bluuebunny/crossref_metadata_2025 huggingface dataset:
Alkaline earth metal ion catalysis of alcoholysis of crown ether aryl acetates. Effect of the base–solvent system
Authors: Cacciapaglia Roberta , Mandolini Luigi , Reinhoudt David N. , Verboom Willem
Published: January 2005

The effect of strontium and barium ions on the acyl transfer reaction to ethoxide ion from 2‐acetoxy‐1,3‐xylyl‐15‐crown‐4, 2‐acetoxy‐1,3‐xylyl‐18‐crwon‐5, 2‐acetoxy‐1,3‐xylyl‐21‐crown‐6 and the model compound phenyl acetate was investigated in ethanol at 25°C. Significant catalytic effects were observed in all of the studied reactions as a result of stronger metal ion associations with the transition states than with reactants. The rate enhancements observed with the crown ether substrates are much larger than those observed with phenyl acetate, the largest acceleration being displayed by 2‐acetoxy‐1,3‐xylyl‐21‐crown‐6, which reacts with EtOBaBr 5 × 105 times faster than with EtONMe4. These findings provide strong evidence that cooperation of electrostatic binding to the negative charge developing at the carbonyl oxygen and coordinative binding to the polyether chain is highly efficient in the metal‐bound transition state. Comparison of rate data reported here with analogous data obtained in methanol solution clearly shows that both modes of interaction contributing to the stability of the metal‐bound transition state are much more efficient in ethanol than in methanol solution.
Guest dynamics and crystal structure of the tri‐o‐thymotide–acetone clathrate studied by solid‐state NMR, X‐ray diffraction and molecular modelling
Authors: Facey G. A. , Ratcliffe C. I. , Hynes R. , Ripmeester J. A.
Published: January 2005

The structure of the acetone complex of tri‐o‐thymotide was solved by single‐crystal x‐ray diffraction. The acetone molecule is situated on the twofold axis of the cage and shows no sign of unusual disorder. Solid‐state 2H and 13C NMR were used to show that, in addition to fast methyl group rotation, there is a rotation of the acetone molecule about the carbonyl bond. The motion is best described as a twofold flip plus the temperature‐dependent population of a secondary site 63 ± 10° away from the minimum energy position. Molecular modelling calculations were used to confirm the potential shape, which was remarkably sensitive to methyl group orientation. 2H spin–lattice relaxation was used to derive an activation energy of 13 ± 6 ± 0 ± 8 kJ mol−1 for the twofold flips, and the secondary site was shown to be energetically less favourable by 4.4 ± 0.5 kJ mol−1 from the temperature dependence of the line shape. Neither motion is seen in the diffraction experiment.
Conformational analysis of calix[n] arenes with chiral substituents by using circular dichroism
Authors: Ikeda Atsushi , Nagasaki Takeshi , Shinkai Seiji
Published: January 2005

Calix [n] arenes (n= 4 and 6) bearing chiral substituents (R*) on the lower rim (1n) or on the upper rim (4n) were synthesized. The isomers derived from 14 were conformationally immobile and showed different circular dichroism (CD) spectra. In the 1H NMR spectra the proton signals in R* shifted to higher magnetic field when the lower rim is sterically crowded and in the CD spectra a strong CD band appeared. Therefore, the change in the CD spectra is rationalized in terms of R*…π interactions. In contrast, compound 44 was not conformationally immobilized and the CD spectra changed in response to the metal binding to the methoxy oxygens arranged on the lower rim. The specific metal interaction enabled the metal binding event to be detected by the CD technique.
Regioselectivity in a benzophenone‐mediated photo‐substitution of some cage‐shaped hydrocarbons
Authors: Chow Tahsin J. , Yan Jeng‐Kuo , Chen Yuh‐Liang
Published: January 2005

Benzophenone‐mediated protolyses of five cage‐shaped compounds in benzene solutions result in the formation of phenyl‐substituted products with high regioselectivity. The regioselectivity is rationalized based on the relative bond strengths and structural parameters of each molecule.
Participation and rearrangement in the gas‐phase elimination kinetics of 3‐(o‐methoxyphenyl)propyl‐1‐methanesulphonate and 4‐(p‐methoxyphenyl)butyl‐1‐methanesulphonate
Authors: Martin Ignacio , Chuchani Gabriel , Dominguez Rosa , Rotinov Alexandra
Published: January 2005

The gas‐phase unimolecular elimination of the methanesulphonates in the temperature range 289.0‐331.4°C and pressure range 18–152.5 Torr follows a first‐order rate law. The rate coefficients for the homogeneous reactions are expressed by the following equations: for 3‐(o‐methoxyphenyl)propyl‐1‐methanesulphonate log[k1(s−1)] = (12.04 ± 0.32) − [(167.8 ± 3.6) kJ mol−1] (2.303RT)−1 and for 4‐(p‐methoxyphenyl)butyl‐1‐methanesulphonate log [k1(s−1)] = (12.82 ± 0.30) − [(175.1 ± 3.4) kJ mol−1] (2.303RT)−1. The oxygen atom of the CH3O substituent in 3‐(o‐methoxyphenyl)propyl‐1‐methanesulphonate appears to participate directly in the C–O bond polarization in order to produce some of the cyclic product dihydrobenzopyran. A parallel reaction occurs with 4‐(p‐methoxyphenyl)butyl‐1‐methanesulphonate where the p‐anisyl substituent participates in the elimination process through a five‐membered spiro intermediate for the formation of the cyclic product 6‐methoxy‐1,2,3,4‐tetrahydronaphthalene. The second pathway of this elimination takes place via normal formation of the corresponding unsaturated aromatic hydrocarbons. These reactions are interpreted in terms of an intimate ion‐pair type of mechanism.
Inclusion compounds of hydroxy hosts. 6. Structures and thermal analysis of 1,2,3,4,5‐pentaphenylcyclopenta‐2,4‐dien‐1‐ol with methanol and ethanol
Authors: Bourne Susan A. , Nassimbeni Luigi R. , Niven Margaret L.
Published: January 2005

