摘要:

AbstractSemi‐empirical SCF calculations using the AM1 Hamiltonian were shown to account for the Hammett σ constants and other properties of a representative series of 22 monosubstituted benzenes in terms of the atomic charges at theipso,ortho,metaandparapositions. The trends of the correlations with experimental σ constants were found to follow the normal expectations of physical organic chemistry. Additional support for the AM1 charge distributions was obtained in correlations with the13C chemical shifts of 12 of the test molecules. On the other hand, no correlations were found between σ constants and computed force constants for bending peripheral H atoms out of the plane of the aromatic ring, supporting the view that Hammett σ constants reffect charge distributions and not secondary effects expressed in the stiffness of

ISSN:0894-3230    

DOI:10.1002/poc.610081002

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractTheo‐iodosobenzoate (IBA−)‐catalyzed hydrolysis of the following esters was studied spectrophotometrically: 4‐nitrophenyl 4‐X‐benzoate (series I, a–e, X = CH3, H, Cl, CN, NO2), 4‐Y‐phenyl 4‐nitrobenzoate (series II, a–d, Y = CH3, H, Cl, CN), 2,4‐dinitrophenyl 4‐X‐benzoate (series III, b–e, X = H, Cl, CN, NO2) and 4‐Y‐phenyl 2,4‐dinitrobenzoate (series IV, a–e, Y = CH3, H, Cl, CN, NO2). Direct detection of a reaction intermediate, viz 1‐(4‐nitrobenzoyloxy)‐1,2‐benzodoxol‐3(1H)‐one, the absence of (general base) catalysis by the leaving group, the small kinetic solvent isotope effect and the fast catalyst ‘turnover’ show that the mechanism of catalysis by IBA−is nucleophilic. Catalytic rate constants, activation parameters and application of the Hammett equation show the following mechanistic features: the first step of the reaction, i.e. that leading to the formation of the above‐mentioned intermediate is irreversible, the leaving group is the phenol and the rate‐limiting step is the collapse of the tetrahedral intermediate that is formed by attack of IBA−on the ester. A comparison of the results of hydrolysis by IBA−and by imidazole (Imz) revealed that IBA−and Imz differ in their sensitivity toward an ester structure and, for the same ester series, IBA−is a much more efficient catalyst. The reasons for the higher catalytic activity of IBA−include nucleophilicity enhancement due to the α‐effect, a smalle

ISSN:0894-3230    

DOI:10.1002/poc.610081003

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractOctaethyltetrathiaporphyrin dication, OTP2+, is electrochemically reduced by two successive electron‐transfer processes to OPT0. OPT2+forms intermolecular complexes with a series of π‐donors. The association constants of the resulting complexes are controlled by the oxidation potentials of the π‐donors and solvent properties. OTP2+was incorporated into a Nafion membrane cast on a Pt electrode. Formation of OTP2+–π‐donor complexes in the polymer membrane was followed by electrochemical and spectroscopic means. The association constants of the OTP2+–π‐donor complexes in the Nafion membrane exhibit lower values than a homogeneous phase owing to the high ionic strength in th

ISSN:0894-3230    

DOI:10.1002/poc.610081004

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe octaethyltetrathiaporphyrin dication, OTP2+, forms 1:1 intermolecular donor–acceptor complexes with NiII‐, VIVO‐ and FeIII‐octaethylporphyrins. The association constants of the complexes are governed by the oxidation potential of the metallo‐octaethylporphyrins and by secondary electrostatic interactions. Octaethylporphyrin, H2OEP, forms intermolecular complexes with OTP2+. Kinetic analyses of the formation of the various complexes revealed the formation of a primary complex exhibiting the stoichiometry (OTP2+)2(H2OEP). This intermolecular complex transforms into a thermodynamically stabilized intermolecular assembly with a stoichiometry corresponding to (OTP2+)4H2OEP. The activation barrier associated with the conversion of the primary complex to the thermodynamically stabilized assembly isEa= 16·5 kcal mol−1(1 kcal = 4·184 kJ). The association constant of the complex (OTP2+)2(H2OEP) isK1= 1·3 × 1010M−2and the equilibrium constant between the two coexisting intermolecular complexes (OTP2+)4(H2OEP) and (OTP2+)2

