摘要:

AbstractThe relative thermodynamic stabilities of the title compounds were determined by iodine catalyzed chemical equilibration in cyclohexane solution. The main point of interest was a determination of the thermodynamic stability of the OCCO moiety found in the exocyclic isomers, i.e. the stabilizing effect of a MeO group on the olefinic linkage of 2‐methylenetetrahydrofuran. All three isomeric compounds have essentially similar enthalpy values, which, in comparison with some previous thermodynamic data, shows that the double bond stabilization energy of the MeO group in the exo isomers is onlyca3 kJ mol−1. The entropy difference between the geometrical isomers is negligible, whereas the endo isomer is favoured by an entropy contribution ofc

ISSN:0894-3230    

DOI:10.1002/poc.610080103

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractA new approach to the long‐standing problem of interrelatingmetaandparasubstituent constants is presented. An analysis of the unified σ0‐scale shows that the interrelation between σ 40and σ 40/σ 30can be modelled by a pair of conjugate rectangular hyperbolae, one for normal (n) and the other for special (s) substituents. The latter are characterized by a lone electron pair in the first atom. The equations σ 4n0(σ 4n0− γ0)/(σ 4n0− 2γ0) = λ0σ 3n0and σ 4s0= γ0+ λ0σ 3s0are derived and discussed in terms of Taft's separation of mesomeric and non‐mesomeric effects. Asymptotic values λ = 0.960 γ = −0.225 were obtained by non‐linear least rectangles fitting. A nonnegligible mesomeric contribution to σ0constants for normal substituents is predicted by the hyperbolic model. The present results are at variance with Exner's analysis of themeta‐parainterrelationship in benzene compounds with normal substituents. This divergence is ascribed to opposite

ISSN:0894-3230    

DOI:10.1002/poc.610080104

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe enthalpies of formation of the condensed phase and gaseousN,N‐dimethyl derivatives of pivalamide, 1‐adamantylcarboxamide and benzamide were determined by combustion calorimetry and the associated enthalpies of vaporization and sublimation. The enthalpies of formation of styrene and its α‐, trans‐β‐ and β,β‐methylated derivatives were determined from measurements of their enthalpies of hydrogenation in dilute hydrocarbon solution. Strain and resonance effects of amides and alkenes are discussed in terms of the exo‐/endothermicity of the following reactions:and in terms of the difference of enthalpies of formation of the isomeric (Z

ISSN:0894-3230    

DOI:10.1002/poc.610080105

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractSlight structural moiecular variations are known to affect different properties of compounds. In solution, different solute‐solvent interactions are known also to alter the properties of numerous compounds. Quantitative structure‐activity relationships (QSAR) are used regularly to analyze and predict the variations of different properties of compounds that are caused by structural variations and significant solute‐solvent interactions. The relative basicities ofn‐propylamine, dipro;ylamine and tripropylamine were determined in nine different solvents from potentiometric titrations. QSAR that were developed from these experimental basicity values were used to evaluate the type and significance of the solute‐solvent interactions. The important interactions that influence basicity variations for the propylamines studied are dipolarity‐polarizability interaction between the solute and the solvent and hydrogen bonds from the propylammonium ions to basic solvents. The role of hydrogen bonds from the propylamines to acidic solven

ISSN:0894-3230    

DOI:10.1002/poc.610080106

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractOn irradiation on a dry silica gel surface, 2‐(N, N‐dialkylamino)ethyl 3‐benzoylacrylates undergo selective E‐Z isomerization. Competing processes, such as remote‐hydrogen abstraction via a charge‐transfer state, with the photoisomerization in solution are completely suppressed on the surface. The yield of theZ‐ from theE‐ isomer increases with increase in the coverage of theE‐isomer on the surface, reaching a limiting value. Thermal isomerization of theZ‐ to theE‐ isomer occurs easily on the silica gels surface when the alkyl group on the nitrogen atom is small. Benzoylacrylates without any amino group also undergo selecti

ISSN:0894-3230    

DOI:10.1002/poc.610080107

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractSubstituent effects were measured for the reactions of substituted benzaldehydes with allytributylin (1) and allyltriethyllead (2) reagents in the presence of BF3.OEt2in CH2Cl2. The Hammettpvalues were small and positive at 25 °C and negative at −78 °C for both 1 and 2. These could be interpreted in terms of the contribution of electrophilic complexation between the aldehyde function and BF3as a rate‐limiting step. A large negativepvalue was observed for the complex‐formation equilibria between substituted benzaldehydes

ISSN:0894-3230    

DOI:10.1002/poc.610080108

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe crystal and molecular structues of six quasi‐phosphonium salts containing phosphorus‐heteroatom heteroatom=oxygen, sulphur or nitrogen0 bonds were determined. Comparison of the molecular parameters obtained with those available for reference structures demonstrated that the ‘double bond’ character for the P+ Y bond decreases in the order Y

ISSN:0894-3230    

DOI:10.1002/poc.610080109

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe associationof C6H5O with NO was studied with the cavity‐ring‐down method by directly monitoring the decay of C6H5O in the presence of varying, excess amounts of NO. The biomolecular rate constant determined in the temperature range 297‐373 K can be effectively rate constant determined in the temperature range 297–373 K can be effectively represented byk1= 10 − 12 · 12 ± 0.24 e(194±185)/rcm3molecule−1with a negative activation energy of 0.8 kcal mol−1(1 kcal = 4.184 kJ). In order to understand better the mechanism of the reaction,ab initiomolecular orbital calculations were also carried out at the MP4(SDQ)/6‐31G* level of theory using the HF optimized geometries. The molecular structues and energetics of five C6H5N1O2isomers were calculated. Among them, the most likely and stable association product, phenyl nitrite (C4H5ONO), was found to be 17 kal mol−1below the reactants, C6H5O + NO. Combining the measured rate constant and the calculated equilibrium constant for the association reaction, C6H5O + NO = C6H5ONO the rate constant for the unimolecular decomposition of C6H5ONO was obtained ask−1= 4.6 × 1015E−8580/Ts−1. The relatively large frequency factor suggests that a loose transition state was involved in the reaction, akin to those of its alkyl anal

ISSN:0894-3230    

DOI:10.1002/poc.610080110

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

Abstract2,3‐Bis(aminocyclohexyl)−6‐methoxy‐1,3,5‐triazine (1a) forms intermolecular H‐bonded complexes with 3,3′‐diacetyl‐cis‐azoenzene (4) and 6,6′‐diethoxy‐cis‐thioindigo (5b), (association constants K = 4.9 × 104and 3.5 × 1051 mol−1, respectively). The thermalcis→transisomerization of 4b and 5b to 3,3′‐diacetyl‐trans‐azobenzene (4a), and 6,6′‐diethoxy‐trans‐thioindigo (5a), is inhibited in the intermolecular complex 1a–4b and 1a–5b. Molecular mechanics calculations support the formation of the inte

ISSN:0894-3230    

DOI:10.1002/poc.610080111

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610080102

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY