摘要:

Large-amplitude solid phase molecular motion has been detected in the macrocyclic ring of the title crown either via13C CPMAS NMR. To study the details of the dynamic processes, two selectively deuteratedd4derivatives have been prepared and examined via2H NMR as a function of temperature. A phase change occurring around 277 K has been verified by differential scanning calorimetry (DSC) and a model for the motional processes has been developed involving equivalent two-site flips of the CD2groups. The amplitude of the CD2motions apparently decreases the closer the group is to the aromatic ring. The influence of KNCS complexation on the13C CPMAS spectrum and on13C spin lattice relaxation times in solution has been explored.Keywords: macrocyclic ethers, solid phase dynamics.

ISSN:0008-4042    

DOI:10.1139/v96-161

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The first examples of dissociating excited state carbon acids were reported by our group. A necessary structural feature is the 5H-dibenzocycloheptene ring system where the incipient carbanion is formally an antiaromatic system in S0. In this work, structure–reactivity studies of the excited singlet state carbon acid dissociation and competing formal di-π-methane rearrangement of several 5H-dibenzo[a,c]cycloheptene derivatives have been carried out in order to gain more insights into the photochemistry displayed by these compounds. Photolysis of 7-deuterio-5H-dibenzo[a,c]cycloheptene (9) in aqueous solution showed that the photogenerated carbanion is allylically delocalized. Derivative7was found to be less reactive than3with respect to carbon acid dissociation whereas8was unreactive. Ethanolamine (in CH3CN) was found to be an effective base in catalyzing carbon acid dissociation for3,7, and9, as indicated by higher yields of deuterium incorporation and rates of fluorescence quenching. Binaphthyl derivatives10and11displayed contrasting photobehaviour. Photolysis of binaphthyl11resulted in only efficient (ΦP = 0.47) formal di-π-methane rearrangement under all conditions and no evidence was found for carbon acid dissociation, even in the presence of ethanolamine as base. On the other hand, the formal di-π-methane reaction was very inefficient for binaphthyl10(Φp < 0.001). Due to the conformational rigidity inherent in10, the methylene protons at the 3-position are NMR resolvable as pseudo-axial and pseudo-equatorial protons. Photolysis in the presence of ethanolamine (in D2O–CH3CN) resulted in stereoselective deprotonation of the pseudo-axial proton (Φex ≈ 0.02), as indicated by deuterium exchange studies. The results show that excited state carbon acid dissociation is an observable general reaction of dibenzo and dinaphtho cycloheptenes only if more favourable photochemical pathways are not competing.Keywords: excited state carbon acid, di-π-methane rearrangement, carbanion, stereoelectronic effect, fluorescence quenchin

ISSN:0008-4042    

DOI:10.1139/v96-162

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

D/H fractionation factors between MeOH and Ph2PH in dilute solutions of tetrachloroethylene, benzene, tetrahydrofuran, pyridine, and acetonitrile and T/H fractionation factors between MeOH and Me2PH vapors were measured. The experimental results agree very well with values calculated from the statistical theory of isotope effects formulated by Bigeleisen and Mayer. There are correlations between observed fractionation factors and solvent polarity, and the interaction energy of methanol with the given solvent. Another correlation has been found between enthalpy of the exchange reactions and the interaction energy between methanol and the given solvent.Keywords: isotope effects, fractionation factor, diphenylphosphine, methanol.

