bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of thermodynamic parameters on higly correlated chemical systems: a spectrophotometric study of the 1:1 and 1:2 equilibria between I2 and R4todit = 4,5,6,7-tetrathiocino[1,2-b:3,4-b′]diimidazolyl-1,3,8,10-tetraalkyl-2,9-dithione; (R = Bu, Me (new data); Et, Ph (reinvestigation)). Crystal and molecular structure of the charge-transfer complex Bu4 todit•2I2
Authors: Bigoli Francesco , Pellinghelli Maria Angela
Published: May 2006

The reactions between Bu4todit and I2 in CHCl3 solution have been investigated by spectrophotometry at different temperatures. Evidence for the stepwise formation of the 1:1 and 1:2 charge-transfer complexes has been obtained by computer analysis of the experimental data. The stability constants of both complexes and the corresponding standard enthalpy and entropy changes have been calculated. The 1:2 complex has been characterized in the solid state by an X-ray structural study: the crystals of Bu4todit 2I2 are monoclinic, space group C2/c, a = 16.352(6), b = 9.376(5), c = 23.470(7) Å, β = 99.78(2)°, Z = 4,R = 0.0579. The molecular structure shows that the two thionic sulfur atoms coordinate two diiodine molecules, the bond distances and angles being similar to those observed previously in Et4todit•2I2. On the contrary, the solution studies disagree with the previous investigations on Et4todit and Ph4todit, where the absorbance data were explained assuming the formation of the 1:1 complex only. A new calculation procedure for the least-squares minimization of spectrophotometric data, based on Powell's direct search method, has been developed. This procedure allows for the refinement of the standard enthalpy and entropy changes from a set of absorbance data at different temperatures. Convergence has been attained for the reaction of diiodine with Bu4todit and Me4todit (new data) and also for Et4todit and Ph4todit (old data). Keywords: equilibrium constants, charge-transfer complexes, Fourier transform – Raman, X-ray, I2-thioamides.
Solvent effects on the reactivity of solvated electrons with ions in tert-butanol/water mixtures
Authors: Zhao Yixing , Freeman Gordon R.
Published: May 2006

Reactions of [Formula: see text] with the ions [Formula: see text] showed different variations of rate with solvent composition in tert-butanol/water mixtures from 0 to 100 mol% water. In pure tert-butanol solvent at 298 K the respective values of k2 (106 m3 mol−1 s−1) are 3.2, 13, and 42. The estimated value of reaction radius Rr depends on the minimum number of solvent molecules needed between [Formula: see text] and the reactant ion to attain the static values of ε of the bulk solvent used in the calculation of the Debye factor f; Rr is assumed to be larger in the alcohol-rich region than in the water-rich region, because the solvent molecules are larger. The Smoluchowski–Debye–Nernst–Einstein model is used to evaluate the effective reaction radius κRr, where κ is the probability of reaction per encounter; κRr decreases from pure tert-butanol to pure water. In the water-rich region the activation energies E2 of the efficient reactions, 11–24 kJ mol−1, are similar to EΛ0 of the reactant electrolyes, 12–23 kJ mol−1. For the inefficient reactant [Formula: see text] E2 = 30 kJ mol−1. The high values of E2 = 43–53 kJ mol−1 in pure tert-butanol solvent are attributed to a high activation energy for diffusion of [Formula: see text] in this solvent. Keywords:tert-butanol/water solvents, solvated electron, ions, reactivity, solvent effects.
Reaction of methylphthalimidoketene with benzylidene- and cinnamylideneanilines: structure and configuration of the products
Authors: Mukerjee Arya K. , Sanjayan Gangadhar J. , Ravikumar Krishnan
Published: May 2006

Methylphthalimidoketene reacts with cinnamylideneanilines to give tetrahydrooxazin-6-ones exclusively, whereas with benzylideneaniline it also forms β-lactam. Configurations are based on X-ray studies. Keywords: β-lactam, tetrahydro-1,3-oxazin-5-ones, ketene–imine reactions, cycloaddition of methylphthalimidoketene, 3-phthaloylamino-3-methylazetidin-2-one.
The reactions of O(3P) with acetonitrile and propionitrile
Authors: Budge Suzanne , Roscoe John M.
Published: May 2006

The reactions of O(3P) with acetonitrile and propionitrile have been studied kinetically in the gas phase as a function of temperature and nitrile concentration. The rate constants obtained experimentally for the consumption of O(3P) in these reactions, in the units L mol−1 s−1, obey the following relations: O + acetonitrile: In k = 20.98 ± 0.23 − 2100 ± 200/T; and O + propionitrile: In k = 21.01 ± 0.19 − 1900 ± 200/T. Mechanisms are discussed in which the reaction is initiated by hydrogen atom abstraction and in which initiation is by addition of O(3P) to the nitrile, followed by decomposition of the addition complex. The kinetic and analytical results are best explained by the mechanism in which the reactions are initiated by RCN + O → R + OCN. In the case of the reaction with acetonitrile, the absolute rate constant is smaller than the experimental value by a factor of 7 ± 20%, independent of temperature, as a result of secondary reactions. Keywords: kinetics, reactions of O(3P) with nitriles.
Solvent effect on the conformational behavior of substituted spiro[4.5]decanes and spiro[5.5]undecanes
Authors: Erdem S. Sağ , Varnali T. , Aviyente V. , Ruiz-Lopez M.F.
Published: May 2006

