bluuebunny/crossref_metadata_2025_split huggingface dataset:
Concentration‐dependent deuterium isotope effects on the quantum yield of arene–alkene meta photocycloaddition: The role of excimers
Authors: De Vaal P. , Lodder G. , Cornelisse J. 
Published: January 2005

AbstractA deuterium isotope effect on the arene–alkene meta photocycloaddition, reported earlier for toluene/toluene‐d8 photoaddition to cyclopentene, was found to depend on the concentration of both arene and alkene. This phenomenon is explained in terms of an unbalanced dissociation of ‘mixed’ excimers, which play a role in the intermolecular competition experiments.
Influence of structure on the isomerization of dialkylaminoalkylbenzotriazoles
Authors: Katritzky Alan R. , Jóźwiak Andrzej , Saczewski Franciszek , Yannakopoulou Konstantina 
Published: January 2005

AbstractThe N‐1 to N‐2 substituent isomerization in a series of 4‐methyl and 4,7‐dimethyl‐N‐(α‐morpholinoalkyl)benzotriazoles and in 1,5‐bis(morpholinomethyl)benzo [1,2‐d:4,5‐d′] bistriazole was studied and the influence of the structure on this process was evaluated.
Configurations and dipole moments of O‐substituted hydroximoyl cyanides. Stereoelectronic effects in the ground state of oximino compounds
Authors: Johnson James Elver , Todd Susan L. , Ghafouripour Abdolkarim , Arfan Mohammad , Hamilton Walter S. , Exner Otto 
Published: January 2005

AbstractThe dipole moments of the Z and E isomers of O‐methylbenzohydroximoyl cyanides [ArC(CN)NOCH3, Ar = C6H5 and 4‐NO2C6H4], the Z isomers of benzohydroximoyl cyanides [ArC(CN)NOH, AR = C6H5 and 4‐NO2C6H4] and the Z isomers of O‐benzoylbenzohydroximoyl cyanides [ArC(CN) = NOCOC6H5, Ar = C6H5 and 4‐NO2C6H4] were measured. An analysis of these dipole moments indicates that anti delocalization from the OCH3 (OH) group has a small but significant effect on the overall direction and magnitude of the dipole moment. Semi‐empirical molecular orbital calculations at the MNDO level support this proposal. The x‐ray crystal structure of (Z)‐O‐methyl‐p‐nitrobenzohydroximoyl cyanide was carried out to confirm the configuration of the compound.
Cycloadditions of 2‐diazopropane to bicyclo[2.2.1]hept‐2‐enes. Direct experimental evidence for an asynchronous mechanism
Authors: Majchrzak Michael W. , Warkentin John 
Published: January 2005

Abstract2‐Diazopropane, generated by photolysis of 2‐methoxy‐2,5,5‐trimethyl‐Δ3‐1,3,4‐oxadiazoline, was trapped in situ by cycloaddition to norbornene, endo‐ and exo‐5‐methylnorbornene, endo‐ and exo‐5‐phenylnorbornene, endo‐ and exo‐5‐methoxycarbonyl norbornene and endo‐ and exo‐dicyclopentadiene. In all cases, only exo addition was observed. In spite of the exo approach of diazopropane to the norbornene double bond, endo substituents at C‐5 and/or C‐6 influenced the regiochemistry of addition whereas exo substituents did not. The results are interpreted in terms of a concerted, asynchronous mechanism in which CC bond formation runs well ahead of NC bond formation, as predicted from theory. The regiochemical control exercised by apparently remote endo substituents provides a new experimental criterion for asynchrony or synchrony of cycloadditions that are known to be concerted.
Thermochemical data of carbonyl compounds via their bond separation energies
Authors: Ibrahim Mustafa R. 
Published: January 2005

AbstractTwo types of equivalents for the aldo and the keto groups have been developed, the Schleyer and the bond separation energy (BSE) equivalents. The equivalents were used to give the heats of formation and the BSE of unstrained models. These equivalents were then used in the calculations of the strain energies for aldehydes and ketones. The results obtained were compared with MM2 values. The equivalents were also used in calculations of the stabilization (destabilization) energies for conjugated carbonyl compounds. The results obtained correlate well with the available experimental data.
Electronic factors in the elimination kinetics of 2‐halosubstituted methanesulphonates in the gas phase
Authors: Alvarez G. Jaime , Chuchani Gabriel 
Published: January 2005

AbstractThe kinetics of the gas‐phase elimination of two 2‐haloethyl methanesulphonates were determined in a static system over the temperature range 310–380°C and pressure range 26–174 Torr. The reactions in seasoned vessels, with the free‐radical inhibitor propene and/or toluene always present, are homogeneous and unimolecular, and follow a first‐order rate law. The rate coefficients are given by the Arrhenius equations: for 2‐bromoethyl methanesulphonate log k1 (s−1) = (11·70 ± 0·43)–(172·8 ± 4·8) kJ mol−1 (2·303RT)−1 and for 2‐chloroethyl methanesulphonate log k1 (s−1) = (11·67 ± 0·50)–(173·9 ± 6·0) kJ mol−1 (2·303RT)−1. The bromo compound was found to be more reactive than the chloro compound. At 340°C, for Br k1 = 9·46 × 10−4 s−1 and for Cl k1 = 7·12 × 10−4 s−1. Apparently, electronic factors seem to be operating in these elimination reactions.
Structure of 1,2,6‐thiadiazine 1,1‐dioxides
Authors: Elguero José , Goya Pilar , Martínez Ana , Rozas Isabel , Mó Otilia , De Paz José Luis G. , Yáñez Manuel 
Published: January 2005

AbstractAb initio theoretical calculations were carried out on the three tautomers, NH, OH and CH, of 1,2,6‐thiadiazine 1,1‐dioxides. Different basis sets were employed in order to obtain an adequate description of these cyclic sulphamide derivatives. A coherent picture is obtained which includes relative stability of tautomers (NH > CH > OH), non‐planarity of NH and CH tautomers and electronic distribution (Boys' localized orbitals).
Kinetics and mechanism of acid hydrolysis of 2‐carboethoxypyridine N‐oxides
Authors: Brycki Bogumil , Brzezinski Bogumil 
Published: January 2005

AbstractRate constants and activation parameters have been measured for the acid hydrolysis of six 2‐carboethoxypyridine N‐oxides. The correlations of log kobs. vs Hammett's constants and chemical shifts of intramolecular hydrogen‐bonded protons in parent acids are analysed. Evidence was found for the participation of intramolecular hydrogen bonding in the hydrolysis of the esters.
Role of exciplex and ion pair in the photosensitized oxygenation of 1,4‐diphenyl‐1,3‐butadiene
Authors: Takahashi Y. , Wakamatsu K. , Kikuchi K. , Miyashi T. 
Published: January 2005

Abstract9,10‐Dicyanoanthracene (DCA) sensitized the photooxygenation of 1,4‐diphenyl‐1,3‐butadiene(1a) and its 4,4′‐dimethoxy derivative (1b) to afford the corresponding endoperoxides 2 and the other oxidized products such as aldehydes and epoxides. The mechanism of the DCA‐sensitized photooxygenation of 1 was diversified by solvent polarity. In non‐polar solvents the reaction involves an exciplex intermediate, which leads to formation of triplet DCA (3DCA*) with an efficiency of 0·64 in the case of 1a. The resulting 3DCA* acts as a sensitizer for the generation of singlet oxygen (1O2) and thus 1O2 reaction occurs exclusively to give 2. On the other hand, single electron‐transfer quenching of 1DCA* by 1 is feasible in polar acetonitrile and a primary ion pair can be formed. Competing with fast back electron‐transfer deactivation, the primary ion pair produces free ions in limited yield to furnish electron transfer oxygenation. In the case of the DCA–1a system, free ions were produced with an efficiency of ca 4%. Thus, in acetonitrile, electron‐transfer oxygenation partly took place to give the other oxidized products, whereas the 1O2 pathway was still valid and responsible for the formation of 2. The effect of solvent polarity was apparently less pronounced in the case of 1b because the reactivity of 1b toward 1O2 is about five times higher than tht of 1a. For such 1O2‐reactive substrates, the electron‐transfer pathway would become dominant only when the substrate concentration is impractically high.
Roles of kinetic and potential energies in conjugation II. Aniline and nitrobenzene
Authors: Ichikawa Hiroshi , Sameshima Keiichiro 
Published: January 2005