The crystal and molecular structures of the methanol and ethanol inclusion compounds of 1,2,3,4,5‐pentaphenylcyclopenta‐2,4‐dien‐1‐ol have been elucidated. (I) C35H26O · CH4O is triclinic, PĪ, a = 10.377, b = 10.778, c = 14.537 Å, α = 102.66(2), β = 90.26(2), γ = 117.33(2)1, V = 1398(15) Å3, Z = 2; (II) (C35H26O)2 · C2H6O is triclinic, P1, a = 10.901(2), b = 13.498(2), c = 19.037(3)Å, α = 101.88(1), β = 93.76(1), γ = 90.84(1), V = 2734(8) Å3, Z = 2. The structures were refined to final residuals of 0.127 for 2207 reflections and 0.073 for 4720 reflections. Thermal analysis of the complexes confirmed the crystallographic results and gave information about their thermal decompositions.
Solvation and metal ion effects on structure and reactivity of phosphoryl compounds. 1. β‐substituted alkylphosphonic esters
Authors: Belciug Marie‐P. , Modro Agnes M. , Modro Tom A. , Wessels Philippus L.
Published: January 2005

NMR spectroscopic and conformational analyses were performed for three β‐substituted β‐phenylethylphosphonic esters, PhCHXCH2PO3Me2 (X = OH, OMe, Cl) in five solvents and in acetone containing sodium and magnesium salts. Strong attractive interactions between the phosphoryl group and the oxygen‐containing substituent X were demonstrated, and were shown to involve intramolecular hydrogen bonding, donor‐acceptor O → P effect and the chelation of a metal ion. These effects led to marked selectivity in the population of individual conformations of the phosphonates. The analogous intramolecular effects were found to be much weaker in the corresponding carboxylic ester systems.
Theoretical studies on the gas‐phase rearrangement of deprotonated allyl phenyl ether
Authors: Kim Chang Kon , Lee Bon‐su , Lee Ikchoon
Published: January 2005

The Wittig and anoinic Claisen rearrangements of deprotonated allyl phenyl ether, PhOC̄HCH=CH2, were investigated by MO theory employing the AM1 method. The most favoured reaction pathway for the Wittig rearrangement is the intramolecularSNAr process proceeding by the addition‐elimination mechanism involving a Meisenheimer complex‐type, three‐membered‐ring intermediate. For the anionic Claisen rearrangement a three‐step mechanism is favoured, in which the intramolecular proton transfer occurs first from theorthoposition of the phenyl ring to the anionic carbon centre, which is then followed by a process involving a six‐membered ring intermediate. The two types of rearrangements can compete, but the Wittig type is more facile and favoured than the Claisen process owing to the lower activation enthalpy. The results are in good agreement with gas‐phase experimental results.
Acid‐catalysed depurination of DI‐, TRI‐ and polydeoxyribonucleotides: Effect of molecular environment on the cleavage of adenine residue
Authors: Hakala Harri , Oivanen Mikko , Saloniemi Eija , Gouzaev Andrei , Lönnberg Harri
Published: January 2005

First‐order rate constants for the acid‐catalysed cleavage of adenine base from 2′‐deoxyadenosine, its 3′ ‐ and 5′‐ monophosphates, various dinucleoside monophosphates, trinucleoside diphosphates and hexameric 2′‐deoxyadenylic acid were measured by the method of initial velocity. The results obtained were compared with the effect that hetroassociation with caffeine has on the pre‐equilibrium protonation and rate‐limiting heterolysis of 2′ ‐deoxyadenosine and its isosteric analogue, 1‐(2‐deoxy‐β‐D‐erythro‐pentofuranosyl) benzimidazole. The kinetics of the depurination of poly (dA‐T), poly(dA) and a mixture of poly(dA) and poly(T) were determined, and the rate variations observed are discussed on the basis of the known structures of these polymers.
Generation of vinyl cations by nitrosation of N‐silylated imines and enamines
Authors: Alvarez Roberto martínez , Hanack Michael , Subramanian L. R.
Published: January 2005

The reaction of monosilylated imines and a disilylated enamine with nitrosyl salts in dichloromethane at −78°C is shown to proceed via intermediate vinyl diazonium salts and vinyl cations.
Precipitation of strong uncharged organic acids with various crown ethers or with acyclic polyethers from aqueous medium. Characterization of adducts in the solid state
Authors: Kolthoff Izaak M. , Chantooni M. K.
Published: January 2005

It was found that substituted picric acids form slightly soluble 1 : 2 : 2 adducts with several simple crown ethers or with tri‐, tetra‐, or pentaethylene glycol dimethyl ether (glymes 4, 5 and 6) in water, whereas 2,4,6‐trinitrobenzenesulfonic acid forms a 1 : 1 : 1 adduct with dicyclohexano‐18‐crown‐6 (cis‐syn‐cis). The ability of a crown ether to precipitate a given acid follows the order dicyclohexano‐18‐crown‐6 (cis‐syn‐cis) > benzo‐15‐crown‐5 > dicyclohexano‐24‐crown‐8 > 18‐crown‐6 > 21‐crown‐7, 15‐crown‐5 ≫ 12‐crown‐4, roughly in the order of its base strength. Precipitation can occur even when the concentration(s) of one or more of the components are < 10−4 M. The solubility products of the 18‐crown‐6 complexes with dichloro‐ or dimethylpicric acids were estimated. In the solid state the 1 : 2 : 2 dicyclohexano‐18‐crown‐6 (cis‐anti‐cis)–dichloropicric acid–water complex, which reportedly has two pseudohydronium ions lacking C3v symmetry of H3O+, exhibits an IR spectrum in the OH stretching region characteristic of an aqua complex.
Elimination kinetics and mechanism of primary, secondary and tertiary α‐hydroxycarboxylic acids in the gas phase
Authors: Chuchani Gabriel , Martin Ignacio , Rotinov Alexandra , Dominguez Rosa M.
Published: January 2005

The rates of elimination of primary, secondary and tertiary α‐hydroxycarboxylic acids were determined in a seasoned, static reaction vessel over the temperature range 280–390°C and the pressure range 30–201 Torr. The reactions, in the presence of a free radical inhibitor, are homogeneous, unimolecular and follow a first‐order rate law. The rate coefficients are given by the following equations: for glycolic acid, log k1 (s−1) = (14·03 ± 0·24) − (209·3 ± 1·5) kJ mol−1 (2·303RT)−1; for lactic acid, log k1 (s−1) = (12·24 ± 0·11) − (182·8 ± 1·3) kJ mol−1 (2·303RT)−1; and for 2‐hydroxyisobutyric acid, log k1 (s−1) = (12·91 ± 0·13) − (174·7 ± 1·5) kJ mol−1 (2·303RT)−1. The basicity and the ease with which the hydroxy group is removed from primary to tertiary α‐hydroxycarboxylic acids are reflected in rate enhancement. The mechanism of these eliminations appears to proceed through a semi‐polar five‐membered cyclic transition state.
α‐Versus β‐carbon nucleophilic attack in vinylic substitution
Authors: Shainyan B. A.
Published: January 2005

The mechanism of substitution of the bromine atom in (Z)‐α‐bromo‐β‐arylthiovinyl phenyl sulphones, PhSO2CBr = CHSAr, with arylthiolates, Ar′S− was studied. The same mixture of the four possible products, PhSO2C(SAr′) = CHSAr (Ar, Ar′ = p‐Tol, p‐CIC6H4), was formed in both cross‐experiments (Ar ≠ Ar′). A mechanism involving 1,2‐intramolecular migration of the arylthio group is suggested.
Phosphonic systems. Part 8. Solution and solid‐state conformation of diastereomeric adducts of diethyl prop‐2‐enylphosphonate to p‐nitrobenzaldehyde
Authors: Müller Elmar L. , Roos H. Marita , Modro Tomasz A.
Published: January 2005

A mixture of siastereomeric adducts of lithiated diethyl prop‐2‐enylphosphonate to p‐nitrobenzaldehyde was separated and the solution and the solid‐state structures of the individual stereoisomers were studied. Conformational analysis demonstrated that in solution the most populated rotamer involves in both cases trans orientation of the PO3Et2 and O2NC6H4 groups and a gauche relationship of the phosphoryl and hydroxyl groups. In the solid state the crystal structure determination demonstrated that the molecules exist in the same conformation as in solution, although the PO ⃛HO hydrogen bonding changed from intramolecular to intermolecular (dimeric) interactions.
Nucleophilic solvent intervention in the solvolysis of 2‐aryl‐2‐Chloropropanes. An example to demonstrate the advantage of using the YBnCl scale in correlation analysis
Authors: Liu Kwang‐Ting , Chen Pang‐Shao , Hu Chia‐Ruei , Sheu Hung‐Chan
Published: January 2005

The advantage of using the YBnCl scale over the YCl scale in Grunwald–Winstein‐type correlation analysis was demonstrated by the kinetic evidence of a significant nucleophilic solvent intervention in the solvolysis of 2‐aryl‐2‐chloropropanes. A depression of log k values measured in ethanol–trifluoroethanol mixtures and a positive azide salt effect were observed.
Versatile packing modes of 9‐substituted 9‐fluorenols. X‐ray crystal structures of 9‐methyl‐, 9‐phenyl‐, 9‐(1‐naphthyl)‐ and 9‐(2‐biphenylyl)fluoren‐9‐ol
Authors: Csöregh Ingeborg , Czugler Mátyás , Weber Edwin
Published: January 2005

The four studied semi‐rigid fluorenol derivatives differ only in the size of the 9‐substituents. All four compounds crystallize with two molecules per asymmetric unit and with relatively low crystallographic symmetry [P1for the 9‐methyl derivative (1) andP21/nfor the 9‐(1‐naphthyl) (3) and 9‐(2‐biphenylyl) (4) derivatives]. Crystal data: 1,a= 8·779(1),b= 11·570(1),c= 11·747(1), α = 91·197(5), β = 104·345(4), γ = 105·848(6)° 2,a= 8·425(1),b= 13·819(1),c= 23·902(2)Å, β = 95·87(1)° 3,a= 15·679(4),b= 8·567(2);c= 23·884(17)Å, β = 92·60(6)° 4,a= 7·770(6),b= 33·766(10),c= 13·649(4)Å, β = 95·30(1)°. Nevertheless, the packing modes are different. The 9‐methylfluoren‐9‐ol molecules, forming H‐bonded tetramers with full H‐bond saturation, give rise to the lowest packing coefficient among the four compounds. The crystals of the 9‐phenyl derivative are built up of H‐bonded dimers, whereas those of the bulky 9‐naphthyl and 9‐diphenylyl derivatives consist of single molecules only. The latter two compounds, however, have a denser packing than the two that contain H‐bonded aggregates. The crystal structures are held together mainly by ordinary Van der Waals forces. In the case of 9‐biphenylyfluoren‐9‐ol, the structure seems to be stabilized also by weak H‐bond‐type interactions from the alcoholic hydroxyl group to the π‐electron cloud of an aromatic ring in the neighbouring molecule the O…centroid distances are 3·182(3) and 3·248(4)Å, and in the 9‐naphthyl derivative the OH groups are involved in some short (˜3·3Å) intermolecular approach, possibly indicating an electrostatically favourable packing for the compound.
Relative stability of crowded isomeric enols: 2‐Mesityl‐2‐phenylethenols and their methyl ethers
Authors: Nadler Ella B. , RÖCk Maik , Schmittel Michael , Rappoport Zvi
Published: January 2005

The structure of 2‐mesityl‐2‐phenylethenol (7) obtained by reduction of mesityl phenyl ketene with LiAlH4 and by acid‐catalysed rearrangement of 1‐mesityl‐2‐phenylethylene glycol was determined by x‐ray crystallography to be Z [(Z)‐7]. In contrast with a literature report, the reduction of 2‐acetoxy‐2‐mesityl‐2‐phenylacetaldehyde did not provide the E isomer [(E)‐7], but a mixture of (Z)‐7 and 2‐mesityl‐2‐phenylethenol. An (E)‐7–(Z)‐7 mixture of 1:5 was obtained starting from pure (Z)‐7 at 80°C in dimethyl sulphoxide. The lower stability of (E)‐7 was ascribed to higher steric effects due to a smaller Ph–C=C compared with Mes–C=C torsional angle and a preferred intramolecular π(Mes)–OH in (Z)‐7 over π(Ph)–OH hydrogen bonding. In order to dissect the effects, the corresponding 2‐mesityl‐2‐phenylvinyl methyl ethers (E)‐15 and (Z)‐15, where hydrogen bonding is absent, were prepared and equilibrated in chlorobenzene. The (Z)‐15: (E)‐15 ratio of ca 3:1 between 58° and 132° (ΔG=0·8 kcal mol−1) gives ΔH ≈ 0·6 kcal mol−1 and ΔS ≈ 0·5 e.u. It was concluded that steric effects contribute ca 1 kcal mol−1 and hydrogen bonding ca 1·5 kcal mol−1 to the higher stability of (Z)‐7 over (E)‐7. The unknown mesitylphenylacetaldehyde 16 was obtained from (Z)‐7 at 135°C in 31% yield.
Calorimetric study of human serum albumin in water–dioxane mixtures
Authors: Borisover Mikhail D. , Sirotkin Vladimir A. , Solomonov Boris N.
Published: January 2005