We studied the relatively complex polar systems 6-substituted-1,4-dioxospiro[4.5]decanes and 7-substituted-1,5-dioxospiro[5.5]undecanes with substituents X = CH3, F, Cl, CN, OH, OCH3, and NO2. Solvent effects on the equilibrium have been analysed by means of a Self-Consistent-Reaction-Field model and the PM3 method. Complete geometry optimizations have been carried out for all the structures in the gas phase and in solution. For some substituents, a set of rotamers have been separately optimized. The discussion of the results is focussed on the effects arising from structural aspects and from steric and electrostatic interactions on the axial/equatorial relative stability. The role played by multipole moment is considered. In general, good agreement with available experimental data and with previous theoretical studies has been obtained. Though the use of semiempirical methods and simple solvent models prevents us from reaching definitive conclusions, this approach seems to be very useful in predicting the main role of solute–solvent interactions in conformational equilibria of complex systems for which ab initio calculations cannot be performed. Keywords: conformational equilibria, spiro decanes and undecanes, cavity model, SCRF, solvent effect, PM3 calculations.
Experimental and theoretical assessments of the substituent and medium dependence of the internal rotational potentials in benzyl fluoride. 3,5-Difluorobenzyl fluoride and 4-fluorobenzyl fluoride
Authors: Schaefer Ted , Schurko Robert W. , Sebastian Rudy , Hruska Frank E.
Published: May 2006

The 1H, 19F, and 13C {H} nuclear magnetic resonance spectra at 300 K of 4-fluoro- and 3,5-difluorobenzyl fluoride, dissolved in CS2–C6D12 and acetone-d6, are fully analyzed. Spin–spin coupling constants over four, five, and six formal bonds are used to derive expectation values of sin2θ and [Formula: see text] and the apparent twofold internal rotational potentials; [Formula: see text] is the angle by which the α C—F(C—H) bond twists out of the ring plane. The conformation of lowest energy has [Formula: see text] for the 3,5-difluoro compound in the polar and nonpolar solutions, whereas it has [Formula: see text] for the 4-fluoro derivative. The magnitudes of the potentials lie between 2 and 4 kJ/mol, that is, comparable to thermal energies. These data are compared with previous results for the parent compound and its 3,5-dichloro derivative. Geometry-optimized molecular orbital computations, including some correlation-gradient procedures, for benzyl fluoride and the two fluoro derivatives have [Formula: see text] for the conformations of highest energy of the free molecules. However, geometry-optimized SCFRF calculations of the solvent perturbations of the potential (dipole terms are insufficient) are in semiquantitative agreement with experiment in the sense that both solvents are predicted to destabilize the conformation with [Formula: see text] For example, the predominant twofold component in the computed (6-31G*) potential is 3.4 (free), −0.7 (CS2), and −3.3 (acetone-d6) kJ/mol for benzyl fluoride, a negative number indicating [Formula: see text] for the stable conformer; the experimental values are −0.8(2) (CS2) and −2.7(2) (acetone-d6) kJ/mol. The agreement between experiment and theory is of a similar quality for the fluoro derivatives. The stabilization of the conformer with [Formula: see text] for the 4-fluoro derivative is tentatively attributed to hyperconjugative electron acceptance by the α C—F bond, enhanced by the π-electron donor at the para position. A number of coupling constants are discussed in terms of the possible mechanisms of their transmission. Future experiments are indicated. Keywords: 1H, 19F, 13C NMR of 4-fluorobenzyl fluoride and 3,5-difluorobenzyl fluoride; MO calculations and internal rotational potentials in benzyl fluoride, 3,5-difluorobenzyl fluoride, and 4-fluorobenzyl fluoride; solvent effects and experimental and theoretical approaches to internal rotational potentials in 3,5-difluorobenzyl fluoride and 4-fluorobenzyl fluoride.
Internal return in the photochemistry of ring-substituted 1-(1-naphthyl)ethyl esters of phenylacetic acid
Authors: Kim J.M. , Pincock J.A.
Published: May 2006

The photochemistry in methanol of the esters 12a–d has been studied in order to determine the importance of internal return of both ion pair and radical pair intermediates. The efficiency of internal return, determined by monitoring 18O exchange between the alcohol and carbonyl oxygens, was shown to be substituent dependent, varying from approximately 10% for the 4,7-dimethoxy substrate to nearly 50% for the 4-cyano case. The corresponding ground state solvolysis reactions gave about 10% internal return and, within experimental error, were substituent independent. Internal return was also examined by racemization of the chiral center in 12a and 12d. In summary, these combined results reveal that internal return probably occurs mainly through a contact (not solvent-separated) radical pair. More important, internal return has little effect on previously calculated electron transfer rate constants for converting the radical pair to the ion pair. Therefore, the previously reported Marcus' correlations are valid. Keywords: photochemistry of benzylic esters, internal return, photosolvolysis, electron transfer, radical pairs.
Synthesis and crystal structure of the allyl(carborane)nickel complex [N(PPh3)2][Ni(η3-C3H5)(η6-7,9-Me2-7,9-C2B10H10)]
Authors: Mullica Donald F. , Sappenfield Eric L. , Stone F. Gordon A. , Woollam Stephen F.
Published: May 2006

Treatment of [Ni2(µ-Br)2(η3-C3H5)2] with Na2[nido-7,9-C2B10H12] in THF (tetrahydrofuran) affords the anionic species [Ni(η3-C3H5)(η6-7,9-C2B10H12)]−, which was isolated as its [NEt4]+ salt 8a. A similar reaction between Na2[nido-7,9-Me2-7,9-C2B10H10] and [Ni2(μ-Br)2(η3-C3H5)2] in thf gives the anionic species [Ni(η3-C3H5)(η6-7,9-Me2-7,9-C2B10H10)]− isolated as [N(PPh3)2]+ (8b) and [NEt4]+ (8c) salts, respectively. The molecular structure of 8b has been established by X-ray diffraction. Crystals are monoclinic, space group P21/c (No. 14) with a = 14.685(3) Å, b = 18.346(2) Å, c = 16.319(1) Å, β = 99.93(1)°, and Z = 4. The nickel atom is ligated by the nido-7,9-Me2-7,9-C2B10H10 cage in an η6 manner, as well as by the allyl group, with the latter adopting an η3-bonding mode (Ni—C = 2.023(5), 1.939(8), and 1.988(5) Å). The palladium analog 8d was similarly prepared from Na2[nido-7,9-Me2-7,9-C2B10H10] and [Pd2(µ-Cl)2(η3-C3H5)2], followed by addition of [NEt4]Cl•H2O, but in contrast with the nickel species this product was unstable, especially its solutions in air. Keywords: nickel, palladium, allyl complex, carborane.
A study of a variety of O,O-alkylene dithiophosphate derivatives of triphenyl-and diphenylgermane. Crystal structures of and
Authors: Drake John E. , Mislankar Anil G. , Ratnani Raju , Yang Jincai
Published: May 2006

