摘要:

This paper presents data showing that in dilute aqueous solutions of butanone, 3-pentanone, cyclohexanone, di-tert-butyl ketone, and acetaldehyde the addition reaction of muonium atoms occurs with a rate constant close to 1 × 108 M−1s−1. The same value was obtained previously for acetone. Thus the reaction rate is virtually independent of the group attached to the C=O, be it a methyl, methylene,tert-butyl, or even a hydrogen atom. This is in sharp contrast to the reactivity of ordinary1H atoms, whose rate constants are much slower and dependent on adjacent groups. In fact muonium and1H react by different mechanisms, to form different products, so their rate ratio represents a complex kinetic isotope effect.Keywords: kinetic isotope effects, muonium atoms, muon spin rotation, ketones, hydrogen atom reactions.

ISSN:0008-4042    

DOI:10.1139/v97-011

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Sodium (Na) clusters in octahedral cages of Na-intercalated fullerides Na6C60and Na11C60are studied within a Thomas–Fermi model. It is shown that the tetrahedral Na4cluster in Na6C60has an electric charge ~ +2.7 (in electron charge units), while the body-centered cubic Na9cluster in Na11C60is almost electrically neutral.Keywords: sodium clusters, alkali fullerides, Thomas–Fermi theory, ionization charge.

ISSN:0008-4042    

DOI:10.1139/v97-012

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

The effects of β-cyclodextrin (β-CD) on the kinetics of the reaction of α-amino acid anions withp-nitrophenyl acetate (pNPA) andp-nitrophenyl hexanoate (pNPH) have been investigated. Pseudo-first-order rate constants obtained with various concentrations of nucleophile and β-CD, in aqueous solution at pH 9.88, were analyzed to yield rate constants for reaction of the nucleophiles with free ester (kN) and with ester bound to β-CD (kcN). For pNPA reacting with amino acids that bind weakly to β-CD, if at all, the ratiokcN/kNis very close to one, but for amino acids that bind appreciably to β-CD this ratio is greater than one (up to 3.5). Generally similar behaviour is observed for reactions with pNPH, but the rate ratios are smaller and all less than one. These findings are comparable to those obtained in this laboratory for the CD-mediated reactions ofp-nitrophenyl alkanoate esters with other nucleophiles, including simple alkylamines that bind quite strongly to CDs, but they differ from those reported in an earlier study of the reaction of pNPA with amino acid anions (Can. J. Chem.69, 1124 (1991)) where the data analysis suggested more substantial catalysis by cyclodextrins.Key words:amino acids, cyclodextrin, ester cleavage, kinetics, catalysis.

ISSN:0008-4042    

DOI:10.1139/v97-013

出版商:NRC Research Press    

年代:1997

数据来源: NRC

摘要:

Several compounds of the type PhCR1R2-TEMPO (where TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxide) have been prepared and examined as potential photosensitized initiators for "living" free radical polymerizations. For example, these compounds are excellent quenchers of triplet xanthone (rate constants in the range (8.0–13) × 108 M−1s−1) and result in the formation of benzylic and TEMPO radicals. The importance of excited state energy and nature of the excited triplet state have also been explored. We also tested pyrene as a possible singlet sensitizer. While pyrene fluorescence is efficiently quenched, the process does not lead to cleavage of the C—O bond. A model is proposed to account for this anomaly in the photosensitized decomposition of these TEMPO adducts.Keywords: TEMPO, laser flash photolysis, time resolved emission spectroscopy, sensitizers, quenching, photoinitiator.

ISSN:0008-4042    

DOI:10.1139/v97-014

出版商:NRC Research Press    

年代:1997

数据来源: NRC