The calorimetric isotherm of water sorption by solid human serum albumin (HSA) was measured in water–dioxane mixtures. This isotherm has been successfully described at low water contents in the solvent within the framework of the presented model. The adsorption equilibrium constant and the monolayer formation energy were estimated from this model. The interaction enthalpies of solid HSA with the solvent in water‐rich mixtures do not differ essentially from the solution enthalpy of HSA in water. This means that the solvent water content may be sufficient for the intermolecular interactions of HSA suspended in a water–organic mixture to be similar to those in aqueous protein solution.
Acid‐catalysed solvolysis of optically active 1‐Phenyl‐1‐methoxythane. The role of ion– molecule pair intermediates
Authors: Thibblin Alf
Published: January 2005

The acid‐catalysed solvolysis of (R)‐1‐phenyl‐1‐methoxyethane was studied in mixtures of dilute aqueous perchloric acid and acetonitrile at 50°C. The rate of loss of optical activity (kα) was found to be the same as the rate of solvolysis (ksolv) at both low and high water contents of the solvent. These results are not in accord with literature data on racemization and oxygen exchange of 1‐phenylethanol in water, which has been discussed in terms of interconversion of chiral ion–dipole pair intermediates. The mechanistic implications of the results are discussed.
Steric deuterium isotope effect in the solvolysis of (Z)‐[Methyl‐D3]‐2‐ethylidene‐1‐adamantyl iodide accelerated by F‐strain
Authors: Ohga Yasushi , Takeuchi Ken'Ichi
Published: January 2005

The deuterium isotope effect in the solvolysis of (Z)‐ and (E)‐[methyl‐d3]‐2‐ethylidene‐1‐adamantyl mesylates [(Z)‐ and (E)‐1d‐OMs] and iodides [(Z)‐ and (E)‐1d‐I] was studied in 2,2,2‐trifluoroethanol at 25·0°C for mesylates and at 50·0°C for iodides. For the mesylates, which show a relatively small F‐strain effect, the (Z/E)H rate ratio (117 ± 1) is essentially identical with the (Z/E)D rate ratio (116 ± 1) at 25·0°C. On the other hand, for the iodides, which show a larger F‐strain effect, the (Z/E)H rate ratio (5413 ± 57) is greater than the (Z/E)D rate ratio (5040 ± 58) at 50·0°C. This indicates that (Z)‐1h‐I has greater F‐strain than (Z)‐1d‐I in the ground state. These results again confirm that the F‐strain effect in the (Z)‐2‐ethylidene‐1‐adamantyl derivatives exists between the (Z)‐methyl group and the leaving group atom directly attached to the reaction centre.
Nucleophilic substitution reactions of isopropyl benzenesulphonates with anilines and benzylamines
Authors: Oh Hyuck Keun , Kwon Young Bong , Lee Ikchoon
Published: January 2005

The nucleophilic substitution reactions of isopropyl arenesulphonates with anilines and benzylamines in acetonitrile at 65·0°C were investigated. The cross‐interaction constants ρxz (and βxz) between substituents in the nucleophile (X) and leaving group (Z) are positive and relatively small. The transition state (TS) variation is consistent with that predicted by the More O'Ferrall–Jencks diagram, as expected from the positive ρxz. The small magnitude of ρxz correctly reflects a relatively loose TS structure, in contrast to the tight TS for ethyl (or methyl) derivatives under similar reaction conditions.
Magnetic field effects upon photochemistry of bifuctional chain molecules
Authors: Nakagaki Ryoichi , Tanimoto Yoshifumi , Mutai Kiyoshi
Published: January 2005

External magnetic field effects are discussed for three typical examples of photoreactions involving biradical intermediates: photo‐induced intramolecular electron transfer in chain molecules D(CH2)nA consisting of an electron donor (D) and an acceptor (A), intramolecular hydrogen abstraction in bifunctional species :X(CH2)nYH containing a hydrogen donor (YH) and an acceptor (:X in the excited triplet state) and photochemical homolytic ring opening of a cyclic ketone which yields a biradical consisting of an acyl and an alkyl group. There has been progress in the elucidation of reaction mechanisms through the analysis of magnetic field effects on lifetimes of reaction intermediates and product yields in radical reactions involving biradicals. Extremely small interactions such as the Zeeman energy and hyperfine interaction may give rise to a large change in the distribution of photo‐products derived from bifunctional chain molecules.
Base catalysed photochemically induced hydrogen isotope exchange in o‐methylbenzophenone
Authors: Michalak Jacek , Wolszczak Marian , Gȩbicki Jerzy
Published: January 2005

Photochemically induced hydrogen–deuterium and hydrogen–tritium exchange between o‐methylbenzophenone and labelled methanol is shown to be catalysed by sodium carbonate. Both the (E) and (Z)‐photoenols participate in the exchange reaction whereas in absence of the catalyst only the (E)‐photoenol is reactive. Analysis of the measured quantum yields for hydrogen isotope exchange reaction allowed the determination of the relative population of transient photoenols and to its comparison with that obtained from the flash photolysis experiments. Both hydrogen kinetic and solvent isotope effects are discussed.
Protonation of 2‐(2‐thienyl)pyrrole and 2‐(2‐thienyl)‐1‐vinylpyrroles
Authors: Sigalov Mark V. , Trofimov Alexander B. , Shmidt Elena Yu. , Trofimov Boris A.
Published: January 2005