The O,O-alkylene dithiophosphate derivatives of triphenyl- and diphenylgermane, [Formula: see text] [Formula: see text] [Formula: see text] [Formula: see text] [Formula: see text] and [Formula: see text] have been prepared and characterized by infrared, Raman, and 1H, 13C, and 31P NMR spectroscopy, mass spectrometry, and, in three cases, X-ray crystallography. [Formula: see text] 1, crystallizes as monoclinic in the space group P21/a (No. 14) with the cell parameters a = 12.868(5) Å, b = 11.354(4) Å, c = 17.207(4) Å, β = 95.42(2)°, V = 2502(1) Å3, Z = 4, R = 0.0640, and Rw = 0.0585. [Formula: see text] 2, crystallizes as orthorhombic in the space group Pbca (No. 61) with the cell parameters a = 23.007(4) Å, b = 16.840(4) Å, c = 12.068(3) Å, V = 4657(3) Å3, Z = 8, R = 0.0502, and Rw = 0.0329. [Formula: see text] 3, crystallizes as monoclinic in the space group C2/c (No. 15) with the cell parameters a = 35.48(2) Å, b = 9.275(5) Å, c = 20.78(1) Å, β = 120.93(3)° V = 5866(5) Å3, Z = 8, R = 0.0674, and Rw = 0.0562. As with their methylgermanium analogues, the environment about germanium is essentially that of a distorted tetrahedron, with the terminal sulfur atoms oriented towards germanium rather than away from it, in contrast with analogous noncyclic dithiophosphatogermanes. The substituents on the atoms in both the five- and six-membered rings maintain their nonequivalence in solution, in contrast with tin analogues. Keywords: structure, germanium, diphenyl, triphenyl, cyclic dithiophosphates
The chemical bonding topology of ternary and quaternary transition metal nitrides containing low-coordinate metal atoms
Authors: King R.B.
Published: May 2006

Solid state ternary and quaternary metal nitrides containing a transition metal and one or two very electropositive metals such as alkali and (or) alkaline earth metals exhibit a number of structures containing low-coordinate transition metals in the transition metal – nitrogen subnetwork. Transition metal – nitrogen double and even triple bonds are found in these structures involving dπ → pπ bonding from a filled transition metal d orbital to an otherwise empty nitrogen p orbital. Such multiply bonded nitrogen atoms function as strong field ligands in contrast to amido ligands such as (Me3Si)2N−, which generally function as weak field ligands in transition metal chemistry. The transition metal environment in these ternary nitrides can be modelled by "banana bonds" from the polyhedron using the atomic orbitals required for both the σ- and π-bonding to the nitride ligands, e.g., the trigonal prism for the trigonal planar derivatives MIIIN36− (M = V, Cr, Mn, Fe) with three M=N double bonds as well as FeIIN24− in Li4FeIIN2 with two Fe≡N triple bonds and the planar square for the discrete linear CoIN25− in LiSr2CoN2. Reasonable electron counts are also obtained for the anionic metal–nitrogen networks in Ca2FeN2, Sr2LiFe2N3, Ba2LiFe2N3, Li3FeN2, CaNiN, Li3Sr3Ni4N4, BaNiN, and Ba8Ni6N7. Keywords: chemical bonding, topology, nitrides, transition metals.
Reactivity of diosmacycles: complementary syntheses and structure Of [(µ-η1,η2-CHCH2)OS2(CO)8][BF4]
Authors: Ball R.G. , Kiel Gong Y. , Kiel William A. , Takats Josef , Grevels Friedrich-Wilhelm
Published: May 2006

The vinyl-bridged cationic complex, [(µ-CHCH2)Os2(CO)8][BF4] (3), is obtained readily and in excellent yield either via hydride abstraction from octacarbonyl diosmacyclobutane, (µ-η1,η1-CH2CH2)Os2(CO)8 (1), or via protonation of octacarbonyl diosmacyclobutene, (µ-η1,η1-CHCH)Os2(CO)8 (2). It is argued that the 1H and 13C NMR data of complex 3 are consistent with the traditional view of bonding in µ-alkenyl complexes, involving substantial σ,π donation. The solid state X-ray structure of the complex lends support for this formulation. Complex 3 crystallizes in the orthorhombic space group, P212121, with a = 12.128(2) Å, b = 14.151(3) Å, c = 9.248(3) Å, V = 1587 Å3, and Z = 4. The final R and Rw values are 0.032 and 0.038, respectively. Keywords: µ-vinyl, diosmacycles, X-ray structure.
Elucidation of the structures of the hydridothiolato complexes WH(SC6H2Pri3-2,4,6)3(L)(PMe2Ph), L = PMe2Ph or pyridine, by NMR and X-ray techniques
Authors: Burrow Timothy E. , Lough Alan J. , Morris Robert H. , Richards Raymond L.
Published: May 2006

