摘要:

AbstractSemiempirical MO (PM3) studies are reported on the reactions of 18 substituted phenolate anions withm‐(MNPF),p‐(PNPF) and 3,4‐dinitrophenyl formates (DNPF). The mechanistic change from rate‐limiting formation to breakdown of the anionic intermediate (T−) can occur in the gas phase, especially for MNPF, at approximately an equal proton affinity (PA) of the nucleophile and leaving group phenolates, i.e. ΔPA= 0 as thePAof the phenolate anion is decreased. In solution, however, owing to the low stability of T and a greater stabilization by solvation of the transition state for the breakdown step (TS2), all three formate esters are predicted to proceed by a concerted mechanism, which is in agreement with the experimental results. The low stability of T can be ascribed to the relatively high electron affinity of the phenol

ISSN:0894-3230    

DOI:10.1002/poc.610080502

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe thermal isomerizations of 1′,3′,3′‐trimethyl‐spiro[2H‐1‐benzopyran‐2,2′‐indoline]‐6‐sulphonic acid (6‐SO 3−‐SP) between its spiro (SP) and merocyanine (MC) forms were followed spectrophotometrically in the presence of cyclodextrins (β‐ and γ‐CDs) at various pressures. From the pressure dependence of the isomerization rate, the activation volumes for the thermal isomerization of 6‐SO 3−‐SP were estimated to be 17·1 cm3mol−1for MC → SP and 8·5 cm3mol−1for SP → MC. The equilibrium constants (K) for the inclusion complex formation of CDs with 6‐SO 3−‐SP were determined at various pressures. It was found that the stability of the 1:1 inclusion complex between 6‐SO 3−‐SP and CDs decreases in the order β–>γ–>α‐CD. From the pressure dependence ofK, the reaction volumes were estimated to be −0·9 cm3mol−1for the 6‐SO 3−‐SP–β‐CD system and 4·0 cm3mol−1for the 6‐SO 3−‐SP–γ‐CD system. Based on the activation volumes, the structure of the transition state for the thermal isomerization is discussed and an explanation for the effect of CDs on the reaction rate is given. The difference in the reaction volumes for the inclusion complex formation of

ISSN:0894-3230    

DOI:10.1002/poc.610080503

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractOscillations of electrical potential differences across the liquid–liquid interface in phase‐transfer catalyst systems are reported. The occurrence of the oscillations is consistent with the Starks' shuttling mechan

ISSN:0894-3230    

DOI:10.1002/poc.610080504

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe reaction in acetonitrile solution of (Z)‐O‐methylbenzohydroximol chlorides [ArC (Cl)NOCH3] with morpholine, pyrrolidine and azetidine gives the corresponding (Z)‐amidoximes [ArC(NR1R2)NOCH3] in acetonitrile. The rates of these reactions were measured under pseudo‐first‐order conditions (excess amine). The reactions were found to follow overall second‐order kinetics (first order in amine). The Hammett ρ‐value (with σ) for the reaction with pyrrolidine is +0·92. The reaction of thep‐nitro compound with pyrrolidine gives a significant element effect [k(p‐nitrobenzohydroximoyl bromide)/k(p)‐nitrobenzohydroximoyl chloride = 10]. A Brϕonsted β‐value of 0·38 was estimated from the reactions of morpholine and piperidine with thep‐nitrohydroximoyl chloride. The slower reaction of (E)‐O‐methyl‐p‐nitrobenzohydroximoyl chloride with azetidine gives primarily the (E)‐amidoxime (E:Z≈ 98:2) This reaction also follows second‐order kinetics. The kinetic observations made in this study are compared with the corresponding results obtained in earlier work in benzene solution. It is suggested that in acetonitrile solution the reactions proceed by an addition–elimination mechanism with rate‐determining loss of chloride ion (AN+ DN#). It is further suggested that acetonitrile is assisting in the breakdown of the

ISSN:0894-3230    

DOI:10.1002/poc.610080505

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractElectrospray mass spectra of a series of substituted 2‐nitrobenzene diazonium cations reveal a surprising stability of the intact ion to the collisionally activated loss of nitrogen, contrary to the behaviour of monosubstituted benzenediazonium ions reported previously. Ions bearing elecron‐releasing groups at theparaposition, e.g. 4‐CH3, 4‐CH3O, and 4‐C6H5O, all experience loss of thisparasubstituent rather than loss of nitrogen. However, ions bearing electron‐withdrawingparasubstituents, e.g. 4‐NO24‐CN, 4‐CF3and 4‐Cl, all undergo nuleophilic displacement of the 2‐nitro group by water to give the corresponding phenol derivative, which subsequently loses nitrogen. The exceptional stability of these substituted 2‐nitrobenzene diazonium salts is attributed to a favourable coulombic interaction between the positive charge on the diazonium group and the partial negative charge on one of the oxygen atoms of the nearby nitro group. Once that nitro group has been removed, the resulting substituted 2‐hydroxybenzenediazonium cation loses n

