bluuebunny/crossref_metadata_2025 huggingface dataset:
1H, 19F, and 13C nuclear magnetic resonance approaches to the barrier to rotation about the Csp2—Csp3 bond in 1,1,1 -trifluoro-2-phenylethane. Proximate coupling constants and molecular orbital computations
Authors: Schaefer Ted , Hazendonk Paul , McKinnon David M.
Published: May 2006

The 1H, 19Fand 13C{1H} nuclear magnetic resonance spectra of 1,1,1-trifluoro-2-phenylethane, 1, in CS2–C6D12, acetone-d6, and benzene-d6 solutions, on analysis, yield long-range coupling constants from which are derived the apparent twofold barriers to rotation about the Csp2—Csp3 bonds. The twofold barrier is 9.0(2) kJ/mol, independent of solvent, 4.0 kJ/mol larger than that of ethylbenzene, also independent of solvent. The theoretical barrier heights for the free molecules at the post-Hartree–Fock level of molecular orbital theory (frozen-core MP2/6-31G*) also differ by 4 kJ/mol, but are about 1 kJ/mol higher than the experimental estimates. The perpendicular conformer is the most stable for both molecules. Comparisons are made with the benzyl halides, in which the internal barriers are remarkably sensitive to solvent. A spin–spin coupling constant over five formal bonds, 5J(H, F), involving the ortho protons in 1, is +0.74(2) Hz and is discussed in some detail in terms of its dependence on intenuclear distances (possible through-space interactions). The solvent perturbations of 3J(H, F) and of 2J(C, F) are of opposite sign. Other long-range coupling constants or their absence are also pointed out. For example, those between 19F and 13C nuclei or protons at the meta position are effectively zero; at the para position they are significant. Keywords: 1,1,1-trifluoro-2-phenylethane; 1H, 19F, and 13C NMR; long-range spin-spin coupling constants; through-space 1H, 19F spin–spin coupling constants; internal rotational potential; molecular orbital computations of internal potential.
Application of median filtering to noisy data
Authors: Stone David C.
Published: May 2006

The properties and application of the median filter are investigated using both simulated and real data for signals evolving with time. Comparison is made with existing numerical techniques for drift compensation and noise reduction in analytical measurements such as the moving average and Savitzky–Golay digital filters. The median filter provides a means for dealing with "spiky" noise and separating peaks from a slowly changing baseline, even when the exact nature of the drift and noise distribution is not known. Median filtering is a useful and complementary addition to existing digital filtering techniques, being mathematically robust and readily implemented on any computer platform. Keywords: median filtering, signal noise, signal processing, digital filters, chemometrics, data smoothing.
The reaction of some endo-5-acetyl norborn-2-enes with benzene thiol: a thermally induced non-allylic [1,3]-acetyl shift, as anticipated by a photochemical precedent
Authors: Leitich Johannes , Heise Ingeborg , Schaffner Kurt
Published: May 2006

The thermal reaction of benzene thiol with rac-1-(4aS*,5R*,8S*,8aR*-5,8-methano-1,2,3,4,4a,5,8,8a-octahydro-4a-naphthyl)ethanone (10b), an endo-5-acetylnorborn-2-ene with a particularly close distance between one olefinic carbon and the keto carbon, and with its 8aS* epimer 10a, has been investigated. Besides the conventional unrearranged adducts 16a, b and 17a, b of the thiol to the C=C bonds of 10a, b, an adduct 18 was formed from 10b that involved a [1,3]-acetyl shift; no other products were formed. The dependence of adduct ratios on thiol concentration is consistent with two competing reactions of an intermediate radical 14b formed by addition of thiyl radical to the C=C bond of 10b, namely, abstraction of hydrogen from thiol (which is the conventional reaction path) vs. intramolecular attack of the radical on the keto group inducing the [1,3] shift of the latter. This shift constitutes an intramolecular variety of the known alkyl transposition reaction of ketones initiated by a reversible attack of an alkyl radical on a keto carbonyl group to generate an intermediate tert-alkoxyl radical; however, it is very much faster than the intermolecular reaction and corresponds to an effective molarity between 107 and 1010. These findings have a bearing on the mechanism of photorearrangements homologous to the ODPM (oxa-di-π-methane) photorearrangement, such as ODPE (oxa-di-π-ethane) and higher homologues, in that they support a two-step mechanism involving this type of acyl shift. Keywords: oxadi-π-methane photorearrangement; oxadi-π-ethane photorearrangement; acetyl shift, non-allylic [1,3]; thiol, addition to C=C bonds; norbornenes.
Optical properties of side-chain liquid crystal copolymers of monosubstituted cholesteryl itaconate and a non-chiral mesogen
Authors: Cowie J.M.G. , Hunter H.W.
Published: May 2006