Measurement of the T1 values of the 31P nuclei in the unusual capped trigonal bipyramidal complexes WH(tipt)3(PMe2Ph)2 (1, tipt = SC6H2Pri3-2,4,6) and W(2H)(tipt)3(PMe2Ph)2 allows the determination of the phosphorus–hydride distance of 2.2 ± 0.2 Å and the P-W-H angle of about 62 ± 7°. An approximate location of the hydride on an S2P face is proposed on the basis of steric interactions. The crystal structure of WH(tipt)3-(NC5H5)(PMe2Ph) (2) is reported; the WS3NP core consists of a distorted trigonal bipyramidal system with the phosphine and pyridine ligands axial and the thiolate ligands equatorial. The hydride ligand was not located by X-ray diffraction but can be positioned on an open S2P face at an angle P-W-H of about 65°. The small P-W-H angle results in large 2J(H,P) couplings for 1 and 2. The 1H NMR spectrum of complex 2 is assigned by use of NOE and homodecoupling NMR methods. This complex has the same crowded structure in solution as in the crystalline state. The pyridine ligand, which is sandwiched between two tipt aryl groups, is hindered from rotating around the W—N bond. Keywords: molybdenum, tungsten, thiolate, hydride, NMR, NOE.
Coordination chemistry of a mixed donor bidentate ligand with nickel. A tetrameric enolate of sodium that binds a phosphine donor
Authors: Fryzuk Michael D. , Gao Xiaoliang , Rettig Steven J.
Published: May 2006

The preparation of a mixed donor ligand consisting of an enolate and neutral phosphine is described and its coordination chemistry with nickel reported. The phosphino ketones R2PCH2COPh (R = Cy or Pri) are prepared by the reaction of the corresponding secondary phosphines with bromoacetophenone to form the corresponding phosphonium salts [R2P(H)CH2COPh]+Br− (R = Cy or Pri); subsequent deprotonation leads to the neutral ketones. The enolates are formed by addition of NaN(SiMe3)2 to the neutral ketone. The crystal structure of the sodium enolate having isopropyl substituents at phosphorus displays a tetrameric structure in the solid state and there is an interaction of the phosphorus donor with the sodium anion: Na—P, 2.852(2) Å. The preparation of a series of nickel complexes was achieved by addition of the sodium enolate to the appropriate Ni(II) starting material. The structures of the nickel derivatives are reported. The catalytic behaviour of one of these complexes in the polymerization and oligomerization of ethylene is briefly described. Crystals of {Na[Pri2PC(H)=C(O)Ph]}4, 2b, are tetragonal, a = b = 20.790(2) Å, c = 13.539(7) Å, Z = 4, space group I41/a1; those of [Cy2PC(H)=C(O)Ph](PPh3)NiPh, 3a, are triclinic, a = 10.742(2) Å, b = 19.811(3) Å, c = 10.621(2) Å, α = 101.88(1)°, β = 119.24(1)°, γ = 91.08(1)°, Z = 2, space group [Formula: see text] and those of [Pri2PC(H)=C(O)Ph](PPh3)NiPh, 3b, are triclinic, a = 10.219(1) Å, b = 17.432(3) Å, c = 9.788(1) Å, α = 101.18(1)°, β = 100.19(1)°, γ = 85.32(1)°, Z = 2, space group [Formula: see text] The structures were solved by direct (2b) and Patterson (3a and 3b) methods and were refined by full-matrix least-squares procedures to R = 0.045, 0.044, and 0.032 (Rw = 0.038, 0.041, and 0.030) for 965, 4483, and 5085 reflections with I > 3σ(I) respectively. Keywords: nickel, sodium enolate, homogeneous catalyst, oligomerization.
1994 Merck Frosst Award Lecture New strategies for the stereoselective synthesis of natural and unnatural products via organometallic reagents and catalysts
Authors: Lautens Mark , Ren Yi , Delanghe Patrick , Chiu Pauline , Ma Shihong , Colucci John
Published: May 2006

Samarium in the presence of diiodomethane selectively cyclopropanates an allenic alcohol to yield a methylenecyclopropane with modest to excellent diastereoselectivity. The effect of substituents at the carbinol carbon and the allenic carbon on the diastereoselectivity was investigated. A palladium catalyst was shown to promote the intramolecular cycloaddition of the methylenecyclopropane with an electron-deficient alkyne to yield a methylenecyclopentane. The reaction was stereospecific with retention of stereochemistry as proven by X-ray diffraction. Nickel catalysts accelerate the hydroalumination of oxabicyclo[3.2.1]alkenes, leading to bicyclic trialkylalanes. Upon treatment with a Lewis acid (diisobutylaluminum chloride), the trialkylalanes fragment to provide cycloheptenols. This reaction, followed by an enantioselective enzyme-catalyzed esterification, was used as a key step in a synthesis of the mevinic acid lactone. Keywords: methylenecyclopropane, palladium, stereocontrol, oxabicyclic, nickel.
A multinuclear NMR study of the cycloaddition reaction of SNSAsF6withtert-butylphosphaacetyline (CH3)3CCP leading to the 5-tert-butyl-1,3,2,4-dithiazaphospholium cation; a preliminary account of the novel exchange of a cyclic phosphorus by nitrogen in the reaction of with azide N3−leading to 5-tert-butyl-1,3,2,4-dithiadiazolyl
Authors: Parsons Simon , Passmore Jack , Sun Xiaoping , Regitz Manfred
Published: May 2006

The cycloaddition reaction of SNS+(as the AsF6−salt) with 5-tert-butylphosphaacetyline (CH3)3CCP in sulfur dioxide solution at −60 °C gave [Formula: see text], the first identified example of a 1,3,2,4-dithiazaphospholium heterocycle [Formula: see text]. The cycloadduct was characterized by multinuclear NMR (1H,13C,14N,15N,31P) spectroscopy at low temperature. Reduction of 6π [Formula: see text] in situ with SbPh3in the presence of Me4NCl led to a radical, which may be the 7π tert-butyl-1,3,2,4-dithiazaphospholyl [Formula: see text]. The reaction of 5-tert-butyl-1,3,2,4-dithiazaphospholium [Formula: see text] with azide N3−surprisingly gave the previously known 5-tert-butyl-1,3,2,4-dithiadiazolyl [Formula: see text]. The conversion of [Formula: see text] to [Formula: see text] by the net replacement of a cyclic phosphorus with nitrogen is unprecedented. Keywords: dithiazaphospholium, dithiazaphospholyl, cycloaddition, multinuclear NMR, ESR.
Volumetric properties of (water + diethanolamine) systems
Authors: Maham Yadollah , Teng Tjoon T. , Mather Alan E. , Hepler Loren G.
Published: May 2006

