bluuebunny/crossref_metadata_2025 huggingface dataset:
The double photodissociation of a geminal dichloride: Evidence for the stepwise formation of a diaryl carbene
Authors: Haider Karl W. , Platz M. S.
Published: January 2005

Photolysis of dichlorodiphenylmethane in glassy 2‐methyltetrahydrofuran at 77 K results in the formation of diphenylcarbene and the diphenylchloromethyl radical, which were detected by their fluorescence emission and excitation spectra. The relative yields of the carbene and biradical are shown to vary dramatically as a function of photolysis time. The photolability of the diphenylchloromethyl radical is also demonstrated. These results were interpreted in terms of a two‐step mechanism, in which the diphenylchloromethyl radical is an intermediate in the formation of diphenylcarbene.
Calorimetric study of the effect of N‐methylation in azoles: Loss of an active centre of solvation
Authors: Catalan Javier , Cabildo Pilar , Elguero José , Gómez Javier , Laynez José
Published: January 2005

Using a series of equations connecting experimental and theoretical values, it is possible to discuss the origin of the N‐methylation effect in azoles dissolved in water and dimethyl sulphoxide. The existence in the azoles studied of a linear relationship between the gas → solution transfer enthalpies and the charge on the pyrrole hydrogen atom demonstrated the fundamental importance of the loss of an active centre for solvation. For the imidazole–N‐methylimidazole pair, the complete thermochemical cycle has been determined, allowing the apparent lack of an effect of N‐methylation on the basicity in solution to be discussed.
Mechanisms of aromatic nucleophilic substitution reactions in ethyl acetate and tetrahydrofuran
Authors: Akinyele Elizabeth T. , Onyido Ikenna , Hirst J.
Published: January 2005

The kinetics of the reactions of 1‐chloro‐, 1‐fluoro‐ and 1‐phenoxy‐2,4‐dinitrobenzene with piperidine, n‐butylamine and benzylamine, and in the case of the ether, morpholine were studied as functions of nucleophile, DABCO and pyridine concentrations in tetrahydrofuran and ethyl acetate. The reactions of the ether with n‐butylamine and benzylamine in benzene were also studied as functions of nucleophile, DABCO and pyridine concentrations. A comparison with results in the literature indicated that the reactions in tetrahydrofuran and ethyl acetate resemble those in dipolar aprotic solvents when primary amines are the nucleophiles and those in aprotic solvents when the nucleophile is a secondary amine. An explanation is suggested for the observation that whereas the reactions of 1‐phenoxy‐2,4‐dinitrobenzene with piperidine and morpholine in both tetrahydrofuran and ethyl acetate are strongly catalysed by the nucleophiles, they are not catalysed by pyridine and there is either extremely weak or no catalysis by DABCO.
The kinetic deuterium isotope effect of proton‐transfer reactions in solution. Comparison of the marcus and the quantum‐statistical mechanical models
Authors: Sühnel Jürgen
Published: January 2005

The free energy dependence of the kinetic deuterium isotope effect of proton‐transfer reactions in solution is usually analysed in terms of the Marcus model. By means of modern computer techniques, the more general quantumstatistical mechanical model is also amenable to routine application. The differnces in the basic assumptions of the two approaches concerning the kinetic isotope effect are discussed. Reaction series for which both the Brønsted relation and the free‐energy dependence of the kinetic isotope effect are available are analysed by means of the two models using one set of parameters for both free‐energy relations.
Gas‐phase nitration of thiophene and N‐methylpyrrole by protonated alkyl nitrates
Authors: Attiná Marina
Published: January 2005

Gas‐phase nitration of thiophene (1) and N‐Methylpyrrole (2) was studied by radiolytic tecniques, supported by chemical ionization mass spectrometry. The substrate and positional selectvities of protonated alkyl nitrtes (CH3NO3H+, CF3CH2NO3H+) were deduced from competitive reactions with bezene performed under different conditions. The apparent kS/kB ratios, which are independent of the pressure and unaffected by the presence of a strong base (NEt3), depend on the nature of the electrophile, passing for 1 and 2 from 7·2 and 6·4, respectively, with CH3NO3H+ to 1·1 and 1·0, respectively, with the more reactive fluorinated caution. Predominant (88%) α‐substitution takes place in 1, whereas no appreciable positional discrimination is displaced by 2. The results suggest that the electrophilic attack can also occur a the heteroatom, and point to the same mechanism postulated for gasphase nitration of aromatic substrates, the reactivity being essentially governed by electrostatic interactions within the ‘encounter pair’.
syn–anti Isomerism in the 1,3‐dipolar cycloaddition to cis‐3,4‐disubstituted cyclobutenes. 4. Intramolecular interactions
Authors: Rastelli Augusto , Burdisso Marina , Gandolfi Remo
Published: January 2005

