bluuebunny/crossref_metadata_2025 huggingface dataset:
Maximally inhibited elimination of kinetics of (2‐bromoethyl)benzene and 1‐bromo‐3‐phenylpropane in the gas phase. Anchimeric assistance of the phenyl group
Authors: Chuchani Gabriel , Martin Ignacio
Published: January 2005

The gas‐phase elimination kinetics of (2‐bromoethyl)benzene and 1‐bromo‐3‐phenylpropane were studied in a static system and seasoned vessels over the temperature range 390–450 °C and the pressure range 32–104 Torr. The reactions, under maximum inhibition of 4‐fold pressure of the free‐radical suppressor cyclohexene and/or propene, are homogeneous, unimolecular and obey a first‐order rate law. The rate coefficients are given by the following Arrhenius equations: for (2‐bromoethyl)benzene, log k1 (s−1) = (13·04 ± 0·10) − (210·8 ± 1·3) kJ mol−1 (2·303RT)−1, and for 1‐bromo‐3‐phenylpropane, log k1 (s−1) = (14·09 ± 0·27) − (227·7 ± 3·6) kJ mol−1 (2·303RT)−1. The phenyl group of (2‐bromoethyl)benzene appears to provide anchimeric assistance in the HBr elimination of this compound. However, neighbouring C6H5 participation at the 3‐position in 1‐bromo‐3‐phenylpropane for a C‐4 conformation is apparently absent. The mechanisms of these reactions are discussed.
MNDOC‐CI calculations for organic photoreactions. I. The α‐cleavage reaction of carbonyl compounds
Authors: Reinsch Manfred , Klessinger Martin
Published: January 2005

MNDOC‐CI results are reported for the photochemical α‐cleavage reaction of carbonyl compounds. Potential energy surfaces for some excited singlet and triplet states and cross‐sections along the bitopic reaction path show that the activation energy of the α‐cleavage reaction is determined by the relative energetic position of the lowest two triplet states. The reactivity of aliphatic ketones depends only on the degree of branching at the α‐carbon, because the triplet barrier is due to an (avoided) crossing of two surfaces corresponding to an nπ* and ππ* excitation respectively, which correlates with the stability of the leaving alkyl radical. For conjugated carbonyl compounds, on the other hand, the position of the crossing point is independent of the 3nπ* excitation energy, but if the 3ππ* surface is below the 3nπ* surface all the way along the reaction coordinate, the reaction is determined by the barrier on the 3ππ* surface and any additional stabilization of the 3ππ* state increases the barrier.
Kinetic and spectroscopy of 2‐naphthylphenylcarbene
Authors: Maloney Vincent , Platz Matthew S.
Published: January 2005

2‐Naphthylphenylcarbene (2‐NPC) was studied by low‐temperature EPR spectroscopy and by solution‐phase laser flash photolysis. 2‐NPC was found to have a triplet ground state with two distinct rotomeric forms. The triplet carbene was found to react with methanol, 1,1‐diphenylethylene, styrene, carbon tetrachloride, isoprene, toluene and 2‐methyltetrhydrofuran.
15N NMR of nitranions: Opposite shift effects of σ and π charge
Authors: Barchiesi Emma , Bradamante Silvia
Published: January 2005

Nitranions of pyrrole, aromatic and heteroaromatic amines, carboxamides and pyridyl carbanions are generated by deprotonation either of NH or CH acids. The positive or negative displacements undergone by the 15N nucleus relative to the neutral precursor depend on whether a σ or π charge is increased. They are associated with the symmetry of the nitrogen orbital in which the electron pair is either generated or that to which it is delocalized.
Topological evidence for an NN bond in cis‐1,2‐dinitrosoethene: The remarkable structure of the di‐N‐oxide of 1,2‐diazacyclobutadiene
Authors: Sarasola C. , Choi S. C. , Ugalde J. M. , Boyd Russell J.
Published: January 2005

Analysis of the topological properties of the electron density distribution of cis‐1,2‐dinitrosoethene (I) shows that there is a normal covalent, albeit very long, single bond between the nitrogen atoms in the molecule, which is in fact the di‐N‐oxide of 1,2‐diazacyclobutadiene (II). Despite its extraordinary length, the NN bond is shown to be consistent with the strong correlation between the electron density at the bond critical points and the calculated bond lengths of NN bonds. The geometry and electronic structure exhibit a number of remarkable features.
Kinetics and mechanism of the oxidation of organic sulphides by N‐chloroacetamide
Authors: Agarwal Anupma , Bhatt Pallavi , Banerji Kalyan K.
Published: January 2005

The kinetics of the oxidation of a number of monosubstituted aryl methyl, alkyl phenyl, dialkyl and diphenyl sulphides by N‐chloroacetamide (NCA) to yield the corresponding sulphoxides were studied. The reaction is first order with respect to the sulphide, NCA and hydrogen ions. There is no effect of added acetamide. Protonated NCA has been postulated as the reactive oxidizing species. The rates of oxidation of meta‐ and para‐substituted phenyl methyl sulphides were correlated with Taft's and Swain's dual substituent parameter equations. For the para‐substituted compounds, the best correlation is obtained with σI and σ; the meta‐substituted compounds correlate best with σI and σR° values. The reaction constants are negative. The oxidation rates of the ortho‐substituted sulphides yield an excellent correlation in a triparametric equation involving Taft's σI and σ values and Charton's steric parameter, V. The oxidation of alkyl phenyl sulphides is susceptible to both polar and steric effects of the alkyl group. A mechanism involving the formation of a halogenosulphonium cation in the rate‐determining step is proposed.
Photochemistry of pyridinium salts 1. Intramolecular charge transfer in N‐amino‐substituted pyridinium cations
Authors: Feygelman Vladimir M. , Tymyanskii Yakov R. , Makarova Nadezhda I. , Knyazhanskii Michail I. , Druzhinin Sergei I. , Uzhinov Boris M.
Published: January 2005

The photophysics and photochemistry of N‐amino‐substituted pyridinium salts were investigated. Electron transfer from the amino group to the pyridinium ring takes place in the singlet excited state. The fluorescence quenching and photoreactivity of these compounds is conditioned by this process. Irradiation of N‐aminopyridinium salts in solution leads to novel fused heterocycles or to triphenylpyridine, depending on the concentration of the initial compound. Free radical intermediates are involved in the photoreaction.
Relationship between substituent electronegativity and field parameter in the aliphatic series. Introduction of charge separation parameter (λ)
Authors: Inamoto Naoki , Masuda Shozo , Niwa Jun
Published: January 2005

The problem of non‐proportionality between the substituent field parameter (σF) and substituent electronegativity (Δι or σχ) was clarified by the introduction of the ‘charge‐separation parameter’ (λ), which is the component for the direction of the HX axis of the distance between positive and negative poles of the dipole in the HX molecule. Thus σF was correlated with Δι or σχ by the equation σF = aΔιλ (or σF = a′σχλ). The above proposal was further supported by performing an energy decomposition analysis for the isodesmic reaction in the isolated molecule system (\documentclass{article}\pagestyle{empty}\begin{document}${\rm X}\raise1pt\hbox{---} {\rm H} \cdots {\rm H}\mathop {\rm N}\limits^ + {\rm H}_3 $\end{document} or XH…︁HCO) at shorter distances (2·0–4·5 Å). It was further shown that field parameters such as σi, F and σF(theor), and the pKa values in the series of aliphatic acids, and spectral data (UPS and 13C‐SCS) were also well correlated with Διλ or σχλ. It is concluded that λ and Δι or σχ are essential parameters for interpreting the electrostatic field effect in aliphatic substituent effects, and that the transmission due to the σ‐inductive effect is considered to be not as significant as the field effect after a few bonds away from the substituent.
β‐Secondary deuterium isotope effect and solvent isotope effects in catalysis by subtilisin BPN′
Authors: Kovach Ildiko M. , Do Son , Schowen Richard L.
Published: January 2005

Subtilisin BPN′ catalyzes the hydrolysis in protium and deuterium oxides of p‐NO2C6H4OCOCL2NHCO2CH2C6H5(L = H, D) in the pH(D) range 5·0–8·5 (H2O) and 5·4–9·0 (D2O), according to simple Michaelis–Menten kinetics. The parameter kcat/Km exhibits pH(D) inflection points of 7·17 ± 0·05 (H2O) and 7·69 ± 0·08 (D2O), and kcat shows 6·88 ± 0·05 (H2O) and 7·50 ± 0·07 (D2O). The ‘normal’ ΔpK values of 0·5–0·6 indicate no unusual effects of D2O on enzyme properties. The solvent isotope effects (H2O/D2O) on the limiting values of the rate constants at high pH(D) are 1·13 ± 0·07 for kcat/Km and 1·29 ± 0·05 for Kcat. These small effects indicate no more than minor contributions of general acid–base catalysis for rate‐limiting events for either kcat/Km or kcat. The β‐deuterium secondary isotope effects (2H/2D) are roughly estimated by extrapolation as 0·95 ± 0·01 for kcat/Km, corresponding to substantial tetrahedral character in the transition state, and 1·03 ± 0·03 for kcat, consistent with no tetrahedral character. Models consistent with these results have as rate‐limiting events for kcat/Km nucleophilic attack by active‐site imidazole and for kcat, among other possibilities, the release of carboxylate product from the imidazolium form of the enzyme.
Theoretical studies on the gas‐phase pyrolysis of acetic anhydride and diacetyl sulphide
Authors: Lee Ikchoon , Cha Ok Ja , Lee Bon‐Su
Published: January 2005

The gas‐phase thermal decomposition of diacetyl compounds, (CH3CO)2X with X = O and S, was investigated theoretically using the semiempirical MO methods MNDO and AM1. The initial decomposition of the diacetyl compounds proceeded through a six‐membered ring transition state involving the keto form with a slightly lower activation enthalpy for diacetyl sulphide (X = S); the process via an enolic form of the transition state was kinetically unfavourable. In the initial decomposition of the diacetyl compounds and in the subsequent pyrolysis of acetic and thioacetic acid, ketene formation was found to be the most preferred path, where the ease of CαX bond cleavage is relatively more important than nucleophilic attack on the β‐hydrogen in determining the overall reactivity. In the methane formation process, the reactivity was entirely dependent on the XH bond strength in CH3COXH where X = S, NH and O.
[4]Paracyclophane: MNDO and STO‐3G molecular structure and strain energy
Authors: Jenneskens Leonardus W. , Louwen Jaap N. , De Wolf Willem H. , Bickelhaupt Friedrich
Published: January 2005

The molecular structure of [4]paracyclophane was optimized at the semiempirical MNDO and the ab initio STO‐3G level. A comparison of the results showed that the benzene ring is much less bent at the STO‐3G level. Although some bond alternation is predicted at both levels of theory, the aromatic carbon–carbon bond lengths are still in the range typical of highly delocalized compounds. The calculated strain energy [SE(tot.)] of [4]paracyclophane is larger for the STO‐3G structure. Nevertheless, the distributions of SE(tot.) over the bent benzene ring [SE(bb.)] and the oligomethylene bridge [SE(br.)] are the same for the MNDO and STO‐3G structures.
Inhibition of peroxidations of lipids and membranes by estrogens
Authors: Komuro Erika , Takahashi Mareyuki , Morita Toshiaki , Tsuchiya Jyunichi , Arakawa Yoshitaka , Yamamoto Yuichi , Niki Etsuo , Sugioka Katusaki , Nakano Minoru
Published: January 2005

The antioxidant activity of 2‐hydroxyestradiol was measured quantitatively and compared with those of other estrogens in oxidations of methyl linoleate in homogeneous solution and in aqueous dispersions, oxidations of soybean phosphatidylcholine liposomal membranes and oxidative haemolysis of rabbit erythrocytes, all induced by free radicals generated from a radical initiator. Similar antioxidant activities of estrogens were observed, irrespective of the oxidation system. 2‐Hydroxyestradiol was found to be a potent radical scavenger and its antioxidant activity was close to that of α‐tocopherol, the strongest natural antioxidant. Other phenolic estrogens such as estrone, estriol and β‐estradiol showed a modest antioxidant activity and testosterone did not act as an antioxidant.
AM1 calculations on the tautomerism of free and hydrated hydroxypyridines: Barriers to proton transfer in 2‐hydroxypyridine–pyrid‐2(1H)‐one and effect of solvation and self‐association
Authors: Fabian Walter M. F.
Published: January 2005

AM1 calculations for the tautomerism of the three isomeric hydroxypyridines are reported and compared with recent ab initio results. Intrinsic stabilities of the various tautomers are predicted by AM1 with an accuracy comparable to or even better than the best available ab initio calculations. Solvation is accounted for by the supermolecule approach and also a continuum model for solvent effects. Except for 3‐hydroxypyridine, AM1 correctly accounts for the observed shift of the tautomeric equilibria due to hydration. Energetics of hydrogen bonding are reasonably described by this semiempirical method. With respect to structures, bifurcated hydrogen bonds are preferred by AM1. In addition, for 2‐hydroxypyridine and its lactam tautomer self‐association in addition to barriers to proton transfer are considered. With respect to tautomerization transition states, AM1 shows serious shortcomings. Compared with both experimental and ab initio results, barriers to proton transfer are considerably overestimated by AM1, especially in the hydrated and associated species. For the dimers AM1 predicts an unsymmetrical transition state which, however, is only slightly lower in energy than the symmetrical structure with two negative eigenvalues of the force constant matrix. Despite several attempts, the transition state for proton transfer in the doubly hydrated species could not be located.
Gas‐phase acidities of acrolein and methyl acrylate
Authors: Bernasconi Claude F. , Stronach Michael W. , Depuy Charles H. , Gronert Scott
Published: January 2005

Gas‐phase acidities of acrolein and methyl acrylate were measured by bracketing in a flowing afterglow/SIFT apparatus. For acrolein ΔG°acid = 369 ± 4 kcal mol−1 and for methyl acrylate ΔG°acid = 373 ± 4 kcal mol−1. These acidities are substantially lower than those of the saturated analogs propionaldehyde and methyl acetate, respectively, even though hydrogens attached to sp2‐hybridized carbon are intrinsically more acidic than those attached to sp3‐hybridized carbon. Resonance stabilization of the neutral form by conjugation of the CC double bond with the carbonyl group and allenic destabilization of the anionic form of the two acids can account for the relatively low acidities.
AM1 studies on the acid hydrolysis of acetamide
Authors: Lee Ikchoon , Kim Chang Kon , Lee Bon‐Su
Published: January 2005

The A2 hydrolysis of acetamide was investigated using the AM1 method and the results were compared with those of MNDO. The two methods agree in the general mechanism but differ in detail; MNDO assigns an unduly high energy for a complex with long bonds in contrast to AM1 and ab initio (at the 3–21G and 6–31G levels) methods, which predict a stable structure for such a complex. Inclusion of solvate water molecules has the effect of lowering activation barriers in general. An increase in the number of solving molecules up to four results in narrowing of the activation energy gap between the two rate determining steps of the A2 hydrolysis of the N‐ and O‐protonated acetamide, but no reversal of the relative order of the barrier occurs; the A2 hydrolysis proceeding through the less stable N‐protonated tautometer is therefore likely to remain as a major reaction path in solution‐phase reactions, in agreement with experimental results.
Synthesis and spectroscopic properties of stereoisomeric 5,7‐oxido‐6‐hydroxyiminocholestane derivatives
Authors: Duddeck Helmut , Frelek Jadwiga , Horvatić Davor , Kojić‐Prodić Biserka , Snatzke Günther , Szczepek Wojciech J. , Wagner Petra
Published: January 2005

The four possible diastereomeric 5,7‐oxido‐6‐hydroxyiminocholestanes and their O‐acetates have been synthesized. The stereochemistry of one of the latter has been proved unequivocally by x‐ray diffraction. Their 1H and 13C NMR and also their circular dichroism spectra are discussed. It is now also possible to determine unequivocally the stereochemistry of the ring‐junction geometry and the oxime E or Z stereochemistry.
Correlation of molecular mechanics and NMR data: Conformation and configuration of the Meerwein lactones
Authors: Stanley Jean , Matallana Astrid , Kingsbury Charles A.
Published: January 2005

A method for relating averaged NMR parameters with point‐by‐point geometries for MM2 calculations is given for mobile molecules, e.g. δ‐lactones. This method permits the identification of cis/trans geometry in cases where a complete set of isomers is not available. 2‐Oxa‐3r,4c,5t‐triphenylcyclohexanone is prepared by condensation of 1,2‐diphenylethanone with 3‐phenylpropenal. The initially formed formed biscarbonyl adduct undergoes an internal Cannizzaro reaction forming a hydroxy ester, which lactonizes during work‐up. The stereochemistry of the hydride transfer in the Cannizzaro reaction is consistent with the Felkin rule for asymmetric induction. The configurations of this and related lactones were studied through observation of NMR coupling constants, reductions to hemiacetals and molecular mechanics calculations. The most stable conformations are found to be a boat form for the parent lactone and a half‐chair form for two isomers. The half‐chair and boat forms lie near in energy for all isomers. The calculation techniques outlined illustrate that it is incorrect to consider only chair and boat forms; many intermediate conformations exist. Difficulties with previous IR methods of analysis are discussed.
23Na nuclear magnetic resonance study of the complexation of sodium tetraphenylborate by [5,5]‐dibenzo‐30‐crown‐10 in nitromethane, acetonitrile, acetone and pyridine
Authors: Brière Kathleen M. , Graves Helen P. , Rana Tariq S. , Maurice Luc J. , Detellier Christian
Published: January 2005

The nature of the (Na–DB30C10)+ complex has been studied by 23Na NMR in four non‐aqueous solvents: nitromethane (NM), acetonitrile (AN), acetone (AC) and pyridine (PY). The equilibrium constant of formation (Kf) of (Na‐DB30C10)+ in AC, AN and PY was determined at 300 K: log Kf = 3·9 ± 0·3, 3·4 ± 0·5 and 3·0 ± 0·1, respectively. In cases such as in pyridine, where exact values of the rate constants for the chemical exchange could not be calculated, limiting values were determined giving k−1 (the rate constant for complex dissociation) > 6·6 × 10−4 s−1 and kc (the rate constant for the formation of the complex) > 5 × 107 l mol−1 s−1. Evidence was found for the formation of a 2 : 1 (Na2–DB30C10)+2 complex in nitromethane. It has been previously shown that for (Na–DB24C8)+ the first coordination sphere of Na+ is exclusively filled by the oxygens of the crown ether, whereas for (Na–DB18C6)+ there is participation of at least one solvent molecule. The first coordination sphere of Na+ in (Na–DB30C10)+ consists of one or several molecules of solvent in addition to a number of oxygens from the crown ether. The soft wrapping of the Na+ by DB30C10 is in constrast with the hard wrapping of Na+ by DB24C8 and of K+ by DB30C10.
Substituent effects in the solvolysis of benzyl tosylates
Authors: Fujio Mizue , Goto Mutsuo , Susuki Toshihiro , Mishima Masaaki , Tsuno Yuho
Published: January 2005

The substituent effect on the rates of solvolysis of substituted benzyl tosylates in acetic acid was analysed based on the Yukawa–Tsuno LArSR equation. Neither the LArSR nor simple σ+treatment was capable of providing any linear correlation plot for the full range of substituents. The σ+plot was not simply bilinear but widely scattered, giving a split pattern of parallel curves with significant gaps. Since any mechanistic transition with substituents should bring about a single continuous curve when plotted against an appropriate substituent constant scale, the split σ+plot is not in line with an interpretation in terms of a mechanistic transition. On the other hand, the LArSR plots withr= 1·3 coalesced into a single smooth curye including themetacorrelation curve. A different resonance demand as high asr= 1·3 is required in order to give a smooth single‐curve correlation for the entire substituent range without splitting. For the reactive substituents down top‐halogens, a sufficiently linear plot can be obtained against a set of substituent constants withr= 1·3 which can be referred to the substituent effect correlation for thekcmechanism of this system. An identicalrvalue was likewise assigned for thekcmechanism of the hydrolysis for a more severely restricted range of activating substituents down to the 4‐MeS‐3‐CN group.
Origin of exo selectivity in norbornene. An ab initio MO study
Authors: Koga Nobuaki , Ozawa Tadahiro , Morokuma Keiji
Published: January 2005

The geometries of the reactants, the reactant π‐complex, the transition state and the product for exo and endo addition of BH3 to norbornene were optimized at the HF/3–21G level, and their energies were calculated at the MP2/6–31G level. The analysis of activation energy and model calculations singled out the endo deformability, a major portion of the norbornene deformation energy, as the origin of exo selectivity; norbornene, with the olefinic hydrogens already bent toward endo in the equilibrium geometry, requires less deformation to reach the exo than the endo transition state. The reason why olefinic hydrogens are bent in norbornene has been elucidated. An analysis indicates that the difference between norbornene exo side and bicyclo [2.2.1] hex‐2‐ene is also dictated by the endo deformability.
Activation barriers in photosensitized pyrimidine dimer splitting
Authors: Kim Sang‐Tae , Rose Seth D.
Published: January 2005

Pyrimidine dimers, which form by a symmetry allowed (π + π) photocycloaddition reaction, are subject to photosensitized cycloreversion by electron donors, such as indoles. In a linked dimer–indole system, photoinitiated electron transfer occurs intramolecularly from indole to dimer to produce a charge‐separated species (dimer−.–indole+.). This species undergoes cycloreversion in competition with back electron transfer. Studies of the temperature dependence and solvent dependence of this competition have allowed the relative values of the activation parameters for the competing processes to be determined. In water (5–65°C) the free energy of activation of splitting minus that of back electron transfer (ΔΔG≠ = ΔG ‐ ΔG) was found to be 1·3 kcal mol−1. The enthalpy of activation difference (ΔΔH≠) was found to be 1·1 kcal mol−1 and the entropy of activation difference (ΔΔS≠) was found to be −0·51 cal mol−1 K−1. In EPA (diethyl ether–isopentane–ethanol, 5:5:2; −85 to 25°C) the value of ΔΔG≠ remained the same, but the entropy and enthalpy contributions were different (ΔΔH≠ = 0·72 kcal mol−1; ΔΔS≠ = −1·8 cal mol−1 K−1). The results have been interpreted in terms of the effect of the polarity of the solvent on the transition states for the two competing processes. Enthalpy effects retard splitting more in water than in EPA, whereas entropy effects favor back electron transfer more in EPA than in water. Potential implications of these results for the mechanism of enzymatic photocycloreversion of pyrimidine dimers in DNA are considered.
Theoretical study of the gas‐phase thermal isomerization of isoxazoles. Part I. Isoxazole
Authors: Davico Gustavo E. , Perez Jorge D.
Published: January 2005

An MNDO study was made of the gas‐phase thermal isomerization of isoxazole to explore theoretically the proposed reaction mechanism. The results showed that isoxazole isomerizes through an azirine, as an intermediate, to oxazole via a nitrile ylide or to ketenimine, with similar activation energies, and that the first step is rate limiting, in agreement with the experimental results. These results also show that isomerization to a nitrile is possible, but in this case the energy barrier is grater than the corresponding isomerization to oxazole. The MNDO study also supports a concerted process for the rate‐limiting step, as reported earlier on the basis of experimental studies.
Stereochemistry and kinetics of addition of amines to acetylenic ketones
Authors: Hamed Ezzat A. , Sharaf Saber M. , Abdel‐Baky Samy A. , Ibrahim Mahmoud F. , Youssef Abdel‐Hamid A.
Published: January 2005

The reactions of a series of 3‐(p‐substituted phenyl)‐1‐phenylprop‐2‐yn‐1‐ones with piperidine and morpholine in methanol were studied and their rates measured. The products were 3‐piperidino‐ and 3‐morpholino‐3‐(p‐substituted phenyl)‐1‐phenylprop‐2‐en‐1‐ones. 1H NMR spectra were used to determine the configurations of the obtained products. A good Hammett correlation was obtained with ρ values of 1·15–1·10 and 1·15–0·53 for piperidine and morpholine, respectively, which suggest a carbanionic character of the transition state. A two‐step mechanism is postulated for these nucleophilic additions.
Effects of CF2 group pyramidalization in the 1,1,3,3‐tetrafluoropropenyl anion
Authors: Hammons James H. , Hrovat David A. , Borden Weston Thatcher
Published: January 2005

Ab initio calculations with the 6–31G* basis set were carried out on planar and pyramidalized geometries of the 1,1,3,3‐tetrafluoropropenyl anion and on two geometries with one CF2 group rotated out of conjugation. Structures with at least one pyramidalized CF2 group are substantially lower in energy than the corresponding unpyramidalized structures for both rotated and unrotated geometries. At the MP2/6–31G* level of theory, three structures of approximately equal energy all lie 17–18 kcal mol−1 below the planar C2v structure. These include a Cs geometry with one CF2 group rotated out of conjugation, an unrotated C1 structure with unequal CC bond lengths and an unrotated, pyramidalized C2 structure with equal CC bond lengths.
Effects of the heteroaromatic moiety on spectroscopic properties, pKa and reactivity of azoles: A chemometric study
Authors: Ebert Cynthia , Elguero José , Musumarra Giuseppe
Published: January 2005

The spectroscopic and physicochemical properties of parent azoles substituted on the nitrogen by a proton, a methyl group, a p‐aminophenyl group and an acetyl group have been gathered. This information on azol‐1‐yl substituents has been treated by principal component analysis and partial least‐squares analysis. The first result of these analyses is that it has been possible to complete some missing properties of azol‐1‐yl substituents, such as pKa for proton loss of some azoles and hydrolysis rates of certain azolides. Another significant result of the chemometric approach is the assignment of the 1‐acetyltetrazole IR and NMR data to one of the two possible isomers.
Formation constants in CH hydrogen bonding. 2. Alkynes and polyhalobenzenes with hexamethylphosphoric triamide in cyclohexane solution
Authors: Lorand John P. , Nelson Jonathan P. , Gilman R. Dean , Staley Kathy L. , Chambers Jeffery R. , Kirk Harold D. , Moeggenborg Kevin J. , Farlow David L.
Published: January 2005

Formation constants, K, for hydrogen bonding of weak CH donors with hexamethylphosphoric triamide were measured in cyclohexane at 35°C by PMR spectroscopy; data were analyzed by the Higuchi method. Donors include terminal alkynes and series of polyfluoro‐, polychloro‐ and polybromobenzenes. The K values for many of these are too small to measure in CCl4. For RCCH, K decreases in the order R = BrCH2 > C6H5 > C2H5O > tert‐butyl. For polyhalobenzenes, K is generally larger for F than Cl or Br, and meta halogens facilitate hydrogen bonding most effectively. Among tetrahalobenzenes, 1,2,3,4‐isomers show the least evidence of 2:1 complex formation. Pentachlorobenzene has a smaller K than its 1,2,3,4‐tetrachlor analog even after statistical correction; for C6HX5, K decreases in the order F > Cl > Br; the K values for 1,3,5‐trihalobenzenes are closely similar; and 1,3,5‐tribromo‐2,4‐dimethylbenzene shows no evidence of hydrogen bonding by PMR. The last three observations suggest that buttressing is important in the C6HX5 structure, at least with Cl and Br. A value of K for benzene was measured, suggesting for the first time that the H atoms of benzene engage in hydrogen bonding. The result compares well with values extrapolated from data for polyfluoro‐ but not polychlorobenzenes, again probably because of buttressing in the latter.
Ionization constants of 5‐pyridylmethylenehydantoins in 80% (w/w) dimethyl sulphoxide–water at 25 °C
Authors: Tan Sau‐Fun , Ang Kok‐Peng , How Gee‐Fung , Yeo Yam‐Khoon
Published: January 2005

The ionization constants of nine amphoteric 5‐pyridylmethylenehydantoins were measured in 80% (w/w) dimethyl sulphoxide–water at 25 °C. The effects of structure and Z/E configuration on both pK and pK values are discussed. The especially low basicity and acidity of the Z‐isomers of the N‐unsubstituted and 3‐methyl‐substituted 5‐(2‐pyridylmethylene)hydantoin are attributed to the formation of intramolecular N(1)H…︁N hydrogen bond in the s‐cis conformation of these compounds. The existence of N(1)CH3…︁N attraction is also postulated for the Z‐isomer of 1‐methyl‐5‐(2‐pyridylmethylene)hydantoin.
Heats of formation of organic molecules by ab initio calculations. 1. Aliphatic amines
Authors: Allinger Norman L. , Schmitz Lawrence R. , Motoc I. , Bender C. , Labanowski J. K.
Published: January 2005

A bond energy scheme has been developed and two previously existing schemes have been extended for the calculation of heats of formation of aliphatic amines using ab initio 6–31G* energies in place of experimental data. The results are in good agreement with those obtained experimentally and suggest that this method can be used to predict heats of formation of molecules of this class with an accuracy competitive with good quality experiments, and with probable errors of less than 1 kcal mol−1.
MO study of the possibility of a concerted mechanism in the pinacol rearrangement
Authors: Nakamura Kensuke , Osamura Yoshihiro
Published: January 2005

