摘要:

The oxidation of ascorbic acid by thioureapentacyanoferrate(III) has been investigated over the pH range 0.8–7.2 at an ionic strength of 1.0M(NaClO4). The variations in the rates of oxidation with hydrogen ion concentration are consistent with a reaction scheme involving three pH-related species; ascorbic acid (H2A) (k1 = 0.111 M−1 s−1, ΔH1≠ = 5.1 kcal mol−1, ΔS≠ = −46 cal deg−1 mol−1), the ascorbic anion, (HA−)(k2 = 66 M−1s−1, ΔH2≠ = 5.4 kcal mol−1, ΔS2≠ = −32 cal deg−1 mol−1) and the ascorbic dianion, (A2−) (k3 = 8.78 × 107 M−1 s−1, ΔH3≠ = 3.1 kcal mol−1, ΔS3≠ = −12 cal deg−1 mol−1). The low reduction potential of Fe(CN)5TU2−(E0 = 0.35 V) permits the measurement of the rate of oxidation of the ascorbate dianion, not previously investigated with metal complexes. The second acid dissociation constant of ascorbic acidhas been measured in 1.0MNaClO4over the temperature range studied. A relationship between the rate of oxidation and the reduction potentials of the ascorbic acid species is shown.

ISSN:0008-4042    

DOI:10.1139/v81-021

出版商:NRC Research Press    

年代:1981

数据来源: NRC

摘要:

Reactions ofL-tryptophan andD,L-homocysteine with mercury(II) halides in acidic ethanol/water solutions result in the formation of bis(L-tryptophanium) trichloromercurate(II) (TPTC) and bis(D,L-homocysteine thiolactonium) tetrabromomercurate(II) (HTTB) respectively. Their crystal structures consist of halide-bridged polymeric chains of quasi HgX3−units. The coordination geometry around Hg(II) in both anions is distorted trigonal bipyramidal. There are no chemical bonds between the amino acid cations and the mercury(II) ions. TPTC is monoclinic, space groupP21, witha = 16.688(5),b = 5.626(1),c = 14.022(5) Å, β = 108.17(3)°. The structure was solved and refined from 2433 independent counter measured reflections to anRfactor of 0.062. The two independent tryptophan molecules per asymmetric unit form a single cation via a strong hydrogen bond between the carboxylate groups. Thus the compound can be represented by MHgCl3where M is the cation. One of the three Cl−ions is strongly bonded to one Hg(II) ion and weakly bonded to two other Hg(II) ions. In addition to the three short Hg—Cl distances of 2.403(3), 2.376(4), and 2.614(4) Å, there are two long Hg—Cl bonds of 2.833(4) and 2.888(4) Å to each Hg(II), from the weakly bridged chloride ions. HTTB is orthorhombic, space groupPnmawitha = 8.068(3),b = 24.985(7), andc = 8.828(3) Å. The structure was refined to anRvalue of 0.066 for 819 observed reflections (I ≥ 3σ(I)), however, inclusion of unobserved reflections did not change the parameters. The homocysteine free base used in the reaction was lactonised under the experimental conditions to a thiolactonium ion. There are three relatively short Hg—Br interactions (2.566(3) twice, 2.662(4) Å) within the distorted trigonal bipyramidal stereochemistry of each mercury atom and two longer Hg—Br bonds (2.917(6) and 3.087(5) Å). The polymeric, anion chains arrange in layers on the mirror planes (y = ±1/4) with these layers sandwiched by two layers of cations.

ISSN:0008-4042    

DOI:10.1139/v81-022

出版商:NRC Research Press    

年代:1981

数据来源: NRC

摘要:

Complexes Hg[CH3SCH2CH(NH2)COO]2and Hg[CH3CH2SCH2CH2CH(NH3COO]2(ClO4)2obtained from reactions of yellow HgO withS-methyl-L-cysteine and ethionine respectively have been characterized by X-ray diffraction methods. The former is monoclinic, space groupP21, witha = 13.790(4),b = 4.994(1),c = 9.898(2) Å, and β = 106.20(2)°. TheRandRwfactors were 0.063 and 0.075 respectively for 980 independent reflections. TheS-methyl-L-cysteine complex is N,O bonded with two Hg—N bond lengths of 2.16(3) and 2.13(3) Å and. There are eightdistances within the range of 2.54–3.75 Å. The three shortest of these are distributed in an equatorial plane with respect to the N—Hg—N moiety. The Hg—O bond length of 2.54(2) Å is associated with the chelate ligand.The ethionine complex is triclinic, space groupwitha = 11.376(7),b = 12.009(8),c = 9.495(6) Å, α = 94.74(6), β = 108.01(5), and γ = 71.78(5)°. TheRandRwvalues for all 3526 reflections were 0.083 and 0.10 respectively butRbecame 0.058 for only observed reflections (I > 3 σ(I)). The Hg2+is bonded to two sulfur, Hg—S = 2.436(5) and 2.537(4) Å, and two oxygen, Hg—O = 2.28(1) and 2.259(9) Å, atoms in a distorted tetrahedron. The Hg2+–carboxylate interactions are stronger in the S-bonded than the N-bonded complex probably indicating a preference for Hg–amine coordination. At low pH when the amino group is protonated, Hg–thioether coordination becomes important for S-substituted amino acids.