ISSN:0894-3230    

DOI:10.1002/poc.610081005

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe original Hammett equation, Δ = ρσ, is transformed in a constrained tetralinear relationship where each straight line with variable intercept term correlates one of the following four groups or subsets of dipolar substituents: normal and special substituents (depending on the absence or the presence of a lone electron pair in their atom next to the aromatic ring) and, in each of these classes, separatingmetaandparaderivatives. There are a total of four fitting parameters in the resulting plurilinear Hammettian transformation (PHT) from which the statistically corrected parameters λ and γ are derived; λ and γ are the asymptotic values in a hyperbolic model for the representation of Δ4vs Δ4/Δ3. Thismeta–parainterrelationship is assumed to hold in the absence of through‐resonance effects which, in turn, are allowed for by the use of alternative sigma scales of substituent constants. By applying the PHT to a large number of selected literature data, parameters λ and γ are determined for the ionization equilibria of 3‐ and 4‐monosubstituted benzoic acids, anilinium ions, phenols and pyridinium ions. In these reactions series, parameter λ, which measures thepara/metaratio of field/inductive effects, is lower than unity and shows a marked dependence on the basic molecular framework. It is best modelled in terms of a through‐space field effect approach. The ratio γ/γ0, where γ0is referred to the unified sigma‐zero scale, is shown to correspond to the original Hammett's reaction constant ρ. It is concluded that the PHT constitutes an improved Hammett equation for the analysis of substituent effects in benzene derivatives taking into account statistical errors and making allowance for different transmission coefficients for the field/inductive effect frommetaandparapositions

ISSN:0894-3230    

DOI:10.1002/poc.610081006

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

Abstract1H,13C and15N NMR data for nitraminopyridines are discussed in terms of tautomeric equilibria in these compounds. The favoured tautomer is determined mainly from15N NMR spectroscopy. The chemical shift of the nitrogen atom of the nitro group in nitraminopyridines andN‐nitroanilines which cannot tautomerize vary from 28·0 to 35·4 ppm in DMSO solution. 3‐Nitraminopyridine and 2‐nitramino‐3‐ and ‐5‐nitropyridines behave similarly. In the15N NMR spectra of nitrimino‐1‐methyldihydropyridines, used as models, an upfield shift of that atom, different from that observed for 2‐nitraminopyridine, indicates the significance of the nitrimino tautomer. In contrast, a downfield shift of the ring nitrogen atom of some other compounds shows an increased weight of the nitramino tautomer. 3‐Nitraminopyridine, when dissolved in DMSO, does not have the zwitterionic structure. Changes in the chemical shift of the amino nitrogen atom are not readily interpreted. Some proton and carbon chemical shifts and hydrogen–hydrogen and carbon–hydrogen spin–spin coupling constants can also be used to determine the predominant tautomer o

ISSN:0894-3230    

DOI:10.1002/poc.610081007

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

Abstractperi‐Bridged naphthalenes in which the C‐1 and C‐8 atoms of the naphthalene ring are bonded to a single atom in a bridging group Z (Z = CH2, CHBr, CO, NH, O, S, SO and SO2) were studied by the AM1 semi‐empirical SCF MO method. Although inperi‐naphthalenes bridged by carbon or sulfur the naphthalene structure is planar, the corresponding nitrogen‐ or oxygen‐bridged molecules are distorted about 15 and 23°, respectively. There are significant variations (from 92 to 106°) of the C‐1–C‐9–C‐8 angle that is calculated in the cycle formed by the bridge. The calculated strain‐energy barrier for pyramidal atom inversion in amine 4 and sulfoxide 7 are 32·

ISSN:0894-3230    

DOI:10.1002/poc.610081008

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610081001

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY