bluuebunny/crossref_metadata_2025 huggingface dataset:
Reactions of the super-electrophile, 2-(2′,4′-dinitrophenyl)-4,6-dinitrobenzotriazole 1-oxide, with methoxide and tert-butoxide: basicity and steric hindrance as factors in σ-complex formation versus nucleophilic displacement
Authors: Dust Julian M. , Buncel Erwin
Published: May 2006

The course of the reactions of methoxide and tert-butoxide with 2-(2′,4′-dinitrophenyl)-4,6-dinitrobenzotriazole 1-oxide (4) clearly shows that the C-7 electrophilic site is significantly more reactive than the C-1′ site of the substrate. The reaction pathways of these alkoxides, which differ in basicity (as a measure of nucleophilicity) and steric bulk, were followed by 400 MHz 1H nuclear magnetic resonance spectroscopy. While both alkoxides lead to immediate formation of the respective C-7 anionic σ-adducts, a greater percentage of C-7 adduct formation occurs with methoxide as attacking nucleophile. Reaction with excess alkoxide results in attack at C-1′ being observed, as well. This leads to formation of metastable C-1′ σ-adducts, whose rapid decomposition results in formation of 2,4-dinitrophenyl ethers and the dinitrobenzotriazole 1-oxyanion in an overall nucleophilic displacement reaction. Under these excess conditions, methoxide also causes a faster rate of displacement than does tert-butoxide as nucleophile. These results are discussed on the basis of the basicity of the nucleophiles, the relative electrophilicity of the positions in the substrate (C-7 versus C-1′), the steric hindrance involved in attack and in the resultant C-7 and C-1′ complexes, and in terms of an activation energy/reaction coordinate profile comparing the pathways for attack at the two electrophilic sites. Key words: anionic σ-complexes, super-electrophiles, aromatic nucleophilic substitution (SN Ar).
Complexes of the crown thioether ligand 2,5,8-trithia[9]-o-benzenophane, (TT[9]OB). Synthesis and molecular structure of [Cu(NCS)(TT[9]OB)]
Authors: Loeb Stephen J. , Shimizu George K. H.
Published: May 2006

The macrocyclic thioether ligand 2,5,8-trithia[9]-o-benzenophane (TT[9]OB) reacts with [Cu(CH3CN)4][ClO4] to yield the Cu(I) complex [Cu(CH3CN)(TT[9]OB)][ClO4]. The coordinated acetonitrile molecule is displaced by the ambidentate, thiocyanate (NCS)− ion to form the neutral complex [Cu(NCS)(TT[9]OB)] which was characterized by infrared spectroscopy, 1H NMR spectroscopy, and X-ray crystallography. The title complex crystallized in the orthorhombic space group Pbca with a = 14.447(3) Å, b = 15.034(4) Å, c = 14.851(4) Å, V = 3232.2(14) Å3, and Z = 8. The structure refined to R = 5.15% and Rw = 5.92% for 1149 reflections with Fo2 > 3σ(Fo2). The Cu(I) ion has a distorted tetrahedral coordination sphere with TT[9]OB occupying three sites while the fourth site is occupied by the thiocyanate ligand which is N-bound to the metal centre. A comparison is made of the molecular conformations of free TT[9]OB and that found for complexed TT[9]OB in [Cu(NCS)(TT[9]OB)] and [Cu(Ph2PMe)(TT[9]OB)][ClO4]. Key words: thioether, macrocyclic metal complex.
Stereochemical requirements for the formation of vanadate complexes with peptides
Authors: Jaswal JASWINDER S. , Tracey Alan S.
Published: May 2006

The condensation reactions occurring between vanadate and a number of amino acids and simple peptides have been studied by 51V nuclear magnetic resonance. Vanadate and ligand stoichiometry has been established for the complexes formed and their formation constants determined. The amino acids were all found to undergo rather weak interactions with vanadate and in general provided two products with 51V chemical shifts near −545 and −555 ppm. The peptides also gave minor products with NMR signals occurring with that of vanadate itself. However, in this case, the major products occur at approximately −510 ppm. These latter complexes are monovanadate, monoligand complexes which require the terminal amino, the carboxylate groups and an unsubstituted nitrogen in the peptide linkage in order for product formation to occur. Sidechains promote product formation, apparently by favouring a more readily complexed peptide conformation. Formation constants vary from about 20 to 50 M−1 depending on the sidechain. The carboxylate group can be replaced by a hydroxymethyl. This, however, leads to a significant decrease in product formation at pH 7. Hydrogen ion concentration studies showed that the complexes, when considered to be formed from VO4H2− and neutral peptide, did not require or give off protons. However, when the carboxylate was replaced by the alcohol functionality a proton was released. On the basis of this study the coordination of the vanadate/peptide complexes has been assigned. Key words: peptide, vanadate, complexes, vanadium magnetic resonance.
Thermal lensing and photoacoustic spectra of gaseous acetylene by pulsed tunable infrared laser
Authors: Takahashi Makoto , Okuzawa Yasuo , Fujii Masaaki , Ito Mitsuo
Published: May 2006

Thermal lensing and photoacoustic spectra of ν3, ν1 + 3, and ν1 + ν2 + ν3 vibrational transitions of gaseous acetylene were observed by a tunable pulsed IR laser converted from a pulsed dye laser. Application of a pulsed IR laser to photothermal spectroscopy is demonstrated. Key words: thermal lensing spectra, photoacoustic spectra, pulsed tunable infrared laser.
Solubility of domoic acid in water and in non-aqueous solvents
Authors: Falk Michael , Seto Ping F. , Walter John A.
Published: May 2006