New mono- and disubstituted cholesteryl derivatives of itaconic acid have been prepared and their thermotropic liquid crystalline behaviour examined. The monosubstituted derivative has been homopolymerized, and also copolymerized with a non-chiral mesogen 4-cyanophenyl-4′-(6-acryloyl oxyhexyloxy) benzoate. Examination of the series of copolymers prepared, using differential scanning calorimetry and hot-stage polarizing microscopy, showed that when the content of the cholesteryl itaconate was high, both a smectic-A phase (SA) and a cholesteric phase (N*) were present. It was found that the SA phase could be eliminated by lowering the content of the cholesteryl itaconate in the copolymers, giving samples that displayed only the N* phase over a much wider temperature range. The samples in the N* phase also exhibited selective reflection of visible light that changed from short to long wavelengths as the samples were cooled from the isotropic melt. These colours can be locked into the glassy state of the polymer by quenching below the glass transition temperature, but only if the SA phase is absent. It was also noted that at high contents of the cholesteryl itaconate the selective reflection appears to occur in the ultraviolet region. Keywords: itaconic acid, copolymers, liquid crystalline polymers, cholesteric phases, selective reflection.
Fluorescence energy transfer studies in a cross-linked polyurethane network
Authors: Yang Jie , Winnik Mitchell A.
Published: May 2006

A series of cross-linked polyurethane samples, labeled with dyes suitable for fluorescence energy transfer experiments, were prepared (donor, phenanthrene; acceptor, anthracene). Fluorescence decay profiles for these samples were measured as a function of acceptor concentration. These decays obey Förster nonradiative energy transfer kinetics, with an energy transfer critical distance (R0) of 26.7 Å. Fluorescence intensities, calculated from the decays by integrating the decay profiles, also fit the Perrin model, with a quenching radius (Rs) of 25.6 Å. The fluorescence decay profiles were further examined with a distribution analysis method, which also revealed uniformly distributed donors and acceptors in the polymer matrices. Keywords: fluorescence quenching, fluorescence decay, phenanthrene, anthracene, polyurethane.
Spectral properties of ionic derivatives of pyrene and their aggregates with anionic surfactant and polyelectrolyte
Authors: Hrdlovič Pavol , Horinová Lubica , Chmela Štefan
Published: May 2006

A multifunctional probe based on pyrene, the chloride of the 2,2,6,6-tetramethyl-4-hydroxy-piperidinium ester of 4-(1-pyrene)butyric acid, at a concentration of 5 × 10−5 mol dm−3 exhibits excimer-like green emission with the maximum at 480 nm in the presence of sodium dodecylsulfate below its critical micelle concentration. A similar effect has been observed for this probe in the presence of the sodium salt of polyacrylic acid in the same concentration range. 1-Pyrenemethylammonium chloride exhibits a broad green excimer-like emission for sodium dodecylsulfate but not for the sodium salt of polyacrylic acid. The quenching constants of the monomeric and excimer-like emissions were determined for acrylamide, 1-oxo-2,2,6,6-tetramethyl-4-hydroxypiperidine, potassium iodide, and copper(II) sulfate, representing neutral, ionic, and free-radical types of quenchers. The rate constants of quenching for N-oxyl radicals indicate that the monomeric emission from both water-soluble probes is diffusion controlled. For other types of quenchers the rate constant of bimolecular quenching is lower. The quenching of the excimer-like emission of an aggregate of the chloride of the 2,2,6,6-tetramethyl-4-hydroxypiperidinium ester of 4-(1-pyrene)butyric acid is less efficient than in the case of 1-pyrenemethylammonium chloride, which indicates a more packed structure of the former aggregate. Keywords: pyrene, ionic derivative, emission, micelle, aggregation, quenching.
Luminescence studies of hydrophobically modified, water-soluble polymers. I. Fluorescence anisotropy and spectroscopic investigations of the conformational behaviour of copolymers of acrylic acid and styrene or methyl methacrylate
Authors: Ebdon J.R. , Hunt B.J. , Lucas D.M. , Soutar I. , Swanson L. , Lane A.R.
Published: May 2006

