摘要:

The1H,19Fand13C{1H} nuclear magnetic resonance spectra of 1,1,1-trifluoro-2-phenylethane,1, in CS2–C6D12, acetone-d6, and benzene-d6solutions, on analysis, yield long-range coupling constants from which are derived the apparent twofold barriers to rotation about the Csp2—Csp3bonds. The twofold barrier is 9.0(2) kJ/mol, independent of solvent, 4.0 kJ/mol larger than that of ethylbenzene, also independent of solvent. The theoretical barrier heights for the free molecules at the post-Hartree–Fock level of molecular orbital theory (frozen-core MP2/6-31G*) also differ by 4 kJ/mol, but are about 1 kJ/mol higher than the experimental estimates. The perpendicular conformer is the most stable for both molecules. Comparisons are made with the benzyl halides, in which the internal barriers are remarkably sensitive to solvent. A spin–spin coupling constant over five formal bonds,5J(H, F), involving theorthoprotons in1, is +0.74(2) Hz and is discussed in some detail in terms of its dependence on intenuclear distances (possible through-space interactions). The solvent perturbations of3J(H, F) and of2J(C, F) are of opposite sign. Other long-range coupling constants or their absence are also pointed out. For example, those between19F and13C nuclei or protons at themetaposition are effectively zero; at theparaposition they are significant.Keywords: 1,1,1-trifluoro-2-phenylethane;1H,19F, and13C NMR; long-range spin-spin coupling constants; through-space1H,19F spin–spin coupling constants; internal rotational potential; molecular orbital computations of internal potential.

ISSN:0008-4042    

DOI:10.1139/v95-172

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

We have determined the heat of formation of 1,1-dimethoxyethene, ΔHf(l) = −75.56 ± 0.87 kcal mol−1, by measuring the heat of hydrolysis. The heat of vaporization, 8.47 ± 0.10 kcal mol−1, was estimated from the boiling points at various pressures. The standard entropy of gaseous 1,1-dimethoxyethene, 82.02 ± 2. cal deg−1mol−1, was calculated using frequencies obtained by MO calculations at the 6-31G** level. The free energy of transfer, 1.02 ± 1, was estimated by additivity with allowance for a distant polar interaction. Thus the free energy of formation in aqueous solution, ΔGf(aq) = −37.07 ± 1.48 kcal mol−1, could be calculated. The free energies of hydrolysis for enol ethers appear to be insensitive to structure; we used an average value and so calculated the values of the free energies of formation of the enols of acetic acid and methyl acetate, −67.90 ± 1.67 and −52.48 ± 1.53 kcal mol−1, respectively. These values are in good agreement with other recent evaluations. This method should be applicable to related enols.Keywords: enols, thermodynamics, heat of formation, entropy, ketene acetal.

ISSN:0008-4042    

DOI:10.1139/v95-173

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

1-Methoxy-2-naphthol was converted by MOM ether formation, lithiation at C-3, reaction with 3,3-dimethylacryloyl chloride, cyclization, and oxidation using cerium ammonium nitrate to 4-keto-α-lapachone (2,2-dimethyl-4-keto-1-oxa-1,2,3,4-tetrahydroanthra-5,10-quinone). Reduction by biotransformation usingMortierellaisabellinaATCC 42613 gave (S)-4-hydroxy-α-lapachone in high (> 95%) optical purity. Reduction of several 2,2-dimethyl-4-chromanones to the corresponding (S)-alcohols byM.isabellinais also reported.Keywords: biotransformation, chemoenzymatic, 4-hydroxy-α-lapachone,Mortierellaisabellina.

ISSN:0008-4042    

DOI:10.1139/v95-174

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Using1H NMR spectroscopy the complex-formation equilibria between halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) and methyltert-butyl ether or tetrahydrofuran in various inert solvents (hexane, heptane, decane, cyclohexane) were measured as a function of temperature. For two different association models (ideal solution and athermal solution), assuming only the formation of a 1:1 H-bonded complex, the equilibrium constants and the standard enthalpies of the complex-formation reaction were calculated. The ideal solution model provides values of the equilibrium constant that differ for different inert solvents. The athermal solution model makes this false solvent effect much smaller. For the low halothane concentration used, its dimerization was neglected. This assumption was verified experimentally.Keywords:1H NMR, association, complex formation, halothane.

ISSN:0008-4042    

DOI:10.1139/v95-175

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

In the last few years, intensive research by several groups has shown that neural networks can be used to analyse spectral data for structural elucidation, and that their performance approaches that of an expert in the field. The construction of such networks, their training and evaluation, requires large structural and spectral databases and significant computational resources and time. However, once the network has been completed it can be used very effectively for practical applications on an ordinary desktop computer. In this article we describe the methodology for creating such a network for infrared and mass spectra, and present a program for use on a personal computer, either connected to a spectrometer or independently. The program accepts data in ASCII format, both for the network description and for the spectral information. This approach permits the use of neural networks in an analytical laboratory with limited computational resources.Keywords: neural networks, infrared spectroscopy, mass spectroscopy, structure determination.

ISSN:0008-4042    

DOI:10.1139/v95-176

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

PolyN-(2-cyanoethyl)pyrrole films have been synthesized by electrochemical polymerization and characterized by cyclic voltammetry, scanning electron microscopy, and X-ray photoelectron spectroscopy. Polymeric coatings prepared on the surface of a thickness-shear-mode acoustic wave sensor have been used to examine response selectivity to a number of gas-phase probe molecules. The responses of the polyN-(2-cyanoethyl)pyrrole based sensor are compared with the parent polypyrrole device and rationalized in terms of the molecular interactions between probes and polymer films. The polar cyano functionality enhances interactions with analytes such as acetonitrile.Keywords: gas sensor, thickness-shear-mode acoustic wave sensor, polyN-(2-cyanoethyl)pyrrole film, polypyrrole film, conducting polymer.

