bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nature of the highest occupied molecular orbitals of trans‐ and cis‐bicyclo[4.1.0]hept‐3‐enes
Authors: Gassman Paul G. , Mlinarić‐Majerski Kata , Kovač Branka , Chen Han , Dixon David A. 
Published: January 2005

AbstractPhotoelectron spectra of trans‐bicyclo[4.1.0]hept‐3‐ene and two methyl derivatives, 1–3, and cis‐bicyclo[4.1.0]hept‐3‐ene and three of its derivatives, 4–7, were measured. Ab initio molecular orbital (MO) and density functional theory (DFT) calculations were performed on these compounds. By combining the results of the experimental and computational trends, it was established that the highest occupied molecular orbitals (HOMOs) of 1–3 are the ‘twist’ bent σ bond at the fusion between the two rings whereas for 4–7 the HOMOs are the π orbital in the six‐membered ring. The DFT ordering is the same as that suggested by the photoelectron spectra whereas at the MO level the HOMO always corresponds to the π orbital. The intensities of the photoelectron spectra were calculated at the DFT level and good qualitative agreement with experiment was found.
Dicarboximide‐based clathrate design. Host synthesis, inclusion formation and x‐ray crystal structures of a free host and of inclusion compounds with 2‐ and 3‐ methylcyclohexanone, 3‐methylcyclopentanone, butyronitrile, propan‐1‐ol and (−)‐fenchone guests
Authors: Weber Edwin , Reutel Christiane , Foces‐Foces Concepción , Llamas‐Saiz Antonio L. 
Published: January 2005

AbstractCrystalline host compounds consisting of a roof‐shaped dicarboxinide framework and pendant diarylethanol analogous subunits were synthesized and shown to form inclusion complexes with small organic molecules such as alcohols, amines, ketones or polar and apolar organic solvents. Clathrate efficiency and selectivity depend on the particular host structure. The crystal and molecular structures of a free host compound (2a) and inclusion compounds [2a·3‐methylcyclohexanone (1 : 1), 1a·3‐methylcyclopentanone (1 : 1), 1a·2‐methylcyclohexanone (1 : 1), 1b·butyronitrile (1 : 1), 1b·propan‐1‐ol (2 : 1) and 1b·(–)‐fenchone (1 : 1)] were determined by x‐ray diffraction analysis. In all the structures, the hydroxyl group is involved in intramolecular hydrogen bonds and the host and guest molecules are held by lattice forces only. The channels and cavities left in the host matrix are large enough to allow disorder or high thermal displacement parameters of the guest molecules. The local packing coefficients for all guests are 0·42 on average.
Kinetics and mechanism of reaction of acridine orange with bromateion at low pH
Authors: Sawunyama P. , Jonnalagadda S. B. 
Published: January 2005

AbstractThe reaction between acridine orange hydrochloride [3,6‐bis(dimethylamino)acridine hydrochloride] and potassium bromate in dilute sulphuric acid was studied by monitoring the absorbance change at 492 nm. The reaction exhibited complex kinetic behaviour. In excess bromate the reaction had an induction period with slow depletion of acridine orange (AC) followed by a fast depletion step. The initial reaction step was found to be first order with respect to both AC and bromate and second order with respect to H+. The stoichiometric ratio of AC to bromate was 3 : 2. At the end of the induction period, the redox potential of the reaction mixture has shown a distinct rise, while the Br− concentration dropped sharply. The dual role of bromide ion both as an inhibitor and autocatalyst is discussed. A 16‐step reaction mechanism is proposed and the simulated curves agreed well with the kinetic curves.
Unusually large kinetic deuterium isotope effects on oxidation reactions. 1. The mechanism of hydroxide‐catalysed permanganate oxidation of PhCD(CF3)OH and PhCD(CH3)OH in Water
Authors: Thibblin Alf 
Published: January 2005

AbstractThe oxidation of 1‐phenyl‐2,2,2‐trifluoroethanol (ROH) with potassium permanganate in 0.2 M aqueous sodium hydroxide solution at 25 °C provides 2,2,2‐trifluoroacetophenone as the sole product. The reaction rate constant, which was measured under pseudo‐first order conditions by following the decrease in substrate concentration as a function of time, was found to be substantially smaller than previously reported. The primary kinetic deuterium isotope effect was measured as k/k = 14·7 ± 1·0. A hydrogen‐transfer mechanism involving significant tunnelling is proposed. The oxidation of 1‐phenylethanol under similar reaction conditions yields acetophenone as the initial product. The kinetic deuterium isotope effect was found to be k/k = 5·2 ± 0·8.
Introduction to living polymeriz. Living and/or controlled polymerization
Authors: Matyjaszewski Krzysztof 
Published: January 2005