The solubility of domoic acid (DA) in H2O and D2O, in aqueous NaCl solution and in several non-aqueous solvents was measured by NMR and UV spectroscopies. The solubility in water is comparable with that of aminoacids such as glutamic acid and aspartic acid. It is markedly pH-dependent, passing through a minimum at the isoelectric point, the increase towards both higher and lower pH values indicating that the anionic and cationic forms are more soluble than the neutral form. The effect of NaCl on the solubility of DA in water is negligible. The solubility of DA in alcohols is lower than in water but it is much higher than the solubility of glutamic acid or aspartic acid. The octanol–water partition coefficient for DA at pH 5.32, Kow = 0.0037, was obtained by a direct UV measurement. The low value of Kow indicates that aquatic organisms cannot take up DA directly from the water and bioaccumulation may proceed only through dietary intake. Key words: solubility of domoic acid, NMR of domoic acid, UV spectra of domoic acid, octanol–water partition coefficient of domoic acid.
The vibrational spectrum, barrier to internal rotation, and ab initio calculations of 2-chloropropane
Authors: Sullivan J. F. , Wang Aiying , Cheng Mei-Shiow , Durig J. R.
Published: May 2006

The Raman spectra (3200–50 cm−1) of gaseous, liquid, and solid 2-chloropropane-d3 (isopropyl-d3 chloride), CH3(CD3)CHCl, and the infrared spectra (3200–50 cm−1) of the gas and solid have been recorded. The torsional transitions observed in the far infrared spectrum of the gaseous sample recorded at a resolution of 0.10 cm−1 between 265 and 135 cm−1 were analyzed in terms of the semirigid rotor model. An effective barrier of 1378 ± 4 cm−1 (3.94 ± 0.01 kcal/mol), cosine–cosine coupling term of 166 ± 10 cm−1 (0.47 ± 0.03 kcal/mol), and sine–sine coupling term of −173 ± 1 cm−1 (−0.49 ± 0.01 kcal/mol) were determined by fitting ten observed frequencies arising from the CH3 and CD3 torsions. The assignment of the 27 fundamentals is given and discussed. A complete equilibrium geometry, barrier to internal rotation, and vibrational frequencies have been determined by ab initio Hartree–Fock gradient calculations employing either 3-21G* or 6-31G* basis sets for both the d0 and d3 species. These calculated results are compared to the experimental values as well as to the corresponding quantities for some similar molecules. Key words: 2-chloropropane, vibrational spectrum; ab initio calculations; barrier to internal rotation.
Structure and conformation of 5-methyoxymethyl-2′-deoxycytidine
Authors: Jia Zongchao , Tourigny Guy , Delbaere Louis T. J. , Stuart Allan L. , Gupta Sagar V.
Published: May 2006

not available
Molecular dynamics simulation of interhalogen compounds. 1. Liquid chlorine trifluoride (ClF3): structure and thermodynamics
Authors: Wadi Ramesh K. , Saxena Vivek
Published: May 2006

The results of a molecular dynamics (MD) simulation study of liquid chlorine trifluoride (ClF3) at 217, 260, and 287 K are reported. The cubic simulation cell consists of 108 ClF3 molecules assumed to be interacting via site–site Lennard–Jones 12–6 pair potential. The parameters for F–F and Cl–Cl interaction are the same as used for the simulation of F2, and Cl2, respectively, and those for the Cl–F cross interaction are calculated using Lorentz–Berthelot rules. These results are then used to calculate various radial distribution functions characteristic of the liquid structure. Thermodynamic properties, namely, configurational energy, constant volume specific heat, and internal pressure are also reported. The time-dependent properties, mean square force and torque, self diffusion coefficient, and the quantum corrections to the free energy, were also obtained. The dimer configuration drawn based on the observed contact distances was found to be in good agreement with the results of matrix isolation infrared and laser Raman spectroscopic studies. Keywords: MD simulation, interhalogens, liquid structure, thermodynamic properties.
Intramolecular nucleophilic participation by the thiol group during amide hydrolysis. Part 2. The imidazole catalysis dilemma
Authors: McDonald Robert S. , Patterson Patricia , Rodwell June , Whalley Ann
Published: May 2006

Catalysis by imidazole and N-methylimidazole buffers of the intramolecular thiolysis of N-n-propyl 2-mercaptomethylbenzamide (forming 2-thiophthalide) has been studied in aqueous solution at 40.0 °C, μ = 1.0. Unlike other buffers previously studied, imidazole and N-methylimidazole are able to catalyze, by a general base route, formation of neutral tetrahedral intermediate; this pathway is rate limiting at pH ≤ 7.5. At higher pH, the previously reported general acid-catalyzed breakdown of this intermediate is rate limiting. The relevance of these observations to the currently accepted pathway for the acylation of papain by amide substrates is discussed. Keywords: amide hydrolysis, intramolecular, thiol participation, imidazole, papain model.
Molecular integrals over Laguerre Gaussian-type functions of real spherical harmonics
Authors: Matsuoka Osamu
Published: May 2006

Molecular integrals are formulated over the Laguerre Gaussian-type functions (LGTF) of real spherical harmonics. They include the overlap integrals and the energy integrals of kinetic, nuclear attraction, and electron repulsion. For the nuclear-attraction integrals the formulations based on the point as well as the Gaussian nuclear charge distribution models are presented. Integral formulas over the LGTFs of real spherical harmonics are found a little more complicated than those of the LGTFs of complex spherical harmonics due to the summations over magnetic quantum numbers. Keywords: molecular integral, Gaussian-type function, spherical harmonic, solid harmonic, Sonine polynomial.
Stationary phase Monte Carlo methods: interference effects in quantum Monte Carlo dynamics
Authors: Doll J. D. , Rick Steven W. , Freeman David L.
Published: May 2006

As summarized by Hamming's motto, "the purpose of computing is insight, not numbers." In the spirit of this dictum, we describe here recent algorithmic developments in the theory of quantum dynamics. Through the use of the somewhat unlikely combination of modern numerical simulations and a visualization device borrowed from 19th century optics, the present efforts suggest the existence of an important, underlying structure in the general problem. This structure, verified for a relatively simple class of model problems, provides broad guidelines for the Keywords: Monte Carlo, stationary phase, quantum dynamics.
An investigation of atomic structure in position and momentum space by means of "ideal shells"
Authors: Schmider Hartmut , Sagar Robin P. , Smith Jr. Vedene H.
Published: May 2006

For atomic systems, the boundaries in position and momentum space that enclose the exact electron number given by the aufbau principle have been numerically calculated. Their relatively irregular behavior as a function of the atomic number Z is demonstrated and explained in terms of corresponding irregularities of the aufbau principle. Direct comparisons of these radii with those derived from density functionals that are known to exhibit atomic shell structure are shown to be useful in judging the ability of the latter to define "physical" shells compatible with the aufbau principle. Keywords: atomic radii, shell structure, aufbau principle, charge density, momentum density.
Higher order exponential split operator method for solving time-dependent Schrödinger equations
Authors: Bandrauk André D. , Shen Hai
Published: May 2006

A new method of splitting exponential operators is proposed for the exponential form of the operator solution to the time-dependent Schrödinger equation. The method is shown to hold for any desired accuracy in the time increment. A comparison of different algorithms is made as a function of accuracy and computation time. Keywords: splitting operator, Fast Fourier Transform (FFT), Schrödinger equations.
A microcolumn method for hexazinone and metabolite residues in soil and vegetation
Authors: Feng Joseph C.
Published: May 2006

A soil cleanup method using a disposable microcolumn and a modified capillary gas chromatographic method is described for the analysis of hexazinone (3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione) and two primary metabolites, 3-(4-hydroxycyclohexyl)-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione (metabolite A) and 3-cyclohexyl-6-(methylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione (metabolite B). The limits of detection (LOD) were 7, 18, and 36 ng g−1 for hexazinone, metabolite B, and metabolite A, respectively; limits of quantitation (LOQ) were 22, 52, and 99 ng g−1, respectively. The mean recoveries were 94 ± 3% and 94 ± 5% for hexazinone and the phytotoxic metabolite B fortified above LOQ and ranging from 0.03 to 6.25 and 0.06 to 12.5 μg g−1, respectively. The recoveries from the non-phytotoxic metabolite A above LOQ increased with concentrations from 0.125 to 25 μg g−1, but were consistent at each level. The final product of cleanup was stable for at least 6 months, and no derivatization was needed before gas chromatography. The use of toluene as the final solvent and the modification of the injection system of the gas chromatograph improved the stability of peak responses. The method is three times more efficient than conventional methods. The cleanup method for soils was modified for the analysis of vegetation samples. Efficient removal of chlorophyll enabled quantitation of hexazinone by gas chromatography. Recoveries of hexazinone ranged from 73 to 96% for forest litter and four different plant species.
Annulations leading to diene systems. Total syntheses of the dolastane-type diterpenoids (±)-(5S,12R,14S)-dolasta-1(15),7,9-trien-14-ol and (±)-amijitrienol
Authors: Piers Edward , Friesen Richard W.
Published: May 2006

Alkylation of the substituted cycloalkanones 14a–d and 30 with (Z)-1-bromo-4-methyl-3-trimethylstannyl-2-pentene (13) produced compounds 15a–d and 33, which were readily converted into the corresponding enol trifluoromethane-sulfonates (triflates) 16a–d and 34. Intramolecular Pd(O)-catalyzed coupling of the vinylstannane and enol triflate functions in 16a–d and 34 provided the dienes 17a–d and 35. The annulation product 35 served as a suitable starting material for the total syntheses of the dolastane diterpenoids (±)-(5S,12R,14S)-dolasta-1(15),7,9-trien-14-ol (2) and (±)-amijitrienol (3). The key steps of the synthesis of (±)-2 involved the stereoselective methylation of the ketone 44 (readily derived from 35) to provide 46 and the Barbier type ring closure of 47 to provide the target compound. For the synthesis of (±)-3, the notable conversions included the reductive transformation of the diene 35 into the alkene 53, the aldol condensation of the ketone 54 with 4-trimethylstannyl-4-pentenal (55), the chemo- and stereoselective reduction of the dione 58, and the one-pot conversion of the keto vinylstannane 63 into the triene 65, via the intermediate 64.
Ring expansion by tandem double Tebbe–Claisen technology
Authors: Paquette Leo A. , Philippo Christophe M. G. , Vo Nha Huu
Published: May 2006

2-Cyclohexenone rings that form part of a larger structural assembly are amenable to peracid oxidation with formation of an epoxy lactone. These intermediates are readily transformed under acidic, basic, or neutral conditions to ring-contracted aldehydo lactones, which are then subjected to condensation with a slight excess of Tebbe reagent. These conditions result in methylenation of carbonyl groups and set the stage for operation of a Claisen rearrangement. When the latter is catalyzed by Tribal, the sigmatropic process occurs at room temperature. With systems typified by 17 and 24, the isomerization is complete within 15 min. The presence of a proximate angular methyl group as in 9b, 29, and 37 exerts a retarding kinetic effect. In such examples, a period of 6 h is required to achieve completion. Swern oxidation completes the conversion to the 4-cyclooctenones, where the two carbons stemming from the Tebbe reagent are inserted between the original carbonyl and α-olefinic carbons. The overall process is tolerant of ether functionality and additional sites of unsaturation.
Enthalpies of solution of naphthalene, N,N-dimethyl-1-naphthylamine, and 1,8-bis(dimethylamino)naphthalene in 16 organic solvents
Authors: Figeys Daniel , Koschmidder Maegorzata , Benoit Robert L.
Published: May 2006

