bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractSubtilisin 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractAM1 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

AbstractGas‐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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractPyrimidine 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

AbstractAn 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

AbstractThe 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

AbstractAb 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

AbstractThe 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

AbstractFormation 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

AbstractThe 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

AbstractA 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

AbstractThe 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.