AbstractAn analysis of the energy components of the conjugation energy between the substituent and the phenyl group in aniline and nitrobenzene showed that there are two types of conjugation: the conjugation energy in aniline, with an electron‐donating substituent, is given by the decrease in the kinetic energy of elctrons, whereas that in nitrobenzene, with an electron‐withdrawing substituent, is caused by the decrease in the potential energy. Physical interpretations are given of the arrows in the organic electron theory of these compounds.
The spin delocalization substitutent parameter σJJ. 5. Correlation analysis of 19F chemical shifts of substituted trifluorostyrenes. The unresolved polar substituent parameter σmb
Authors: Ji Guo‐Zhen , Jiang Xi‐Kui , Zhang Yu‐Huang , Yuan Shen‐Gan , Yu Chong‐Xi , Shi Yi‐Qun , Zhang Xiu‐Lian , Shi Wen‐Ting 
Published: January 2005

AbstractThe 19F NMR spectra of 30 substituted α,β,β‐trifluorostyrenes (Y‐TFSs, 1) covering a diverse range of substituents are reported and discussed. Values (δF1, δF2, Δδ3 − 1 and Δδ3 − 2) derived from the 19F NMR chemical shifts of the fluorine atoms of 1 are found to correlate very well with σI and σ. The results of the correlations of the 19F chemical shifts with dual (σF and σR) and triple parameter (σF, σR and σα) treatments are compared. An unresolved substituent parameter σmb, is proposed for applications to systems in which the substitutent Y interacts with a multiple bond and is compared with σ+.
Electrochemical and photoelectronic spectral study of compounds with high ionization potentials: Anodic oxidation of vinyl triflates in aprotic solvents
Authors: Kowalski Mark H. , Pons Joseph W. , Stang Peter J. , Pons Stanley , Werstiuk N. H. , Ashley Kevin 
Published: January 2005

AbstractMicroelectrodes can be used to measure redox half‐wave potentials in aprotic solvents containing no purposely added supporting electrolyte. By employing an electrode of sufficiently small size, the accessible potential range in solution is considerably extended. The electrochemical oxidation of vinyl (enol) triflates, which are oxidized at high electrode potentials, can therefore be studied using an ultramicroelectrode. Oxidation and ionization potentials, determined by ultramicroelectrode voltammetry and He I photoelectron spectroscopy, respectively, of 2‐methylprop‐1‐enyl, cyclohexenyl, cyclopentenyl, 1,1‐diphenylethenyl and prop‐2‐enyl triflate are reported. The results from electrochemical measurements and photoelectron spectra were compared.
Direct measurement of the relative rates of C‐protonation at unsubstituted and substituted α‐and/or β‐positions of a pyrrole ring
Authors: Terrier François , Chatrousse Alain‐Pierre , Jones John R. , Hunt Stephen W. , Buncel Erwin 
Published: January 2005

AbstractDirect kinetic measurements of the C‐protonation rates of 1,2,5‐trimethyl‐ and 3,4‐dimethyl‐pyrroles allow the first comparison of the relative kinetic proton affinities of unsubstituted and substituted positions of the pyrrole ring.
Benzyloxyfluorocarbene: Resistance to fragmentation
Authors: Moss Robert A. , Zdrojewski Tadeusz 
Published: January 2005

