摘要:

The notion of division between active and dormant electrons has been well received and widely used in the chemists' way of thinking. The core–valence separation in atoms is the best-known example. This paper describes a theoretical and computational method called the model potential method, which deals only with active electrons in molecular and solid state calculations. The method is capable of reaching computational accuracy of testing the validity of the separation of active and dormant electrons in individual cases.Keywords: separability of electrons, model potential method, valence orbitals, relativistic effects.

ISSN:0008-4042    

DOI:10.1139/v95-080

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Two kraft lignins, one precipitated with carbon dioxide and the other with sulphuric acid, have been fractionated into three distinct fractions by sequential extraction using organic solvents of increasing hydrogen-bonding capacity. The lignins and fractions were comparatively characterized in terms of yield, methoxyl group content, molecular weight distribution, and13C NMR spectroscopy. For a given lignin, the fractions showed differences in yield, composition, and chemical structure. A significant portion (21%) of the carbon dioxide precipitated kraft lignin is composed of ethyl acetate soluble material of low molecular weight. This fraction, which is richer in guaiacyl moieties than the other fractions, is virtually absent in the sulphuric acid precipitated kraft lignin. The occurrence of "tails" in the high molecular weight fraction, especially from the carbon dioxide precipitated kraft lignin, suggests the presence of material of very high molecular weight. Data from13C NMR spectroscopy does little to elucidate the nature or origin of the material causing these tails.Keywords: kraft lignins, heterogeneity, fractionation, characterization.

ISSN:0008-4042    

DOI:10.1139/v95-081

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Treatment of [M(cod)Cl]2(M = Rh, Ir) with iminophosphoranophosphine ligands of the type Ph2PQP(Ph2)=NSiMe3(Q = CH2, CH2CH2, 1,2-C6H4) afforded the new metallacyclesthrough elimination of Me3SiCl, ultimately "anchoring" the ligand to the metal through a metal–nitrogen σ bond. The phosphine functionality completes the chelate. Cationic complexes of the type [M(cod)L2]+PF6−(M = Rh or Ir; L2 = Ph2PQPPh2NR where Q = CH2, CH(CH3), 1,2-C6H4, and R = p-C6F4CN, C6H2F(NO2)2, and C6H3(NO2)2or L = Ph2PN=C(H)Ph) were readily prepared by treating [M(cod)Cl]2with L2and KPF6. The detection and chemical shift determination of the insensitive103Rh nucleus was easily accomplished via31P detected indirect two-dimensional31P,103Rh{1H} NMR spectroscopy. The103Rh,1H, and31P NMR data are discussed.Keywords: rhodium, iridium, phosphoranimine, iminophosphine, Rh NMR.

ISSN:0008-4042    

DOI:10.1139/v95-082

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Thirteen new cembranolide diterpenoids, uprolides3–15, have been isolated from the Caribbean gorgonianEuniceamammosacollected off the West coast of Puerto Rico. Several known metabolites, such as eupalmerin acetate (1) and eupalmerin (2), were also isolated from the same organism. The structures of the new compounds, which also showed modest in vitro cytotoxic activity, were assigned on the basis of extensive nuclear magnetic resonance analysis, chemical correlation studies, and by comparison with analogous spectral data from known cembranolide diterpenoids. One structure (3) was confirmed by X-ray diffraction analysis.Keywords: uprolides,Euniceamammosa, cytotoxicity, Caribbean, gorgonian.

ISSN:0008-4042    

DOI:10.1139/v95-083

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The structures and stereochemistry of 14 taxanes isolated fromTaxuscanadensishave been rigorously established by high-resolution NMR techniques and mass spectrometry.Taxuscanadensisis the only yew that accumulates 9-dihydro-13-acetylbaccatin III (also called 7,9-deacetylbaccatin VI) as the most abundant taxane. Five 9-dihydrotaxanes derived from the Canadian yew are novel natural products not reported in the needles of other species of yew (8–11,14, Fig. 1).Keywords:Taxuscanadensis, Taxaceae, taxanes, 9-dihydro-13-acetylbaccatin III, taxol.

ISSN:0008-4042    

DOI:10.1139/v95-084

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The reactions of O(3P) with acetonitrile and propionitrile have been studied kinetically in the gas phase as a function of temperature and nitrile concentration. The rate constants obtained experimentally for the consumption of O(3P) in these reactions, in the units L mol−1s−1, obey the following relations: O + acetonitrile: Ink = 20.98 ± 0.23 − 2100 ± 200/T; and O + propionitrile: Ink = 21.01 ± 0.19 − 1900 ± 200/T. Mechanisms are discussed in which the reaction is initiated by hydrogen atom abstraction and in which initiation is by addition of O(3P) to the nitrile, followed by decomposition of the addition complex. The kinetic and analytical results are best explained by the mechanism in which the reactions are initiated by RCN + O → R + OCN. In the case of the reaction with acetonitrile, the absolute rate constant is smaller than the experimental value by a factor of 7 ± 20%, independent of temperature, as a result of secondary reactions.Keywords: kinetics, reactions of O(3P) with nitriles.

ISSN:0008-4042    

DOI:10.1139/v95-085

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The first known lithiated 1-azafulvene derivatives, e.g.,8and13a, were generated by a low-temperature bromine–lithium exchange procedure withtert-butyllithium. These lithio species show substantial stability at ≤ −90 °C because, at these temperatures, the sterically demanding 6-diisopropylamino moiety, unlike the dimethylamino group, completely inhibits nucleophilic addition to C-6. At higher temperatures, the addition oftert-butyllithium to C-6 is significant and it can become the dominant process. The lithio species8is a useful formal equivalent of 5-lithiopyrrole-2-carboxaldehyde since, on reaction with electrophilic reagents and subsequent hydrolysis, a wide variety of regiochemically pure 5-substituted pyrrole-2-carboxaldehydes is formed. The 6-dialkylamino-1-azafulvenes described herein exist predominantly or exclusively as thesynconformer in solution at room temperature. This conformational preference is confirmed by a significance NOE effect between H-4 and H-6 in the parent diisopropylamino compound3f. The origin of thesynconformational preference stems from a substantial contribution of the charge-separated form16to the ground state structure of these compounds, a phenomenon that is strongly supported by variable temperature NMR measurements on 2-bromo-6-diisopropylamino-1-azafulvene (3c).Keywords: 2-bromo-6-diisopropylamino-1-azafulvenes, stereochemistry, lithiation, pyrrole-2-carboxaldehydes.

ISSN:0008-4042    

DOI:10.1139/v95-086

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Syntheses of tetraazamacrocycles have been carried out by usingp-toluenesulfonyl chloride as protective group. The [14(ane) N4] and [15(ane) N4] were also obtained by the template synthesis. Mono-N-functionalization of tetraazamacrocycles was accomplished by reaction of a fivefold excess of the free macrocycles with 1 equivalent of a suitable alkylating or arylating reagent. The key point of the synthesis lies in the use of an excess of the macrocycle over the substituting reactants to reduce the formation of polysubstituted derivatives, and in the easy separation of the excess of unreacted macrocycle. All the products were characterized on the basis of spectral studies (1H andl3C NMR, including 2D NMR and NOE difference studies) and mass spectrometry.Keywords: tetraazamacrocycles, improved synthesis, protective group, N-tosylation, template synthesis, NOE difference spectroscopy.

ISSN:0008-4042    

DOI:10.1139/v95-087

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The model potential, in which the Coulomb and exchange core operators are converted to the matrix elements by use of the spectral representation technique, should be equivalent to the frozen-core operator, if the basis set of the spectral representation is complete. Its fidelity to the frozen-core operator is examined in this paper through a thorough study of basis set convergence in both the spectral space and the atomic orbital space, along with comparison with the Sakai–Huzinaga and the Huzinaga–Seijo–Barandiaran–Klobukowski model potentials. The RHF molecular calculations on N2, P2, As2, Sb2, PN, and AsN show that the present model potential can approximate, with reliable accuracy, the results obtained from the frozen-core calculations.Keywords: model potential, spectral representation, molecular orbital calculation, electronic structure.

ISSN:0008-4042    

DOI:10.1139/v95-088

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

We studied the relatively complex polar systems 6-substituted-1,4-dioxospiro[4.5]decanes and 7-substituted-1,5-dioxospiro[5.5]undecanes with substituents X = CH3, F, Cl, CN, OH, OCH3, and NO2. Solvent effects on the equilibrium have been analysed by means of a Self-Consistent-Reaction-Field model and the PM3 method. Complete geometry optimizations have been carried out for all the structures in the gas phase and in solution. For some substituents, a set of rotamers have been separately optimized. The discussion of the results is focussed on the effects arising from structural aspects and from steric and electrostatic interactions on the axial/equatorial relative stability. The role played by multipole moment is considered. In general, good agreement with available experimental data and with previous theoretical studies has been obtained. Though the use of semiempirical methods and simple solvent models prevents us from reaching definitive conclusions, this approach seems to be very useful in predicting the main role of solute–solvent interactions in conformational equilibria of complex systems for which ab initio calculations cannot be performed.Keywords: conformational equilibria, spiro decanes and undecanes, cavity model, SCRF, solvent effect, PM3 calculations.

ISSN:0008-4042    

DOI:10.1139/v95-089

出版商:NRC Research Press    

年代:1995

数据来源: NRC