2‐(2‐Thienyl)pyrrole and its 3‐alkyl‐1‐vinyl derivatives have been shown (1H NMR) to be protonated by HSO3F, CF3COOH, HCl and HBr at the pyrrole C‐5 atom. Reaction with the superacid system HSO3FSbF5–SO2 results in the equilibrium between pyrrolium and thiophenium ions. The heats of formation (ΔH), charges and HOMO partial electron densities for 2‐(2‐thienyl)pyrrole and its protonated forms have been calculated by the MNDO method. The calculated ΔH values of thienylpyrrolium and pyrrolythiophenium ions are in agreement with the experimentally observed ratio.
Zero‐field splitting versus interelectronic distance in triplet electron spin resonance spectra of localized dinitrenes
Authors: Minato Masaki , Lahti Paul M.
Published: January 2005

Triplet zero‐field splitting parameters were obtained in 2‐methyltetrahydrofuran glass at 77 K for phenylene‐1,4‐dinitrene (1), biphenyl‐4,4′‐dinitrene (2), (E)‐1,2‐bis(4′‐nitrenophenyl)ethene (3), 1,4‐bis(4′‐nitrenophenyl)buta‐1,3‐diene (4) and 1,8‐bis(4′‐nitrenophenyl)octa‐1,3,5,7‐tetraene (5). The results were (1) |D/hc| = 0·169 cm−1 |E/hc| = 0·004 cm−1, (2) |D/hc| = 0·189 cm−1, |E/hc| = 0·00 cm−1, (3) |D/hc| = 0·122 cm−1, |E/hc| = 0·00 cm−1, (4) |D/hc| = 0·0865 cm−1, |E/hc| = 0·00 cm−1 and (5) |D/hc| = 0·0442 cm−1, |E/hc| = 0·00 cm−1. All these biradicals are ground‐state singlets. Based on the observed decrease in triplet signal intensities as temperature decreases. The substantial magnitudes of |D/hc| for 3–5, despite the large distance between localized nitrene electrons, is much more than can be explained by a simple dipolar interaction between localized electrons, and is attributed at least partly to spin polarization effects on the π‐electron clouds of these systems.
Micellar catalysis of organic reactions. Part 34. Nucleophilic aromatic substitution reactions in micelles with bulky head groups
Authors: Broxton Trevor J. , Christie John R. , Theodoridis Dorothy
Published: January 2005

Observed second‐order rate constants for the hydroxydechlorination of 1‐chloro‐2,4‐dinitrobenzene (2), 2‐chloro‐3,5‐dinitrobenzoate ions (3) and 4‐chloro‐3,5‐dinitrobenzoate ions (4) in micelles of cetyltrialkylammonium bromide (C16H33NR3Br, where R = Me, Et, n‐Pr and n‐Bu) are reported. For substrate 2, the observed catalysis increased as the size of the micellar head group was increased. This was shown to be primarily due to an increase in the rate of reaction in the micellar pseudo‐phase (k). For substrates 3 and 4, the observed catalysis decreased as the size of the micellar head group was increased. Much smaller changes in k were observed in these reactions, which lead to a dianionic intermediate which is more sensitive to polarity effects at the micelle surface than is the monoanionic intermediate formed from 2. These results support the contention that as the size of the micellar head group is increased, water is squeezed away from the micelle surface, resulting in a less polar reaction environment.
Attractive interactions between negative charges and polarizable aryl parts of host–guest systems
Authors: Schneider Hans‐Jörg , Werner Frank , Blatter Thomas
Published: January 2005

NMR measurements of associations between simple organic host and guest molecules bearing alternatively negatively charged groups and electroneutral π‐moieties show in aqueous solutions weak but distinct attractions reaching approximately 2 kJ mol−1 and X−/arene unit. A similar value is observed for the complex between a calixarene with 4‐sulfonato groups and toluene, for which the NMR shifts indicate an orientation with the guest methyl substituent upwards. The conformational fit of the selected complexes is checked by molecular mechanics simulations.
Structure–odour relations for bitter almond odorants
Authors: Zakarya Driss , Yahiaoui Mohamed , Fkih‐Tetouani Souäd
Published: January 2005

Structure–bitter almond odour relations are established for a set of 65 organic compounds (40 having the bitter almond character) belonging to benzene, pyrrole, thiophene, acyclic and cyclic compounds. Compounds are described using components of autocorrelation method, and the odour is described with a binary variable. Data were analysed using principal component analysis followed by a linear discriminant analysis. The obtained model gives satisfactory classification and prediction of the training set and the test set, respectively. In addition, obtained structure–odour relations were translated to structural elements (and rules) necessary for a stimulus to have bitter almond character.
Classification of the solubility behaviour of β‐cyclodextrin in aqueous–CO‐solvent mixtures
Authors: Coleman Anthony W. , Munoz Marc , Chatjigakis Alexis K. , Cardot Philippe
Published: January 2005

The solubility behaviour of β‐cyclodextrin in aqueous mixtures was studied by determining the interactions between the co‐solvent and water. Three clear classes of co‐solvent are evident. In the first, including ethanol, tetrahydrofuran and acetone, there is formation of clathrate hydrates by the co‐solvent. In these systems the solubilization is a maximum at the mole fraction corresponding to the minimum in the partial excess molar volume. The second class, including such solvents as dimethyl sulphoxide and pyridine, forms strong molecular complexes with water, and here a valley and a plateau region occur in the solubility values. In the third class, including formamide and urea, a steady increase in the solubility is observed. For these highly polar solvents no maxima are observed in intrinsic solvent properties such as the partial excess molar volume.
Acidities of 4‐substituted benzoic, bicyclo[2.2.2]oct‐1‐yl and bicyclo[2.2.2]oct‐2‐enyl carboxylic acids
Authors: Koppel I. A. , Mishima M. , Stock L. M. , Taft R. W. , Topsom R. D.
Published: January 2005

Experimental gas‐phase acidities are reported for a series of 4‐substituted bicyclo [2.2.2] oct‐1‐yl carboxylic acids and for a limited number of the corresponding bicyclo [2.2.2] oct‐2‐enyl carboxylic acids. Similar acidities are also reported for additional 4‐substituted benzoic acids, allowing a comparison of field and resonance effects between the three series. Ab initio molecular orbital calculations for these series of acids confirm the conclusion that the aromatic acids display direct field and resonance effects, whereas the acidities in the aliphatic series acids are largely determined by direct field effects.
First determination of an absolute rate constant for a 1,2‐phenyl migration to a carbene
Authors: Liu Michael T. H.
Published: January 2005

