摘要:

An evaluation is given of approximate density functional theory as a practical tool in studies on organometallic energetics and kinetics. The evaluation covers electronic excitations and ionizations, electron capture, conformational changes, molecular vibrations, and bond energies, as well as reaction profiles.Keywords: density functional theory, energetics, kinetics, organometallic, catalysis.

ISSN:0008-4042    

DOI:10.1139/v95-095

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction of methanol, 2,6-dimethyl-1,6-heptadiene, and 1,4-dicyanobenzene yields three distinct types of 1:1:1 adducts: an acyclic product, 4-(1-methoxymethyl-1,5-dimethyl-5-hexenyl)benzonitrile (8, 5%); acis–transpair of cyclohexanes, 4-(3-methoxymethyl-1,3-dimethylcyclohexyl)benzonitrile (9cis(12%) and9trans(11%)); and acis–transpair of cycloheptanes, 4-(4-methoxy-1,4-dimethylcycloheptyl)benzonitrile (10cis(12%) and10trans(10%)). Variation in the concentration of the nucleophile, methanol, and codonor, biphenyl, affects the product ratio and it has been possible to establish the mechanisms for the formation of these products. The acyclic product is formed by a typical photo-NOCAS reaction, that is, addition (anti-Markovnikov) across one of the heptadiene double bonds. The cyclohexane products are formed following 1,6-endocyclization of the intermediate β-alkoxy radical. And the cycloheptane products result from 1,7-endo,endocyclization of the initially formed 2,6-dimethyl-1,6-heptadiene radical cation. Comparison of the relative rates of these cyclization processes can be made with those of the next smaller homolog, 2,5-dimethyl-1,5-hexadiene.Keywords: photochemistry, photoinduced electron transfer, radical ions, radicals, cyclization.

ISSN:0008-4042    

DOI:10.1139/v95-096

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Transition-metal-bound imidazoles are suitable models for evaluating the roles of metal ions in biomolecules having the imidazole moiety and similar heterocyclic residues as part of their structure. Such studies provide useful insights into metal–biomolecule interactions in biological systems, especially when the lability of the metal–ligand bond is substantially reduced, such that the identity of the metal–ligand complex is preserved during the course of the reaction under investigation. The present paper reports on a kinetic study of tritium exchange from the C(2) position of the imidazole moiety in the substitution-inert complex cations [Co(NH3)5[2-3H]-imidazole]3+(1) and [Co(NH3)5-1-methyl-[2-3H]-imidazole]3+(2). Rate–pH profiles have been determined in aqueous solution at 60 °C. Both substrates are believed to react through rate-determining attack of hydroxide ion (kM+pathway) at C(2)-T. Dissection of the kinetic data reveals an additional pathway for1consequent upon deprotonation of its pyrrole-like N-H(T) to yield3, which is then attacked by hydroxide at C(2) (kMpathway). The ratiokM+/kM = 103that is obtained is in accord with the expected reduced reactivity of3. Comparison of the present data with those reported for a variety of heterocyclic substrates shows that the order of reactivity, protonatedmetal ion coordinatedneutral form of substrates, prevails. The superiority of the proton over metal ions in catalyzing isotopic hydrogen exchange is attributed to its larger ground state acidifying effect coupled with the greater transition state stabilization it affords, relative to metal ions. The exchange reaction of3via thekMpathway is the first example of a reactive anionic species in which the negative charge is located α to the exchanging C-H.Keywords: tritium exchange, cobalt (III)-coordinated imidazoles.

ISSN:0008-4042    

DOI:10.1139/v95-097

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The Onsager–Samaras version of Wagner's theory of surface tension of solutions of 1:1 salts has been modified to include ion-induced dipole terms in the image potential. Effective solvated ionic polarizabilities are used, which are large and negative owing to local dielectric saturation around the ions. The modified theory agrees with the order of magnitude of the variation among various alkali halides (e.g., Δσ for 0.1 molar aqueous KI is about 70% of the corresponding value for KCl). Polarizabilities were estimated from Hasted's dielectric decrements.Keywords: surface tension, salt solutions, symmetric electrolytes, effective polarizability, theory.

ISSN:0008-4042    

DOI:10.1139/v95-098

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Ice freezing temperatures and hydrate formation temperatures have been measured by the dynamic cooling method for aqueous ethylene oxide (EO) solutions containing from 0 to 95 mol% EO. The ice and the congruent hydrate freezing temperatures exhibited standard errors on a single measurement of 0.004 °C and 0.013 °C, respectively. The ice–hydrate eutectic temperature was observed at −2.107 °C with standard error 0.001 °C and composition 1.991 mol% EO with standard error 0.008 mol% EO. The congruent hydrate was found to freeze at 11.083 °C with standard error 0.002 °C and composition 12.64 mol% EO with standard error 0.02 mol% EO. The formula of the congruent hydrate was EO•6.91H2O with standard error 0.013 mol water/mol EO. Only a single hydrate was found over the whole composition range down to −26 °C: the shoulder of the hydrate freezing curve above 40 mol% EO resulted from the high activity coefficients to dilute water in concentrated EO solutions. Equations and best values for the ice freezing temperatures and the hydrate formation temperatures together with their standard errors were evaluated by the method of least squares.Keywords: clathrate hydrate of ethylene oxide, freezing of water – ethylene oxide, ethylene oxide hydrate.

ISSN:0008-4042    

DOI:10.1139/v95-099

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The bulky amino-siloxide hybrid ligand−OSi(tBu)2((CH2)3NMe2) allows isolation of dinuclear, hydrocarbon-soluble barium complexes. The structures of two mono(ligand) complexes, {Ba[OSi(tBu)2((CH2)3NMe2)][N(SiMe3)2]}2(1) and {Ba[OSi(tBu)2((CH2)3NMe2)][N(SiMe3)2][THF]}2(3), have been determined by X-ray diffraction (crystal data:1, monoclinic,C2/c,a = 20.451(4) Å,b = 14.277(3) Å,c = 22.258(5) Å, β = 101.14(2)°,V = 6376.1(2) Å3,Z = 4 (dimers),R = 0.069,Rw = 0.100;3, monoclinic,P21/c,a = 11.025(4) Å,b = 22.414(3) Å,c = 13.925(2) Å, β = 104.05(2)°,V = 3338.0(1) Å3,Z = 2 (dimers),R = 0.090,Rw = 0.117). Complex1is stable towards ligand redistribution ind6-benzene ord8-toluene, but ind8-THF both1and3exist in equilibrium with the ligand redistribution products Ba[N(SiMe3)2]2[THF]2and Ba[OSi(tBu)2((CH2)3NMe2)]2(2). Complex2was prepared independently and appears to exist as a monomer–dimer equilibrium ind6-benzene solution. Complexes1–3do not sublime when heated under high vacuum (200 °C, 10−4Torr).Keywords: siloxide, barium, synthesis, structure, bis(trimethylsilyl)amide.

ISSN:0008-4042    

DOI:10.1139/v95-100

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The electrocatalytic hydrogenation (ECH) of limonene (1) and carvone (4) at a Raney nickel cathode was studied both in hydroorganic medium (methanol–water) and aqueous micellar and emulsified solutions. In methanol–water, the ECH of1stopped atp-menthene (2) and proceeded with low current efficiencies (18–24%). In micellar solutions containing a cationic surfactant (CTAB), the ECH of1was more efficient, givingp-menthane (3) as the main product with current efficiencies up to 30%. The pH of the solution (hydroorganic and micellar) had little influence. With anionic (SDS) and nonionic (Brij 35) surfactants, somep-menthane (3) was formed but the conversion rate and current efficiency were much lower. The ECH of carvone (4) was slightly less efficient in methanol–water than in micellar and emulsified solutions. The saturated alcohols neocarvomenthol (8), carvomenthol (9), isocarvomenthol (10), and neoisocarvomenthol (11) were the main products with current efficiencies of 75% (hydroorganic medium) to 90% (micellar solutions). The effect of surfactant concentration and pH on the efficiency and stereochemistry of the ECH of carvone (4) was also studied.Keywords: electrocatalytic hydrogenation, limonene, carvone, Raney nickel, micellar medium.