The ab initio SCF‐MO method was employed to examine the possibility of a reaction pathway without a carbocation intermediate in the pinacol rearrangement. The molecular geometries of the transition states were obtained for the 1,2‐hydride shift together with H2O elimination, starting from the various methyl‐substituted protonated 1,2‐diols. It was found that the activation energies depend strongly on the substitutents. A comparison of the relative energies between β‐hydroxycarbonium ions and the transition states of the concerted mechanism suggests that the stepwise mechanism is less favourable than the concerted path in each case.
Cross interaction constants as a measure of the transition state structure. 13. Steric effects of the N,N‐dimethyl group on the transition state structure in aminolysis of alkyl benzenesulphonates
Authors: Lee Ikchoon , Rhyu Keun Woo , Lee Hai Whang , Shim Chang Sub , Oh Hyuck Keun
Published: January 2005

Kinetic studies on the reactions of methyl (MBS) and ethyl benzenesulphonates (EBS) with N,N‐dimethylanilines (DMA) in methanol and acetonitrile are reported. The cross interaction constants ρXZ and βXZ, between the substituents in the nucleophile (X) and the leaving group (Z) indicated that the transition states (TS) are looser than those for the reactions with anilines, but the relative tightness between the two substrates was the same; the TS was tighter for EBS despite the increase in steric effect leading to looser TSs for MBS and EBS alike. The TS variation between two different reaction series expected from the simple Hammett and Brønsted coefficients, ρX, ρZ, βX and βZ, was incompatible with that predicted by the cross interaction constants, demonstrating again the unreliability of the simple parameters.
Microenvironmental effects in solid‐state reactions. Dispersive kinetics of conformation‐dependent charge delocalization in aliphatic diamine radical cations
Authors: Marcinek Andrzej , Gȩbicki Jerzy , Płonka Andrzej
Published: January 2005

The kinetics of conformation‐dependent charge delocalization in radical cations generated from a series of α,ω‐diaminoalkanes, (CH3)2N(CH2)nN(CH3)2, (n = 1–3) in a 3‐methylpentane glassy solution at 90 K are discussed in terms of dispersive kinetics with the use of the time‐dependent rate constant in the form k(t) = Btα − 1. It was found that the activation energy distribution function became broader as the number of methylene groups separating the two nitrogen atoms increased, and that this process was accompanied by a substantial increase in the mean value of the activation energy.
Mechanisms of acylation of 2,3‐dihydrobenzoxazol‐2‐one and 2H‐1,4‐benzoxazin‐3(4H)‐one
Authors: Cotelle Nicole , Cotelle Philippe , Capelle Sylvie , Lesieur Daniel
Published: January 2005

The kinetics of the acylation of 2,3‐dihydrobenzoxazol‐2‐one with dodecanoic acid in polyphosphoric acid were investigated. Two competitive reactions occurred at the 3‐ and 6‐positions. The acylation reactions in polyphosphoric acid of 2,3‐dihydrobenzoxazol‐2‐one and 2H‐1,4‐benzoxazin‐3(4H)‐one give 6‐ and 7‐acyl derivatives. 4‐Acetyl‐2H‐1,4‐benzoxazin‐3(4H)‐one rearranged in polyphosphoric acid exclusively to the 7‐acetyl derivative. Two mechanisms are proposed to explain the different reaction sites for the acylation of 2H‐1,4‐benzoxazin‐3(4H)‐one.
Reactivity and reaction patterns of alkyl alkoxybenzene radical cations. Mechanistic pathways of the reactions between 2,5‐DI‐tert‐Butyl‐1, 4‐Dimethoxybenzene and perfluorodiacyl peroxides
Authors: Jiang Xi‐Kui , Zhao Cheng‐Xue , Gong Yue‐Fa
Published: January 2005

In the reactions of 2,5‐di‐tert‐butyl‐1,4‐dimethoxybezene (1) with different oxidants, the radical cation 1+. is always detectable by EPR. However, the observed reactivity of 1+. depends greatly on the oxidation systems employed. In S2OCu2+HOAc and Ce4+HOAc systems (HOAc = acetic acid), 1+. appears to have long lifetimes and does not undergo fragmentation spontaneously. In contrast, in (RFCO2)2CF2CICFCI2 (F113) systems, the readily formed 1+. is short‐lived, and large amounts of de‐tert‐butylation products have been isolated. Experimental results imply that the CC bond cleavage involved in de‐tert‐butylation could be a consequence of an attack by perfluoroacyloxy radical on 1+. in their original solvent cage. The fact that addition of methanol to the reaction mixture leads to the formation of a large amount of tBuOCH3 (46%) and other evidence suggest that the tert‐butyl group leaves as a carbocation. On the basis of these results, we conclude that the reactions of 1 with (RFCO2)2 are initiated by electron transfer and followed by a fast coupling of various radical species, namely, 1+. with RFCO2 or with RF in the solvent cage, to form σ‐complexes which collapse or react with nucleophiles to yield the final products.
1,5‐Sigmatropic shifts of bromine over a cyclopentadiene ring
Authors: Minkin Vladimir I. , Mikhailov Igor E. , Dushenko Galina A. , Yudilevich Joseph A. , Minyaev Ruslan M. , Zschunke Adolf , Mögge Klemens
Published: January 2005