Fluorescence spectroscopy and anisotropy measurements have been used to study a series of styrene – acrylic acid, STY–AA, and methyl methacrylate – acrylic acid, MMA–AA, copolymers in dilute methanolic and aqueous solutions. Copolymerization of either STY or MMA with AA has little effect upon the rate of intramolecular segmental motion in methanol solutions. In aqueous media, intramolecular hydrophobic aggregation occurs and restricts the macromolecular dynamics to an extent dependent upon pH, nature of the comonomer, and copolymer composition. The hydrophobic domains formed in these copolymer systems can solubilize organic guests. In this respect, STY is a more powerful modifier of AA-based polymer behaviours than is MMA. In general, the hydrophobic modification increases the solubilization power of the resultant polymer. Furthermore, the copolymers retain their solubilization capacities to higher values of pH the more hydrophobic the comonomer and the greater its content in the copolymer. The interiors of the hydrophobic aggregates reduce the mobilities of occluded guests: the microviscosities of the domain interiors depend upon the nature of the hydrophobe, pH, and copolymer composition. Keywords: fluorescence, anisotropy, water-soluble polymers, acrylic acid, hydrophobic modification.
Photochemistry of nonconjugated diketones: internal self-quenching and energy transfer
Authors: Wagner Peter J. , Frerking Jr. Harlan W.
Published: May 2006

The triplet state behavior of nine α,ω-dibenzoylalkanes indicates the occurrence of a rapid quenching interaction between the two carbonyl groups. This quenching is fastest (k = 3 × 107 s−1) in dibenzoylbutane, is slightly slower (~107 s−1) in dibenzoylethane, dibenzoylpentane, and 2,2-dibenzoylpropane, but is absent in 1,3-dibenzoylpropane. It also occurs in several "mixed" 1,4-diaroylbutanes incorporating p-ethylbenzoyl or p-methoxybenzoyl chromophores This internal self-quenching is interpreted as the intramolecular counterpart of the well-known bimolecular self-quenching of aryl ketones, although no exact mechanism can be proposed. Such internal quenching does not occur as rapidly, if at all, in three "turned around" diketones: δ-(p-acetylphenyl)valerophenone, δ-(p-acetylphenoxy)valerophenone, and γ-(p-acetylphenoxy)butyrophenone. This fact, together with the varying rates of internal self-quenching in the dibenzoylalkanes, indicates the necessity for a very specific and close orientation of the two carbonyl groups for self-quenching. In the mixed diketones containing a p-alkylbenzoyl group, triplet excitation appears to be fully equilibrated between the two chromophores. However, in those containing a p-methoxybenzoyl group, excitation does not fully equilibrate before triplet decay, as evidenced by different quenching efficiencies for products from the two carbonyls. Analysis indicates intramolecular energy transfer rate constants ≤ 108 s−1. These are sufficiently lower than in other bichromophoric systems to suggest relatively slow energy hopping in the polymers of phenyl vinyl ketone. Keywords: nonconjugated diketones, dibenzoylalkanes, self-quenching, energy transfer, triplet ketones.
Synthesis, NMR and ultraviolet spectroscopy of [10] (N6,9)-6-aminopurinophane; calculation of the chemical shifts in the [n](N6,9)-6-aminopurinophane series
Authors: Capretta Alfredo , Bell Russell A.
Published: May 2006

[10](N6,9)-6-Aminopurinophane was prepared via a Mitsunobu reaction involving 6-chloropurine and 10-azido-1-decanol. Reduction of the azido moiety to an amine allowed for subsequent cyclization to the cyclophane. N6-Nonyladenine, 9-nonyladenine, and N6,9-dinonyladenine were also prepared using the established chemistry. Heating the [10](N6,9)-6-aminopurinophane to 90 °C allowed for a complete assignment of the proton and carbon spectra, while cooling the sample to −77 °C revealed additional isomers likely due to the anti–syn isomerization about the N6—C6 bond. A variable temperature proton NMR study revealed a ΔGc≠ of 59.4 ± 5.5 kJ mol−1 associated with the interchange between the two major conformers. The ultraviolet spectra of the [n](N6,9)-6-aminopurinophane series show a bathochromic shift and a hypochromic effect in the transitions as the methylene chain length decreases (i.e., n = 10 to 9 to 8). The aminopurinophanes are used to assess the relative merits of a semi-empirical model that allows for the calculation of the chemical shifts of the methylene bridge protons. Keywords: synthesis of heterocyclophanes, variable temperature NMR, cyclophane conformational analysis, ultraviolet spectroscopy, calculation of chemical shift.
Ion chemistry of second-row transition metals in hydrocarbon flames: cations and anions of Y, Zr, Nb, and Mo
Authors: Chow Christine C.Y. , Goodings John M.
Published: May 2006