ISSN:0008-4042    

DOI:10.1139/v95-101

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The1H,19F, and13C {H} nuclear magnetic resonance spectra at 300 K of 4-fluoro- and 3,5-difluorobenzyl fluoride, dissolved in CS2–C6D12and acetone-d6, are fully analyzed. Spin–spin coupling constants over four, five, and six formal bonds are used to derive expectation values of sin2θ andand the apparent twofold internal rotational potentials;is the angle by which the α C—F(C—H) bond twists out of the ring plane. The conformation of lowest energy hasfor the 3,5-difluoro compound in the polar and nonpolar solutions, whereas it hasfor the 4-fluoro derivative. The magnitudes of the potentials lie between 2 and 4 kJ/mol, that is, comparable to thermal energies. These data are compared with previous results for the parent compound and its 3,5-dichloro derivative. Geometry-optimized molecular orbital computations, including some correlation-gradient procedures, for benzyl fluoride and the two fluoro derivatives havefor the conformations of highest energy of the free molecules. However, geometry-optimized SCFRF calculations of the solvent perturbations of the potential (dipole terms are insufficient) are in semiquantitative agreement with experiment in the sense that both solvents are predicted to destabilize the conformation withFor example, the predominant twofold component in the computed (6-31G*) potential is 3.4 (free), −0.7 (CS2), and −3.3 (acetone-d6) kJ/mol for benzyl fluoride, a negative number indicatingfor the stable conformer; the experimental values are −0.8(2) (CS2) and −2.7(2) (acetone-d6) kJ/mol. The agreement between experiment and theory is of a similar quality for the fluoro derivatives. The stabilization of the conformer withfor the 4-fluoro derivative is tentatively attributed to hyperconjugative electron acceptance by the α C—F bond, enhanced by the π-electron donor at theparaposition. A number of coupling constants are discussed in terms of the possible mechanisms of their transmission. Future experiments are indicated.Keywords:1H,19F,13C NMR of 4-fluorobenzyl fluoride and 3,5-difluorobenzyl fluoride; MO calculations and internal rotational potentials in benzyl fluoride, 3,5-difluorobenzyl fluoride, and 4-fluorobenzyl fluoride; solvent effects and experimental and theoretical approaches to internal rotational potentials in 3,5-difluorobenzyl fluoride and 4-fluorobenzyl fluoride.

ISSN:0008-4042    

DOI:10.1139/v95-102

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

A series of nitropolychlorodibenzo-p-dioxins (NPCDDs) was synthesized by condensation between catechols and 2,6-dinitrohalobenzene derivatives. In the presence of sodium ethoxide in anhydrous ethanol, these underwent photochemical SN2Ar* substitutionsmetato the nitro group in high chemical yield and moderate quantum yield. Both ring-opening and chloride replacement reactions were observed. The reactions involved the triplet excited state of the NPCDD, and showed a linear relationship between Φ−1and [nucleophile]−1. Analogous reactions with KCN in methanol showed similar behaviour, but the products could not be isolated.Keywords: photosubstitution, SN2Ar*, dibenzo-p-dioxins.

ISSN:0008-4042    

DOI:10.1139/v95-103

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The diphenylboron chelates of ketoxime adducts to formaldehyde and trichloroacetaldehyde (chloral) have been synthesized. The reactions were carried out as three-component one-pot reactions, and also as two-step reactions for the ketoxime chloral adducts, which were isolated and characterized before reaction with oxybis(diphenylborane) to form the diphenylboron chelates. One of the intermediate adducts and two of the diphenylboron chelates have been characterized by X-ray crystallographic analysis. Crystals ofO-(2,2,2-trichloro-1-hydroxyethyl)-2-propanonoxime,8d, are monoclinic,a = 7.010(2),b = 11.832(3),c = 11.684(2) Å, β = 90.48(2)°,Z = 4, space groupP21/c; those of 4-isopropylidene-2,2-diphenyl-1,3-dioxa-4-azonia-2-boratacyclopentane,9a, are orthorhombic,a = 11.098(2),b = 15.364(1),c = 8.715(2) Å,Z = 4, space groupP212121; and those of 5-trichloromethyl-2-isopropylidene-3,3-diphenyl-1,4-dioxa-2-azonia-3-boratacyclopentane,10d, are monoclinic,a = 9.164(2),b = 15.304(3),c = 13.859(2) Å, β = 105.28(1)°,Z = 4, space groupP21/n. The structures were solved by direct methods and refined by full-matrix least-squares procedures toR = 0.037, 0.044, and 0.040 (Rw = 0.041, 0.016, and 0.042) for 1082, 1256, and 3062 reflections withI ≥ 3σ(F2), respectively. The X-ray analyses show that the products from the reaction between acetonoxime, oxybis(diphenylborane), and formaldehyde or trichloroacetaldehyde, respectively, possess different boron chelate structures. With formaldehyde a COBON chelate ring is obtained and with trichloroacetaldehyde a BOCON chelate ring is formed.Keywords: boron chelates, boron compounds, organboron compounds, crystal structures.

ISSN:0008-4042    

DOI:10.1139/v95-104

出版商:NRC Research Press    

年代:1995

数据来源: NRC