The first determination of a 1,2‐phenyl shift in α,α‐dimethylbenzylchlorocarbene was achieved by nanosecond laser flash photolysis, τ = 50 ns.
Magnetic coupling between two phenoxyl radicals attached to the phenyl rings of CIS‐ and trans‐stilbenes
Authors: Mitsumori Teruyuki , Koga Noboru , Iwamura Hiizu
Published: January 2005

Magnetic coupling between two sterically protected phenoxyl radicals through the cis‐ and trans‐stilbene chromophores was studied by means of their EPR fine structures. While the zero‐field splitting parameters D, which are governed by the magnitude of dipolar coupling, were dependent on the geometrical isomerism, the sign of exchange coupling was independent of it and dictated by the topology of the π‐conjugated systems: ferromagnetic in o,m‐isomers and antiferromagnetic in m,m'‐isomers.
Estimation of marcus λ for p‐phenylenediamines from the optical spectrum of a dimeric derivative
Authors: Nelsen Stephen F. , Yunta Maria J. R.
Published: January 2005

The radical cation of N,N′‐bis‐(4‐dimethylaminophenyl)pyridazine (3˙+) has a near‐IR band with a transition energy of 29·7 ± 0·5 kcal mol−1 (1 kcal = 4·184 kJ) in acetonitrile. This band is assigned to a Hush‐type charge‐transfer band, and the transition energy to the Marcus λ value for electron transfer. Such a large λ is inconsistent with previous estimates of λ for intermolecular electron transfer between mono‐p‐phenylenediamines and their radical cations. It agrees well with estimates of λ based on AM1 semi‐empirical MO calculations, which gives the enthalpy contribution to λin at about 21 kcal mol−1 for 3·+. It is suggested that the traditional method of estimating λin by summing bond displacements weighted by force constants produces values which are significantly too low for systems such as phenylenediamines, where pyramidalization changes are important. The results suggest that λ for tetramethyl‐p‐phenylenediamine self‐electron transfer (10/·+) has been significantly underestimated, and that this prototype organic electron transfer reaction is not as strongly diabatic as was previously concluded.
Single‐parameter versus dual‐parameter correlation for radical reactions. Addition of bromine atoms to α‐methylstyrenes
Authors: Jiang Xi‐kui , Liu Wayne Wei‐zhong , Wu Shi‐hui
Published: January 2005

A rigorous procedure was developed for measuring the relative rates of addition of bromine atoms to eleven substituted α‐methylstyrenes (1‐Y, with Y = CF3, NO2, F, CN, Cl, Br, CO2Me, Me, COMe, OMe and SMe). The reaction was run in tetrahydrofuran in the presence of HBr, O2and dibutyl peroxyoxalate at 30°C. All products were derived from the YC6H4CMeCH2Br adduct radicals, which were immediately intercepted by O2. Correlation analysis of all the data confirmed the proposition that in the absence of measurable steric effects, the relative rates for radical additions can be correlated only by a dual‐parameter equation and not by a single‐parameter equation. Among various combinations of σ* and σx, the (σJJ* + σmb) combination yields the best correlation.
Estimation of ionization constants of azo dyes and related aromatic amines: Environmental implication
Authors: Hilal S. H. , Carreira L. A. , Baughman G. L. , Karickhoff S. W. , Melton C. M.
Published: January 2005

Ionization constants for 214 dye molecules were calculated from molecular structures using the chemical reactivity models developed in SPARC (SPARC Performs Automated Reasoning in Chemistry). These models used fundamental chemical structure theory to predict chemical reactivities for a wide range of organic molecules from molecular structure. The energy differences between the protonated state and the unprotonated state for a molecule of interest are factored into mechanistic components including the electrostatic and resonance contributions and any additional contributions to these energy differences. The RMS deviation was found to be less than 0·62 pKa units, which is similar to the experimental error.
Estimation of the OH ⃛O interaction energy in intramolecular hydrogen bonds: A comparative study
Authors: Dziembowska T. , Szczodrowska B. , Krygowski T. M. , Grabowski S. J.
Published: January 2005

An empirical formula relating directly H ⃛O interatomic distance to H ⃛O interaction energy is used to estimate this energy in intramolecular OH ⃛O hydrogen bonds calculated for systems of known precise geometry. The values obtained were correlated with spectroscopic characteristics of the H‐bond (Δv for OH bands in IR spectra and NMR chemical shifts, δ, for protons) and CNDO and INDO estimations of H‐bond energy. The regressions obtained had good or very good correlation coefficients.
Enolimine and geminaldiamine forms in the reaction of pyridoxal phosphate with ethylenediamine. An electrochemical and spectroscopic contribution
Authors: Hidalgo Carmen , Sevilla Jose M. , Pineda Teresa , Blazquez Manuel
Published: January 2005

The enol–keto tautomerism of the Schiff bases formed by pyridoxal 5'‐phosphate (PLP) with ethylenediamine (Etd) and ethylamine (Et) was studied by electrochemical and spectrophotometric methods. The spectroscopic results revealed differences between the two reactions, i.e. the enolimine/ketoenamine ratio observed in PLP–Etd mixture is higher than that of PLP–Et. The differences observed in the electroreduction mechanism and stability of the Schiff bases formed provide additional evidence for an unusually high concentration of enolimine in PLP–Etd in buffered aqueous solutions.

The results are consistent with a cyclic structure such as that previously proposed by Robitaille et al. [J. Am. Chem. Soc. 111, 3034–3047 (1989)] on the basis of spectroscopic data. Protonation of the terminal amino group of the Etd moiety in the Schiff base involves the formation of the cyclic species. The low basicity of ethylenediamine favours the formation of similar concentrations of the enol and keto tautomers of the Schiff base at neutral pH. This behaviour, also observed in the Schiff base formed by PLP and polylysine, may be involved in coenzyme–protein linkages.
Relative thermodynamic stabilities of the isomeric propenylbenzenes
Authors: Taskinen Esko , Lindholm Nina
Published: January 2005

The relative thermodynamic stabilities of 2‐propenylbenzene (allylbenzene), and the E and Z forms of 1‐propenylbenzene were determined over the temperature range 50–170°C by chemical equilibration in DMSO solution with t‐BuOK as catalyst. The values of the thermodynamic parameters ΔG⊖, ΔH⊖ and ΔS⊖ at 298·15 K for each isomerization reaction between the title compounds were evaluated.
Solvolysis of 3‐substituted 4‐homoadamantyl methanesulphonates. Can the β‐substituent effect distinguish between classical and non‐classical ion intermediates?
Authors: Okazaki Takao , Kitagawa Toshikazu , Takeuchi Ken'Ichi
Published: January 2005

The rates of the solvolysis of 3‐R‐4‐homoadamantyl methanesulphonates (mesylates) (3) were determined in 80% aqueous ethanol. The relative first‐order rate constants at 25 °C were 1·0 (R = H), 2·29 (R = Ph), 3·26 (R = p‐anisyl), 73·6 (R = Me) and 209 (R = Et). The methanolysis of 3 gave rearranged methyl ethers and rearranged olefins as major products together with small amounts (0·9–3·4%) of unrearranged products. The order of the accelerating effect suggests that the transition states involve significant σ‐participation, despite the fact that 3 (R = H) solvolyses via a classical ion intermediate. The logarithms of the solvolysis rate constants of 3 showed linear correlations with those of 1‐R‐2‐adamantyl tosylates (1) and 1‐R‐exo‐2‐norbornyl tosylates (2), indicating that the linear free‐energy relationship between the β‐substituent effects on the solvolysis rate is not a definite measure to distinguish between classical and non‐classical intermediates.
Nucleophilic substitution reaction of benzyl bromide with N, N‐Dimethylaniline: Significance of equilibrium cross‐interaction constant
Authors: Lee Ikchoon , Park Yong Kyun , Huh Chul , Lee Hai Whang
Published: January 2005

Kinetic studies on the reversible reactions of benzyl bromides with N, N‐dimethylanilines were carried out for both the forward (kf) and reverse (kr) directions. The equilibrium constants, K were calculated using the ratio kf/kr and the equilibrium cross‐interaction constant, ρeXY was determined. The ρeXY value was shown to represent a maximum intensity of interactions between substituents X and Y through covalent bonds within a molecule. The normalized values of ρX (ρnuc) and ρXY indicate that in the transition state the fractional development or loss of polar and resonance interactions in the benzyl system are imbalanced or non‐perfectly synchronized. In the forward reaction the fractional loss of resonance interaction becomes enhanced, whereas in the reverse reaction the fractional development of resonance interaction lags behind the corresponding changes of polar interactions by ca 45%.
Directional hydrogen bonding in the MM3 force field. I
Authors: Lii Jenn‐Huel , Allinger Norman L.
Published: January 2005

The MM3 molecular mechanics program calculates a fair representation of hydrogen bonding interactions, but to improve the MM3 hydrogen bond potential, a directional term has been added to the hydrogen bonding function. The resulting total function was reoptimized. Comparisons of the hydrogen bonding potential functions from ab initio, the original MM3, the current MM3(92) force field and the reoptimized MM3 force field MM3(94) for a variety of C,N,O systems are described.
Carbonyl oxygen as a hydrogen‐bond super‐base: The amidates
Authors: Chardin Aurelie , Berthelot Michel , Laurence Christian , Morris David G.
Published: January 2005

An infrared spectroscopic study of the 1 : 1 hydrogen‐bond association of amidates with both methanol and 4‐fluorophenol showed that the site of complexation is the oxygen of the amidate function. However the formamidate HCON2Me3 forms a second 1 : 1 complex on the nitrogen of the amidate. The formation constants of the hydrogen‐bond complexes of the amidates with the reference hydrogen‐bond donor 4‐FC6H4OH indicate that the amidates are stronger hydrogen‐bond bases than are amides and amide vinylogues. As such, the amidates constitute the strongest carbonyl bases hitherto investigated on the hydrogen‐bond basicity scale.
Rechecking of the equilibrium gas‐phase basicity scale for low‐basicity compounds using fourier transform ion cyclotron resonance spectrometry
Authors: Koppel Ilmar A. , Anvia Frederick , Taft Robert W.
Published: January 2005

Fourier transform ion cyclotron resonance (FT‐ICR) spectrometry was used to study and recheck the proton transfer equilibria for a large number of low‐basicity compounds, particularly those which are less basic than H2O. The data obtained were used for the construction of a continuous scale of the relative gas‐phase basicities between H2S and SO2F2. The present results are compared with the results obtained by McMahon and co‐workers using ICR spectrometry and high‐pressure mass spectrometric (HPMS) techniques. Satisfactory agreement is found with the existing ICR spectrometric data. The results, however, show unexplained variances with earlier and recent HPMS results that also not internally self‐consistent. The substituent effects for some families of low‐basicity compounds (nitriles, carbonyl compounds, ethers and some others) are discussed.
Distinguishability of liquid environments in the presence of brownian noise
Authors: Grunwald Ernest , Steel Colin
Published: January 2005

This paper extends previous work on the dynamic averaging of distinct liquid environments by cage exchange to include potential energy fluctuations at the cage centers due to Brownian motions of molecules in the cage walls. After a brief review of the effect of cage exchange on the Schrödinger wave train associated with the electronic ground state of the caged molecule, typical magnitudes and time‐scales for the Brownian potential energy fluctuations are estimated. Then a zig‐zag model for the resulting noise in the wave trains is developed, and applied to analyze distinguishability in the presence of noisy cage exchange. When distinguishability survives the noise, the distinct caged species are fully fledged environmental isomers. Applications of these concepts are discussed.
Model solvent systems for QSAR. Part IV. The hydrogen bond acceptor behaviour of heterocycles
Authors: Leahy David E. , Morris Jeffrey J. , Taylor Peter J. , Wait Alan R.
Published: January 2005

