摘要:

AbstractAn empirical MM2 force‐field was developed for the calculation of steric or strain energies of carbenium ions, and applied to the rationalization of the rates of solvolysis of bridgehead derivatives. The latter constitute a homogeneous series of model compounds for solvolysis, spanning a rate range ofca20 log units. Their rate constants correlate with the calculated steric energy differences between bridgehead bromides and the corresponding carbenium ions. The rate constants of tertiary derivatives of general structure may similarly be rationalized in terms of strain changes, although the correlation exhibits more scatter than that for the bridgehead derivatives alone. In the bridgehead series, the relative free energies of activation for solvolysis correlate with the heterolytic bond dissociation energiesD° (R+– Br−) in the gas phase. However, this correlation breaks down when simple mono‐ and acyclic substrates are included. This is attributed in part to the proximity of the leaving group in the transition state of solvolysis, which stabilizes the developing positive charge at the cationic centre in comparison with the charge of the free ion. The significance of the force‐field calculations with respect to the structure of bridgehead carbenium ions was tested by comparison of structural data obtained fromab initiocalculations. The structures of cations suffering strong distortions owing to C–C hyperconjugation are poorly reproduced by the molecular mechanics calculations, the parameters of which are based on solvolytic reactivity and not on ca

ISSN:0894-3230    

DOI:10.1002/poc.610080802

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractEquilibrium acidities and estimates of homolytic bond dissociation enthalpies (BDEs) in DMSO of the O–H bondsfor α‐tocopherol, 2,6‐di‐tert‐butylphenol, six 4‐substituted‐2,6‐di‐tert‐butylphenols, and ten related phenols are reported. The presence of a 2,6‐di‐tert‐butyl group in a phenol increases its acidity and makes substituent effects on the acidity caused by aparaelectron‐withdrawing group larger. TheBDEs of the O–H bonds in 2,4,6‐tri‐tert‐butylphenol, 4‐methoxy‐2,6‐di‐tert‐butylphenol, 4‐methyl‐2,6‐di‐tert‐butylphenol and α‐tocopherol, estimated by combining their pKHAvalues with the oxidation potentials of the conjugate anions,Eox(A−), according to the equationBDE= 1·37pKHA+ 23·1Eox(A−) +Chave been found to agree to within 2 kcal mol−1with literature values (1 kcal = 4·184 kJ). Introduction of 2,6‐di‐tert‐butyl groups into phenol and six 4‐substituted phenols weakens the O–H bonds by amounts ranging from 3·6 to 10·3 kcal mol−1. These effects are attributed to increases in ground‐state energies which introduce strains that are relieved when homolytic cleavage of the O–H bond forms an o

ISSN:0894-3230    

DOI:10.1002/poc.610080803

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe molecular and electronic structures of the cyclic 1,2‐dithiolium ylides (2) and their acyclic dithiocarbonyl valence isomers (4) were studied byab initiomethods. All molecular structures were fully optimized at the R(U)HF/6–31G* and R(U)MP2/6–31G* levels of theory. The results of the restricted and unrestricted Hartree–Fock (HF) calculations are less reliable since the RHF wavefunctions of the cyclic compounds proved to be singlet/triplet unstable. A marked ground‐state energy depression occurs on passing from restricted to unrestricted HF methods. The title compounds are of diradicaloid nature and belong to the non‐classical class of structures. The heterocycles are planar and displayC2vsymmetry, while the cisoid open‐chain dithiocarbonyl isomers are slightly distorted (C2symmetry). According to the calculated bond lengths and charge distributions, the title compounds are in fact more or less zwitterionic. The thiolate is the most polar compound of this series. According to RMP2 calculations, the thiolate (2, X = S) and olate (2, X = O) should exist in the single ground rather than in the triplet state. However, in these cases the isomeric acyclic compounds are predicted to be almost isoenergetic with the cyclic compounds. Since the cyclic ylides have higher dipole moments than the acyclic dithiocarbonyls, the ylides will gain stability in polar solvents. However, the continuum quantum chemical model used in estimating the solvent effect does not favour the ring compounds sufficiently to exclude valence isomerization between the ring‐closed and ring

ISSN:0894-3230    

DOI:10.1002/poc.610080804

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe solid–solid chemical reaction of benzopinacol (1) withp‐toluenesulphonic acid monohydrate (2) to give a quantitatively proton‐catalysed pinacol rearrangement with formation of triphenylacetophenone (3) in the absence of a solvent was studied preparatively and mechanistically using atomic force microscopy (AFM) measurements and known crystal structure data. The reaction rate is dramatically enhanced if the water of reaction is continuously removed. AFM reveals that no reaction occurs on (001) of 1 whereas (100) exhibits distance dependent craters and volcano‐like mounds over areas extending to 1·5 mm from the contact edge of the crystals with 1 undergoing phase rebuilding while reacting. A mechanism resembling the formation of membrane potentials is seen for the first time in crystals. Thus it appears that catalytic protons migrate without their counterions from one molecular compartment to the next by proton consumption at its inside and proton liberation at its outside, which is the inside of the next compartment. The uppermost molecular layer determines reactivity or non‐reactivity. On (001) of 1 the hydroxyl groups occur with their hydrogens up. Hence no AFM features are found and crystals of 2 do not become adhered to 1. However, on (100) the hydroxyl hydrogens point down (free electron pair up). Protonation is possible, chemical reaction is indicated by the formation of the AFM features and crystals of 2 adhere firmly to the surface of 1 afte

ISSN:0894-3230    

DOI:10.1002/poc.610080805

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractRates of acid‐catalysed hydrolysis of phenyldiazoacetic acid were measured in aqueous solution at 25°C across the entire acidity range pC H += 1–13. The reaction was found to occur through both the non‐ionized and inonized forms of the substrate, with the ionized form reacting faster by a factor of 650. Appreciable solvent isotope effects in the normal direction (kH/kD>1) and the occurrence of general acid analysis show that hydrolysis occurs by rate‐determining proton transfer to the diazocarbon atom of the substrate. Analysis of the rate profile for the reaction provides an estimate of the acidity constant of phenyldiazoacetic acid, pKa= 3·70, which is apparently the first determination of the acid strength of an α‐diazocarboxylic acid. Comparison of this result with the pKaof phenylacetic acid shows the diazo group to have a mild (4×) acid‐strengthening effect, whose modest magnitude is attributed to opposing resonanc

ISSN:0894-3230    

DOI:10.1002/poc.610080806

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe monocation–dication equilibria of 6‐nitro and 6‐sulphonic acid derivatives of betacarboline (9H‐pyrido[3,4‐b]indole) were investigated mostly by UV‐visible absorption spectroscopy. Dications are formed in very highly concentrated sulphuric acid solutions (>15M). Ionization constants, pKavalues and solvation parameters obtained from the excess acidity method are reported. The unusually highm* parameters are discussed and compared with other r

ISSN:0894-3230    

DOI:10.1002/poc.610080807

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractAs a rebuttal to recent criticisms aimed at the mechanistic significance of the determination of the mesitylene/durene relative reactivity, it is argued that this ratio does provide reliable information about the structure of the transition state of manySEAr reactions, supporting the operation of the ‘conventional’ ionic mechanism, as opposed to the ET mechanism. Not even in the case of competitive experiments does it appear that the criticisms are justif

ISSN:0894-3230    

DOI:10.1002/poc.610080808

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610080801

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY