摘要:

The completeness profile is used to provide a visual assessment of the quality of one-electron basis sets. The approach is illustrated by examples of a number of basis sets for the carbon atom. New insight is gained regarding the superiority of spin density calculations using Chipman's basis sets.Keywords: One-electron basis sets.

ISSN:0008-4042    

DOI:10.1139/v95-011

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Two new and rare unsaturated, extracellular fatty acids were identified in the culture filtrates of the biocontrol fungi,SporothrixflocculosaandS.rugulosa. 16-Methyl-9E-nonadecenoic acid (1) and (Z,Z)-10,14-eicosadienoic acid (2) were characterized on the basis of infrared (IR) and nuclear magnetic resonance spectra (13C and1N NMR), and gas chromatographic – mass spectrometric data (GC–MS). The structure of compounds1and2was confirmed by oxidative degradation to the known standards, dimethyl azelate, dimethyl succinate, dimethyl sebacate, and methyl caproate.Keywords: unsaturated fatty acids, fungi.

ISSN:0008-4042    

DOI:10.1139/v95-012

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The value for the equilibrium constant representing the ratio of equatorial to axial conformers for 2-methylthio- and 2-methoxycyclohexanone has been measured in five solvents using 500 MHz1H nuclear magnetic resonance. In cyclohexane solvent, the methylthio group is found to have a large axial preference while that for the methoxy is negligible. For both compounds the equilibrium shows a strong solvent dependence. Comparison of these results with those known for the halo derivatives shows the amount of axial to increase in the order: fluoro, methoxy, chloro, bromo, and methylthio. With the aid of molecular mechanics calculations it is concluded that the order of conformational preference can be attributed to a variation in the van der Waals interaction between the substituent and the carbonyl group in the equatorial conformer.Keywords: α-substituted cyclohexanones, conformation, MMX calculations, steric and dipolar effects.

ISSN:0008-4042    

DOI:10.1139/v95-013

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The title compound was synthesized as part of an effort to produce a more effective fungicide to combat Dutch Elm Disease (DED), which is caused by the fungusCeratocystisulmi. A full X-ray structural analysis of the 1:1 adduct has been carried out and the results are reported along with the Mössbauer data for the compound. The crystals are monoclinic, space groupP21/awitha = 19.240(3) Å,b = 9.1463(24) Å,c = 19.3512(24) Å, β = 118.874(8)°,V = 2982.0(10) Å3,z = 4, andDcalc = 1.427 Mg m−3. The final discrepancy factors areRF = 0.056 andRw = 0.058 for 1915 significant reflections. The QS and IS values in the Mössbauer spectrum of the complex are 3.08 mm s−1and 1.28 mm s−1, respectively. The 2,3-diphenylthiazolidin-4-one behaves as a monodentate ligand and coordinates to the tin through the oxygen atom. The complex exhibits a trigonal bipyramidal configuration with the three phenyl groups in equatorial positions and the chloride and ligand oxygen occupying the apical sites.Keywords: triorganotin, fungicide, Dutch Elm Disease, thiazolidin-4-one.

ISSN:0008-4042    

DOI:10.1139/v95-014

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The X-ray crystal structure of the title material indicates that the molecule possesses a pseudo-centre of inversion. A pair of O-C-C-O bonds have atransconformation in contrast to normalgauchestereochemistry for such units in crown ethers. For the C-O-C-C networks, which are normallytransoidin crown ethers, all four such units involving methylene carbons of the 20-membered ring exhibit unusual geometries. Two units possessgaucheconformations and the other two have torsion angles near 120°. Solution1H and13C NMR spectra have been recorded as a function of temperature and13C solid state spectra are included.Keywords: crown ether, solid state conformation.

ISSN:0008-4042    

DOI:10.1139/v95-015

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The free energies of activation at 110 K for rotation about the exocyclic C—C bonds in 2,6-difluorobenzaldehyde and 2,4,6-trifluorobenzaldehyde, in dimethyl ether solutions, are 18.8 ± 0.5 and 20.0 ± 0.5 kJ mol−1, respectively, as determined from19F{1H} dynamic nuclear magnetic resonance measurements. For the parent compound ΔG≠is 32.2 kJ mol−1in the same solvent. These free energy barriers, the lowest available for benzaldehyde derivatives, are likely a result of steric and electrostatic repulsions between the C+—O−and C+—F−bonds. Computations of the spectroscopic barrier in the 2,6-difluoro compound at various levels of molecular orbital theory imply that the barrier is predominantly twofold, with a fourfold component of opposite sign, whose magnitude is about 10% of the twofold component. A correlation-gradient computation, MP2/6-31G*, finds a barrier height of 18.6 kJ mol−1for this compound, lower by 3.0 kJ mol−1than found with the 6-31G* basis and 2.9 kJ mol−1with 6-31G**. Similar computations are compared for the parent compound and the 4-fluoro, 2,4,6-trifluoro, and 3,5-difluoro derivatives. Linear relationships exist between the computed spectroscopic barriers (ΔEvalues at absolute zero for the free molecules) and the free energy barriers for benzaldehyde and the four fluoro derivatives; the theoretical barriers utilize 6-31G** and correlation-gradient MP2/6-31G* procedures. For the 2,6-difluoro derivative, the computed frequencies of the torsional motions about the exocyclic C—C bond yield spectroscopic twofold barriers. These barriers are much lower than the computed energy differences between the planar and perpendicular conformers, perhaps because the negative fourfold components flatten the potential at its minimum. A rough estimate of the relationship between ΔG≠and ΔE0for the 2,6-difluorobenzaldehyde suggests that the solvent increases the internal barrier by only about 3 kJ mol−1. By way of contrast, the AM1 barriers, scaled by a factor of 1.9 (as previously recommended) range from 17.3 to 22.6 kJ mol−1, the ΔG≠values from 18.8(5) to 34.4 kJ mol−1, and the MP2/6-31G* (correlation-gradient) barriers span 18.6 to 36.8 kJ mol−1for benzaldehyde and the four fluorine derivatives. It seems likely that the internal barrier in benzaldehyde is considerably larger than that modeled on torsional frequencies.Keywords: Free energies of activation, internal rotational barriers in 2,6-difluoro- and 2,4,6-trifluorobenzaldehyde; molecular orbital computations, internal rotational barriers in 2,6-difluoro- and 2,4,6-trifluorobenzaldehyde; correlation gradient computations on internal barriers in benzaldehyde and four of its fluorine derivatives.

ISSN:0008-4042    

DOI:10.1139/v95-016

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The stepwise synthesis of two new selenium coronands containing an odd number of selenium atoms, starting from the fragments HO-(CH2)3-Cl, HO-(CH2)3-Br, HO-(CH2)3-SeCN, and NCSe-(CH2)3-SeCN, is described. The key diol containing one selenium atom, 4-selenaheptan-1,7-diol4, was obtained in 97% yield from the reaction of 3-bromo-1-propanol3and the selonolate anion derived from 3-selenocyanato-1-propanol2by sodium metal reduction in liquid ammonia. The ditosylate5, obtained from4, was then treated with the bisselenolate anion derived from 1,3-bisselenocyanatopropane6to give the target selenium coronand, 1,5,9-triselenacyclododecane [12Se3]7, in 96% yield. The ditosylate5was treated with two equivalents of the selenolate anion derived from2to give a second key acyclic diol8now containing three selenium atoms. The ditosylate9, obtained from8, was then treated with the bisselenolate anion derived from6to afford the second target coronand, 1,5,9,13,17-pentaselenacycloeicosane [20Se5]11in 44% yield.Keywords: selenium coronands, stepwise synthesis, macrocycles, heterocycles.

ISSN:0008-4042    

DOI:10.1139/v95-017

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The optical absorption spectrum of the solvated electron (es−) in liquid hexane-1,2,6-triol has been measured by nanosecond pulse radiolysis at different temperatures (10–40 °C) to investigate the influence of high solvent viscosity values on the spectral and kinetic properties of es−. The wavelength at the absorption maximum, λmax, is equal to 560 nm, and its variation with temperature, if it exists in the considered zone, is less than the experimental error. At 20 °C and 150 ns, the value of the productof the yield of es−and the molar extinction coefficient at λmaxis 2.55 × 104molecule/(M cm 100 eV). In the context of this work, we have compared results obtained with both a linear accelerator and a Febetron, a comparison that has allowed us to evaluate the influence of variations of the dose per pulse and to extend measurements to short times. In the case of experiments performed with the linear accelerator, es−is found to decay at all wavelengths by a first-order reaction (or by a pseudo-first-order reaction) with an activation energy of ~45 kJ mol−1. By contrast, kinetic curves obtained with the Febetron seem to show a competition in which a second-order law is followed at short times. The fact that the shape of the spectra seems to vary as a function of the dose per pulse indicates the possible intervention of another species whose formation is favored by the use of high radiation doses. In other respects, the kinetics of electron solvation does not seem to be controlled by the viscosity of the solvent in our experimental conditions.Keywords: liquid hexane-1,2,6-triol, pulse radiolysis, linear accelerator and Febetron, solvated electron, optical absorption spectrum, kinetic properties, solvent viscosity, dose and temperature effects.

ISSN:0008-4042    

DOI:10.1139/v95-018

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The photoinduced addition of O2onto the unsaturated cluster Pd3(dppm)3CO2+(as a CF3CO2−salt) in acetonitrile is reported. The final product Pd3(dppm)3(O2)22+(v(O2) = 838 and 866 cm−1) is formed in a multi-step fashion in which two photochemical intermediates are observed (presumably Pd3(dppm)3(O2)(CO)2+and Pd3(dppm)3(O2)2+. No X-ray structure could be obtained, but numerous spectroscopic findings demonstrate that O2binds the Pd3center as a peroxo-O2, and acts as a two-electron donor that triply bridges the metal atoms (forming a 44-electron cluster). The very small excited state lifetimes (between 25 and 35 ± 10 ps) obtained by picosecond flash photolysis indicate that the primary photoreaction is unimolecular, and demonstrate that the first photochemically added O2molecule must be preassembled in the excited statepriorto any photoinduced transformation. Thisground state complex is responsible for the photoinduced production of the bisdioxygen compound and can be observed by UV–visible spectroscopy. The low efficiency of the photoreaction (quantum yield (Φ) = 0.033 ± 0.004) is explained by the very short excited state lifetime at 298 K, and the competition of O2with solvent molecules to occupy the unsaturated site of the empty cavity in Pd3(dppm)3CO2+(i.e., ground state guest–host chemistry). The binding constant for O2with Pd3(dppm)3CO2+is roughly estimated to range between 1 and 730 M−1in the ground state and is considered to be weak.Keywords: clusters, photochemistry, guest–host, oxidation, dioxygen.

ISSN:0008-4042    

DOI:10.1139/v95-019

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Thiophosgene, Cl2CS, has been excited in the near-uv and the resulting quantum yields offluorescence and of Cl2CS consumption have been measured as a function of solvent composition (perfluoro-n-hexane,n-hexane, CCl4), temperature, and excitation wavelength. In agreement with previous work it is shown that (i) nonradiative processes dominate the decay of thestate in solution, and (ii) perfluoroalkane solvents act as inert "heat baths". The process by which thestate is apparently "quenched" by CCl4andn-hexane has been discovered. The nonradiative process leading todecay involves activation and crossing a barrier, the height of which is a function of the nature and composition of the solvent. CCl4andn-hexane do not quench the excited state, but instead accelerate its rate of relaxation by lowering the barrier between the bound, radiative portion of the surface and a dark, unbound region. There is evidence that CCl4orn-hexane form clusters around a Cl2CS solute molecule in mixed perfluoroalkane–CCl4orn-hexane solutions. Barrier crossing leads to photodecomposition via at least two parallel paths, one free radical (Cl + ClCS) and the other likely molecular (Cl2 + CS), the relative contributions of which are a function of excitation wavelength.Keywords: thiophosgene, photochemistry, solvent-mediated barrier crossing.

ISSN:0008-4042    

DOI:10.1139/v95-020

出版商:NRC Research Press    

年代:1995

数据来源: NRC