Densities of completely miscible (water + methyldiethanolamine) and (water + ethyldiethanolamine) systems have been measured over the full range of compositions at temperatures from 25 to 80 °C. Results of these measurements have been used in calculating excess molar volumes and partial molar volumes of each component. We have also identified different measures of the thermal expansion of these systems and have calculated some of these derivative quantities. The partial molar volumes and their derivatives with respect to temperature provide a basis for extending our understanding of molecular interactions in these (water + organic) systems. Keywords: diethanolamines, excess molar volumes, partial molar volumes, expansivities, aqueous solutions.
Peptide models XI. Substitution effects on peptide chains. The magnitude of side-chain–backbone interactions in oligopeptides HCO-(NHCHRCO)n-NH2 for R=CH3. An ab initio study
Authors: McAllister M.A. , Endredi G. , Ladik J. , Viviani W. , Perczel A. , Csaszarau P. , Rivail J.-L. , Csizmadia I.G.
Published: May 2006

The stabilization effect of the Me group with respect to H has been determined for peptides of the type HCO-(NH-CHR-CO)n-NH2 for 1 ≤ n ≤ 3. It was found that for the Me group the stabilization energy was in the vicinity of 5.4 kcal/mol. The site of substitution had no appreciable effect on the stabilization energy. The stabilization energy for monomethyl substitution (R=CH3) ranged from 5.21 to 5.60 kcal/mol. For dimethyl substitution the values were between 10.53 and 10.81 kcal/mol, and for trimethyl substitution it was 15.99 kcal/mol. Keywords: stabilization effects of substituents, oligopeptide conformations, ab initio study on HCO-(NHCHRCO)n-NH2.
Raman and resonance-Raman spectra of polypyrrole with application to sensor – gas probe interactions
Authors: Vigmond Stephen J. , Ghaemmaghami Vida , Thompson Michael
Published: May 2006

Raman spectra of the oxidized and neutral forms of polypyrrole show a relative intensity shift from the high wave number region to the low wave number region as the source wavelength is increased from 514.5 to 1064 nm. Variations in band positions are also observed with changes in excitation source; particularly, two C=C stretches at 1500 and 1605 cm−1 can be observed with the neutral polymer but only a single band is observed at 1575 cm−1 when sources of higher energy are used. These changes arise from differences in the electronic states of the two forms that can greatly alter the intensities of the numerous bands. The oxidized and neutral forms of the polymer both act as electron acceptors towards the gas phase species tested here (toluene, water, methanol, and ammonia). The increased electron density induces an upward shift of the peaks at 915 and 1035 cm−1 of the neutral polymer and increases the relative intensity of the 1575 cm−1 peak of the oxidized polymer using 1064 and 514.5 nm sources, respectively. Keywords: polypyrrole, Raman spectroscopy, gas probe interactions.
Anticipating molecular polarity 1: a back-of-the-envelope approach to topologically induced polarity in non-alternant monocycles
Authors: Baum James Clayton , Martin Eric Donald , Ginsburg Jack Leon , Langler Richard Francis
Published: May 2006

Perturbation theory is applied to fully π-bonded non-alternant monocycles at the Hückel level. An effective "back-of-the-envelope" approach is developed that permits one to anticipate Hückel charge patterns for these monocycles. Very little modification is required to alter the basic approach so that it will permit rapid anticipation of AM1 -calculated charge patterns for such compounds. Keywords: hydrocarbon polarity, perturbation theory.
A study of the vacuum pyrolysis of 4-diazo-3-isochromanone with Hel ultraviolet photoelectron spectroscopy
Authors: Werstiuk N.H. , Ma J. , Roy C.D. , Kresge A.J. , Jefferson E.
Published: May 2006

A newly developed ultraviolet photoelectron spectrometer – CO2 laser instrument that utilizes a 50-W CW laser as a directed heat source was used to study the vacuum pyrolysis of 4-diazo-3-isochromanone (1). Analysis of the pyrolysate with ultraviolet photoelectron spectroscopy and photoionization mass spectrometry established that 1 undergoes a facile, unexpected pyrolysis at a laser power level of 26 W yielding N2, CO, and benzocyclobutenone (6). A multistep mechanism beginning with the formation of 4-carbena-3-isochromanone (2), which rearranges to oxaketene 3, can be written for the reaction. If 3 is an intermediate, it must be unusually thermally labile for it readily decarbonylates to 2-carbena-3,4-benzotetrahydrofuran (4). The ring opening of 4 into the ortho-quininoid ketene 5 and the cyclization of 5 into 6 are possible final steps in the conversion of 1 into 6. Keywords: vacuum pyrolysis, 4-diazo-3-isochromanone, HeI ultraviolet photoelectron spectroscopy.
Micellar properties of two-headed surfactant systems: the disodium 1,2-alkanedisulfates
Authors: Comeau Ellen K. , Beck Ellen J. , Caplan Jennifer F. , Howley Calvin V. , Marangoni D. Gerrard
Published: May 2006

A family of two-headed surfactants, the disodium 1,2-alkanedisulfates, has been synthesized by the addition of chlorosulfonic acid to the corresponding 1,2-alkanediol. The ability of these salts to form micelles in aqueous solution is demonstrated from solution conductance measurements and counterion activity measurements of the sodium ion. The critical micelle concentrations (cmc's) and apparent degrees of counterion binding (β′ values) of these surfactants indicate that the addition of the second headgroup has a profound effect on the micelles formed by the disulfates when compared to their monosulfate analogues. Keywords: surfactants, synthesis, micellization, cmc values, counterion binding.
Non-equilibrium particle morphology in dispersion-polymerized polystyrene particles
Authors: Paine Anthony J. , Shivers Richard R.
Published: May 2006

