97-004

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

trans-[ReO2(BzimH)4]I is generated by reacting ReO2I(PPh3)2with excess BzimH in methanol, but free ligand is difficult to eliminate from the product. Pure samples of thetrans-[ReO2(BzimH)4]BPh4salt and of the protonated oxo-hydroxo form [ReO(OH)(BzimH)4](BF4)2are isolated. Recrystallization of the dioxo complex in acetone–ether yields a pink solid. Red crystals suitable for X-ray diffraction are obtained by recrystallization of this solid in ethanol. The crystallographic study reveals a hemiprotonated [ReO2(BzimH)4][ReO(OH)(BzimH)4](ReO4)3phase (Acam,a = 10.794,b = 21.520,c = 27.196 Å,R = 0.049). The O=Re=O fragments are parallel to one another along the unit cellcaxis, forming layers of dimeric units consisting of one dioxo and one oxo-hydroxo species connected by a very shorthydrogen bond of 2.44 Å. The ReO4−anions form hydrogen bonds with ligand N-H groups. The complexes are characterized by solution (1H and13C) and solid-state (13C CP-MAS) NMR, UV–VIS, and IR spectroscopies.1H NMR reveals that the benzimidazole protons in close proximity to the rhenium–oxo multiple bond or adjacent benzimidazole ligands are very sensitive to nonbonded magnetic effects.Keywords: rhenium, oxo complexes, benzimidazole, X-ray diffraction.

ISSN:0008-4042    

DOI:10.1139/v97-005

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The main component of the solid originally believed to be a peroxosilicate with pulp-brightening properties has been shown to be Na2O2•8H2O. The solid crystallizes in the monoclinic space groupC2/c, with an empirical formula H8O5Na, and witha = 14.335(3),b = 6.461(1),c = 11.432(2) Å, β = 118.28(3)°, andZ = 8. The centrosymmetric structure consists of a peroxide anion with an O—O distance of 1.499(2) Å. Each of these oxygen atoms is at the apex of an approximate square-based pyramid, the base of which consists of four oxygen atoms of water molecules. The bases of the two pyramids are staggered when viewed down the peroxide bond. Each sodium is at the centre of an approximate octahedron of water molecules, four of which bridge other sodium atoms and two bridge to the peroxide anions. One hydrogen atom of each of these two water molecules is terminal and the other two are hydrogen bonded to peroxide oxygen atoms. The compound reacts very rapidly with CO2in moist air to form Na2CO3, but in drier conditions, formation of the carbonate can take many days and proceeds via a percarbonate, believed to be Na2CO4. This has been identified by infrared spectroscopy and X-ray powder diffraction and can persist for long periods in dry air.Key words: sodium peroxide hydrate, sodium percarbonate, pulp brightening, X-ray diffraction, infrared.

ISSN:0008-4042    

DOI:10.1139/v97-006

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Equilibrium constants of complexes between β-cyclodextrin and glucose, galactose, or mannose have been determined by fluorometric competition titrations in water to be as low asK = 1 – 0.5 M−1, in sharp contrast to recently published values for glucose aroundK = 400 M–1. The pentoses ribose, xylose, and arabinose show association constants between 6.3 and 1.5 M−1, in agreement with published values. Preliminary computer-aided molecular modelling studies suggest that the small values observed for the hexoses are not due to steric hindrance of an intracavity inclusion but to a smaller number of intermolecular hydrogen bonds and to some strain as a result of cyclodextrin deformation. Calorimetric measurements of the glucose–β-cyclodextrin complex show, again in contradiction with the literature, anendothermic reaction, or a free enthalpy value close to zero.Key words: cyclodextrin complexes, sugar complexation, glucose complexation, calorimetry, computer-aided molecular modelling.

ISSN:0008-4042    

DOI:10.1139/v97-007

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The acid-catalyzed hydrolysis of the cyclic diazothiolactone, 4-diazoisochroman-3-one (3) was found to occur with the hydronium-ion isotope effect,and to give the ring-contracted product, 1,3-dihydrobenzo[c]thiophene-1-carboxylic acid (4). This shows that protonation of the diazo carbon atom occurs in the rate-determining step and that the reaction also involves migration of the thio group. The hydronium-ion catalytic coefficient for this reaction,, is 45 times less than that for hydrolysis of its acyclic thio ester analog,S-methyl phenyldiazothioacetate (5). Semiempirical AM1 molecular orbital calculations support the idea that this difference in reactivity is the result of increased delocalization of negative charge into the aromatic ring in the case of the cyclic substrate, which reduces the negative charge on the diazo carbon atom and makes it less susceptible to protonation.Key words: hydrolysis, diazoalkanes, charge delocalization, AM1 calculations, thio group migration.

ISSN:0008-4042    

DOI:10.1139/v97-008

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Ab initio calculations on a series of trisubstituted amines, phosphines, and arsines are presented at the MP2/6-311G(d,p) level. Specifically, the species studied are XH3, XF3, and X(Me)3, where X = N, P, or As. The influence of the substituents on the proton affinity is analyzed in terms of the charge distribution, its topology, some one-electron properties (dipole moments, electric field gradients), and dipole polarizabilities. An atoms-in-molecules. decomposition of the charge distribution, energetics, and polarizabilities also proves informative. There seems to be no straightforward way of rationalizing the basicities of these compounds on the basis of electrostatic properties (i.e., properties of the charge distribution in the unprotonated bases). However, substituent effects on basicities can be correlated with response properties, such as the molecular (and atoms-in-molecules) polarizability tensors, and the amount of charge that a substituent group or atom donates on protonation.Key words: quantum chemistry, proton affinity, electron density, lone pairs, polarizability, basicity, dipole moment, electric field gradient.

ISSN:0008-4042    

DOI:10.1139/v97-009

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The C-3—C-9 main fragment of aspicilin was prepared via a fructose 1,6-diphosphate aldolase reaction. The same fragment was also synthesized by chemical transformation starting fromD-arabinose.Key words: aldolase, aspicilin, arabinose, synthesis.

ISSN:0008-4042    

DOI:10.1139/v97-010

出版商:NRC Research Press    

年代:1997

数据来源: NRC