AbstractBenzyloxyfluorocarbene resists fragmentation to benzyl fluoride, and can be intercepted by water, fluoride ion, methanol or acrylonitrile. In contrast, benzyloxybromocarbene and benzyloxychlorocarbene fragment to the corresponding benzyl halides, and are not efficiently trapped.
Electrochemical reduction of substituted α,α,α‐trifluoroacetophenones. Linear relationship between cyclic voltammetric peak potentials and Hammett substituent constants
Authors: Yang Jye‐Shane , Liu Kwang‐Ting , Su Y. Oliver 
Published: January 2005

AbstractThe peak potentials of substituted α,α,α‐trifluoroacetophenones were measured in acetonitrile with 0·1 M tetrabutylammonium perchlorate. The cyclic voltammograms indicated an irreversible electrochemical process for the first wave (Epc1) and a partially reversible one for the second wave (Epc2) when the scan rate was slower than 100 mV s− 1. Excellent linear correlations were observed for Epc1 with σ constants (ρ = 0·526, r = 0·999) and for Epc2 with σ− constants (ρ = 0·605, r = 0·998), respectively. Therefore, unknown σ values such as −0·36 for 3, 4‐ethyleneoxy can be estimated from these correlations. The mechanism of the electrode process probably involves a single electron transfer and the formation of a pinacol.
The current status of isomer enumeration of useful benzenoids and a new topological paradigm
Authors: Dias Jerry Ray 
Published: January 2005

AbstractThe enumeration and development of conceptual tools for understanding the chemical properties of practical benzenoid hydrocarbons is stressed. Currently known isomer numbers for benzenoids having up to 60 carbons are summarized. Although there are well over 2 million isomers in this range, only approximately 500 benzenoid hydrocarbons have been characterized.
Kinetics of oxidation of amino acids by alkaline hexacyanoferrate(III)
Authors: Laloo Didcy , Mahanti Mahendra K. 
Published: January 2005

AbstractThe kinetics of the oxidation of α‐amino acids (glutamic acid and aspartic acid) by alkaline hexacyanoferrate(III) were studied at constant ionic strength and over the temperature range 323–348 K. The rate was dependent on the first power of the concentrations of substrate and oxidant, but was independent of the concentration of alkali in the range studied. The value of kH/kD was in the range 8·1–8·3 for the slow step, indicating the loss of a hydrogen atom from the CH bond, giving a radical species which was characterized by ESR spectroscopy. The reaction proceeds by way of the α‐imino acid, formed in a fast step, which undergoes hydrolysis to give the corresponding α‐keto acid.
Development of a model to explain the influence of the solvent on the rate and selectivity of diels–alder reactions
Authors: Cativiela C. , Garcia J. I. , Mayoral J. A. , Avenoza A. , Peregrina J. M. , Roy M. A. 
Published: January 2005

AbstractSolvophobic and polarity parameters are used to explain the endo/exo selectivity of the reaction between cyclopentadiene and methyl acrylate. A good linear regression model with Sp and E is obtained. The existence of an intrinsic correlation between the Sp and E values, which makes the interpretation of the results difficult, is shown. A comparison of the results with other previously reported showed that the relative influence of Sp and E on selectivity depends on the nature of the reagents.
Crystal and molecular structure of pyrylium salts. IV. Crystal and molecular structure of 2,6‐diphenyl‐4‐(4‐carboxyphenyl)pyrylium perchlorate. Interrelations between structural parameters due to substituent effects
Authors: Krygowski Tadeusz Marek , Anulewicz Romana , Pniewska Barbara , Milart Piotr 
Published: January 2005

AbstractThe crystal and molecular structure of 2,6‐diphenyl‐4‐(4‐carboxyphenyl)pyrylium perchlorate was solved by x‐ray diffraction, yielding structural parameters of high precision (estimated standard deviations for bond length ≤ 0.5 pm, for bond angles ≤ 0.2°). Comparison with other 4‐substituted derivatives of 2,6‐diphenyl‐4(4‐carboxytphenylpyrylium) perchlorate shows regularities in the variations of structural parameters following classical views of substituent effects.
Structure and dynamics of dicyandiamide: A theoretical study
Authors: Bach Robert D. , McDouall Joseph J. W. , Owensby Amy L. , Schlegel H. Bernhard , Holubka Joseph W. , Ball James C. 
Published: January 2005

AbstractAb initio MO methods have been used to study the structures and energetics of dicyandiamide, [(NH2)2CNCN], its isomers, protonated species, radical anions, transition structures for internal conformational change and transition structures for isomerization. Structures were optimized at the HF/STO‐3G, HF/3–21G and HF/6–31G* levels; selected barrier heights for smaller analogues were also computed at the MP4SDTQ/6–31G* level. The most stable isomer of dicyandiamide has the cyano group on the imine nitrogen [1, (NH2)2CNCN]; the other isomer [2, HNC(NH2)NHCN] lies 12.8 kcal mol−1 higher. Inversion at the imino nitrogen proceeds by a linear, in plane process with a barrier of 32.5 kcal mol−1. The amino rotation barriers are 19 kcal mol−1 (single NH2) and 40 kcal mol−1 (both NH2 in a conrotaory or a disrotatory fashion; if the NH2 groups are allowed to pyramidalize the disrotatory barrier drops to 20 kcal mol)−1. Protonation occurs preferentially on the imine nitrogen (PA = 219.7 kcal mol−1 for 1); the proton affinities PA of the amino nitrogens are 25–30 kcal mol−1 lower. Isomerization between 2 and 1 would go via a 1,3‐sigmatropic hydrogen shift, but the barrier is high (48.3 kcal mol−1); protonation reduces the hydrogen shift barrier by ca 15 kcal mol−1. However, the most likely mechanism for isomerization involves protonation of the imine nitrogen in 2 followed by deprotonation of the cyano‐substituted nitrogen to form 1, circumventing the energetically costly 1,3‐sigmatropic hydrogen shift. When an electron is transferred to dicyandiamide, a sizeable fraction of the resonance stabilization of the guanidine moiety is lost.
Theoretical study of simple push–pull ethylenes in solution
Authors: Pappalardo Rafael R. , Marcos Enrique Sánchez , Ruiz‐Lóapez Manuel F. , Rinaldi Daniel , Rivail Jean‐Louis 
Published: January 2005

AbstractThe influence of solvation on the Z—E isomerization process of three representative molecules of simple push‐pull ethylenes [H2N(H)C1=C2(H)R = NO2, COH and CN] derived from aminoethylene was investigated by means of RHF‐SCF ab initio calculations at the 3–21 + G level. Solute‐solvent interactions were modelled by a cavity model. The shape of the cavity is based on electronic isodensity surfaces. By using an ellipsoidal cavity very close to the isodensity surface, the perturbation due to the solvent takes an analytical form which is incorporated into the Hartree‐Fock equations and leads to efficient quantum chemical computations. The polarization of the solutes under the influence of the solvent is noticeable and was analysed in detail. Similarly, the barriers to internal rotations are substantially modified by the solvent: the barrier around the CC double bond is appreciably decreased in the thermal mechanism whereas its lowering is less important in the anionic mechanism; in contrast, the barrier around the C‐1N bond is slightly increased. The variation of the barriers with the nature of the acceptor group is fairly well reproduced by the computations. The electronic structure of the push‐pull ethylene molecules and the modifications of this structure under the influence of the solvent are analysed in detail.
Isotope effects in acid‐catalyzed aromatic hydrogen exchange
Authors: Cox Robin A. 
Published: January 2005