An LSER analysis based on the partitioning of 15 proton acceptor heterocycles has succeeded in extracting Σβ values, but only at the cost of demonstrating solvent dependence for some of them. As noted by Abraham, the division lies between protic and aprotic organic phases. His observation that pyridine and quinoline are less effective acceptors when surrounded by solvent than in 1 : 1 association was confirmed, and possible reasons for this are discussed. Two other such cases are N‐methylimidazole and pyridazine, both of which give lower Σβ values in octanol than in PGDP. For both, Σβ in PGDP is what would be expected on the basis of log Kβ. The value for pyridazine in octanol suggests that, here, the ‘α‐effect’ is no longer operative; it is possible that this result can be generalized to other such heterocycles. Elsewhere, the most remarkable finding is that, where there are two proton acceptor sites in one heterocyclic ring, Σβ is the simple unattenuated sum of the separate βf values. If this result is general, it leads to a very simple way of estimating Σβ for heterocycles by calculation where data are unavailable. Evidence was also found, in certain cases, for hydrogen bonding to the π‐donor heteroatom or the aromatic ring. The QSAR implications of these results are discussed.
Control of the thermal cis to trans isomerizations of azobenzene and thioindigo derivatives by the formation of supramolecular H‐bonded assemblies
Authors: Rosengaus Jacqueline , Willner Itamar
Published: January 2005

2,3‐Bis(aminocyclohexyl)−6‐methoxy‐1,3,5‐triazine (1a) forms intermolecular H‐bonded complexes with 3,3′‐diacetyl‐cis‐azoenzene (4) and 6,6′‐diethoxy‐cis‐thioindigo (5b), (association constants K = 4.9 × 104 and 3.5 × 105 1 mol−1, respectively). The thermal cis→trans isomerization of 4b and 5b to 3,3′‐diacetyl‐trans‐azobenzene (4a), and 6,6′‐diethoxy‐trans‐thioindigo (5a), is inhibited in the intermolecular complex 1a–4b and 1a–5b. Molecular mechanics calculations support the formation of the intermolecular H‐bonded complexes between 1a and 4b or 5b.
Oil‐in‐water microemulsions based on chemodegradable surfactants as reaction media
Authors: Bieniecki Albert , Wilk Kazimiera A.
Published: January 2005

The reaction of 2‐(p‐nitrophenyl)ethyl bromide with hydroxide ion was studied in oil‐in‐water (o/w) microemulsions at 50°C. The octane‐in‐water microemulsion systems were stabilized by chemodegradable cyclic acetal‐type cationic surfactants as [(2‐alky‐1,3‐dioxolan‐4‐yl)methyl]trimethylammonium bromides Ia–c (alkyl: a = n‐C9H19; b = C11H23; c = C13H27) and butan‐1‐ol as co‐surfactant. The rate constants were also determined in the corresponding aqueous micelles and in water. The hydrolytic lability of Ia–c in the microemulsion systems was achieved at 50°C by acid hydrolysis of the 1,3‐dioxolane ring. Systematic synthetic studies of the dehydrobromination were performed in order to develop a preparative approach for the product separation without unfavourable process such as foaming and emulsification.
A remarkably bent allene. X‐Ray crystal structure and ab initio calculations
Authors: Weber Edwin , Seichter Wilhelm , Hess Bernd , Will Georg , Dasting Hans‐Jürgen
Published: January 2005

X‐ray data on the crystal and molecular structure of bis(biphenyl‐2,2′‐diyl)allene, reveal a remarkably strong deviation from linearity (170·1°) of the allene unit of the molecule. Ab initio calculations involving dimers of molecules yield good correlation with this geometry, showing that the non‐linearity is due to packing effects in the crystal.
Are kekulene, coronene, and corannulene tetraanion superaromatic? Theoretical examination using hardness indices
Authors: Zhou Zhongxiang
Published: January 2005

The superaromaticity concept is examined and developed, taking into account what is known about aromaticity. Three new hardness indices indices are defined, and shown to be excellent parameters for characterizing superaromaticity. High superaromaticity indicates significant global annulenoid conjugation relative to local benzenoid conjugation in circularly annealed benzenoid molecules. Kekulene, coronene and corannulene tetraanion all are predicted to be superaromatic. The prediction for the corannulene tetraanion strongly supports the “annulene‐within‐an‐annulene” structure derived from NMR data.
Several mechanisms in the elimination kinetics of ω‐chlorocarboxylic acids in the gas phase
Authors: Chuchani Gabriel , Martin Ignacio , Rotinov Alexandra , Dominguez Rosa M. , Milagros Perez I.
Published: January 2005

The kinetics of the gas‐phase pyrolysis of ω‐chlorocarboxylic acids were examined in a seasoned static reaction vessel and in the presence of at least twice the amount of the free radical inhibitor cyclohexene or toluene. In conformity with the available experimental data on rate determination, these reactions proved to be unimolecular and obeyed a first‐order rate law. The presence of the primary chlorine leaving group in Cl(CH2)nCOOH (n = 1–4) showed a change in mechanism from intramolecular displacement of the Cl leaving group by the acidic hydrogen of the COOH to anchimeric assistance of the carbonyl COOH to the CCl bond polarization in the transition state. This mechanistic consideration is nearly the same for the series of 2‐, 3‐, and 4‐chlorobutyric acids. The chlorine atom at the 2‐position of acetic, propionic and butyric acids is dehydrochlorinated through a prevailing reaction path involving a polar five‐membered cyclic transition state.
Nature of the highest occupied molecular orbitals of trans‐ and cis‐bicyclo[4.1.0]hept‐3‐enes
Authors: Gassman Paul G. , Mlinarić‐Majerski Kata , Kovač Branka , Chen Han , Dixon David A.
Published: January 2005

Photoelectron spectra of trans‐bicyclo[4.1.0]hept‐3‐ene and two methyl derivatives, 1–3, and cis‐bicyclo[4.1.0]hept‐3‐ene and three of its derivatives, 4–7, were measured. Ab initio molecular orbital (MO) and density functional theory (DFT) calculations were performed on these compounds. By combining the results of the experimental and computational trends, it was established that the highest occupied molecular orbitals (HOMOs) of 1–3 are the ‘twist’ bent σ bond at the fusion between the two rings whereas for 4–7 the HOMOs are the π orbital in the six‐membered ring. The DFT ordering is the same as that suggested by the photoelectron spectra whereas at the MO level the HOMO always corresponds to the π orbital. The intensities of the photoelectron spectra were calculated at the DFT level and good qualitative agreement with experiment was found.