Ab initio (4‐31G) molecular geometry optimizations on cis‐3,4‐disubstituted cyclobutenes show that the extent and direction of their ground‐state pyramidalization depend on the type of substituents and on whether these substituents are part of a ring condensed on to the cyclobutene moiety. Thus, electron‐withdrawing substituents such as Cl and OMe bring about an out‐of‐plane anti bending of the olefinic hydrogens, whereas cyclobutenes condensed on to small carbocyclic rings exhib high syn bending. Intramolecular interactions are analysed in the frame of a new model where the interactions arise from the energy matrix elements between the hybrid atomic orbitals involved in the different bonds. The analysis is carried out by C‐INDO calculations on deformed molecules (with the olefinic hydrogens forced out‐of‐plane by ± 20°). These latter calculations reveal that non‐planarity of the double bond occurs in the direction of increasing interactions between the hybrid atomic orbitals involved in the σ‐allylic bonds and in the π‐bond. The increase in this kind of interaction can also explain non‐planarity in norbornene, 7‐oxanorbornene, 5,6‐dioxanorbornene and 5,6‐dioxabicyclo [2.2.2] oct‐2‐ene. The results strongly suggest that pyramidalization and the related energy asymmetry of out‐of‐plane bending of olefinic hydrogens are important factors in governing facial selectivity of 1,3‐dipolar cycloadditions to cis‐3,4‐disubstituted cyclobutenes.
Lone‐pair charges and structural effects
Authors: Catalán Javier , De Paz José Luis G.
Published: January 2005

The lone‐pair charge, Qlpf, of a base B is a (theoretically calculated) measure of the amount of charge on the lone pair of B that binds to an acid Av (v = 0, 1) in an acid–base reaction. It is shown how they can be used for the quantitative study of structural effects on gas‐phase proton affinities, vertical ionization potentials and hydrogen‐bonding basicities.
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

A 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

The 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

The 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

2‐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

Two 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

The 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

Ab 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

Rate 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

9,10‐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

An 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

The 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

Microelectrodes 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

Direct 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

Benzyloxyfluorocarbene 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

The 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

The 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

The 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

Solvophobic 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

The 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

Ab 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

The 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

Deuterium‐ 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

The 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

Oxygen‐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

Polycrystalline 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

Gas‐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.
Origin of apparent swain–schaad deviations in criteria for tunneling
Authors: Huskey W. Phillip
Published: January 2005

Vibrational analysis calculations have been conducted in an effort to understand the origins of apparently anomalous relationships reported between kH/kT and kD/kT kinetic isotope effects. The actual isotopic substitutions used in previous work are more complex than a simple comparison between H/T and D/T isotope effects. The relationships between the actual isotope effects determined in these studies amount to tests of two standards of conventional isotope effect theory, the rule of the geometric mean (or the lack of isotope effects on isotope effects) and the Swain—Schaad rule. Model calculations illustrate the importance of violations of the rule of the geometric mean (over Swain—Schaad deviations) arising in models that incorporate both explicit reaction‐coordinate coupling of two isotopic sites and reaction‐coordinate tunneling. Implications for experimental studies of turneling using apparent Swain—Schaad exponents are discussed.
Influence of some steric and electronic effects on the mechanism of SNAr reactions in dimethyl sulphoxide
Authors: Onyido Ikenna , Hirst Jack
Published: January 2005