A pair of laminar, premixed, CH4–O2 flames above 2000 K at atmospheric pressure, one fuel-rich (FR) and the other fuel-lean (FL), were doped with ~10−6 mol fraction of the second-row transition metals Y, Zr, Nb, and Mo. Since these hydrocarbon flames contain natural ionization, metallic ions were produced in the flames by the chemical ionization (CI) of metallic neutral species, primarily by H3O+ and OH− as CI sources. Both positive and negative ions of the metals were observed as profiles of ion concentration versus distance along the flame axis by sampling the flames through a nozzle into a mass spectrometer. For yttrium, the observed ions include the YO+•nH2O (n = 0–3) series, and Y(OH)4−. With zirconium, they include the ZrO(OH)+•nH2O (n = 0–2) series, and ZrO(OH)3−. Those observed with niobium were the cations Nb(OH)3+ and Nb(OH)4+, and the single anion NbO2(OH)2−. For molybdenum, they include the cations MoO(OH)2+ and MoO(OH)3+, and the anions MoO3− and MoO3(OH)−. Not every ion was observed in each flame; the FL flame tended to favour the ions in higher oxidation states. Also, flame ions in higher oxidation states were emphasized for these second-row transition metals compared with their first-row counterparts. Some ions written as members of hydrate series may have structures different from those of simple hydrates; e.g., YO+•H2O = Y(OH)2+ and ZrO(OH)+•H2O = Zr(OH)3+, etc. The ion chemistry for the production of these ions by CI in flames is discussed in detail. Keywords: transition metals, ions, flame, gas phase, negative ions.
Electrochemical investigation of reduction of mercury complexes of 2-aminocyclopentene-1-dithiocarboxylic acid and some of its derivatives at mercury electrodes
Authors: Safavi A. , Gholivand M. B.
Published: May 2006

Electrochemical techniques of polarography, cyclic voltammetry, and controlled potential electrolysis at mercury electrodes have permitted a detailed investigation of the reduction reactions associated with mercury 2-aminocyclopentene dithiocarboxylate complexes, Hg(ACD)2, in dimethyl sulphoxide (DMSO). As a complementary study, the electrochemistry of the ligands themselves was investigated in DMSO solutions and at mercury electrodes. The lability of mercury(II) complexes and their rapid interaction with elemental mercury strongly influence the nature of the redox processes observed at mercury electrodes. Reduction of Hg(ACD)2at a mercury electrode occurs in an overall two-electron step as:[Formula: see text]although mercury(I) is implicated as an intermediate. Key words: reduction, electrochemical techniques, mercury complexes.
Decomposition of tosylhydrazones of benzoin, benzoin acetate, and benzoin benzoate with alkali and metal complex hydrides
Authors: Iwadare Tsukasa , Ichinohe Yoshiyuki , Orito Kazuhiko
Published: May 2006

Treatment of tosylhydrazones of benzoin, benzoin acetate, and benzoin benzoate with alkali under protic and aprotic conditions yielded diphenyl acetylene together with desoxybenzoin. An increase in leaving aptitude of the adjacent group enhanced the formation of diphenyl acetylene. By treatment with LiAlH4 and with NaBH4, the tosylhydrazones gave stilbenes in good yields. Selective formation of cis- or trans-stilbene was observed in some cases. Key words: tosylhydrazone, benzoin derivatives, decomposition, metal complex hydrides.
2,3-Pyrazinedicarboxylates of cobalt(II), nickel(II), and copper(II); magnetic properties and crystal structures
Authors: Mao Long , Rettig Steven J. , Thompson Robert C. , Trotter James , Xia Shihua
Published: May 2006

Complexes of singly and doubly deprotonated 2,3-pyrazinedicarboxylic acid (pyzdcH2) have been prepared and studied. Crystal data: [Ni(pyzdcH)2(H2O)2], 1, monoclinic, P21/n, a = 9.2897(8) Å, b = 7.6931(8) Å, c = 10.1044(9) Å, β = 93.378(7)°,Z = 2; [Cu(pyzdcH)2]x•2xH2O, 2, monoclinic, P21/n, a = 6.598(1) Å, b = 14.198(2) Å, c = 8.586(2) Å, β = 97.65(2)°, Z = 2;• [Co(pyzdc)(H2O)2]x•2xH2O, 3, monoclinic, C2/c, a = 12.5777(9) Å, b = 7.5191(9) Å, c = 11.839(1) Å, β = 110.754(4)°, Z = 4; [Ni(pyzdc)(H2O)2]x•2xH2O, 4, monoclinic, C2/c, a = 12.4772(7)Å, b = 7.452(1) Å, c = 11.9166(5) Å, β = 112.302(2)°, Z = 4; [Cu(pyzdc)(H2O)2]x•2xH2O, 5, monoclinic, C2/c, a = 12.784(2) Å, b = 7.527(2) Å, c = 12.323(2) Å, β = 116.150(2)°, Z = 4. The structures were solved by Patterson (1), direct (2, 3), or isomorphous replacement methods (4 and 5 are isomorphous with 3) and were refined by full-matrix least-squares procedures to R = 0.028, 0.030, 0.029, 0.030, and 0.030 (Rw = 0.026, 0.029, 0.027, 0.025, and 0.027) for 1894, 1694, 2476, 1867, and 1412 reflections with I ≥ 3σ(F2), respectively. The structure of 1 consists of monometallic [Ni(pyzdcH)2(H2O)2] units linked by hydrogen-bonding interactions in a three-dimensional network. In 2 extended chains of copper ions are linked by double-bridging pyzdcH− groups while 3, 4, and 5 have structures in which pyzdc2− ligands bridge metal centers to form extended linear chains. Magnetic susceptibility studies reveal antiferromagnetic interactions in 1, 2, 3, 4, and 6 (6 is Cu(pyzdc)). The magnitude of exchange (−zJ = 0.033 cm−1 for 1 and −J = 0.11, 0.74, 0.049, and 1.14 cm−1 for 2, 3, 4, and 6, respectively) is discussed in terms of the structures and d-electron configurations involved. Key words: metal 2,3-pyrazinedicarboxylates, synthesis, X-ray structure, antiferromagnetism.
Estimation of the fines content of Athabasca oil sands using instrumental neutron activation analysis
Authors: Donkor Kingsley K. , Kratochvil Byron , Duke M. John M.
Published: May 2006

Knowledge of the "fines" or clay-rich mineral fraction is necessary for the efficient extraction of bitumen from bulk oil sands, and has environmental implications in tailings disposal. Current methods for estimating the fines in oil sand are laborious and of questionable accuracy. We propose here their estimation directly in bulk oil sand by instrumental neutron activation analysis (INAA) of the trace and major elements determinable. Correlations between fines content and individual elements fall in the range of 0.80 to 0.81 for dysprosium, samarium, and europium, and 0.75 to 0.77 for sodium, potassium, aluminum, and titanium. These results follow the expected geochemical relationships between major and trace elements and the mineral fraction of the oil sands. Principal component analysis (PCA) using a combination of elements gave correlations with fines content no better than those obtained from individual elements. However, the PCA approach can be considered more robust. Advantages of INAA over conventional methods to estimate the fines content of unprocessed oil sand include minimal sample preparation (including elimination of the use of organic solvents), rapid turnaround time, and the potential for automation. Key words: instrumental neutron activation analysis, oil sand, elemental correlations, fines.
An ab initio study on the insertion reactions of CH [X2II] with NH3, H2O, and HF
Authors: Wang Zhi-Xiang , Liu Ruo-Zhuang , Huang Ming-Bao , Yu Zhonghua
Published: May 2006

The mechanisms of the reactions of CH (X2II) with NH3, H2O, and HF have been studied by means of ab initio molecular orbital calculations incorporating electron correlation with Møller–Plesset perturbation theory up to the second order. For each of the three CH reactions, the insertion path has been found in the potential energy surface; in the calculated insertion path there exists an intermediate complex prior to the transition state that has a lower energy than the reactants. Energetic results indicate that insertion paths are favourable channels for these CH reactions, which is in line with proposals based on kinetic experiments. Key words: CH radical, ammonia, water, hydrogen fluoride, reaction mechanism.
Cyclobutene photochemistry. Adiabatic photochemical ring opening of alkylcyclobutenes
Authors: Leigh William J. , Postigo J. Alberto , Zheng K.C.
Published: May 2006

The photochemistry of the cis and trans isomers of a series of dimethylbicyclo[n 2.0]alk-(n + 2)-enes (n = 2–5) (bicyclic cyclobutene derivatives in which the C=C bond is shared by the two rings) in pentane solution is described. Irradiation of these compounds using monochromatic 193- or214-nm light sources results in ring opening to yield the corresponding 1,2-bis(1-ethylidene)cycloalkanes (C4–C7) in high chemical and quantum yields. In all cases, the reaction proceeds with a high (70–90%) degree of disrotatory stereoselectivity. Quantum yields for direct cis,trans photoisomerization of the isomeric E,E-and E,Z-1,2-bis(1-ethylidene)cycloalkanes have also been determined. The product distributions from irradiation of the cyclobutenes are wavelength dependent, but for 214-nm excitation the isomeric diene distributions obtained from cyclobutene ring opening agree fairly closely with those calculated from the quantum yields for cis,trans photoisomerization of the isomeric dienes on the assumption that the process involves purely disrotatory ring opening to yield a single diene isomer in the lowest excited singlet state. The results are consistent with an orbital-symmetry-controlled, adiabatic mechanism for ring opening. Key words: photochemistry, cyclobutene, electrocyclic, adiabatic, conical intersection, orbital symmetry.
Local energy and chemical potential equations and the exchange-correlation potential
Authors: March N.H.
Published: May 2006

Local energy and chemical potential equations are considered in some detail in relation to low-order density matrices. Some asymptotic properties can be extracted in exact form. The spatial derivative of the chemical potential equation referred to above yields the external force, defined as the (negative of the) gradient of the potential energy of the nuclear framework. This quantity, by utilizing the differential virial theorem, can be expressed as a sum of three terms: (i) a Laplacian contribution known explicitly in terms of the ground-state electron density; (ii) a kinetic part derivable from the "near-diagonal" behaviour of the first-order density matrix; and (iii) a term from electron–electron interactions, that involves the electronic pair correlation function. Following the work of Holas and March, this allows the exchange-correlation potential of density functional theory to be expressed in terms of low-order density matrices. Finally, scaling of electron–electron interactions is briefly considered, as well as the adiabatic connection formula in density functional theory. Such scaling arguments lead to a kinetic correction to the Harbola–Sahni form of the exchange-only potential. Key words: external force, first-order density matrix, electronic pair function.
Coherent control of electrons in molecules
Authors: Bandrauk Andre D. , Yu Hengtai , Aubanel Eric E.
Published: May 2006

Coherent superposition of electronic states can be achieved by simultaneous laser excitation at different frequencies. As an example, the three-level system is examined in order to demonstrate the possibility of phase control of electron transfer in molecules. Ab initio calculations are used to illustrate the principle in a charge transfer molecule DMBAN, 4-(N,N-dimethylamino)benzonitrile. Key words: laser control, charge transfer electrons.
Topological studies of the charge density of some group 2 metallocenes M(η5-C5H5)2 (M = Mg or Ca)
Authors: Bytheway Ian , Popelier Paul L.A. , Gillespie Ronald J.
Published: May 2006

Ab initio quantum chemical calculations using both the Hartree–Fock and the B3LYP density functional theory methods have been performed for the group 2 metallocenes M (η5-C5-C5H5)2 (M = Mg or Ca). The topology of the calculated charge density (ρ) and its Laplacian [Formula: see text] have been analysed using a new critical point search algorithm in order to understand why Mg(η5-C5-H5)2 is linear while Ca(η-C5H5)2 is bent. The Laplacian of the core of the Ca atom in Ca(η5-C5H5)2 is perturbed by the polarizing field of the cyclopentadienyl ligands and the bent geometry is a consequence of the interactions between the distorted core and the ligand atoms. In the case of Mg(η5-C5H5)2, charge concentration maxima in the Mg core occur along the vectors connecting the metal to the centroids of the cyclopentadienyl ligands irrespective of whether or not the molecule is linear, and the preferred geometry is linear as expected. The results of these calculations demonstrate that the geometries of the group 2 metallocenes can be understood in terms of the repulsive interactions between the ligands and between the ligands and the distorted core of the metal atom. Key words: atoms in molecules, group 2 metallocenes, VSEPR, charge density, Laplacian of ρ.
Anisotropic and isotropic triple-dipole dispersion energy coefficients for all three-body interactions involving He, Ne, Ar, Kr, Xe, H2, N2, and CO
Authors: McDowell Sean A.C. , Kumar Ashok , Meath William J.
Published: May 2006

Formulae for the computation of isotropic and anisotropic dipolar dispersion energy coefficients, for two-body and three-body interactions involving H2, N2, CO, and the rare gases, are presented in an average energy approximation. These coefficients are computed to within 1% of the reliable values for these coefficients, which are obtained by using the relevant dipole oscillator strength distributions, with the exception of a few that are recorded in tabular form. The input data required for these formulae are the isotropic and anisotropic polarizabilities and average energies for the interacting species. The results provide the first reliable anisotropic triple-dipole dispersion energy coefficients for interactions involving molecules. Key words: non-additive, anisotropic, interaction energies, triple-dipole dispersion energies.
Oxidative damage to the glycyl α-carbon site in proteins: an ab initio study of the C—H bond dissociation energy and the reduction potential of the C-centered radical
Authors: Armstrong D.A. , Yu D. , Rauk A.
Published: May 2006

The C—H bond dissociation energies (DC—H) of a series of model glycyl proteins were derived from selected isodesmic reactions based on high level ab initio calculations. At 298 K, the recommended values of DC—H, in kJ mol−1 are: NH2CH2CHO, 308; NH2CH2C(O)NH2, 336; HC(O)NHCH2CHO, 331; HC(O)NHCH2C(O)NH2, 350; CH3C(O)NHCH2C(O)NH2, 347; HC(O)NHCH2C(O)NHCH3, 349; and CH3C(O)NHCH2C(O)NHCH3, 346. The average of the last four values, 348 kJ mol−1, is the predicted bond dissociation energy of the α-C—H bond of a glycyl protein. The reduction potential in aqueous medium at 298 K and pH 7 for the process, R• + H+ + e− = RH, where R• = XNHCH•C(O)Y and X and Y represent extension of the protein chain, is E0′ 0.8 V. This result suggests that the α-C—H bond of a glycyl protein is susceptible to attack by RS•, ROO•, tyrosyl, and OH• radicals, whose reduction potentials for the analogous process are higher. The present study has established that the molecule HC(O)NHCHRC(O)NH2 (R = an amino acid side chain) serves as an accurate model for the α-C environment of an amino acid residue in a protein, and that a reliable DC—H value for the α-C—H bond may be obtained from calculations on this model at the B3LYP/6-31G(D) level of theory in conjunction with an isodesmic reaction using neutral glycine as reference. Key words: glycine, peptide, amino acid, bond energy, radicals, ab initio, computation.
Interpretation of neighboring group interactions in crystal structures. A solid state and quantum-chemical study of "incipient nucleophilic attack" in 2-diazonium benzoic acid and its benzoate
Authors: Glaser Rainer , Horan Christopher J.
Published: May 2006

The concept of the Bürgi–Dunitz angle of attack on carbonyls is compatible with the electronic structure of carbonyls. However, it is argued here that the generalization asserted to describe the interaction of nucleophiles with diazonium ions is inappropriate. Distortions in crystal structures of diazonium ions with proximate nucleophiles were interpreted by an incipient nucleophilic attack (INA) on the formally positive-charged Nα. This "Nα attraction model" relies on the assumption that the formal charge in the most commonly used Lewis structure of diazonium ions represents actual charge. We proposed that the close approach of the proximate nucleophile to the diazonium group occurs to enhance attractive 1,3-(C,Nβ)-bridging interactions and despite repulsion between Nα and the proximate oxygen (Opr). The present study combines theoretical analysis of rotamers of 2-diazonium benzoic acid and its conjugate base with experimental results on the same system to provide compelling evidence that the more general conclusions drawn from analyses of neighboring group interactions in propenoic acid models are fully warranted. The crystallographic record is more fully consistent with the "1,3-bridging attraction model." Combined analysis of solid state and gas phase structures reveals the intrinsic features due to INA. Both electrostatic models can account for these features but with different postulates about the electron density distribution. While the structural analysis alone cannot distinguish between the alternative interpretations, the study of the electronic structure allows one to clearly differentiate between these competing interpretations. A method (ESI) for the quantitative evaluation of electrostatic neighboring group interactions has been devised and this ESI concept employs atomic electrical moments (charges, dipoles, and quadrupoles) determined via topological electron density analysis. The results of the ESI analysis support the 1,3-bridging attraction model and eliminate the Nα attraction model. Key words: electrostatic interactions, electron density analysis, atoms in molecules. X-ray crystallography, ab initio molecular orbital theory, incipient nucleophilic attack, bonding models, ESI analysis.
Furo-fused 2H-chromenes: synthesis and photochromic properties
Authors: Pozzo Jean-Luc , Samat André , Guglielmetti Robert , Lokshin Vladimir , Minkin Vladimir
Published: May 2006

New photochromic chromenes annulated with a furan ring have been synthesized. Thus, suitable heterocyclic phenols react with different propargylic alcohols in acidic medium to give the corresponding ethers, which cyclize into benzopyrans by thermal Claisen rearrangement. This synthetic approach was found to lead to a mixture of linear and angular chromenes that is strictly related to the nature of the phenol. However, regiospecificity could be obtained by reacting β-phenylcinnamaldehyde, in refluxing aprotic nonpolar solvents, with titanium(IV) salts of the former phenols. Electrocyclization of intermediately generated o-quinoid structures occurs on the α position towards the heterocyclic junction. All compounds exhibit photochromic behavior at room temperature. Furo-fused benzopyrans are particularly interesting with respect to naphthopyran parents in view of the bathochromically shifted and broadened absorption spectra of photoinduced forms. This trend is confirmed by the spectral data of several heterocyclospiro(7H-furo[3,2-f] chromenes). The achievable color depends significantly on the relative position of annulation on the chromenic moiety and substitution on the sp3 carbon atom. Key words: photochromism, heterocycle, regiospecificity, 2H-chromene, furan.
Chiral synthesis of (+)-8′-demethyl abscisic acid
Authors: Rose Patricia A. , Lei Bo , Shaw Angela C. , Abrams Suzanne R. , Walker-Simmons M. K. , Napper Scott , Quail J. Wilson
Published: May 2006