AbstractThe effects of various imperfections such as slow initiation, termination, transfer and slow exchange on kinetics, molecular weights and polydispersities in chain growth polymerizations are simulated. The simukations demonstrate that well defined polymers polymers can be prepared in systems with chain‐breaking reactions. Thus, under carefully selected conditions, non‐living polyumerizations may provide controlled polymers. On the other hand, polymers with unpredictable molecular weights and broad and even polymodal molecular weight distributions can be formed in living systems without irreversible transfer and termination. In some living systems molecular weights may stay constant or even decrease with conversion. Therefore, a clear distrinction should be made between living and polymerizations.
Living carbocationic copolymerizations. I. Synthesis and characterization of isobutylene/p‐methylstyrene copolymers
Authors: Országh I. , Nagy A. , Kennedy J. P. 
Published: January 2005

AbstractThe isobutylene (IB)–p‐methylstyrene (pMeSt) monomer pair readily produces by living copolymerization nearly random copolymers with up to M̄n ≈ 50,000 g mol−1 and M̄w/M̄n ≈ 1·4. The livingness of the copolymerization process and the compositional homogeneity of the copolymers have been demonstrated by the following four diagnostic plots: (A) M̄n (number‐average molecular weight) and corresponding N (number of moles of copolymer formed) vs Wp (weight of copolymer formed) for the diagnosis of chaintransfer; (B) −ln (1 – C) (C = conversion of monomers) vs time for the diagnosis of termination; (C) copolymer composition vs molecular weight; and (D) cumulative copolymer composition vs conversion for the diagnosis of true homogeneous copolymers. The theory for the use of the −ln(1 – C) vs time plot for the quantitation of irreversible termination in copolymerization has been developed. The exact conditions (i.e. nature of reagents, reagent concentrations, temperature, etc.) which lead to desirable products in terms of conversion, molecular weight, molecular weight distribution (MWD) and copolymer composition, were developed systematically by analyzing the results of a large number of experiments. The following combination of chemicals was found to lead to living copolymerization, and uniform high molecular weight, narrow MWD copolymers: IB–pMeSt 97 : 3 mol/mol with 5‐tert‐butyl‐1,3‐dicumyl methyl ether (5‐tBu‐1,3‐DiCumOMe) initiator, TiCl4 coinitiator, ethyl chloride (EtCl) solvent, triethylamine (TEA) electron pair donor (ED) and 2,6‐di‐tert‐butylpyridine (DtBP) proton trap at −52, −65 and −85°C.
Polymerization of acrylates and methacrylates to make homopolmers and block copolymers initiated by N‐alkoxyphthalimides and succinimides
Authors: Druliner J. D. 
Published: January 2005

AbstractVarious N‐alkoxyphthalimides were prepared and tested as initiators for polymerization of a variety of monomers. Acrylates and methacrylates polymerized at 40–50°C. Vinyl acetate and styrene were moderately reactive at 60–65°C. Rates of reaction were sensitive to the choice of solvent. THF generally gave the highest conversions, with much reduced reactivities in solvents of higher polarity, such as acetonitrile, acetone, DMSO or DMF. In aliphatic hydrocarbon solvents or di‐n‐butyl ether no polymers were obtained. Bulk polymerizations proceeded well, depending on the monomer. Rates of polymerization were sensitive to the structure of the group attached to nitrogen. OR groups, where R contained α‐electron‐withdrawing esters, benzylic or allylic groups, were more reactive than when R was an aliphatic group. A–B block copolymers were readily prepared, starting with PMA, PBA or PMMA. However, because the rates of initiation were slow, relative to rates of propagation, N‐alkoxyphthalimide initiators survived mostly intact. Consequently, final A–B block copolymers were invariably contaminated with indeterminate amounts of homopolymer from polymerization of the second monomer.
Ab initio molecular orbital study of the O + C6H5O reaction
Authors: Lin M. C. , Mebel A. M. 
Published: January 2005

AbstractAn ab initio molecular 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 C6 ring in the ortho‐ or para position 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 are o/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 of o/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 C5H5 and 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 as ktot = 5·52×10−17 T1·38 e+148/T cm3 mol−1 s−1.
Kinetics and mechanism of oxidation of (arylthio)acetic acids by pyridinium hydrobromide perbromide
Authors: Karunakaran K. , Elango K. P. 
Published: January 2005