The reactions of 1‐chloro‐ and 1‐fluoro‐2,4‐dinitrobenzenes with aniline and 4‐methylaniline in dimethyl sulphoxide are not base catalysed. When the nucleophile is 2‐methylaniline, the reaction of the fluoro compound is base catalysed whereas that of the chloro compound is not. The reactions of 1‐bromo‐2‐fluoro‐ and 1‐bromo‐2‐chloro‐3,5‐dinitrobenzenes with both aniline and 2‐methylaniline are not base catalysed. These results indicate that, in certain cases, the incidence or absence of base catalysis in aromatic nucleophilic substitution (SNAr) reactions in dipolar aprotic solvents results from an interplay of steric and electronic factors as they affect the magnitude of the (k2 + k3 [B])/k−1 ratio.
Factors affecting the reversibility of the electrophilic step in olefin bromination. The case of 5H‐dibenzo [a, d] cycloheptene
Authors: Bellucci Giuseppe , Chiappe Cinzia , Marioni Franco , Marchetti Fabio
Published: January 2005

The tendency of ion‐pair intermediates generated by treatment of trans‐10‐bromo‐10, 11‐dihydro‐11‐hydroxy‐5H‐dibenzo [a, d] cycloheptene (5) with HBr to release Br2 giving 5H‐dibenzo [a, d,] cycloheptene (1) has been evaluated. The observed 9:1 ratio between ion‐pair collapse to the trans‐dibromide (7) and Br2 release to olefin 1, as compared with the 3:7 ratio found with the 5H‐dibenz [b, f] azepine‐5‐carbonyl chloride system, has been interpreted as indicating a much lower propensity for reversibility of ion‐pair formation in the bromination of 1. The structural parameters obtained by x‐ray diffraction of the dibromide 7 show for this compound an angle strain at C‐10 and C‐11 analogous to that found in trans‐10, 11‐dibromo‐10, 11‐dihydro‐5H‐dibenz [b, f,] azepine‐5‐carbonyl chloride. Olefin 1 adds Br2 in 1,2‐dichloroethane at 25°C according to a third‐order rate law, with k3 = 30(3) M−2 s−1. About 10% of 5‐bromo‐5H,‐dibenzo [a, d] cycloheptene, arising from collapse of a dibenzo [a, d] tropylium ion intermediate (9), is formed in this reaction besides the expected trans‐dibromide 7. Spectral evidence for the formation of 9, arising by rearrangement of a first formed weakly bridged ionic intermediate in the reaction of 1 with Br2, is presented. the results obtained with the 5H‐dibenzo [a, d,] cycloheptene system have been rationalized on the basis of the formation of very weakly bridged intermediates having a much lower tendency to revert back to olefin and Br2 with respect to the corresponding symmetrically bridged bromonium ions.
Correlations between empirical lewis acid–base solvent parameters and the thermodynamic parameters of ion solvation. Part II. Acidity parameters of cations and basicity parameters of anions
Authors: Wrona Piotr K. , Krygowski Tadeusz M. , Galus Zbigniew
Published: January 2005

The parameters α, β and γ of the equation WΔsGf(MX) = αBKT + βET + γ were calculated and analysed for different salts (MX) in pure solvents and in non‐aqueous solvent–water mixtures (S). Among 109 different equations, only 64 were taken for the final analysis since in the case of 38 equations the parameters BKT and ET were highly correlated. After a statistical analysis and normalization, on the basis of the coefficients α and β obtained, it was found that the ions analysed may be divided into three classes characterized by the different dependences of α on β: class I, H+, Li+, Na+, Rb +, Cs+, Me4N+, Cl−, Br− and I−, where αN = 1.074βN + 1.639; class II, Ag+, Cl−, Br−, I−, OAc−, SCN− and N where αN = 2.057βN + 2.147; and class III, Ph4As+, BPh, Ph4C and Ph4Ge, where αN is variable and βN is almost constant. Two main conclusions can be drawn out from the results obtained: the behaviour of ions considered as Lewis acids and bases reflects different hard–soft properties of the ions, and the behaviour of the Ph4As+ ion is different from that of BPh.
Short Communications. Biphenyl‐3,4′‐Dinitrene
Authors: Minato Masaki , Lahti Paul M.
Published: January 2005

Photolysis of biphenyl‐3,4′ ‐diazide in a rigid glassy matrix at 77 K yields quintet state biphenyl‐3,4′ ‐dinitrene with zero‐field splitting parameters of |D/hc‐ = 0.153 cm−1 and |E/hc| = 0.019 cm−1 determined by electron spin resonance spectroscopic studies. Curie plot studies are consistent with assigning the quintet to be the ground state in this species. This finding confirms qualitative connectivity‐based predictions for this general connectivity type of openshell system, and is in qualitative agreement with spectral INDO‐CI computational predictions for both planar and twisted geometries of the dinitrene.
Experimental and theoretical studies of the effects of hydration on proton exchange equilibria
Authors: Bromilow J. , Marriott S. , Partridge A. , Taft R. W. , Topsom R. D.
Published: January 2005

Theoretical calculations with the ab initio molecular orbital STO‐3G basis set on the effect of trihydration on the relative acidities of 3‐ and 4‐substituted phenols, 4‐substituted quinuclidinium ions and 4‐substituted bicyclo [2.2.2]octylammonium ions are reported. These results are contrasted with calculations on non‐hydrated species and compared with results in aqueous solution and in the gas phase. Unlike results for smaller molecules such as substituted acetic acids and methylammonium ions, the solvation effects of three molecules of water are far short of that observed in going from the gas phase to aqueous solution. Reasons for this are discussed in the context of solvent attenuation factors for various molecules.
Theoretical models for the reduction of arylmethyl halides by triorganotin hydrides
Authors: Mahiou Belaid , Clapp Gary E. , Gleicher Gerald J. , Freeman Peter K. , Camaioni Donald M.
Published: January 2005

Two mechanistic pathways have been proposed for halogen atom transfer from the benzylic positions of halomethylarenes to triorganotin radicals. These are direct atom abstraction, which might involve an extremely polar transition state, and single electron transfer followed by bond cleavage. AM1 semi‐empirical calculations have been utilized to model the rate‐determining step of these processes. A wide range of related families of compounds have been studied, including substituted halomethylbenzes, selected halomethyl‐substituted polycyclic aromatic hydrocarbons and oxygen‐ and nitrogen‐containing chloromethyl‐substituted heteroaromatic systems. Although these calculations are relatively simple, the present results corroborate the view that chlorine and bromine atom transfer from the benzylic position to triorganotin radicals involves a direct atom abstraction in the rate‐determining step whereas reduction of the corresponding iodides proceeds via an electron‐transfer mechanism.
Conformations of pentan‐1‐OL, hexan‐1‐OL and their thio analogues
Authors: Sepiol Jadwiga , Pietrzycki Władysław , Tomasik Piotr
Published: January 2005

Empirical linear relationships between standard Gibbs free energy, ΔG298°, and parachor and between standard entropy, S298°, and parachor are presented for the aliphatic series H(CH2)nX with X = CH3, CHCH2, CCH, Cl, OH, SH, SCH3 and SC2H5. In the series with X = OH and SH significant deviations of points for n = 4 and 5 are observed. INDO quantum‐chemical analysis points to the formation of cyclic conformers of butan‐1‐ol, pentan‐1‐ol and their corresponding thio analogues. The formation energy of the cyclic conformers (higher for thiols than for alcohols) is proportional to the sum of deviations from the linearity of the points under discussion.
Photochemical transformations. 49. Solvent effects on singlet ionic and triplet photoreactions of some bridged polycyclic chlorides: Comparison with ground‐state reactions
Authors: Cristol Stanley J. , Plas Bart J. Vanden
Published: January 2005

Direct irradiations of 2‐chloro‐6,7 : 8,9‐dibenzotricyclo [3.2.2.02,4] nona‐6,8‐diene (5‐Cl) were conducted in cyclohexane, acetic acid and wet acetonitrile. The products are 1‐methylfluoranthene (8) in all three solvents, the allylic chlorides 8‐chloro‐7‐methylene‐2,3 : 5,6‐dibenzobicyclo [2.2.2] octa‐2,5‐diene (3‐Cl) and 7‐chloromethyl‐2,3 : 5,6‐dibenzobicyclo [2.2.2] octa‐2,5,7‐triene (2‐Cl) in acetic acid and acetonitrile and solvolysis product amides in wet acetonitrile. Compound 5‐Cl had previously been shown to be the product of triplet sensitization of 2‐Cl and 3‐Cl, so that the singlet reaction reverses that of the triplet. The formation of 8 from 5‐Cl was quenched with piperylene, whereas that of the allylic chlorides was not. Quantum yields of products and singlet lifetimes in the three solvents were measured. The solvent effects are discussed. Deuterium‐labeling results on the formation of 5‐Cl from 2‐Cl and of 8 from 5‐Cl are reported.
Protein immobilization by aminolysis of cellulose xanthate esters
Authors: Humeres Eduardo , Oliveira Célia Ma. Da Silva , Osellame Volnei T. , De Souza Iolanda
Published: January 2005