AbstractDeuterium‐ or tritium‐substituted aromatics undergo isotope exchange in fairly concentrated aqueous sulfuric acid media. The rate constant for this process is complex, being composed of that for the slow electrophilic attack, and the isotope effect on the breakup of the Wheland intermediate. Using experimental rate constant data for both deuterium and tritium exchange on the same substrate, the excess acidity method and the Swain–Schaad relationship, true protonation rate constants and isotope effects have been separated out for several positions in several simple aromatic substrates. Most of the electrophilic attack rate constants are described by a Hammett‐type linear free energy relationship with a ρ+ of − 6.5. The average observed deuterium isotope effect on Wheland intermediate breakup was 5.3. Using rate constants obtained at different temperatures, enthalpies and entropies of activation were also obtained for benzene, naphthalene and thiophene; the activation entropy averaged − 8 cal mol−1 K−1. Some evidence was found for general acid catalysis at the higher acidities.
Structure, topological electron density analysis and aromaticity of 4‐heterosubstituted methylenecyclopropenes: X XCH2, NH, O, SiH2, PH and S
Authors: Bachrach Steven M. , Liu Meixiao 
Published: January 2005

AbstractThe structures of a series of heterosubstituted methylenecyclopropenes and methylenecyclopropanes were optimized at the HF/6–31G* level. All methylenecyclopropenes are planar except for the silicon analogue, which is bent at both C‐3 and Si. The planar silicon structure is a transition state. The relative aromaticity of these compounds were evaluated using the C‐1C‐3 bond length, the integrated charge on C‐3 and the heteroatom, and delocalization energy. Second‐row systems have slightly larger delocalization energies than their first‐row counterparts, owing to their larger polarizability. Using these criteria, methylenecyclopropene and the silicon analogue are not aromatic and the N, O, P and S analogues are moderately aromatic. The planar silicon analogue is antiaromatic and bends from planarity to reduce this antiaromaticity.
Oxygen‐17 NMR of selected N‐Nitrosamines. A preliminary report
Authors: Cerioni Giovanni 
Published: January 2005

AbstractOxygen‐17 NMR spectra of some N‐nitrosamines are discussed in terms of steric and conjugative effects. Relationships have been observed between rotational barriers, conformations and 17O chemical shifts. A tentative linear correlation has been determined between repulsive Van der Waals interactions and some N‐nitrosopiperidines.
NMR study of polycrystalline azoles
Authors: Zavelsky Vladimir O. , Babin Valerii N. , Vysokovskaya Mariya S. , Fedin Erlen I. 
Published: January 2005

AbstractPolycrystalline pyrazole, imidazole and benzotriazole were investigated by 1H and 2H NMR spectroscopy. In addition to the usual broad line of the 1H NMR signal due to the dipole‐dipole interaction, a significantly narrowed component (δν = 1–2 kHz) of a complex triplet structure was detected in all three compounds. To interpret this observation, a mechanism with virtually complete averaging of the dipole‐dipole interaction for a small part of the sample protons is suggested. The mechanism is based on proton interaction with the unpaired electron of a short‐lived radical of the type Az· or (H–Az·–H).
Theoretical studies on the gas‐phase smiles rearrangement
Authors: Kim Chang Kon , Lee Ikchoon , Lee Bon‐Su 
Published: January 2005

AbstractGas‐phase Smiles rearrangements and SNi processes of ions C6H5X(CH2)nY− (X, Y = 0 or S, and n = 2–4) have been studied by the AM1 MO method. The Smiles rearrangement to an ortho position of the phenyl ring is diffcult owing to involvement of a high‐energy barrier process involving a 1,2‐hydrogen shift. The reactivity of the Smiles rearrangement to the ipso position increases in the order X  O and Y  S < X  S and Y  O < X  Y  0 as a result of steric and electronic effects in the transition state (TS). The reactivity order with respect to the side‐chain length, n = 3 < 4 < 2, is mainly determined by ring strain in the TS. For the SNi process, in which Y attacks the β‐carbon (β to phenyl), the enthalpy (δH≠) factor becomes more favourable but the entropy (δS≠) factor becomes unfavourable with an increase in the side‐chain length, in agreement with a general trend for cyclization processes. The product ratio in the Smiles rearrangement and the SNi process is largely controlled by the thermodynamic stabilities of the products, as observed in gas‐phase experiments.