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 of para and meta‐substituted (phenylthio)acetic acids were correlated with Hammett's substituent constants. The ρ value is −1·60 at 35°C. The rates of oxidation of ortho substituted 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 is proposed.
Competitive product inhibition in a clay‐catalysed Diels–Alder reaction
Authors: Collet Christine , Laszlo Pierre 
Published: January 2005

AbstractThe normal electron‐demand Diels–Alder cycloaddition between cyclopentadiene and methyl vinyl ketone is inhibited by the cycloadduct, when catalyzed by kaolinite‐supported zinc chloride. This inhibition stems from occupation of the catalytic sites by both the cycloadduct and methyl vinyl ketone. Catalysis occurs in this case exclusively by a decrease in the enthalpy of activation.
Theoretical studies of the identity allyl transfer reactions
Authors: Lee Ikchoon , Kim Chang Kon , Lee Bon‐Su 
Published: January 2005

AbstractAb initio studies on the SN2 identity exchange reactions RCH2X + X− → X− + RCH2X for R = CH2CH with X− = H, NH2, OH, F, PH2, SH and Cl, and for R = CH3 and CHC with X− = Cl were carried out at the HF and MP2 levels using the 6–31 ++ G** basis sets. The activation barriers, ΔE≠, and major structural changes, Δd≠ (C–X), in the activation process are closely related to the electronegativity of the R and X groups. The effect of electronegativity is twofold: a stronger electronegativity of R and/or X leads to a lesser electronic as well as structural reorganization required in the activation and to a greater correlation energy in the transition state. The former effect lowers the energy barriers at both the HF and MP2 levels whereas the latter lowers only the correlated (MP2) activation energies. Results with R = CH2CH, as a model for R = C6H5, indicate that ‘benzylic effect’ arises mainly from the relatively stronger electron acceptor ability of the phenyl group.
Atomic force microscopy and Solid‐State rearrangement of benzopinacol
Authors: Kaupp Gerd , Haak Michael , Toda Fumio 
Published: January 2005

AbstractThe solid–solid chemical reaction of benzopinacol (1) with p‐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 after reaction.
Mechanism of electrophilic aromatic substitutions: Comments on a recent criticism concerning the use of the mesitylene/durene reactivity ratio as a probe to distinguish between ionic and et pathways
Authors: Baciocchi Enrico , Galli Carlo 
Published: January 2005

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 many SEAr 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 justified.
ortho‐Substituent effects in N‐arylacetamides. NMR and molecular mechanics investigation
Authors: Kolehmainen Erkki , Laihia Katri , Kauppinen Reijo , Rasała Danuta , Puchała Agnigeszka 
Published: January 2005

Abstract1H, 13C, 15N and 17O NMR spectra of N‐phenylacetamide (acetanilide) and 21 ortho‐substituted acetanilides were measured and assigned. The observed NMR parameters are related to the Hammett substituent parameters and conformational characteristics of the acetamido moiety estimated by molecular mechanics calculations. Significant relationships were found for the 13C NMR chemical shifts of C‐5 (para to substituent) and the direct spin–spin coupling constant, 1J(C, H), of C‐3 (ortho to substituent) with Hammett substituent parameters. For 15N NMR chemical shifts of the amido nitrogen, no general correlation with the Hammett substituent parameters was found. The interactions between functionalities contiguous to the carbonyl group and the amino nitrogen in acetanilides are effectively hampered owing to the increased twist angle between the planes containing the phenyl ring and the H–N–C fragment in derivatives bearing bulky ortho substituents. Especially in ortho‐halogen‐substituted derivatives the 15N NMR chemical shift of the amino nitrogen is very clearly related to the twist angle between the phenyl ring and H–N–C fragment. For 17O NMR chemical shifts of the acetamido carbonyl, the observed variations can be related to the steric inhibition of resonance between the benzene ring and the acetamido group.
Radiolytic studies of the reductive cyclization of 2‐nitroarylamides: Cyclization via hydroxylamine intermediates
Authors: Sykes Bridget M. , Atwell Graham J. , Denny William A. , O'Connor Charmian J. 
Published: January 2005

AbstractThe reductive cyclization of several 2‐nitroarylamides was studied by radiolytic reduction, examining the effects of substituents on the nitrophenyl ring and on the leaving aniline and variations in the nature of the link between the nitrophenyl ring and the leaving aniline. The stoichiometry of the reduction and the identification of N‐hydroxylactam and aniline products suggest that the major initial products of such a reduction of the nitroamides are the corresponding hydroxylamines. Under anaerobic conditions, cyclization via the hydroxylamines was considerably faster (up to 160‐fold) than via the corresponding amines under comparable conditions, but was similarly influenced by changes in geometry. Unlike cyclization via the amines, rates of cyclization via the hydroxylamines were sensitive to substitution on the leaving aniline, being accelerated by electron‐withdrawing groups. The rate‐determining step in the cyclization of the hydroxylamines is proposed to be breakdown of the tetrahedral intermediate.
Mono‐, bis‐ and tetra‐acridine ligands: Synthesis, X‐ray structural determination and dynamic fluorescence microscopic studies on the modification of the higher order structure of DNA
Authors: Atwell Graham J. , Denny William A. , Clark George R. , O'Connor Charmian J. , Matsuzawa Yukiko , Yoshikawa Kenichi 
Published: January 2005