ISSN:0894-3230    

DOI:10.1002/poc.610080506

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe photostimulated reaction ofN,N‐dibutyl‐p‐toluenesulfonamide with diphenylphosphide ions in liquid ammonia leads to the corresponding phosphinic amides after oxidation Inhibition experiments withp‐dinitrobenzene and 2,2,6,6‐tetramethyl‐1‐piperidinyloxy free radical (TEMPO) and the fact that the reaction is retarded in the dark suggest that it proceeds by theS

ISSN:0894-3230    

DOI:10.1002/poc.610080507

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe potential energy surface describing the unimolecular thermolysis of 1,2‐dioxetane into two formaldehydes is explored via semi‐empirical calculations using the PM3 Hamiltonian with multi‐electron configuration interaction. An active space of the four highest occupied orbitals and the two lowest virtual orbitals is used. Several reaction coordinates were examined, and the in‐plane OO distance was used most extensively. The results indicate that the activation barrier to fission is on the ground state surface and is about 18 kcal mol−1, in good agreement with experimental results. Furthermore, calculations of the vertical triplet‐state energies show that the ground and triplet states are nearly degenerate for OO distances beyond that corresponding to the 18 kcal mol−1barrier, until the molecule abruptly dissociates into two formaldehydes (atca2·55 A). This picture is indicative of a diradical pathway, where the activation energy is associated with motion of the system on the ground‐state surface, not the triplet surface, but where generation of a triplet product can be expected. Similar results were obtained for dimethyl‐ and tetramethyldioxetanes, which require higherEafor dissociation, and for dimethyldioxetanone. Thermochemical calculations pertaining to these reactions and to the formation of triplet produ

ISSN:0894-3230    

DOI:10.1002/poc.610080508

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractAn approach to the quantitative evaluation of XOH acidity in the gas phase and in dimethyl sulphoxide solution is described. It is based on one‐parameter correlations between the acidities of XOH family members and the electron affinities of the XO radicals or highest occupied molecular orbitals (HOMO) energies of the correspondging XO−anions. The approach is based on Koopmans' theorem and provides familial correlations for the acidities of alcohols, carboxylic acids, phenols and mineral acids with calculated (MNDO, AM1 or PM3) HOMO energies of their ani

ISSN:0894-3230    

DOI:10.1002/poc.610080509

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractEPR techniques, including spectral double integration, were utilized to measure the equilibrium contants in dimethoxyethane at 195 K for electron transfer between perdeuteriatedp‐xylene and xylene, α‐d3‐p‐xylene and α,α′‐d6‐p‐xylene. The results, coupled with the law of Hess, yield all the relative solution electron affinities (EA). It was found that methyl deuteriation lowers the solutionEAby 160 J l mol−1per deuterium, whereas ring deuteriation in these systems lowers the solutionEAby about 370 J l mol−1per deuterium, which is about the same effect as is observed in the benzene system. The results are discussed in terms of hyperconjugative, inductive

ISSN:0894-3230    

DOI:10.1002/poc.610080510

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe molecular complex formation of six macrocyclic and six acyclic polyethers with tropylium and 4‐methoxyphenyltropylium tetrafluoroborates was observed and characterized in the gas phase by fast atom bombardment mass spectrometry (FABMS), to obtain information on intrinsic molecular interactions in the absence of the complicating effect of solvation. The stoichiometry of the complexes was assessed on the basis of corresponding peaks in FAB mass spectra. In addition to the expected 1 : 1 complexes between polyether and tropylium ion, some 2 : 1 complexes were also formed. The stability order of selected complexes was studied in competition experiments. The complexation equilibrium constant and thermodynamic parameters for the interaction of dibenzo‐18‐crown‐6 with tropylium ion were determined in 1,2‐dichloroethane solution by UV‐visible spectrophotometry. Complexation of polyethers with tropylium salts has not been reported

ISSN:0894-3230    

DOI:10.1002/poc.610080511

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY