bluuebunny/crossref_metadata_2025_split huggingface dataset:
Theoretical studies of the singlet and triplet potential energy surfaces of cyclobutanediyl
Authors: Pranata Julianto , Dougherty Dennis A. 
Published: January 2005

AbstractCyclobutanediyl (2) has been studied in both its singlet and triplet states by ab initio electronic structure theory. The triplet, which is the ground state of the molecule, exists in both C2h and C2v forms which interconvert via a Cs transition state. For the singlet, only a C2h form is found. It passes, via a Cs transition state, onto the C2v surface on which bicyclobutane (3) is the only minimum. The ring‐flipping (inversion) process in 3 includes the singlet biradical as an intermediate, and involves a novel, non‐least motion path similar to one previously proposed by Gassman. Semiclassical periodic orbit theory indicates that the various minima on both the singlet and triplet surfaces can interconvert via quantum mechanical tunneling.
Reassignment of the fundamental vibrations of azetidine from ab initio calculations
Authors: Nielsen Per Halfdan , Gajhede Michael 
Published: January 2005

AbstractThe harmonic vibrational frequencies were calculated analytically at the 6‐31** level for azetidine using the GAUSSIAN 82 program. The results strongly indicate the presence of several errors in a recent assignment of the fundamentals of azetidine based on a normal coordinate analysis and a revised assignment is suggested. It is concluded that reliable vibrational data for azetidine in the gas phase are needed in order to resolve the remaining ambiguities in the interpretation of the spectra.
Mechanism of the Grignard reaction
Authors: Maruyama Kazuhiro , Katagiri Toshimasa 
Published: January 2005

AbstractMechanism of the Grignard reactions of aromatic ketones in THF was studied by spectroscopic and kinetic methods. The stable radical intermediates generated in the initial electron transfer from Grignard reagent to ketones are in a state of aggregated dimer of corresponding ion–radical pairs; in which two ketone anion radicals are bridged by a dimer di–cation of Grignard reagent. Subsequent alkyl radical transfer from dimeric Grignard reagent cation moiety to ketone anion radical aggregated each other are promoted by a participation of another neutral Grignard reagent. Proposed mechanism by present authors is able to explain well addition products/reduction products ratios in the Grignard reactions.
Alkylating properties of phosphate esters. 3. Reactivity of dialkyl 2‐(dimethylamino) ethyl phosphates
Authors: Modro T. A. , Moorhoff C. M. 
Published: January 2005

AbstractReactivity of dialkyl 2‐(dimethylamino)ethyl phosphates, (RO)2P(O)OCH2CH2NMe2 (R = Me, PhCH2) was studied in aqueous solutions. Depending on the initial substrate's concentration, reaction can involve the unimolecular fragmentation to N,N‐dimethylaziridinium dialkylphosphate, or the bimolecular isomerization to the zwitterionic derivative. The latter reaction proceeds via two consecutive SN2 steps and involves the formation of two ionic intermediates which were synthesized independently and allowed to react to give the zwitterionic product. Rate constants for the isomerization of the dimethyl ester (R = Me), as well as rate constant for the reaction between the corresponding intermediates have been determined, and the reactivity of the dimethyl ester has been compared with that of the dibenzyl derivative.
Crystal and molecular structures of dimesitylketene and bis(3,5‐dibromo‐2,4,6‐trimethylphenyl)ketene. Intrinsic torsional angles and buttressing effects in 1,1‐dimesitylvinyl propellers
Authors: Biali Silvio E. , Gozin Michael , Rappoport Zvi 
Published: January 2005

AbstractThe crystal and molecular structures of dimesitylketene (1) and bis(3,5‐dibromo‐2,4,6‐trimethylphenyl)ketene (2), the first free ketenes to be studied by X‐ray diffraction, have been determined. The molecules of 1 and 2 have crystallographic C1 and C2 symmetry, respectively, and exist in a propeller conformation. Molecular mechanics calculations of 1 and 2 using a modification of Allinger's new allene parameters reproduce satisfactorily the structural parameters of the ketenes. The torsional angles of the aryl rings in 1 and 2 are similar (ca. 50°) and are regarded as ‘intrinsic’ values for a 1,1‐dimesitylvinyl propeller. From the similarity in aryl torsional angles in 1 and 2 it is concluded that the buttressing effects in 2 (if any) are not markedly manifested in the structural parameters of 2.
A theoretical and experimental investigation of acid catalyzed rearrangements of small [n]cyclophanes
Authors: Kostermans Gerardus B. M. , Kwakman Pieter J. , Pouwels Petra J. W. , Somsen Gus , De Wolf Willem H. , Bicklehaupt Friedrich 
Published: January 2005

AbstractThe difference in reactivity of small [n]cyclophanes towards CF3CO2H is discussed in terms of charge densities, strain energies and proton affinities. These data are calculated with MNDO and MINDO/3 for para‐, meta‐ and ortho‐cyclophanes and for their ipso‐protonation products; an attempt is made to transform gas phase ΔH values into liquid phase ΔH0f values. Experimental evidence is presented that the acid catalyzed rearrangement of [5]paracyclophane to its ortho‐isomer proceeds via two consecutive 1,2‐carbon shifts without deprotonation; intermediate adducts were identified by NMR‐spectroscopy. Thus, a gradual shift in reaction pattern in the series [4]‐, [5]‐ and [6]paracyclophane is observed experimentally, in line with the calculational results.
Oxygen‐17 NMR study on substituted α, α, α‐trifluoroacetophenones
Authors: Liu Kwang‐Ting , Wu Tseng‐Rong , Lin Ying‐Chih 
Published: January 2005

AbstractAn excellent linear correlation of oxygen‐17 substituent chemical shifts (SCS) for twelve α, α, α‐trifluoroacetophenones with single σ+ constants, δ = 24·6σ+ + 556·3 (correlation coefficient 0·998) has been observed. However, from this plot, an SCS of 34·3 ppm for 1,1,1‐trifluoroacetone will give a corresponding σ+ (γ+) value of 1·44 for a methyl group, which is not in agreement with 0·63–0·79 obtained from solvolytic rate data.
Free‐radical reactions of cyclic ethers and sulfides with bromotrichloromethane
Authors: Hallen Richard T. , Gleicher Gerald Jay , Mahiou Belaid , Clapp Gary E. 
Published: January 2005

AbstractThe relative rates of hydrogen atom abstraction from a series of twelve saturated cyclic ethers and sulfides were determined at 70°C. The abstracting radical could be generated from bromotrichloromethane both photolytically or by the thermal decomposition of AIBN. The reaction rates did not show a dependence upon method of radical generation. Reaction occurred only at the position adjacent to the heteroatom. The reactivity of the cyclic ethers was in the order C4H8O > C6H12O > C3H6O > C5H10O. This trend would indicate appreciable influence by ring strain, however, the slightly greater reactivity of tetrahydrofuran relative to oxepane suggests a contribution by stereoelectronic factors as well. The reactivity of the cyclic sulfides, which reacted faster than the corresponding ethers, was in the order C4H8S > C5H10S > C6H12S. This would imply little influence of ring strain. The major structural effect would be that of variable electron donating ability of the sulfur atom. The rate of reaction of thietane was also determined. It was found to preferentially undergo SH2 attack at the sulfur atom followed by ring opening rather than hydrogen abstraction. The reactivities of both series of compounds were decreased by the inductive effect of a second heteroatom beta to the reaciton site.
Photochemistry of matrix‐isolated norbornadiene
Authors: Gȩbicki Jerzy , Kuberski Sławomir , Kamiński Rafał 
Published: January 2005

AbstractNorbornadiene, isolated in argon, xenon and nitrogen matrices at 20 K, was irradiated with UV light. Characterized by UV–VIS–IR absorption spectroscopy the photoproduct, representing a single species, was assigned to quadricyclane. This result is compared with the literature data on photochemistry of norbornadiene in the gas and liquid phase. The role of the rigid matrix environment on selectivity of the photochemical reaction is discussed.
Bifunctionally catalyzed 1,3‐proton transfer of a propene by a sec‐amidine studied by deuterium isotope effects. A stepwise two‐proton transfer mechanism
Authors: Ahlberg Per , Janné Kjell , Löfås Stefan , Nettelblad Folke , Swahn Lena 
Published: January 2005

AbstractA two‐hydron transfer mechanism involving hydron transfers from carbon to nitrogen and from nitrogen to carbon was studied. The rearrangement of 1,3,3‐triphenylpropene (1) into 1,1,3‐triphenylpropene (2) catalyzed by 2,10‐diazabicyclo[4.4.0]dec‐1‐ene (3) in benzene at 25·00°C was studied by 2H‐labeling experiments and kinetic 2H‐isotope effects. The synthesis and purification of [6,10‐2H2]‐2,10‐diazabicyclo[4.4.0]dec‐1‐ene ([6,10‐2H2]‐3), [3‐2H]‐1,3,3‐triphenylpropene ([3‐2H]‐1), [3‐2H]‐1,1,3‐triphenylpropene ([3‐2H]‐2) and [3,3‐2H2]‐1,1,3‐triphenylpropene ([3,3‐2H2]‐2) together with their precursors are reported. Partial reaction of [3‐2H]‐1 with [6,10‐1H2]‐3 gave 42% conversion into product 2, which was shown by 1H NMR to be composed of 88% [3‐1H]‐2 and 12% [3‐2H]‐2. Partial reaction of [3‐1H]‐1 with [6,10‐2H2]‐3 gave 43% of 2, composed of 73% [3‐1H]‐2 and 27% [3‐2H]‐2.These results clearly show that a substantial fraction of the reaction takes place in a bifunctional manner but isotope exchange and/or monofunctionally catalyzed reactions interfere. The following kinetic deuterium isotope effects on the rearrangement 1 → 2 were measured: kHH/kDH = 6·56; kHH/kHD = 1·19; kHH/kDD = 7·08; kHD/kDD = 5·94; and kDH/kDD = 1·08.On the basis of these results, a concerted two‐hydron transfer mechanism is excluded. Instead, a stepwise mechanism is favored, in which at first the 3‐hydron of 1 is abstracted by 3 yielding an ion pair(s), the carbanion of which in a separate step is then hydronated to yield the product 2.The abstraction of the 3‐hydron from 1 might be hydrogen bond assisted. The two hydron transfer transition states are together rate limiting, although they limit the rate to different extents. A detailed mechanistic analysis is presented together with the results of an investigation of the nature of the catalyst. The dimerization constant for 3 was determined by 1H NMR to be 1·67 l mol−1 at 25·0°C. Isotopomer composition was measured by 1H NMR and GLC was used for the separation of the substrate and products. Computer‐assisted capillary GLC was used for the kinetics.
Dynamic isotope dilution kinetic study of the thermal conversions of bicyclo[3.2.0]hept‐2‐ene to bicyclo[2.2.1]hept‐2‐ene and to cyclopentadiene and ethene
Authors: Baldwin John E. , Belfield Kevin D. 
Published: January 2005

AbstractMixtures of bicyclo[3.2.0]hept‐2‐ene and bicyclo[2.2.1]hept‐2‐ene, with one isomer labeled with two deuterium atoms, were isomerized in the gas phase at 276 °C. By following the concentration ratios of d0 and d2 versions of bicyclo[2.2.1]hept‐2‐ene as functions of time one finds that the partitioning of bicyclo[3.2.0]hept‐2‐ene between the primary products bicyclo[2.2.1]hept‐2‐ene and cyclopentadiene plus ethene is 2:1. This and earlier stereochemical results permit the calculation of the partitioning of the [3.2.0] olefin between the orbital symmetry allowed [1,3] carbon shift‐with‐inversion product and several forbidden or non‐concerted reaction channels (approximately 1:1) and the stereochemistry of the ethene d2 formed from exo,exo‐[6,7‐2H2]bicyclo[3.2.0]hept‐2‐ene directly (1:1 E: Z).
Aromatic substitution by free benzyl cations from the nuclear decay of multi‐tritiated toluene. Competitive benzylation of benzene and toluene in the gas phase and in solution
Authors: Giacomello Pierluigi , Angelini Giancarlo , Sparapani Cinzia , Speranza Maurizio 
Published: January 2005

AbstractBenzyl cations, free of counter ions, have been generated from the nuclear decay of tritium atoms contained exclusively in the side‐chain of multi‐labelled toluene and allowed to react in competition experiments with benzene and toluene. The aromatic substitution was studied in both the gaseous and liquid phase, employing pure aromatics or aromatics dissolved in n‐hexane, carbon tetrachloride and nitromethane as the reaction medium. In the gaseous systems, at 20 Torr total pressure, High meta substitution of toluene indicates extensive isomerization of the excited intermediates, while the increase of the kT/kB value in the Presence of NH3 suggests that selective transalkylation competes with proton transfer to ammonia from the benzyl cation adducts with benzene, but not with toluene. High positional and low substrate selectivities were measured in solution, the decrease in kT/kB (from 2·8 to 1·7) in the different solvents being accompanied by an increase in the o/2p ratio. Competition experiments between benzene and methanol indicate that the reaction is not diffusion controlled. The influence of the reaction medium on the substrate selectivity is explained by postulating the formation of a cation–solvent adduct, where the solvent molecule undergoes displacement by the substrate. Positional selectivity would be controlled in a subsequent step.
Influence of aromatic substituents on the configuration and conformation of calix[4]areneoctols
Authors: Mann G. , Weinelt F. , Hauptmann S. 
Published: January 2005

AbstractAcid‐catalysed condensation of resorcinol with aromatic aldehydes results in 2,8,14,20‐tetrasubstitued calix[4] areneoctols. Sixteen compounds of this type containing different aromatic substituents were synthesized. The ring closure step, under the conditions of the reaction, is a reversible process. Four configurations are possible for every constitution, viz. cccc, cctt, ccct and ctct, but only cccc and cctt were formed in detectable amounts. In seven cases the thermodynamically more stable cccc isomers (1a–5a, 7a, 8a) and also the kinetically controlled cctt isomers (1b–5b, 7b, 8b) could be isolated. The configurations of the compounds were assigned by temperature‐dependent 1H NMR analysis. The conformational motions of the macrocyclic ring permit only the cccc isomers to show coalescence for the signals of the aromatic Hb protons. The coalescence temperature was determined for isomers 1a (ΔG = 83·5 kJmol−1) and 3a (ΔG = 83·7 kJ mol−1). For steric reasons the ‘chair–chair’ conformers B and the ‘quasi‐boat–chair’ conformers F are favoured. 1H and 13C NMR shifts show that compounds 1a–5a and 1b–5b have a quasi‐axial arrangement of the aromatic substituents, whereas 6–9 have a quasi‐equatorial arrangement of the substituents.
Decarboxylation of 6‐nitrobenzisoxazole‐3‐carboxylate ion in cationic micelles: Effect of head group size
Authors: Germani Raimondo , Ponti Pier Paolo , Romeo Tiziana , Savelli Gianfranco , Spreti Nicoletta , Cerichelli Giorgio , Luchetti Luciana , Mancini Giovanna , Bunton Clifford A. 
Published: January 2005

AbstractThe rate of decarboxylation of 6‐nitrobenzisoxazole‐3‐carboxylate ion increases sharply with increasing head group size in a series of cetyltrialkylammonium bromides (C16H33NR3Br: R = Me, CTABr; R = Et, CTEABr; R = n – Pr, CTPABr; R = n – Bu, CTBABr) with rate enhancements of 102 (CTABr) and 2·8 × 103 (CTBABr). Micellized tetradecylquinuclidinium bromide and hexadecyl‐N‐methylmorpholinium bromide are slightly better catalysts than CTABr, as is 1,3‐bis(N‐cetyl‐N,N‐dimethylamino)propane dibromide, but p‐octyloxybenzyltrialkylammonium bromides (alkyl = Me, n – Bu) are less effective than the corresponding CTA+ surfactants. These differences in catalytic efficiency depend on the head group structure and the extent to which the cationic head groups become less accessible to water rather than the overall micellar structure.
Anionic nucleophile–cation radical combination reactions. The super‐electrophilic properties of cation radicals in solution
Authors: Parker Vernon D. , Reitstöen Björn , Tilset Mats 
Published: January 2005

AbstractRate constants were determined for the combination reactions of a series of cation radicals derived from substituted anthracenes with acetate, p‐nitrobenzoate, trifluoroacetate, nitrate and perchlorate ions. Rate constants, depending on the identities of the cation radicals and the nucleophiles, ranging from about 200 to 2 × 1010 1 mol−1 s−1 were observed in acetonitrile at 293 K. The key steps in the reaction are (1) reversible complex formation between the anion and the cation radical followed by (2) irreversible bond formation

.The preliminary results show that cation radical–anionic nucleophile reactions can be very facile. The cation radicals of 9‐nitro‐ and 9‐cyanoanthracene are particularly reactive, giving rise to rate constants close to the diffusion‐controlled limit with all anionic nucleophile studied and even react moderately rapidly with perchlorate ion. The reaction with perchlorate ion can be compared to the behavior of stable carbenium ions that coexist with the anion in solution and in crystalline salts.
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

AbstractPhotolysis 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

AbstractUsing 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

AbstractThe 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

AbstractThe 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

AbstractGas‐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

AbstractAb 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

AbstractThe 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.