P‐Nitrobenzyl cellulose xanthate (CelXNB) was obtained with a degree of substitution (DS) in the range 0.5–3. The first order rate constant for hydrolysis of CelXNB at 25°C, extrapolated to zero buffer concentration, is hydroxide ion catalysed, whereas the water‐catalyed path is about 2000 times faster than that observed for p‐nitrobenzyl ethylxanthate (EXNB), probably owing to the highly ordered cybotactic region of cellulose. Aminolysis of CelXNB produces the corresponding thioncarbamate ester; for simple alkylamines, the second‐order rate constants are similar to those obtained for EXNB. The secod‐order rate constants are similar to those obtained for EXNB. The second‐order rate constants at pH 11 for immobilization of bacterial α‐amylase and bovine serum albumin were 13.4 and 112 1 mol−1 s−1, respectively, unexpectedly high values when compared with simple alkylamines, even considering the concentration of external reactive groups of the proteins. CelXNB with low DS should release 1 mol of p‐nitro‐α‐toluenethiol for every mole of protein that becomes immobilized. The net weight increase of the cellulose matrix allows the calculation of the absolute molecular weight of the protein. Preliminary results support this assumption.
Vinyl ether hydrolysis. 25. Effect of α‐ and β‐trimethylsilyl substitution
Authors: Kresge A. J. , Tobin J. B.
Published: January 2005

Rates of hydrolysis catalyzed by dilute perchloric acid were measured for the vinyl ethers MeOC(SiMe3)CH2, EtOC(SiMe3)CH2, cis‐EtOCHCHSiMe3, MeOC(t‐Bu)CH2 and EtOC(t‐Bu)CH2 in wholly aqueous solution and for MeOCHCH2, MeOC(SiMe3)CH2 and MeOC(t‐Bu)CH2 in ethanol—water (4:1), in order to assess the effects of silyl substitution on the stability of the carbocations formed in the rate‐determining step of these reactions. The results for α‐substitution give the reactivity order H < SiMe3 < t‐Bu, with a greater spread (103) in wholly aqueous solution than in the mixed solvent (102). The β‐trimethylsilyl substituent shows a modest 100‐fold acceleration over hydrogen; the diminished magnitude of this effect relative to the 1012‐fold acceleration found recently in a cyclohexyl solvolysis reaction is attributed to the imposition of a conformation in the transition state of vinyl ether hydrolysis that is far from optimum for hyperconjugative electron donation.
The effect of pressure on swern–moffatt oxidations
Authors: Isaacs Neil S. , Laila A. H.
Published: January 2005

Rates of oxidation of cyclohexanol by dimethyl sulphoxide in conjunction both with acetic anhydride and with dicyclohexylcarbodiimide have been measured with variation of both temperature and pressure. Activation volumes were found to be −25 and −34 cm3 mol−1, respectively, and the absence of an αd PKIE was noted. The reaction mechanism is discussed in the context of these values.
Short communication. Synthesis and properties of 4‐cyano‐1‐(7‐carboxyheptadecyl)pyridinium bromide: A probe of the ionic composition near the surface of positively charged micelles
Authors: Zanette Dino , Chaimovich Hernan
Published: January 2005

4‐Cyano‐1‐(7‐carboxyheptadecyl)pyridinium bromide (CCHP) was synthesized using standard procedures. The rates and product distribution of the reaction between OH− ion and CCHP are very similar to those previously obtained with alkyl‐substituted 4‐cyanopyridinium ions. The effect of hexadecyltrimethylammonium bromide micelles on the rate and product distribution of CCHP indicated that the cyanopyridinium ring of this compound is distal to the surface of the micelle. CCHP constitutes a probe for ionic composition in the vicinity of positively charged micelles.
Protolytic equilibria of several 4‐acyl‐substituted 1‐phenyl‐3‐methylpyrazol‐5‐ones in dioxane–water mixtures
Authors: Shukla J. P. , Nagar M. S. , Arora S. K.
Published: January 2005