The enthalpies of solution of naphthalene, N,N-dimethyl-1-naphthylamine, and 1,8-bis(dimethylamino)naphthalene (proton sponge) were determined at 298.15 K in 16 organic solvents (n-hexane, cyclohexane, carbon tetrachloride, chloroform, 1,2-dichloroethane, benzene, chlorobenzene, dimethyl sulfoxide, N,N-dimethylformamide, ethyl acetate, 1,4-dioxane, anisole, nitrobenzene, benzonitrile, methanol, ethanol). Additional determinations were made with benzene. Useful linear relationships are observed between the molar enthalpies of solution of the four compounds in the solvents. The molar enthalpies of solution were correlated with the solvatochromic parameter of the solvents. The presence of N(CH3)2 groups on naphthalene does not significantly contribute to the solute–solvent interactions.
A pulse radiolysis study of the reaction of OH with I2 and the decay of I2−
Authors: Elliot A. John
Published: May 2006

The rate constant for the reaction of the hydroxyl radical with iodine was estimated to be (9.5 ± 1.0) × 109 dm3 mol−1 s−1 at 20 °C from fitting the time dependence of the decrease in the absorption of iodine at 460 nm. A number of rate constants associated with the decay of I2− that were required for this fitting process were also determined. They were the reaction of I2− with I2− (2k = (4.6 ± 1.6) × 109 dm3 mol−1 s−1) at an ionic strength of 1.2 × 10−2 mol dm−3; I2− with I (k = (4.6 ± 1.6) × 109 dm3 mol−1 s−1); and I with I (2k = (3.0 ± 1.0) × 1010 dm3 mol−1 s−1).
Structure effects in copper coordination compounds of ligands with thioether, aromatic nitrogen, and ether donors
Authors: Liu Shuang , Lucas C. Robert , Hynes Rosemary C. , Charland Jean-Pierre
Published: May 2006

Preparations of the ligands 1,9-bis(2-pyridyl)-2,5,8-trithianonane (L1) and 1,9-bis(2-pyridyl)-5-oxa-2,8-dithianonane (L2) and their copper(II) complex cations [Cu(L1)]2+ and [Cu(L2)]2+ as ClO4−, CF3SO3−, or BF4− salts are described. X-ray structures of both complex cations as perchlorates are reported. For [Cu(L1)](ClO4)2: space group Pbca, a = 10.4896(5), b = 12.9376(6), c = 33.3943(13) Å, Z = 8, Rf = 0.067, Rw = 0.074 for 1769 reflections. For [Cu(L2)](ClO4)2: space group P21/c, a = 14.388(2), b = 9.990(5), c = 16.076(3) Å, β = 107.919(12)°, Z = 4, Rf = 0.069, Rw = 0.089 for 2813 reflections. Both are slightly distorted square pyramidal species having interactions in the solids between their ClO4− anions and one of the pyridine rings in the L1 complex and the copper atom in the L2 complex. The complexes have been examined by electron spin resonance, electronic spectroscopy, and cyclic voltammetry. The effects of anion coordination, solvation, and removal or alteration of the apical donor atom are assessed by comparisons involving the new complexes and published data on related substances.
Laser flash photolysis studies of the formation and reactivities of phenyl(naphthyl)methyl carbocations generated from phosphonium salt precursors
Authors: Alonso E. O. , Johnston L. J. , Scaiano J. C. , Toscano V. G.
Published: May 2006

The photolysis of several substituted phenyl(naphthyl)methyl triphenylphosphonium chlorides has been examined using a combination of laser flash photolysis experiments and product studies. Both carbocation and radical intermediates have been characterized in the transient experiments, with the relative yields depending strongly on the solvent. For example, in alcohols, acetonitrile, or aqueous solvents cation formation predominates while acetonitrile/dioxane mixtures (5–10%) are required for the observation of radicals. Quantum yields for cation formation vary from 0.79 in methanol to 0.093 in 1:4 acetonitrile/dioxane, as measured by product studies and transient experiments, respectively. The addition of perchlorate salts leads to dramatic enhancements in the cation lifetimes; the effects are particularly pronounced for acetonitrile/dioxane mixtures where the cation yields also increase by factors of 3–4. In this case the effects are attributed primarily to replacement of chloride by perchlorate in the initial ion pairs. The combined data from both solvent and perchlorate salt effects on the cation lifetimes and yields suggest that the excited state of the phosphonium salt cleaves homolytically, followed by electron transfer within the initial radical/triphenylphosphine radical cation pair to generate carbocation, as opposed to direct heterolytic cleavage. The cation yields also indicate that back reaction to regenerate starting material, as well as product formation within the initial geminate cage, occur in some solvents. The effects of solvent and added perchlorate salts on the rate constants for reaction with nucleophiles have been examined. For example, rate constants that vary by an order of magnitude have been measured for quenching by azide ion in various aqueous acetonitrile and trifluoroethanol mixtures.
Crystal structures of (n-Pr4N)2SnCl6, (n-Pr4N)[TeCl4(OH)], (n-Pr4N)2[Te2Cl10] (nominal), and (n-Pr4N)2[Se2O2Cl6], with observations on Z2L102n− and Z2L82− dimers in general
Authors: James Margaret Ann , Knop Osvald , Cameron T. Stanley
Published: May 2006

The crystal structures of the following four compounds have been determined: (n-Pr4N)2SnCl6 (A; Pbca, a = 29.073 Å, b = 15.825 Å, c = 14.851 Å, Z = 8), (n-Pr4N)[TeCl4(OH)] (B; C2/c or Cc, a = 10.800 Å, b = 17.429 Å, c = 10.604 Å, β = 93.81°, Z = 4), (n-Pr4N)2[Te2Cl10] (C, nominal composition; [Formula: see text], a = 9.635 Å, b = 10.900 Å, c = 11.547 Å, α = 73.34°, β = 70.44°, γ = 68.15°, Z = 1), and (n-Pr4N)2[Se2O2Cl6 (D; [Formula: see text], a = 9.252 Å, b = 11.691 Å, c = 18.304 Å, α = 110.18°, β = 92.57°, γ = 97.72°, Z = 2). B contains a square-pyramidal TeCl4OH)− anion. The actual composition of the centrosymmetric μ2-dimer in C is Te2Cl8(Cl, OH)22−, the replacement of Cl by OH being at the axial ligand positions of the TeCl5− halves of the dimer. In the centrosymmetric μ2-Se2O2Cl62− dimer in D the Se—O bonds are perpendicular to the Se(Cl)2Se bridge plane and trans to the presumed lone-electron pairs on the Se atoms. It is shown that the Z—Hal bonds in the known centrosymmetric homoligand μ2-Z2Hal102n− (Z = Se, Te) and μ2-Z2Hal82− (Z = As, Sb, Bi) as well as in the μ2-Se2O2Hal62− dimers exhibit a uniform stereochemistry, in which the d(Z—Hal) bond lengths in pairs of opposing bonds trans to each other are consistently related by the trans effect (TE). Empirical quantification of Te for the Se and Te dimers is exploited to authenticate the composition of the dimer in situations where the nature of the ligand is in doubt (in the present case on partial hydrolysis). The asymmetry of the Z(Hal)2Z double bridge is examined in relation to the overall stereochemistry of the dimer. Also examined are circumstances that may lead to the occurrence, in a crystal, of a Z2Hal102n− dimer on the one hand and of a discrete ZHal5n− monomer on the other, in situations which otherwise are chemically closely similar. TLS analysis of the thermal motion is used to judge the strength of association of the two monomer halves in a μ2-dimer, with results that are important when correcting bond lengths for the effects of thermal motion. A variety of general observations on μ2-dimers in crystals is proffered for good measure.
Electron transport in clay-modified electrodes: study of electron transfer between electrochemically oxidized tris (2,2′-bipyridyl) iron cations and clay structural iron(II) sites
Authors: Xiang Yan , Villemure Gilles
Published: May 2006

The cyclic voltammograms of tris (2,2′-bipyridyl) iron(II) ([Fe(bpy)3]2+) adsorbed in clay-modified electrodes made from a range of different smectites were recorded. In all cases, at low scan speed (1 mV/s), the initial anodic peak current was much larger than the initial cathodic peak current. Partial reduction of the clay structural iron further increased the initial anodic to cathodic current ratio, suggesting that the discrepancy between the charge transferred in the anodic and cathodic scans was due to a slow electron transfer between the clay structural Fe(II) and the oxidized bipyridyl cations. However, no clear quantitative relation was found between the measured FeO contents of the different clays and the observed excess anodic currents. In fact, of all the clays tested only one, montmorillonite SWy-1, contained enough Fe(II) for it to account for all of the excess anodic charge transferred in the initial scan.
L'ion positif géminé intervient-il sur le destin de l'électron piégé incomplètement relaxé dans les liquides polaires irradiés?
Authors: Jay-Gerin J.-P. , Ferradini C.
Published: May 2006

A model is proposed concerning the influence of the parent positive ion on the fate of the incompletely relaxed trapped electron (eir−) in irradiated polar liquids. This model is based on the release, by a tunneling and (or) a trap-hopping mechanism in the Coulomb field of the cation, of the electrons captured in preexisting shallow localized states below the bottom of the conduction band of the solvent. The released electrons would either recombine with the parent positive ion or get retrapped. The net effect would be an accumulation of electrons in deeper traps. The removal of weakly trapped electrons would contribute to the decrease of the infrared part of the optical absorption spectrum during the very early time dynamics of electron solvation. Such a process would imply, as a consequence, the existence of a maximum of the eir− absorption spectrum.
Kinetics and mechanism of aminolysis of (Z)-4-arylidene-2-phenyl-5(4H)oxazolones
Authors: Alkaabi Sharifa S. , Shawali Ahmad S.
Published: May 2006

The kinetics of the reactions of a series of (Z)-4-arylidene-2-phenyl-5(4H)oxazolones 1 with n-butylamine and piperidine were studied spectrophotometrically in dioxane, ethanol, and cyclohexane under pseudo-first-order conditions and at different temperatures. The relation k1(obs) = k2[amine] + k3[amine]2 was found applicable for all reactions studied in either dioxane or ethanol. However, in cyclohexane the n-butylaminolysis of 1 followed only third-order kinetics k1(obs) = k3[n-BuNH2]2. The kinetics of the reaction of 1 with n-butylamine in the presence of catalytic amounts of triethylamine in dioxane followed the equation: k1(obs)k2 = [n-BuNH2] + k3[n-BuNH2]2[Formula: see text] [Et3N]. The rate constants k2 and k3 correlated well with the Hammett equation and the corresponding activation parameters were determined. The results were interpreted in terms of a mechanism involving solvent- and amine-catalyzed processes.
Second nearest-neighbor interactions in ternary regular solutions
Authors: Nagamori Meguru
Published: May 2006

The concept of classical regular solutions has been expanded by considering both first and second nearest-neighbor interactions between randomly distributed molecules. While the present model requires an ideal entropy of mixing, as does the classic regularity model, its heat of mixing is expressed by a more flexible equation which attributes the second-order terms of the Margules formalism to first nearest-neighbor interactions, and the third-order terms to second nearest-neighbor interactions. The activity–composition relations have been expressed by a single equation of the grand partition function, which converges to that of the classical regularity with decreasing contributions from second nearest-neighbor molecules.
The trans effect: methodological musings
Authors: Knop Osvald , Choi S. C. , Hamilton David C.
Published: May 2006

The trans effect (TE) in the present context refers to the electronic compensation which in collinear homoligand L—Z—L* trans bond pairs lengthens the Z—L* bond when the Z—L bond is shortened. The existence of a functional relation d* = f(d) between the conjugated bond lengths d(Z—L) and d*(Z—L*) (d and d* not equivalent by symmetry; population A) has been demonstrated for a variety of Z-L combinations, with Z mostly from Groups VI and V and L mostly a halogen. The two model functions investigated in detail are the empirical DPF (difference power fit), d* – d0 = K(d − d0)−c, and the semiempirical CSBO (constant sum of bond orders) based on a modified 3-centre 4-electron bond concept, d* − d0 = −B ln {1 − expt[−(d − d0)/B]}, where B = b0 + b1(d − d0). Fitting DPF and CSBO to experimental d,d* data sets involves 3-parameter nonlinear optimization; in this CSBO differs from the 2-parameter treatment of Sheldrick etal., in which the limiting bond length d0 was supplied externally. Modified versions of DPF and CSBO have been devised to accommodate, along with A, d,d* pairs in which d = d* by symmetry (population S).The relative merits of DPF and CSBO and the various aspects of TE quantification are discussed at length, among these the effect of the oxidation state of Z and of the presence of heteroligands on Z. The meaning of the parameters of optimization and the existence of "chemical" trends between them are examined as well as the importance of the symmetrically balanced bond length de = d = d* and of the total d range Δ = de−d0 resulting from the d,d* regressions. Attempts to extend TE quantification to collinear heteroligand L1—Z—L2trans bond pairs have provided insight into the nature of the bond-length variation in such systems. The very good DPF and CSBO fits to d,d* sets obtained from 6-31G* optimizations of the equilibrium geometries of the OBOX, XOCN, and OCNY (X, Y = H, F, Cl, Li, Na, or no ligand) molecules and ions support the validity of the modified 3c4e model in accounting for the TE bond-length relationships.
The HLM method: a simple way to get the solid–liquid phase diagrams and enthalpies of transition of pure components and mixtures
Authors: Quirion François , Lambert Daniel , Perron Gérald
Published: May 2006

A simple method of thermal analysis is described which gives the same information as differential scanning calorimetry. The method is based on the Heat-Leak-Modulus, HLM, of a sample cell placed in a constant temperature reservoir. In the present study, the HLM method is used for the investigation of pure components and mixtures from −190 to 50 °C. The method allows the determination of glass-transition, crystallizations, solid–solid transition, eutectic, and melting temperatures with a reproducibility better than ±0.1 °C. The enthalpy of a transition can be determined with a reproducibility of ±5%. The simplicity, the low cost, and the precision of the HLM method fills the gap between standard cooling curves and sophisticated differential scanning calorimetry experiments. The HLM method has numerous applications in physical chemistry, polymer science, metallurgy, and chemical engineering.
Organometallic intermediates in the group 4 metallocene-catalyzed dehydrocoupling of organosilanes. 2. Variations in the intermediates of dimethyltitanocene reactions with several silanes
Authors: Harrod J. F. , Mu Ying , Samuel E.
Published: May 2006

As previously described, organosilanes react with dimethyltitanocene (DMT) to give three reduced titanium intermediates, Cp2Ti(μ-SiRR′H)(μ-H)TiCp2, 1, [Cp2Ti(μ-SiRR′H)]2, 2, and [Cp2Ti(H)]2(μ-H), 3. When R = Me and R′ = n-Bu, the DMT is quantitatively converted to 1, which is persistent at room temperature. When R′ = H, DMT is initially converted quantitatively to 1, which then rapidly declines to give an equilibrium mixture of 1, 2, and 3, with 2 as the dominant species. When R = Ph and R′ = Ph or Me, 1 is again produced quantitatively, but it rapidly declines to produce an unidentified paramagnetic product, rather than 2 or 3. It is believed that 2 is unstable with respect to dissociation in these cases due to the steric bulk of the substituents.
The synthesis and X-ray structures of the geometric isomers of 1-amino-1,2-cyclopentanedicarboxylic acid
Authors: Curry Kenneth , McLennan Hugh , Rettig Steven J. , Trotter James
Published: May 2006

The geometric isomers of 1-amino-1,2-cyclopentanedicarboxylic acid were isolated as the hydrolysis products of 2-cyclopentanecarboxylic acid-5,5′-hydantoin. The hydantoin ester was formed from ethyl -2-oxocyclopentanecarboxylate via a Bucherer–Bergs reaction. Hydrolysis of the hydantoin at elevated temperatures gave a mixture of trans- and cis-1 -amino-1,2-cyclopentanedicarboxylic acids indicating epimerization during hydrolysis. The trans product has not been described previously and the cis not extensively characterized. X-ray crystallography reveals that both isomers are zwitterionic in the solid state and the cis isomer molecules are linked by unusually strong hydrogen bonding. Structures were confirmed by 13C NMR, X-ray crystallography, and elemental analysis. Physical data are also presented.
A model calculation on structures and excitation energies of the hydrated electron
Authors: Tachikawa Hiroto , Lund Anders , Ogasawara Masaaki
Published: May 2006

Model calculations were made on the hydrated electron by using the ab initio MO method combined with the MR-SD-Cl method and the coupled cluster theory. The models used in the calculations were water clusters denoted by [e−(H2O)n(H2O)m], where n = 2,3,4, and 6 for the first solvation shell and m = 0–28 for the second and third solvation shells. In these model calculations, the interactions between the excess electron and the water molecules in the first solvation shell are explicitly calculated by ab initio MO methods and the water molecules in the second and third solvation shells were represented by the fractional charges obtained at the MP2/D95V** level. The stabilization energies and the solvation radius r(e−–O), in terms of the distance between the center of the cavity and an oxygen atom of the surrounding water molecules, increased monotonically with the number of water molecules in the first solvation shell. On the other hand, the first excitation energy was not dependent on the number of water molecules in solvation shells, but constant, with the value of ca. 2.0 eV. On the basis of the present calculations, we suggest that (1) the energetic stability of excess electrons depends on both short-range interaction and long-range interaction, (2) the first excitation energy is critically affected by only the short-range interactions, and the excitation is theoretically attributed to the1s→2p transition of the excess electron.
Solvation and metal ion effects on structure and reactivity of phosphoryl compounds. Part 2. Alkali metal ion catalysis in the demethylation of methyl di(3-nitrophenyl)phosphinate by iodide ion
Authors: Modro Agnes M. , Modro Tomasz A.
Published: May 2006