The intrinsic mechanism of circumambulatory rearrangements of 5‐bromo‐5‐methyl‐1,2,3,4‐tetramethoxycarbonylcyclopentadiene, 5‐bromo‐1,2,3,4 5‐pentamethoxycarbonylcyclopentadien and 5‐bromo‐ 1,2,3,4,5‐pentaphenylcyclopentadiene due to sigmatropic shifts of bromine over the cyclopentadiene ring was proved, using the dynamic 13C and 1H NMR technique, to be governed by successive intramolecular 1,5‐sigmatropic shifts. Semi‐empirical AM1 and MINDO/3 calculations of reactions paths performed for fluoro‐, chloro‐ and bromocyclopentadienes are in accord with the conclusion of a preference for a 1,5‐ over a 1,3‐shift reaction path of halogen migration over a cyclopentadiene system. Intramolecular 1,5‐sigmatropic shifts of chlorine in 5‐chloro‐5‐methyl‐1,2,3,4‐tetramethoxycarbonylcyclopentadiene with the free energy barrier of ΔG = 26.1 kcal mol−1 were studied using 1H NMR spectroscopy.
Specific micellar rate effects on unimolecular decarboxylation and cyclization
Authors: Cerichelli Giorgio , Mancini Giovanna , Luchetti Luciana , Savelli Gianfranco , Bunton Clifford A.
Published: January 2005

Decarboxylation of 6‐nitrobenzisoxazole‐3‐carboxylate ion and cyclization of o‐3‐halopropyloxyloxyphenoxide ion (PhY7) (Y = Br, I) are accelerated by aqueous cationic and zwitterionic micelles. For cationic micelles the rate enhancements (kM′kW′) increase with increasing bulk of the surfactant head groups but are largest for decarboxylation. There are good linear free energy relationships between micellar effects on these reactions and the plots of log kM′ for cyclization against log kM′ for decarboxylation have slopes of 0.46 and 0.64 for Y = Br and I, respectively. Surfactants that have twin tail or tri‐N‐alkylbenzyloxy head groups do not fit the relationship. Despite mechanistic differences between decarboxylation and cyclization (an intramolecular SN2 reaction), similar factors control micellar effects on these reactions. In the absence of surfactant there is only a qualitative relationship between free energies of activation of cyclization and decarboxylation in different solvents.
Evaluation of solvent effects on the dissociation of aliphatic carboxylic acids in aqueous N,N‐dimethylformamide mixtures according to the scaled particle theory
Authors: Gonzalez A. Gustavo , Rosales D. , Gomez Ariza J. L. , Fernandez Sanz J.
Published: January 2005

The standard Gibbs energies of transfer, ΔGt° from water to water–N,N‐dimethylformamide mixtures for the dissociation process of several organic acids (formic, acetic, propionic, n‐butyric, isobutyric, n‐valeric, isovaleric, 2‐methylbutyric and trimethylacetic acids) obtained from the pKa* values are discussed in terms of the extended Scaled Particle Theory from quantum mechanical calculations in order to estimate the cavity and electrostatic contributions. The residual energy term was correlated with the hydrogen bond acceptor density and the Kamlet–Taft β parameters. The main contribution to ΔGt° was due to dipolar and specific interactions.
Internal rotation of CH3 groups and entropy difference between diastereoisomers: A 13C NMR study via relaxation time measurements
Authors: Brunel Y. , Coulombeau C. , Lavaitte S. , Reisse J.
Published: January 2005

Overall and internal rotation can be studied by means of relaxation time measurements on carbon‐13. Diffusion constants associated with overall rotation and internal rotation can be used to quantify the intermolecular and intramolecular motional constraints. During the course of the study, a very careful examination of monomethyl and dimethylnorbornanes was performed and the results were compared with data obtained by neutron inelastic scattering. This study led to a clear conclusion concerning the real usefulness of relaxation time measurements as a very good alternative method for the determination of entropy contributions, which are usually calculated on the basis of vibrational studies.
Reactivity in radical abstraction reactions: Application of the curve crossing model
Authors: Pross Addy , Yamataka Hiroshi , Nagase Shigeru
Published: January 2005

The curve crossing model was applied to a series of hydrogen abstraction reactions from a family of alkanes, RH (R = methyl, ethyl, isopropyl, tert‐butyl) by alkyl, hydrogen and chlorine radicals. The analysis was based on quantitative data obtained from an ab initio MO study. Schematic reaction profiles for the reaction of RH with alkyl and hydrogen radicals are built up from just two configurations: reactant, DA, and product D3* A. For the Cl atom reaction, however, a significant contribution of D+ A−, a charge‐transfer configuration, is also shown to be present. A simple explanation for differences in the intrinsic barrier for the identity radical abstraction reaction based on the initial gap size between DA and D3* A configurations is provided. The influence of the D+ A− configuration on the nature of the transition state of the Cl atom reaction and its intrinsic barrier is described. It is the D+ A− configuration that is responsible for the polar character often observed in radical abstraction and addition reactions.
AB initio MO study of the halogen cation basicities of some organic bases
Authors: Alcamí M. , Mó O. , Yáñtez M. , Abboud José‐Luis M.
Published: January 2005

Hartree—Fock calculations were performed to investigate the structure and relative stabilities of complexes between halogen cations and first‐ and second‐row bases. It is shown by means of both a qualitative perturbation molecular orbital treatment and a topological analysis of the electronic charge density that second‐row bases present enhanced halogen cation basicities compared with first‐row bases. In this respect the results predict that although the fluorine cation basicity of water is smaller than its proton affinity, the fluorine cation basicities of SH2, thioether, phosphine and trimethylphosphine are considerably higher than their proton basicities. Similarly, phosphine and trimethylphosphine should have chlorine cation basicities greater than their proton affinities, in contrast with ammonia and trimethylamine. The results also show that fluorine and chlorine cation basicities are more sensitive to methyl substitution than proton affinities.
Kinetics and mechanism of the alcoholyses of ethyl fluorosulphate
Authors: Cafferata Lázaro F. R. , Svartman Esther L.
Published: January 2005

The alcoholyses of ethyl fluorosulphate in methyl, ethyl, n‐propyl, isopropyl, n‐butyl, isobutyl, tert‐butyl and n‐pentyl alcohols in the initial concentrations and temperature ranges of (1–10) 10−3 M and − 15.00 to 22.00 °C, respectively, were studied. The reactions follow pseudo‐first‐order kinetic laws up to at least ca 75 per cent ester conversions. At 10°C the experimental rate constant for ethanolysis is similar to that of ethyl triflate but much smaller than for its hydrolysis. The reactivity of the ester is ca 104–105 times higher than that of alkyl halides and benzenesulphonates. In alcohol–benzene and alcohol–acetone mixtures the ΔH≠ and ΔS≠ activation parameters are only slightly affected by the media composition. The effect of the ‘lyate ion’ on the kinetics of these reactions were investigated and the effects of changes in the dielectric constant of the media were evaluated. It is concluded that the alcoholyses of the ester fulfil the requirements for predominant SN2‐type mechanism, where the reaction ground states are more polar than the corresponding transition states.
Gas‐phase acidity of some α‐keto aldoximes: Experiment and theory
Authors: Bouchoux G. , Jaudon P. , Decouzon M. , Gal J.‐F. , Maria P.‐C.
Published: January 2005

Experimental gas‐phase acidities of 2‐oxopropanaloximes, XCH2COCHNOH (X  H, CH3S, CH3SO, CH3SO2; compounds 1–4), were determined by Fourier transform ion cyclotron resonance (FT‐ICR) spectrometry. The values are δ Gacid° = 1401, 1381, 1360 and 1351 kJ mol−1 for 1, 2, 3, and 4, respectively. Molecular orbital calculations using the semi‐empirical AM1 method provided information on the geometry and relative energy of neutrals species 1–4 and their conjugate bases, together with charge distributions and entropies of deprotonation. It is demonstrated that the proton abstraction occurs preferentially at the oxime function; the formation of an enolate as a conjugate base is unfavourable by 70–140 kJ mol−1. The large variation of the gas‐phase acidities for 1–4 is explained in terms of the field/inductive empirical substituent constant σF. The variation of solution acidities appears to be comparatively strongly attenuated. This attenuation is attributed mainly to charge delocalization in the anion, which was confirmed by charge density calculations.
Kinetic modelling of the low‐temperature photo‐oxidation of hexafluoropropene
Authors: Faucitano A. , Buttafava A. , Comincioli V. , Marchionni G. , Pasquale R. J. De
Published: January 2005

By kinetic simulation, the significant features of the mechanism of the low‐temperature photo‐oxidation of hexafluoropropene were elucidated and the rate constants for the major elementary reactions of the intermediate fluorinated peroxy and alkoxy radicals were determined through a best‐fit procedure. Comparison with analogue reactions in non‐fluorinated systems showed a significant increase in reactivity for the self‐reaction and double bond addition by peroxy radicals and β‐scission by alkoxy radicals, which are discussed in terms of fluorine substituent effects.
Two reintroduced solvatochromic indicators for hydrogen bond donation and acceptance
Authors: Migron Y. , Marcus Y.
Published: January 2005

The behaviour of cis‐dicyanodi‐1‐10‐phenanthrolinoiron(II) and acetylacetonato‐N, N,N′,N′‐tetramethylethylene‐diaminocopper(II) perchlorate as solvatochromic indicators for the ability of solvents to donate a hydrogen atom or a pair of electrons, respectively, towards the formation of a hydrogen bond is examined by multivariable linear regressions and principal component analysis.
Crystallographic studies of intra‐ and inter‐ molecular interactions. Part VI. Crystal and molecular structure of N,N′‐dimethyl‐N′‐phenylsulphonylformamidine. Equalization of CN bond lengths in the amidine fragment as a result of substituent effects due to push–pull
Authors: Anulewicz Romana , Krygowski Tadeusz Marek , Raczyńska Ewa D. , Laurence Christian
Published: January 2005

The crystal and molecular structure of N,N‐dimethyl‐N′‐phenylsulphonylformamidine is solved by direct methods and refined to R = 0.041 for 935 reflections, Equalization of CN bond lengths in the amidine fragment is discussed in terms of a push—pull effect operating between the N,N‐dimethyl and SO2Ph groups. A significant role of the substituent at the functional carbon on the degree of π‐electron delocalization on the NCN fragment is interpreted by use of the HOSE model.
The schiff base between pyridoxal‐5′‐phosphate (PLP) and hexylamine. Formation of the unprotonated form of the imine by reaction of the unprotonated PLP and free amine
Authors: Blázquez Manuel , Pineda Teresa , Sevilla Jose M. , Domínguez Manuel , García‐Blanco Francisco
Published: January 2005

Electrochemical and spectrophotometric studies of the pyridoxal‐5′ ‐phosphate hexylamine Schiff base (PHSB) in strongly basic media were carried out. The equilibrium constant and rate constants of the formation and hydrolysis of the unprotonated imine from the unprotonated pyridoxal‐5′ ‐phosphate and free amine were determined by linearscan cyclic voltammetry. The acid‐base dissociation constant of the imine group was obtained by spectrophotometric measurements. Absorption bands for the free imine and the conjugate acid are characterized by a log‐normal distribution. These studies are for general application to the quantitative characterization of non‐enzymatic model compounds.
Short communication. Solvolysis of some benzylic and non‐benzylic tertiary p‐nitrobenzoates. A new YBnOPNB scale
Authors: Liu Kwang‐Ting , Chen Hung‐I , Chin Chien‐Pu
Published: January 2005

A new Y scale for the correlation of solvolytic reactivities for benzylic ρ‐nitrobenzoates was established based on rate data for 2‐phenyl‐2‐adamantyl ρ‐nitrobenzoate, and from which the solvent assistance in the solvolysis of tert‐cumyl ρ‐nitrobenzoate is suggested.
Methyl substituent effects on the dissociation energies of SiX bonds
Authors: Luo Yu‐Ran , Pacey Philip D.
Published: January 2005

Empirical expressions are obtained for bond dissociation energies (BDEs) of alkylsilanes and their derivatives. It is pointed out that the sensitivity of the BDEs of SiX bonds to methyl substitution depends on the electronegativity of the substituent X. When X is an atom or group with low or moderate electronegativity, the SiX BDE is insensitive to methyl substitution, but when X is a halogen atom or OH, the SiX BDEs increase with increasing methyl substitution.