Thermodynamic proton ionization constants, TpKa, of several 4‐acyl‐substituted pyrazol‐5‐ones [acyl = trifluoroacetyl (HPMTFP), acetyl (HPMAP), hexanoyl (HPMHP)] were determined in various dioxane–water mixtures at 25 and 35 ± 0.1°C. The TpKa values were determined by glass‐electrode potentiometry and refined by using the extensive weighted least‐squares FORTRAN program TPKA. Both extrapolation and leastsquares methods were used to obtain TpKa values in pure water (0%). All three acyl derivatives are weak monoprotic acids with TpKa values between 2.5 and 3.9, following the order HPMTFP < HPMAP ≤ HPMHP. The TpKa values do not vary linearly with the reciprocal of the dielectic constant of the medium; however, a plot of TpKa versus the mole fraction of dioxane, n2, is linear at a given temperature. Values of standard free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) changes associated with their protolytic equilibria were also calculated. Temperature, medium and substituent effects are briefly discussed.
Molecular and electronic structure of substituted bithiophenes
Authors: Novak I. , Ng S. C. , Huang H. H. , Mok C. Y. , Khor E. , Kovač B.
Published: January 2005

Several substituted bithiophenes were prepared and their electronic structure studied by photoelectron spectroscopy and semi‐empirical MO calculations. Comparison of the spectra of related molecules and analysis of calculated molecular geometries gave an indication of molecular conformations in the gas phase.
Topological aspects of chemical reactivity. Evans–dewar principle in terms of molecular similarity approach
Authors: Ponec Robert , Strnad Martin
Published: January 2005

The Evans‐Dewar concept of aromaticity of transition states is given new theoretical support in terms of the similarity approach to chemical reactivity. The principal goal of this approach consists in providing a simple, chemically lucid justification for the legitimacy of the original intuitively formulated concept.
Comparison of the cleavage reactions of αnitrocumene radical anion and α,p–dinitrocumene radical anion
Authors: Wu Fangfang , Guarr Thomas F. , Guthrie Robert D.
Published: January 2005

The rates of loss of NO2− from the radical anions of 2‐(4‐nitrophenyl)‐2‐nitropropane (α,p‐dinitrocumene) and 2‐phenyl‐2‐nitropropane (α‐nitrocumene) were compared by three different methods. The rate constant for the apparently heterolytic process (DN) undergone by the radical anion of the dinitrocumene was measured by pulse radiolysis to be 2 × 103 s−1 in dimethylformide at 25°C. This value is in reasonable agreement with values estimated from an indirect chemical method and from cyclic voltammetry at low temperature. The rate for the formally homolytic dissociation process (DR) undergone by the radical anion of the mononitrocumene could not be determined by pulse radiolysis but could be shown to be between 50 and 1000 times faster than the dinitro case using the other two methods. Possible reasons for the faster rate for the DR process are discussed with reference to related systems for which the DR process had been found to be unexpectedly slow.
Generation and rearrangement of spirocyclopropane‐substituted 2‐norbornyl cations
Authors: Kirmse Wolfgang , Landscheidt Heinz , Schleich Annette
Published: January 2005

Access to spiro(bicyclo [2.2.1] heptane‐2,1' ‐cycloprop‐6‐yl) derivatives was gained from the alkene spiro(bicyclo [2.2.1] hept‐5‐ene‐2,1'‐cyclopropane) via separation of positional isomers. Spiro(bicyclo [2.2.1]‐2,1'‐cycloprop‐exo‐6‐yl) p‐toluenesulphonate (10) and spiro(bicyclo [2.2.1] heptane‐2,1'‐cycloprop‐exo‐6‐yl) trifluoroacetate were found to solvolyse faster than the analogous exo‐2‐norbornyl esters, as predicted by theory. Ion‐pair recombination, with the formation of tricyclo [4.2.1.03,7] non‐3‐yl p‐toluenesulphonate, accounts for previous failures to assess the true reactivity of 10. An intervening bridged carbocation (3), labelled with deuterium, was shown to achieve equivalence of C‐1 and C‐6 prior to ring expansion. The rate of the formal Wagner–Meerwein rearrangement is estimated to be of the order of molecular vibrations, thus supporting the symmetrical bridged structure of 3. Methyl substitution at C‐6 was found to direct nucleophilic attack exclusively to the tertiary carbon, and ring expansion preferentially to the secondary carbon. An equilibrating pair of 6(1)‐methylspiro(bicyclo [2.2.1] heptane‐2,1′‐cycloprop‐6‐yl) carbocations is thought to explain these observations most reasonably.