ISSN:0008-4042    

DOI:10.1139/v81-023

出版商:NRC Research Press    

年代:1981

数据来源: NRC

摘要:

The equilibria RNH3+(H2O)n−1 + H2O = RNH3+(H2O)nwere measured for R = CH3, C2H5, and CF3CH2fromn = 1 ton = 3 with a pulsed electron beam high ion source pressure mass spectrometer. The proton and hydrate transfer equilibria CH3NH3+(H2O)n + C2H5NH2 = CH3NH2 + C2H5NH3+(H2O)nwere measured forn = 0 ton = 3. These data allow the evaluation of ΔH0and ΔG0for the reactions: R0NH3+(H2O)n + RNH3+ = R0NH3+ + RNH3+(H2O)n. ΔH0 = δΔH00,n(RNH3+), ΔG = δΔG00,n(RNH3+). These data are compared with δΔE0,3(STO-3G) evaluated by Hehre and Taft. In general good agreement is observed atn = 3. The δΔH00,3(RNH3+) ≈ δΔE0,3(RNH3+) are also found close to the ion hydration free energy difference in aqueous solution

ISSN:0008-4042    

DOI:10.1139/v81-024

出版商:NRC Research Press    

年代:1981

数据来源: NRC

摘要:

Electron spin resonance spectra are reported from a number of radicals derived from 2,4-pentadione substituted at the 3 position with nitroxide-containing groups. If the substituent ist-butyl nitroxide no metal complexes are formed. This is attributed to steric factors which prevent the formation of the enol form of the β-diketone. If the substituent is trifluoromethyl nitroxide two types of metal complex have been observed. The esr spectra of the first type are very similar to that of the uncomplexed radical. Such complexes are formed with Co(III) and Al(III). The esr spectra of the second type show considerably increased14N and19F hyperfine coupling constants and in some cases large couplings to the metal nucleus. Complexes for the second type have been observed with Pd(II), Pt(II), and Rh(III). The possible structures of these radicals are discussed.

ISSN:0008-4042    

DOI:10.1139/v81-025

出版商:NRC Research Press    

年代:1981

数据来源: NRC

摘要:

It is shown that a conveniently placed internal nucleophile (carboxyl group) is not involved with the rearrangement of a dichlorocyclopropane into an allylic chloride. This result appears to support a concerted mechanism of a σs2 + σa2type for this rearrangement. In the products obtained, the allylic chloride may undergo displacement either by solvent (H2O), leading to alcohols, or by the internal carboxyl group, leading to a lactone. [Journal translation]

ISSN:0008-4042    

DOI:10.1139/v81-026

出版商:NRC Research Press    

年代:1981

数据来源: NRC

摘要:

Hydroxyurea, in its oxanion form NH2—CO—NHO−reacts in basic medium by a 1,4-addition to α-acetylenic esters leading to theEandZethylenic β-ureidoxy ester isomers. TheEisomer can be isolated and identified from reaction with ethyl tetrolate and ethyl propiolate. TheZisomer undergoes cyclisationin situto give the 2-carboxamido-5-methyl-3-isoxazolone. With phenyl propiolate, the 3-hydroxy-5-phenyl isoxazole is directly obtained without isolation of the intermediateN-carboxamido derivative. In the case of ethyl propiolate two cyclic compounds are obtained: 3-hydroxyisoxazole and 2-N-carboxamido-5-ethoxy-3-isoxazolidinone. The formation of the former is due to thein situcyclisation of theZisomer. The isolation of the latter compound can be explained by solvent addition to theEβ-ureidoxyacrylate intermediate in a Michaël addition followed by cyclisation.

ISSN:0008-4042    

DOI:10.1139/v81-027

出版商:NRC Research Press    

年代:1981

数据来源: NRC