bluuebunny/crossref_metadata_2025_split huggingface dataset:
Origin of apparent swain–schaad deviations in criteria for tunneling
Authors: Huskey W. Phillip 
Published: January 2005

AbstractVibrational 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractPhotolysis 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

AbstractTheoretical 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

AbstractTwo 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

AbstractEmpirical 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

AbstractDirect 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

AbstractP‐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

AbstractRates 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

AbstractRates 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

Abstract4‐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

AbstractThermodynamic 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

AbstractSeveral 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

AbstractThe 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

AbstractThe 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

AbstractAccess 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.
Evaluation of the conformational, hydrogen bonding and crystal packing preferences of acyclic imides
Authors: Reutzel Susan M. , Etter Margaret C. 
Published: January 2005

AbstractThe molecular recognition properties of 18 acyclic imides were studied to evaluate the relative contributions of conformational, hydrogen bonding and crystal packing forces to the stabilization of specific aggregate patterns in the solid state. The crystal structure of diisobutyramide and the aggregate patterns of the 18 imides are presented. The stabilization by hydrogen bonding was found to override the conformational preferences of imides, while packing forces often precluded the formation of the most stable hydrogen bonded aggregate. The aggregate patterns of imides were also found to be a function of the type of substituents present as R groups. Imides with R groups of similar shape and size prefer to pack as bifurcated hydrogen bonded chains, whereas dimers or singly hydrogen‐bonded chains form when the R groups have significantly different spatial requirements. Analysis of imide aggregate patterns revealed the similar spatial requirements of isopropyl and phenyl groups. The molecular recognition properties of acyclic imides are summarized as a set of hydrogen bond rules, which can be used to design new imide aggregates.
Hydrolyses of terpenoid diphosphates. Effects of azide ion on products of hydrolysis
Authors: Alarcon Maritza , Cori Osvaldo , Rojas M. Cecilia , Pavez Hernan , Bacaloglu Radu , Bunton Clifford A. 
Published: January 2005

AbstractHydrolysis of geranyl diphosphate (GPP) at pH 7 in water gives largely linalool (LOH) + geraniol (GOH) in the ratio of 3:1. Added N−3 generates mixed acylic allylic azides and increases the LOH GOH ratio to 15:1 in 2 M NaN3, but does not speed up the overall reaction. Hydrolysis of neryl diphosphate (NPP) gives largely α‐terpineol (TOH) +p LOH, but their ratio is not very sensitive to NaN3 concentration although acyclic azide and small amounts of α‐terpinyl azide (TN3) are formed. Hydrolysis of α‐terpinyl diphosphate (TPP) gives large amounts of the cyclic alkenes, limonene and terpinolene. Added N−3 does not change the amount of elimination, but increases the ratio of limonene to terpinolene, and diverts some substitution product to TN3. Trapping of carbocationic species from GPP by N−3 is sharply increased by addition of Mn2+, which also catalyzes the overall reaction. Products of reaction of GPP are derived from acyclic intermediates and of NPP from acyclic and cyclic intermediates, and ionizations of the three substrates do not generate common carbocationic species.
Molecular modelling of organophosphorus podands and their complexes with alkali metal cations
Authors: Varnek A. A. , Morosi G. , Gamba A. 
Published: January 2005

AbstractA molecular mechanics study on some organophosphorus podands, with both rigid and flexible terminal groups, and on their complexes with Li+ and Na+ was carried out. Na+ causes smaller deformations of the ligands than Li+, but its complexes are less stable, as the interaction energy with the podands is weaker. The ligands with rigid terminal groups are, in general, more pre‐organized for complex formation. The cation‐ligand interaction energies, including also the changes in steric energies, are always higher in complexes of flexible molecules than in those containing rigid ligands. The lower entropy losses which accompany the formation of complexes of rigid molecules with alkali metal cations probably account for the observed higher stabilities of the complexes.
Cycloaddition of heterofunctionalized allenes with tert‐butylthioacrylonitrile. Determination of activation parameters for diradical intermediate formation and the detection of reversible ring closure
Authors: Pasto Daniel J. , Kong Wei 
Published: January 2005

AbstractThe cycloaddition reactions of 4‐methylphenylthio‐, methoxy‐, 4‐methoxyphenyl‐, phenyl‐, chloro‐ and cyanoallene with tert‐butylthioacrylonitrile (BTA) in toluene‐d8 solution were investigated. The relative reactivity sequence of these substituted allenes is 4‐methylphenylthio ∼ methoxy‐ > 4‐methoxyphenyl‐ > phenyl‐ > chloro‐ > cyanoallene. Activation parameters were measured for diradical intermediate formation of BTA with 4‐methylphenylthio‐, methoxy‐ and 4‐methoxyphenylallene, giving average values of Ea and ΔS± of 14.2–16.8 kcal mol−1 and ca. ‐33 eu, respectively. The relative reactivity sequence is consistent with a FMO allene‐HOMO, BTA‐LUMO dominant interaction for diradical intermediate formation. However, the regioselectivity of attack on the substituted allene appears to be thermodynamically controlled. The formation of the diradical intermediates in these cycloaddition processes appears to be irreversible. However, the ring closure of the diradical intermediates formed from 4‐methylphenylthio‐, methoxy‐ and 4‐methoxyphenylallene is reversible under the conditions of the kinetic experiments. The kinetically controlled ring closure of the diradical intermediates is allyl radical SOMO controlled, while the final cycloadduct distribution is thermodynamically controlled.