The interior particle morphology of dispersion-polymerized polystyrene particles was investigated by freeze fracture TEM (FFTEM) in order to compare the morphology of particles formed under two extreme mechanistic conditions: (a) particle growth by scavenging of dead polymer chains formed in solution (which gives rise to large particles of 5–10 μm) and (b) particle growth by oligomeric radical capture and polymerization inside the particle (giving rise to small particles of 1 μm or less). Although not generally recognized in the literature, ascribing mechanistic significance to an observed morphology requires a demonstration that the morphology is of the non-equilibrium type, i.e., the morphology depends upon how the particle was formed. An equilibrium morphology, by definition, must be independent of the particle formation route, and therefore carries no mechanistic information. In this case, large PS particles of 6.5 μm were found to have a nodular morphology that was unchanged on heating well above Tg, indicating an equilibrium structure (the nodularity of amorphous polymers is briefly rationalized by comparison with literature results over the past 20 years). On the other hand, small PS particles of 1.3 μm had a pronounced radial structure that could be converted into the nodular morphology by the same heat treatment, or by dissolving and precipitating from a suitable solvent combination. This means that the small particles have a non-equilibrium morphology, from which we may attempt to draw mechanistic inferences consistent with the suspected growth mechanism. Keywords: dispersion-polymerized polystyrene, non-equilibrium polystyrene particle morphology, freeze fracture transmission electron microscopy.
Photomodulation of the hydrophilic properties of acrylic polymers containing side-chain azobenzene chromophores
Authors: Tirelli Nicola , Altomare Angelina , Solaro Roberto , Ciardelli Francesco
Published: May 2006

Partially hydrophilic polymers have been prepared by free-radical copolymerization of trans-4-acryloyloxyazobenzene with N-tert-butylacrylamide and N-isopropylacrylamide. A turbidimetric method has been adapted to investigate the dependence of the polymer hydrophilic–hydrophobic balance on chemical composition and extent of photoisomerization of side-chain azobenzene chromophores. Irradiation at 366 nm of polymer suspension in THF–water induces the isomerization of azobenzene groups from the planar apolar trans form to the nonplanar polar cis form. Correspondingly, the solution turbidity is appreciably modified. Experimental results are discussed in terms of polymer structural parameters. Suspension stability and the possibility of carrying out photochemical cycles have been also investigated. Keywords: photochromic polymers, photoresponsive polymer, photomodulation of polymer properties, azobenzene-containing polymers, acryloyloxyazobenzene, N-tert-butylacrylamide.
The effect of transformation products of the antioxidant BHT on the initial stages of thermo- and photo-oxidation of LDPE
Authors: Kovářová J. , Rotschová J. , Brede O. , Burgers M.
Published: May 2006

The effect of individual transformation products of 2,6-di-tert-butyl-4-methylphenol (1, BHT), namely, 2,6-di-tert-butyl-4-hydroxy-2,5-cyclohexadienone (3), 2,6-di-tert-butyl-1,4-benzoquinone (4), and 3,5,3′,5′-tetra-tert-butylstilbenequinone (5), in the course of thermal- and photo-degradation of LDPE has been examined. Oxidative DSC studies combined with oxygen uptake measurements, IR spectroscopy data, and embrittlement tests proved to be sufficiently sensitive to detect differences in the stability of the above-mentioned samples. A comparative study revealed a pronounced stabilizing effect of 5 in LDPE thermooxidation at moderate temperatures (90–100 °C). The stabilizing action of BHT is due in part at least to its coloured transformation product. Keywords: LDPE, thermooxidation, photooxidation, transformation products of phenolic antioxidants.
Unusually strong emission from an exciplex formed between benzenoid solvents and dibenzoylmethanatoboron difluoride. Formation of a triplex
Authors: Valat Pierre , Wintgens Véronique , Chow Yuan L. , Kossanyi Jean
Published: May 2006

Benzene derivatives give, in cyclohexane solution, a strongly emitting (Φf up to 0.8) excited complex with dibenzoylmethanatoboron difluoride (DBMBF2). The rate constant kex for the complex formation varies between 3.5 × 109 and 2.6 × 1010 M−1 s−1 depending on the benzene derivative, and the complex emission can already be detected at very low concentrations of the aromatics. This behaviour is different from that found in acetonitrile where the DBMBF2 emission disappeared completely before the new emission of the complex could be detected. The position of the complex emission and its lifetime depend upon the redox potential of the aromatics: the lower the latter, the longer the lifetime and the more the emission is red-shifted. The monotonous increase of the lifetime and of the fluorescence quantum yield of the emission with the concentration of the aromatics has been interpreted on the basis of an equilibrium between the exciplex and excited DBMBF2. The equilibrium constant varies with the electron-donating property of the aromatics, from 2.7 for benzene to 560 M−1 for isodurene. A decrease in the excited complex lifetime or in its fluorescence quantum yield is observed in some cases when the aromatics concentration is close to that of the pure solvent. Such behaviour is believed to arise from a new equilibrium resulting from the interaction of the exciplex with a second molecule of the aromatics to form triplex species. A detailed kinetic study has been undertaken in case of p-xylene, and the rate constants of all the processes involved in the double equilibrium have been determined. No basic difference is observed in the rate constant for exciplex formation when a single or double equilibrium is assumed; on the other hand, large discrepancies are found in the exciplex deactivation processes, specially for the back reaction from the exciplex to excited DBMBF2. The equilibrium constants between DBMBF2 and the exciplex, on one hand, and between the latter and the triplex are 56.6 and 0.68 M−1, respectively, when the concentration of p-xylene in cyclohexane is 3.3 M. Keywords: fluorescence, exciplex, triplex, dibenzoylmethanatoboron difluoride, kinetics.
Fluorescence spectroscopy of mechanical pulps III: Effect of chlorite delignification
Authors: Olmstead Jennifer A. , Zhu Jian H. , Gray Derek G.
Published: May 2006

Many paper and wood samples fluoresce, but the sources of the emission are not well understood. Fluorescence excitation and emission spectra of paper sheets prepared from thermomechanical pulp (TMP) and bleached chemithermomechanical pulp (BCTMP) showed that the emission from the BCTMP was significantly higher than that from the TMP. Removing almost all of the lignin from both pulps by means of an acid chlorite treatment did not reduce the fluorescence significantly. By means of an approximate correction for changes in sheet reflectivity caused by the chlorite treatment, the fluorescence intensity was found to increase with lignin removal. Clearly, fluorescence is not simply related to lignin content. Keywords: wood pulp, lignin, cellulose, fluorescence spectra, acid chlorite delignification.
The synthesis and polymerization behaviour of silicon-bridged [1]- and [2]ferrocenophanes with sterically demanding trimethylsilyl substituents attached to the cyclopentadienyl rings
Authors: Peckham Timothy J. , Foucher Daniel A. , Lough Alan J. , Manners Ian
Published: May 2006

The silicon-bridged [1]ferrocenophane Fe(η-C5H3SiMe3)2(SiMe2) (5) was synthesized via the reaction of Li2[Fe(η-C5H3SiMe3)2]•tmeda (tmeda = tetramethylethylenediamine) with Me2SiCl2 in hexanes. The disilane-bridged [2]ferrocenophane Fe(η-C5H3SiMe3)2(Si2Me4) (7) was prepared using a similar route from the disilane ClMe2SiSiMe2Cl. Despite the presence of sterically demanding SiMe3 substituents on the cyclopentadienyl rings, compound 5 was found to undergo thermal ring-opening polymerization at 170 °C to produce very soluble, high molecular weight poly(ferrocenylsilane) 6 with Mw = 1.4 × 105, Mn = 8.4 × 104. However, the [2]ferrocenophane 7 was found to be resistant to thermal ring-opening polymerization even at 350 °C and decomposed above 380 °C. A single-crystal X-ray diffraction study of 7 revealed that the steric interactions between the bulky SiMe3 groups are relieved by a significant twisting of the disilane bridge with respect to the plane defined by the centroids of the cyclopentadienyl ligands and the metal atom. The angle between the planes of the cyclopentadienyl rings in 7 was found to be 5.4(6)°, slightly greater than that in the non-silylated analogue Fe(η-C5H4)2(Si2Me4) (4a) (4.19(2)°), and dramatically less than the corresponding tilt angle of the strained, polymerizable, silicon-bridged [1]ferrocenophane Fe(η-C5H4)2(SiMe2) (1) (20.8(5)°). The length of the Si—Si bond in 7 (2.342(3) Å) was found to be close to the sum of the covalent radii (2.34 Å). Crystals of 7 are monoclinic, space group C2/c, with a = 23.689(3) Å, b = 11.174(1) Å, c = 31.027(3) Å, β = 109.16(1)°, V = 7758(2) Å3, and Z = 12. Keywords: ring-opening polymerization, ferrocenophane, organometallic polymers.
Effect of the degree of neutralization on the micellization of block ionomers
Authors: Khougaz Karine , Nguyen Diep , Eisenberg Adi , Williams Claudine E.
Published: May 2006

The effect of neutralization on the micellization of polystyrene-b-poly(acrylic acid) (PS-b-PAA) was investigated by static light scattering (SLS) and small-angle X-ray scattering (SAXS). The block copolymers, present initially in single chain form in either dioxane or benzene–methanol (90/10 (v/v)), were neutralized to different degrees by the addition of cesium hydroxide dissolved in methanol. The solutions were simultaneously monitored by SLS. It was found that micelle formation began near 5% neutralization. The normalized scattered light intensity at 90° (I90/c) increased dramatically in the neutralization range between ca. 10 and 60%. For neutralization degrees between 60 and ca. 100%, I90/c did not change significantly with a further increase in the percent of neutralization. In the second part of the study, two block copolymers, PS(600)-b-PAA(34) and PS(600)-b-PAA(45) (the number of monomer units is given in parentheses), were prepared from benzene–methanol (90/10 (v/v)) with different degrees of neutralization: 5, 25, 60, 100, and 150%. The resulting solutions were freeze-dried and the dried samples were redissolved in toluene. These samples were investigated by SLS and SAXS. By SLS, the aggregation numbers (f) were found to be the same between 5 and 60% neutralization. An increase in f was observed in going from 60 to 100%; i.e., f increased from 82 to 92 for PS(600)-b-PAA(34), and from 79 to 110 for PS(600)-b-PAA(45). The f values for the two block copolymers at 150% neutralization were found to be the same as those for 100% neutralization. The core radii values (Rcore) were measured for the neutralized PS(600)-b-PAA(45) samples at 0.05 g/mL in toluene by SAXS. The Rcore values increased from 5 to 60% neutralization, remained constant from 60 to 100%, and increased again in the range from 100 to 150%. The results are explained by dynamic considerations. For low percent neutralization, between 5 and 60%, the micelles and single chains are proposed to be in a dynamic equilibrium state. For higher percent neutralization, between 60 and 100%, the dynamics are probably much slower. Keywords: block copolymers, micelles, ionomers, light scattering, small angle X-ray scattering (or SAXS).
Arene-functionalized polyisocyanides: photophysics of well-defined homopolymers and block copolymers for efficient light harvesting
Authors: Hong Bo , Fox Mary A.
Published: May 2006