ISSN:0008-4042    

DOI:10.1139/v95-177

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The synthesis and crystal structure of the 1:1 complex formed between triphenyltin chloride and isoquinoline-1-carboxylic acid ([Ph3SnCl:C10H7NO2]) is described. The tin(IV) atom is found to be five coordinate, being bound to three phenyl groups, the chlorine atom, and an oxygen from isoquinoline-1-carboxylic acid. The geometry around the tin atom is that of a trigonal bipyramid, with the three phenyl groups occupying the equatorial positions, and the apical sites taken by the chlorine atom and a carboxyl oxygen from the isoquinoline-1-carboxylic acid. The crystals are triclinic, space groupwitha = 17.932(12) Å,b = 17.935(15) Å,c = 18.686(12) Å, α = 63.16(5)°, β = 81.70(6)°, γ = 68.45(6)°,V = 4986(6)(10) Å3Z = 8, andDcalc = 1.486 Mg m−3The final residual isRF = 0.050, on 8375 reflections withInet > 2.5σ(Inet). Two of the cell dimensions are equivalent and there are four molecular units in the unit cell, so precautions are described for ensuring that correct space-group determination had been made and that some concealed symmetry in the final structure had not been overlooked.Keywords: triorganotin, complex, isoquinoline-1-carboxylic acid, fungicidal activity.

ISSN:0008-4042    

DOI:10.1139/v95-178

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The title compounds have been studied in CH2Cl2by cyclic voltammetry. Nonacarbonyldi(μ3-methylidyne)triiron, [Fe3(CO)9(μ3-CX)2] (X = F, Cl, Br, H), undergo chemically reversible 1e−reduction (−0.72 to −0.88 V vs. SCE) and irreversible oxidation (+1.59 to +1.72 V); the compound for which X = F displays a second, irreversible reduction (−1.96 V). Using reduction potentials of comparable μ3-E (E = S, Se, NPh, PPh) clusters, μ3-methylidyne ligands are shown to be strongly basic, comparable to PPh and NPh and more basic than S or Se. The methylidyne clusters are both more difficult to oxidize and reduce than isomeric alkyne clusters [Fe3(CO)9(C2R2)], indicative of greater thermodynamic stability. The complexes previously formulated as [Fe2(CO)6(S2N2CC6H4X)] (X = H, CF3, OCH3) are reversibly reduced (−1.59 to −1.70 V) and irreversibly oxidized (+1.0 to 1.3 V), unlike free dithiadiazole radicals, which are reversibly oxidized around +0.8 V. This behaviour is not consistent with the ring-centered free radical claimed for the case where X = H; instead we find conclusive evidence that these are diamagnetic complexes of the 3-H-1,2,3,5-dithiadiazolines, [Fe2(CO)6(S2N{NH}CC6H4X)] (νNH = 3376−3381 cm−1;δNH = 6.55−7.1 ppm). Dicyclopentadienyl-dithiadiazoledinickel, [Ni2Cp2(S2N2CC6H5)], is both reversibly reduced (−0.79 V) and oxidized (+0.45 V); and irreversibly oxidized at +1.9 V. This is consistent with an unpaired electron, and only the nickel cluster has an ESR signal (g = 2.0410, no resolved hyperfine splitting). Extended Hückel theory locates the free electron in a SOMO restricted to Ni, C, and S atoms.Keywords: electrochemistry, organometallic, methylidyne, dithiadiazole, dithiadiazoline.

ISSN:0008-4042    

DOI:10.1139/v95-179

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

A general method has been developed for the preparation of acetylenedicarboxylic acid esters and applied to the synthesis of both achiral and chiral esters. Diels–Alder reactions of one of the chiral esters, the bis(methyl (S)-lactyl) ester, with various types of dienes, such as phenylbutadiene, orthoquinodimethanes, and isobenzofuran, have been investigated. Moderate asymmetric induction was observed in reactions with dienes bearing an α-hydroxyl group. In one case, an unusual solvent-induced reversal of asymmetric induction was observed.Keywords: acetylenedicarboxylic acid esters, phenylbutadiene, orthoquinodimethanes, isobenzofurans, Diels–Alder, asymmetric.

ISSN:0008-4042    

DOI:10.1139/v95-180

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Three unsymmetrical chiral dienophiles have been synthesized and their reactivities with α-hydroxy-ortho-quinodimethanes have been assessed. Thus, the methyl (S)-lactyl esters oftrans-4-bromocrotonate andtrans-4-acetoxycrotonate (1and2) and mono (methyl (S)-lactyl) fumarate3were synthesized. Each was reacted with α-hydroxy-o-quinodimethane and α-hydroxy-α-phenyl-o-quinodimethane. While reaction with α-hydroxy-o-quinodimethane gave synthetically useful yields of cycloadducts with high regioselectivity and diastereoselectivity, the corresponding reactions with α-hydroxy-α-phenyl-o-quinodimethane were less successful. The reaction of3with α-hydroxy-α′-phenyl-o-quinodimethane also gave a mixture of cycloadducts.Keywords:ortho-quinodimethanes, lignans, Diels–Alder, asymmetric, lactate.

ISSN:0008-4042    

DOI:10.1139/v95-181

出版商:NRC Research Press    

年代:1995

数据来源: NRC