ISSN:0008-4042    

DOI:10.1139/v96-163

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Tris(trimethylsilyl)-2,2,2-trifluoroethoxysilane6, tris(trimethylsilyl)-2-fluoroethoxysilane7, and tris(trimethylsilyl)-2-chloroethoxysilane8were synthesized and characterized by1H,13C and29Si NMR, IR spectroscopy, and EI and CI mass spectrometry. Thermodynamic considerations would suggest that, as a result of the driving force provided by the formation of a Si—F or Si—CI bond, the thermolyses of these compounds would lead to the formation of bis(trimethylsilyl)silanone4. To examine this question, gas chromatography–mass spectrometry was as used a detection technique for products resulting from the high-pressure thermolyses of6–8. The elimination of (Me3Si)3SiCl appears to be the major thermolytic pathway of decomposition for8at ambient or higher pressures, although it is accompanied by the formation of other products, some of which could have arisen from the addition of various halosilanes to a silanone. Neither6nor7thermolyzed cleanly; the former compound was essentially unreactive under the thermolysis temperatures used (850 °C). Of the products produced in the thermolysis of7, no evidence for the formation of the silanone was obtained. Independently, mass spectrometry was used to study unimolecular reactions of molecular ions derived from6–8. The major route to solitary ions appears to involve a 1,2-trimethylsilyl migration from Si to O (9 → 10) prior to decomposition, for example, of them/z346 parent ion in the decomposition of6. The preparation of the ionized silanone may be a minor pathway. Some of the other fragmentation pathways for6–8are discussed.Keywords: gas phase thermolysis, ion rearrangements, silyl group migration, silanone, halosilane.

ISSN:0008-4042    

DOI:10.1139/v96-164

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Four 2-methoxy-5,5-dimethyl-Δ3-1,3,4-oxadiazolines bearing an alkylthio or arylthio group at C2 were prepared. The oxadiazolines undergo thermolysis at 60–80 °C in solution to afford the corresponding oxythiocarbene intermediates. In the absence of carbene traps, dimers of the carbenes were formed. The carbenes were trapped with ethyl crotonate, with dichloromaleic anhydride, with dimethyl acetylenedicarboxylate, and with phenyl isocyanate. Phenyl isocyanate traps methoxy(methylthio)carbene to form two types of adducts, both fundamentally different from the product obtained from reaction of dimethoxycarbene with phenyl isocyanate. All of the adducts have structures consistent with nucleophilic behaviour of the carbenes.Keywords: oxythiocarbene, oxadiazoline, thermolysis, nucleophilic.

ISSN:0008-4042    

DOI:10.1139/v96-165

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Four new thallium-containing salts with different alkyl ammonium cations have been prepared and characterized by X-ray crystallography:1: bis(p-toluidinium) pentachlorothallate(III), [CH3C6H4NH3]2[TlCl5],M = 597.9, monoclinic,P21/c,a = 7.515(2),b = 21.376(6),c = 12.769(2) Å, β = 92.13(2)°,V = 2050(1) Å3,Z = 4,Dcalc = 1.937 Mg m−3, µ = 8.512 mm−1,T = 293(1) K,R = 0.046;2: 1,5-pentanediammonium pentachlorothallate(III), [NH3(CH2)5NH3] [TlCl5],M = 485.83, orthorhombic,P212121,a = 13.226(2),b = 13.595(2),c = 7.628(2) Å,V = 1371.6(3) Å3,Z = 4,Dcalc = 2.352 Mg m−3, µ = 12.84 mm−1,T = 173(1) K,R = 0.025;3: pentakis(2-adamantaneammoniumn) bistetrachlorothallate(III) hexachlorothallate(III), [C10H18N]5[TlCl4]2[TlCl6],M = 1870.75, monoclinic, P21a = 16.466(5),b = 25.985(2),c = 7.724(3) Å, β = 90.14(3)°,V = 3305(2) Å3,Z = 2,Dcalc = 1.880 Mg m−3, µ = 7.978 mm−1,T = 173(1) K,R = 0.056;4: bis(piperazinium) decachlorodithallate(III) trihydrate, [NH2C4H8NH2]2[Tl2Cl10]•3H2O,M = 993.62, monoclinic,C2/c,a = 14.154(3),b = 15.445(3),c = 13.228(3) Å, β = 114.44(1)°,V = 2632.7(9) Å3,Z = 4,Dcalc = 2.506 Mg m−3, µ = 13.39 mm−1,T = 173(1) K,R = 0.026. For most of the salts examined, formation of [TlCl5]2−is possible; however, no isolated [TlCl5]2−anions were observed. In most cases, high coordination numbers at thallium were observed (CN = 6), and these can be achievedviaformation of anionic chains or through dimerization, giving [Tl2Cl10]4−. Of particular interest is the solid state structure of salt3in which there are three geometrically different chlorothallate ions. Extensive hydrogen-bonding networks are observed in each structure.Keywords: thallium, thallium(III) chlorides, chlorothallate, hydrogen bonding, alkyl ammonium cations.

ISSN:0008-4042    

DOI:10.1139/v96-166

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

A potential agent for the imaging of tumours has been synthesized from the antineoplastic agent chlorambucil. Standard peptide coupling techniques were used to synthesize a tripeptide covalently coupled to chlorambucil in 10 steps. The final product was characterized by high-resolution NMR spectroscopy.Keywords: ethylN-triphenylmethylthioethanoyl-O-{4′-[4″-(1"-bis(2″′-chloroethyl)amino)phenyl]butanoyl}-L-seryl-S-benzyl-L-cysteine, NMR spectroscopy, synthesis, radio-imaging agent.

ISSN:0008-4042    

DOI:10.1139/v96-167

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

A gas-to-solid phase methanolysis method for the analysis of oligosaccharides is presented. The advantages of using this new gas-to-solid phase method, as opposed to conventional bulk phase techniques, are described, along with comparisons of results obtained from both techniques. The reliable bulk phase methanolysis methods are used as benchmarks for assessing the extent of completion of the gas-to-solid reactions. Gas chromatographic – mass spectrometric (GC–MS) data show that, in general, higher temperatures and longer reaction times are required for completion of the gas-to-solid methanolysis process than for completion of the bulk phase reaction. On the other hand, the gas-to-solid procedure requires only minimal amounts of substrate that would be difficult to characterize using bulk phase methanolysis due to losses during clean-up procedures. Gas-to-solid methanolysis reactions of permethylated di- and trisaccharides were investigated (GC–FID and GC–MS), following initial experiments performed in order to characterize the GC retention times and mass spectra of permethylated standard monosaccharides. Conversion of neutral disaccharides, as well as neutral and acidic trisaccharides, to their respective monosaccharides was successful using the gas-to-solid method. The GC–FID and GC–MS traces show that the gas-to-solid method gives a cleaner reaction than the bulk phase method.Keywords: oligosaccharides, methanolysis, permethylation, gas chromatography – mass spectrometry, mass spectrometry.

ISSN:0008-4042    

DOI:10.1139/v96-168

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

An excellent linear correlation (r = 0.9999) exists between the spin–spin coupling constants1J(1H,13C), in benzene dissolved in four solvents (R. Laatikainen et al. J. Am. Chem. Soc.117, 11006 (1995)) and Ando's solvation dielectric function, ϵ/(ϵ – 1). The solvents are cyclohexane, carbon disulfide, pyridine, and acetone.1J(1H,13C)for gaseous benzene is predicted to be 156.99(2) Hz at 300 K.Keywords: spin–spin coupling constants,1J(1H,13C) for benzene in the vapor phase; spin–spin coupling constants, solvent dielectric constant dependence of1J(1H,13C) in benzene; benzene, estimate of1J(1H,13C) in the vapor; nuclear magnetic resonance, estimate of1J(1H,13C) in gaseous benzene.

ISSN:0008-4042    

DOI:10.1139/v96-169

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

We applied a discretized version of the generator coordinate Hartree–Fock (GCHF) method to generate Gaussian- and Slater-type functions for mono positive and mono negative ions of the atoms H through Xe. The basis sizes for Slater-type functions are (12s, 10p, 10d) for positive ions, and (13s, 11p, 10d) for negative ions. In the case of Gaussian-type functions the bases are (18s, 12p, 11d) for both positive and negative species. Ground and excited state Hartree–Fock energies are calculated with these bases and the results compared with the best atom-optimized calculations and numerical HF results available. A discussion on the role of weight functions in the evaluation of electronic energies emphasizes the integral character of the GCHF method.Keywords: Slater-type bases, Gaussian-type bases, generator coordinate Hartree–Fock, atomic ions.

ISSN:0008-4042    

DOI:10.1139/v96-170

出版商:NRC Research Press    

年代:1996

数据来源: NRC