摘要:

AbstractThe rate of proton/deuteron transfer reactions of 2,4,6‐trinitrotoluene (TNT), 2,2′4,4′6,6′‐hexanitrobibenzyl (HNBB) and 2,4,6‐trinitrobenzyl chlorides (TNB) with tertiary amines was followed spectrophotometrically. The rate constants obtained under various conditions were interpreted in terms of isotope, solvent and steric effects. The Arrhenius plots curved upward at low temperatures for TNT and TNB, indicating clearly the intervention of tunnelling. The tunnelling parameteres were calculated semi‐empirically on the basis of Bell's theory. The observed rate constants at low temperatures agreed well with the predicted values. In the present systems, some side and consecutive reactions involving the formation of 2,2′,4,4′6,6′‐hexanitrostilbene took place, and in some cases interrupted the rate measurements in the proton/de

ISSN:0894-3230    

DOI:10.1002/poc.610080602

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractFree energies of solvation were calculated for the four most stable gas‐phase tautomeric forms of 2‐thiocytosine (TC) using a local field SCF procedure with the solvation model SM2. The calculated changes in the free energies for each pair of tautomers reveal that all six possible equilibria tend to produce the thione‐amino TC(1, 8, 8) and TC(3, 8, 8) species. This is in agreement with UV and ionization constant

ISSN:0894-3230    

DOI:10.1002/poc.610080603

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractBase‐catalysed substrate‐enantioselective 1,3‐hydron transfer (kinetic resolution) was studied in the indene system. A series of 1‐methylindenes substituted in the aromatic ring [5‐methoxy‐ (2), 5‐fluoro‐ (3), 5‐nitro‐ (4) and 7‐nitro (5)] and 1‐methylindene (1) were employed as substrates. The rate constants, the enantioselectivities and the kinetic isotope effects (KIEs) for the enantioselective reactions [(kH/kD)+and (kH/kD)−] were determined at 20°C using (+)‐(8R, 9S)‐dihydroquinidine as chiral catalyst in the solvento‐dichlorobenzene. The rate constants vary according to the elecronic effects of the substituents. The primary deuterium KIE, ranging from 4·73 [for (+)‐(S)‐2] to 11·3 [for (−)‐(R)‐5], is correlated with the rate constants as expected on the basis of the Melander–Westheimer postulate. The introduction of a substituent in the aromatic ring decreases the enantioselectivity. All compounds except 5 show the same sense of the enantioselectivity [k+/k−>1; all substrates used are (+)‐(S)/(−)‐(R)]. The enantiomer dependence of the KIE is most pronounced for 1 [(kH/kD)+= 5·71 and (kH/kD)−= 6

ISSN:0894-3230    

DOI:10.1002/poc.610080604

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractAnab initiomolecular orbital study of the potential energy surface of the C6H5O + O reaction was performed at the (PUMP3/6‐31G*//UHF/6‐31G*) level of theory. Various reaction channels were considered. The most favorable mechanism, la and Ib, start from the attachment of the oxygen atom to the carbon atom of the C6ring in theortho‐ orparaposition with respect to CO, taking place without activation energy. Then, either hydrogen elimination by mechanism Ia or 1,2‐H shift from the C(H)(O) group takes place; the latter process leads to the formation of the very stable C6H4(O)(OH) radical, which can also eliminate H by mechanism Ib. Thus, the main products of the C6H5O (2B1) + O(3P) reaction areo/p‐benzoquinones and the hydrogen atom. At low temperatures, however, the system may be trapped in the potential well of the C6H4(O)(OH) intermediate. At high temperatures, the reaction may proceed by the formation and decomposition ofo/p‐benzoquinones. Because of their higher activation energies, the reaction mechanisms giving rise to other products–the attachment of the oxygen atom to the bridging position to form an epoxy intermediate, followed by insertion of O into the CC bond and dissociation to give C5H5and CO2(channel IIc), in addition to the attachment of oxygen to the terminal O atom of C6H5O followed by elimination of O2(channel III) – cannot compete with channel Ia or Ib. RRKM calculation was carried out for the total and individual rate constants for channels Ia and Ib. The three‐parameter expression for the total rate constant, fitted by the least‐squares method for the temperature range of 300–3000 K, is given asktot= 5·52×10−

ISSN:0894-3230    

DOI:10.1002/poc.610080605

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractMeasured molecular dipole moments and theoretical percentages of mesomeric zwitterionic forms were used to calculate the attractive electrostatic potentials at short,V(s), medium,V(m), and long,V(I), distances. Values of an attraction power function Φ (H) =A+BV(s)V(m)V(I) was then deduced for 18 nitriles and cyanamide vinylogues or iminologues along the C≡N direction. The satisfactory agreement observed between Φ (H) and the hydrogen bond basicity scale pKHBindicates that the method can be a useful tool for the prediction of hydrogen bonding, needing only simple calculations and allowing easy interpretat

ISSN:0894-3230    

DOI:10.1002/poc.610080606

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractOxidation of several monosubstituted (phenylthio)acetic acids (PTAA) by pyridinium hydrobromide perbromide (PHPB) was studied in aqueous acetic acid. The reaction is first order with respect to PHPB. Michaelis–Menten type kinetics are observed with respect to (arylthio)acetic acid. The effect of solvent composition indicates that the transition state is more polar than the reactants. The formation constants of the intermediate substrate–PHPB complexes and the rates of their decomposition were determined at different temperatures. The rates of oxidation ofparaandmeta‐substituted (phenylthio)acetic acids were correlated with Hammett's substituent constants. The ρ value is −1·60 at 35°C. The rates of oxidation oforthosubstituted compounds are correlated with Charton's triparametric equation. A mechanism involving the decomposition of the intermediate complex in the slow rate‐determining step affording a sulphonium ion which hydrolyses in a subsequent fast step to the sulphoxide

ISSN:0894-3230    

DOI:10.1002/poc.610080607

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractRing proton affinities (PAs) in fluorobenzene and toluene were examined by the MP2(fc)/6–31G**//HF/6–31G* +ZPE(HF/6–31G*) model. The calculatedPAs are in good accordance with the available experimental evidence, their order beingPA(p)>PA(o)>PA(m)>PA(i), wherep,o,mandistand forpara,ortho,metaandipsopositions, respectively. The relative values of the proton affinities can be interpreted in terms of the ground‐state charge distribution (initial state effect) and the characteristic π‐bond fixation produced by protonation (final state effect). The influence of the latter is either concerted with the initial charge distribution leading to higherPAs (orthoandparapositions) or disconcerted as inmetaprotonation, which has a lowerPAvalue. Finally, it is shown thatPAs in difluorobenzenes and fluorotoluenes are additive and can be reduced to the characteristicPAs of fluorobenzene and toluene with goo

ISSN:0894-3230    

DOI:10.1002/poc.610080608

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractNovel mechanistic criteria forSNreactions were proposed on the relative magnitudes of ρZand interaction terms, ρijorPijvalues. The mechanistic change by the sign of the product of ρijs orPijs was also proposed. These criteria were applied to the reaction between substituted phenyl benzoates and amines and indicated that the sign of ρZis positive and the interaction termPXZis larger thanPYZ, hence these reactions proceed via anSANmechanism and a change in the substituent of the substrate leads to a change in the substituent of the substrate leads to a change in the transition state towards the Hammond eff

ISSN:0894-3230    

DOI:10.1002/poc.610080609

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY

ISSN:0894-3230    

DOI:10.1002/poc.610080601

出版商:John Wiley&Sons Ltd.    

年代:1995

数据来源: WILEY