Second-order rate constants for the demethylation of methyl di(3-nitrophenyl)phosphinate by iodide in acetone-d6 at 25 °C have been measured. For a series of iodide salts the rate increased in the order: NBu4I < KI ≤ NaI < LiI. The addition of crown ethers or D2O has a retarding effect on the reaction rate. The results are discussed in terms of the increase of the nucleofugality of the phosphinate group via the complexation of metal ions with the phosphoryl function.
Squaraine chemistry. A new approach to symmetrical and unsymmetrical photoconductive squaraines. Characterization and solid state properties of these materials
Authors: Law Kock-Yee , Bailey F. Court
Published: May 2006

A new approach to 1-(p-dimethylaminophenyl)-2-hydroxycyclobutene-3,4-dione (4), a precursor for the synthesis of symmetrical and unsymmetrical photoconductive squaraines, is described. In situ generation of p-nitrophenyl ketene, by the reaction of p-nitrophenylacetyl chloride and triethylamine in the presence of tetraethoxyethylene in anhydrous ether, results in a (2 + 2) cycloaddition between the ketene and the electron-rich olefin. The cycloadduct generated, 6, was hydrolyzed to give 1-(p-nitrophenyl)-2-hydroxycyclobutene-3,4-dione (5) in 51% yield. Reductive alkylation of 5 by hydrogen in the presence of formaldehyde in DMF at 50–60 °C affords 4 in 81% yield. The overall yield of 4 is 41% from tetraethoxyethylene and is significantly better than other preparative procedures of 4 based on a partial Friedel–Crafts reaction or an arylation reaction of a squaric acid derivative with an aniline, where yields ranging from 8.5% to 24% were reported. Condensations of 4 with 2 equivalents of N,N-dimethylaniline and its derivatives in refluxing 2-propanol in the presence of 6 equivalents of tributyl orthoformate yield symmetrical squaraine 1a and unsymmetrical squaraines 1b–1e. The synthesis is accomplished by a (2 + 2) cycloaddition – reductive alkylation – condensation reaction sequence and represents a new, cost-effective synthesis for photoconductive squaraines where the expensive squaric acid reactant is bypassed. The spectroscopic and the solid state properties of 1a–1e have been studied. Results show that the properties exhibited by 1a–1e are essentially identical to those exhibited by other photoconductive, symmetrical squaraines that are synthesized by other known processes. From the resemblance in solid state properties, 1a–1e are expected to be photoconductive. This has indeed been observed in preliminary experiments.
1H nuclear magnetic resonance and molecular orbital studies of the internal rotational potential and electron delocalization in triphenylphosphine
Authors: Schaefer Ted , Sebastian Rudy , Hruska Frank E.
Published: May 2006

The 1H nuclear magnetic resonance spectral parameters are reported for triphenylphosphine as solutions in CS2/C6D12 and acetone-d6 at 300 K. The internal rotational potential opposing torsion about the P—C bond is computed by AMI and STO-3G MO methods. The computed potentials are used to calculate the average shielding of the para protons caused by the diamagnetic anisotropies of the neighbouring phenyl groups. The results are used to estimate the degree of electron delocalization from the lone pair on phosphorus. The extent of delocalization depends on the internal motions and comparisons are made with phenylphosphine. The maximum possible screening of the para protons in phenylphosphine is calculated as 0.19 ppm for a conformation in which the lone pair on phosphorus is oriented perpendicular to the aromatic plane. The intramolecular rotational potentials then yield 0.029 ppm as the shielding of the para protons in triphenylphosphine due to electron delocalization, just as found for the CS2/C6D12 solution after correction for diamagnetic anisotropy fields.
A kinetic study on the reactivity of chromium(IV)
Authors: Perez-Benito Joaquin F. , Arias Conchita
Published: May 2006

The inhibition effect of Mn2+ on the rates of the Cr(VI) oxidations of 13 different organic reductants (RH) in aqueous perchloric acid has been studied. The reductants were formaldehyde, formic acid, maleic acid, and 10 alcohols (8 primary and 2 secondary). The second-order rate constants decrease with increasing [Mn2+] according to the law[Formula: see text]Parameter b′ is interpreted as the ratio between the rate constants for the Cr(IV)–Mn2+ and Cr(IV)–RH reactions. Cr(IV) is reduced by Mn2+ (and also by Ce3+) much faster than by organic reductants RH. The Cr(IV) oxidations of primary alcohols follow Taft's equation with a negative reaction constant (ρ* = −0.80 ± 0.06). The Cr(IV) present in aqueous perchloric acid is postulated as an oxo species (CrO2+) capable of reacting by one-electron transfer with both Mn2+ and Ce3+, and by hydride-ion transfer with organic reductants. Relevance of these results to the general mechanism for the Cr(VI) oxidations of organic substrates is discussed.
Large perturbations of long-range nJ(1H,1H) and nJ(1H,19F) by the intramolecular hydrogen bonds in 2-mercaptobenzaldehyde, salicylaldehyde, and some derivatives. Reference structures for intramolecular hydrogen bonds
Authors: Schaefer Ted , Sebastian Rudy , McKinnon David M. , Spevack Perry W. , Cox Kerry J. , Takeuchi Craig S.
Published: May 2006