AbstractA series of mono‐, bis‐ and tetra‐acridine ligands were prepared and their effects on the higher order structure of DNA were studied by dynamic fluorescence microscopy. The single‐crystal structure of the bis‐acridine derivative N‐[2‐(dimethylamino) ethyl]‐4‐[2‐(9‐acridinylamino)benzamido]‐2‐(9‐acridinylamino)benzamide trihydrochloride (4) was determined, and shows that the molecule is sufficiently flexible to fold into an intramolecular stacking interaction in the crystal, supporting earlier hydrodynamic evidence that this compound can bis‐intercalate into DNA forming a single base pair (bp) sandwich complex. The corresponding tetra‐acridine analogue 1,11‐bis[4‐[2‐(9‐acridinylamino)benzamido]‐2‐(9‐acridinylamino)phenyl]‐1,11‐dioxo‐6‐methyl‐2,6,10‐triazaundecane pentahydrochloride (6) was synthesized, and dynamic fluorescence microscopy was used to study the effects of 4 and 6 on the higher order structure of large T4 DNA molecules (166 kbp), by measuring the average long‐axis legth (persistence length, l) of the complexes. The mono‐intercalating ligand acridine orange (5) increase l, whereas the bisintercalating diacridine 4 has no apparent effect and the putative multi‐intercalating tetracridine derivative 6 decreases l by compacting the higher order DNA structure. These results demonstrate the usefulness of the technique for directly observing ligand–DNA complexes, and show that ligands with suitably positioned multiple binding sites can influence the higher order structure of DNA (and thus possibly gene expression).
Aminolysis and hydrolysis of sulphamate esters: Substantial NS bonding in the transition state leading to Nsulfonylamines
Authors: Spillane William J. , Hogan Geraldine , McGrath Paul 
Published: January 2005

AbstractThe aminolysis and hydrolysis of several sulphamate esters, RNHSO2ONp (R = PhCH2, Ph, 4‐MeC6H44, 3‐MeC6H4, 4‐FC6H4, 4‐ClC6H4, 3‐ClC6H4, H; Np = 4‐NO2C6H4) were been studied in 50% (v/v) aqueous acetonitrile at various temperatures. Reaction of the esters with an amine (R1NH2) gives −ONp and both sulphamide, (RNHSO2NHR1) and sulphamate (RNHSO2O−R1NH3+) products. First‐order rates were determined by the appearance of −ONp and sometimes also by the disapperance of ester. The reaction was found to be independent of amine type and concentration and at the high pHs that obtain the substrate esters are fully ionized. A Hammett ρacyl of −;1·8 was obtained for the decomposition of the sulphamate anions and this is consistent with substantial NS bonding in the transition state leading to N‐sulphonylamine, RNSO2. This intermediate then partitions very rapidly, reacting with R1NH2 and H2O respectively. ΔH‡, ΔS‡ and a deuterium solvent isotope effect were determined and were also interpreted in favour of the proposed mechanism. The dimethyl sulphamate ester (Me2NSO2ONp) does not react under the conditions used.
Molecular complexes between octaethyltetrathiaporphyrin dication and electron donors: A spectroscopic and electrochemical study
Authors: Mahammed Atif , Rabinovitz Mordecai , Willner Itamar , Vogel Emanuel , Pohl Michael 
Published: January 2005

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 the polyelectrolyte.
Molecular aggregates between octaethyltetrathiaporphyrin dication (OTP2+) and octaethylporphyrin (H2OEP) and its metal complexes
Authors: Mahammed Atif , Rabinovitz Mordecai , Hoffman Roy E. , Willner Itamar , Vogel Emanuel , Pohl Michael 
Published: January 2005

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 is Ea = 16·5 kcal mol−1 (1 kcal = 4·184 kJ). The association constant of the complex (OTP2+)2(H2OEP) is K1 = 1·3 × 1010 M−2 and the equilibrium constant between the two coexisting intermolecular complexes (OTP2+)4(H2OEP) and (OTP2+)2(H2OEP) is K2 = 7·4.