Steady-state and time-resolved fluorescence spectroscopy of the homopolymers and di- and triblock copolymers of 2-naphthylethylisocyanide (7), 9-anthrylethylisocyanide (10), and 2-phenanthrylethylisocyanide (11) exhibit emission from the isolated chromophores, i.e., naphthyl, anthryl, or phenanthryl, upon excitation at 284 or 354 nm. The absence of excimer emission in the fluorescence spectra of all these homopolymers is indicative of a rigid polymeric backbone. Fluorescence quenching in diblock copolymers containing a dimethylaniline donor block and a naphthalene or anthracene acceptor block takes place through directional energy migration to the acceptor–quencher interface. The migrating excited state is then quenched at the interface either by photoinduced electron transfer in the anthracene–dimethylaniline diblock copolymer or by exciplex formation in the naphthalene–dimethylaniline diblock copolymer. Upon incorporation of an intervening block derived from pentamethylphenylethylisocyanide 8, exciplex formation is suppressed in the related triblock copolymer. Transient absorption spectra of this family of di- and triblock copolymers reveal the formation of radical ion pairs, with a lifetime of 1.1 μs in the anthracene–dimethylaniline diblock copolymer. Keywords: electron transfer, energy migration, excimer emission, polyisocyanides, block copolymers.
Photochemistry of N-(pyrimidin-2-one-4-yl)pyridinium derivatives. The ring contraction of pyrimidinone into imidazolinone
Authors: Wenska Grazyna , Skalski Bohdan , Paszyc Stefan , Gdaniec Zofia
Published: May 2006

Photochemical reactions (λ > 300 nm) of N-(1-methylpyrimidin-2-one)-and N-(1,5-dimethyl-pyrimidin-2-one)pyridinium chlorides were studied in deoxygenated aqueous solution at various pH's. Only the former compound was found to be reactive under these conditions to give pyrimidine ring contraction photoproducts 1-methyl-4-imidazolin-2-one and 1-methyl-4-imidazolin-2-one-5-carboxyaldehyde, with pH-dependent chemical yields. The photochemical pyrimidine ring contraction reaction does not occur for other photochemically reactive pyrimidin-2-ones bearing 3-methylimidazolium-1,1,2,4-triazol-1-yl, or imidazol-1-yl as substituents at the C-4 position. The suggested mechanism of the reaction involves the addition of water to the pyrimidinone part of the N-(1-methylpyrimidin-2-one)pyridinium salt in the excited triplet state as the primary photochemical step. Addition of alcohol to the pyridinium ring was found to be the major reaction under irradiation of N-(1-methylpyrimidin-2-one-4-yl)pyridinium chloride in methanol. Keywords: photochemistry, N-(pyrimidin-2-one-4-yl)pyridinium compounds, pyrimidine ring contraction, 1-substituted-4-imidazolin-2-ones, 4-imidazolin-2-one-5-carboxyaldehydes.
Properties of atoms in molecules: nuclear magnetic shielding
Authors: Keith T.A. , Bader R.F.W.
Published: May 2006

This paper analyzes the nuclear magnetic shielding tensors underlying the chemical shift in NMR spectroscopy in terms of the field generated at the nucleus by the current J(1)(r) induced by an external magnetic field. The magnetic field at nucleus [Formula: see text] resulting from an element of the induced current density at a distance [Formula: see text] is proportional to [Formula: see text] which defines the shielding density [Formula: see text] The magnetic shielding of a nucleus is fundamentally an atomic property, a feature brought to the fore by using the theory of atoms in molecules and the integration of [Formula: see text] over the individual atomic basins relates the shielding tensor [Formula: see text] to a sum of atomic contributions. The shielding of nucleus ** is primarily determined by the flow of current within the basin of atom [Formula: see text], a contribution that varies from the approximate diamagnetic limit, given by the atomic Lamb value for the atom in the molecule, to values that are greatly reduced by the presence of paramagnetic current flows associated with particular bonding effects. Whether the contribution of a neighbouring atom is shielding or deshielding is readily understood by relating the form of the current flow within its basin to the magnetization density. [Formula: see text]. A study of the currents induced in benzene shows that the extent to which a proton, bonded to a ring of atoms, is deshielded by the field exerted by its bonded neighbour provides a direct diagnostic test for a ring current and an accurate relative measure of its strength. The theory of atoms in molecules isolates transferable atomic properties and because of this ability one finds, in addition to the anticipated result that a given functional group contributes identical amounts to the isotropic shielding [Formula: see text] of a nucleus external to it through a series of molecules, the more remarkable result that the whole of the variation in [Formula: see text] can have its origin in the basin of atom [Formula: see text], the contribution from external groups remaining constant. For example, the external contribution to [Formula: see text] for a carbon nucleus in a normal hydrocarbon is independent of chain length and position of [Formula: see text] within the chain, the methyl group in ethane contributing the same shielding to a methyl carbon as does the butyl group in pentane. This constancy in external contributions to the shielding is also found for N, O and F nuclei in substituted, saturated hydrocarbons. Key words: NMR, magnetic shielding, current density, magnetic shielding density.
Rheological and interfacial properties of nucleic acid films studied by thickness – shear mode sensor and network analysis
Authors: Su Hongbo , Thompson Michael
Published: May 2006

Complete characterization of the thickness – shear mode (TSM) acoustic wave sensor coated with films of nucleic acid and subjected to conductive solution loading is achieved through network analysis of impedance measurements. The responses of coated sensors with respect to series resonant frequency can be separated into two regions. When 0 < σ < 0.5Ω−1 m−1 the frequencies for sensors with films of single strand (ss) DNA or polycytidylic (5′) (Poly C) in place rise significantly with increasing conductivity. The motional resistances for sensors coated with both species peak at the same value of conductivity where the frequency has the most rapid shift. In the second region (0.5 < σ < 12Ω−1 m−1), the frequency response for the ss DNA system exhibits a bell-shaped curve, with a minimum at σ = 4Ω−1 m−1. The motional resistances for both nucleic acid layers show reversible behaviour, whereas TSM sensors with bare PdO and Au electrodes yield little change. Models that consider only the bulk properties of solutions fail to explain the observed changes in frequency. On the other hand, an equivalent circuit model incorporating the rheological properties of nucleic acid layers on the sensor surface is successful in explaining the various responses. Key words: DNA sensor, network analysis, thickness – shear mode sensor, DNA interfacial properties.