Precise 1H nuclear magnetic resonance spectral parameters are reported for salicyladehyde and its 3-fluoro and 5-fluoro derivatives in nonpolar solutions. Such data are also given for the 2-mercapto, 2-methylthio, and 2-methoxy derivatives of benzaldehyde. Comparison of the long-range coupling constants in the various compounds and their conformers shows a large perturbation of their magnitudes by hydrogen bond formation. For the salicylaldehyde system, the perturbation is particularly large for couplings involving the aldehyde proton and protons or fluorine nuclei placed ortho to the hydroxyl group. For example, 5Jt (F, CHO) is reduced by about 50%. The perturbation, as expected, is much smaller for coupling constants of nuclei remote from the site of the hydrogen bond. In 2-mercaptobenzaldehyde the long-range coupling constants are also sensitive to hydrogen bond formation, those involving the sulfhydryl proton markedly so compared to the hydroxyl proton in salicylaldehyde. The strength of the [Formula: see text] bond is discussed. It is argued that the reference conformer for the mercapto compound in such a discussion is less easily defined than for salicylaldehyde because [Formula: see text] are similar to [Formula: see text] energies. The experimental data for the CCl4 solutions imply a free energy of formation of the [Formula: see text] bond of 4.8(5) kJ/mol at 300 K. Molecular orbital computations on the four planar conformers of each salicylaldehyde and 2-mercaptobenzaldehyde with the 6-31 G**(5D) basis are reported. For salicylaldehyde, the [Formula: see text] arrangement is taken as the reference conformer, with a computed energy of 25.7 kJ/mol relative to the hydrogen-bonded structure. For 2-mercaptobenzaldehyde, the [Formula: see text] and [Formula: see text] conformers are calculated to be isoenergetic, at 5.1 kJ/mol relative to the hydrogen-bonded conformer. Hence either arrangement serves as a reference structure in computations of the strength of the hydrogen bond. The computations are consistent with the experimental results for solutions of the molecules under discussion. An appendix gives the computed geometries of the eight planar conformers, as well as some atomic charges, allowing a rationalization of the relative energies of the conformers.
Proton nuclear magnetic resonance studies of nucleosides: hindered internal rotation in N-dimethylaminomethylene derivatives of adenosine, guanosine, and cytidine
Authors: Osei-Twum Emmanuel Yaw
Published: May 2006

Studies of the internal rotation about the C—N bond of the dimethylaminomethylene (DMAM) group of DMAM-adenosine, 1, DMAM-guanosine, 2, and DMAM-cytidine, 3, was determined by high-resolution 1H NMR spectroscopy. Arrhenius plots (ln k = ln A − Ea/RT) were used to determine the barriers to internal rotation, Ea, of the compounds. They were found to be 58.0 ± 2.4 kJ mol−1, 71.2 ± 1.4 kJ mol−1, and 62.8 ± 2.4 kJ mol−1 for 1, 2, and 3, respectively. At the same time the coalescence temperatures, tc, for the compounds were determined and found to be 93.5 °C for 1, 124.7 °C for 2, and 119.6 °C for 3 in DMSO. In D2O, compound 1 was found to have a tc of 104.5 °C. The compound decomposed so rapidly at this temperature that the barrier to internal rotation could not be determined. The results indicate that the oxo function in the nucleobase has a considerable effect on the barrier to internal rotation and coalescence temperature.
Spruce budworm (Choristoneura fumiferana) antifeedants 3. Structure–activity relationship among some angularly functionalized decalin compounds
Authors: Schwerdtfeger A.E. , Chan T.H. , Thomas A.W. , Strunz G.M. , Salonius A. , Chiasson M.
Published: May 2006

Several angularly functionalized decalins, synthethized during the preparation of three keto diacetate decalins, models for clerodane diterpenoids, were tested for antifeedant activity against the spruce budworm, Choristoneura fumiferana. Definite biological activity was observed for some of these compounds. A structure–activity relationship was discussed on the basis of these results.
Synthesis and fluorescence studies of oligoethylene glycols labeled with naphthalene by a phosphate triester linkage
Authors: Lee Sam , Winnik Mitchell A.
Published: May 2006

A series of oligoethylene glycols, incorporating two pendant naphthyl groups along the interior of the chain using a phosphate triester linkage, have been prepared. The synthetic methodology has been discussed in the context of eventually providing a route to polymers labeled in a similar fashion. The steady state fluorescence behavior of these compounds is reported briefly. Relatively little excimer emission is observed. The synthetic intermediates to the phosphates, the 2-(1-naphthyl)ethyl N,N-diisopropylphosphoramidites of triethylene glycol and tetraethylene glycol, show exciplex fluorescence. The fluorescence from these latter compounds exhibits some sensitivity to solvent quality.
Crystalline complexes of 18-crown-6 with methyl sulfonates
Authors: Chênevert Robert , Gagnon René , Chamberland Daniel , Simard Michel
Published: May 2006

The macrocyclic polyether 18-crown-6 forms crystalline complexes with methyl sulfonates. Most complexes have a 2:1 (sulfonate:crown) stoichiometry whereas small aliphatic sulfonates give a 1:1 ratio. In the 2:1 ratio complexes, the guest molecules are coordinated above and below the crown in such a way that the dipoles are compensated. In the 1:1 ratio complexes, there is a polymeric type association with the two ends of the sulfonate interacting with different crown molecules. A crystal structure is reported for each type of complex. The complex 18-crown-6•CH3SO3CH3 (7) crystallizes in the monoclinic system, space group P21/c with Z = 4. The unit cell dimensions are as follows: a = 8.525(4), b = 16.401(7), c = 15.071(6) Å, β = 113.92(3)°. Final Rw = 0.064 for 3650 reflections. The complex 18-crown 6•(C6H5SO3Me)2 (1) crystallizes in the monoclinic system, space group P21/c with Z = 2. The unit cell dimensions are as follows: a = 8.101(3), b = 17.136(5), c = 12.990(5) Å, β = 121.30(3)°. Final Rw = 0.048 for 2913 reflections.
The infrared and Raman spectra of methoxycarbonyl and thiomethoxycarbonyl isocyanates
Authors: Balfour Walter J. , Fougere Scott G. , Klapstein Dieter , Nau Werner M.
Published: May 2006

The gas phase infrared and liquid phase Raman spectra of the title compounds have been recorded and vibrational analyses performed. The spectra show evidence for the presence in both the liquid and vapour phases of two rotational isomers: cisoid-Z and transoid-Z with reference to orientation of isocyanato and methyl groups relative to the carbonyl group.