An enantioselective synthesis of (+)-8′-demethyl ABA (2) is described. The chiral intermediate 7 was prepared by yeast reduction of a substituted monoprotected cyclohexa-2,5-dien-1,4-dione (9) synthesized through a phenol oxidation. The scope and limitations of the phenol oxidation is described. 8′-Demethyl ABA shows ABA-like activity in wheat embryo germination inhibition, showing that the 8′-methyl group is not essential for biological activity. Key words: abscisic acid, phenol oxidation, yeast reduction.
Electrochemical study of the reaction between progressively alkylated thiazine leucodyes and Fe(III) on a glassy carbon electrode
Authors: Ahmed S. , Saha S.K.
Published: May 2006

An electrochemical investigation on five progressively alkylated thiazine dyes in the presence of Fe(III) ions is reported. The theory of the catalytic regeneration mechanism involving an electrode reaction followed by a coupled chemical reaction is applied to derive kinetic parameters of homogeneous reaction. The second-order rate constant for the reaction of thiazine leucodyes with Fe(III) ions was found to increase from 0.25 × 104 to 1.6 × 104 dm3 mol−1 s−1 upon monomethylation and to vary from 0.7 × 104 for the dimethyl derivative to 1.4 × 104 dm3mol−1 s−1 for the tetramethyl one. The electron-donating nature as well as the hydrophobic characteristics of the methyl group influence the kinetics of the homogeneous reaction. Key words: cyclic voltammetry, thiazine dyes, ferric ion, catalytic regeneration, kinetics
The energetics of the oxidative addition of trisubstituted silanes to photochemically generated (η5-C5R5)Mn(CO)2
Authors: Palmer Bentley J. , Hill Ross H.
Published: May 2006

The rates for the oxidative addition reaction of trisubstituted silanes (Et3SiH, Et2MeSiH, EtMe2SiH, Et2SiH2) to photochemically generated (η5-C5R5)Mn(CO)2(R5 = H5, Me5, H4Me) species have been measured for the temperature range 70–125 K. The reactions were carried out in either neat silane or a 50/50, by volume, mixture of methylcyclohexane and silane. The activation energies, determined using Arrhenius law, varied from 2 to 35 kj/mol. The kinetic data fit an isokinetic relationship with an isokinetic temperature of 102 ± 6 K. The results are interpreted in terms of a variation in the loss of solvation prior to the oxidative addition. When the solvating molecule is methylcyclohexane, then loss of the solvent molecule precedes oxidative addition. In cases where solvation is by the silane, the incomplete loss of this silane precedes the oxidative addition. Key words: mechanism, oxidative addition, solvation.
Absorption spectroscopy of (Ind)Ni(PPh3)X (Ind = indenyl, 1-Me-indenyl; X = Cl, Br, Me) and M(Ind)2 (M = Ni, Ru; Ind = indenyl)
Authors: Bayrakdarian Malken , Davis Martin J. , Reber Christian , Zargarian Davit
Published: May 2006

The electronic structures of two types of transition metal indenyl complexes have been studied. The first type, a series of (Ind)Ni(PPh3)X compounds (Ind = indenyl, 1-Me-indenyl; X = Cl, Br, Me) was investigated by absorption spectroscopy and Extended Hückel Molecular Orbital calculations. The energy differences between calculated levels are in good agreement with experimental band positions. For example, the lowest energy singlet–singlet band maximum for (Ind)Ni(PPh3)Cl is at 19 500 cm−1 and the calculated HOMO–LUMO difference is 19 817 cm−1. For X = Me, the calculated energy difference increases to 21 930 cm−1 and the corresponding absorption band is at 22 500 cm−1. The influence of the metal–ligand interactions on the molecular orbitals is discussed. The second category of indenyls, the bis(indenyl) compounds of Ni and Ru, show absorption spectra that are markedly different from those of nickelocene and ruthenocene. For example, in comparison to nickelocene, the first absorption band of Ni(Ind)2 is 5700 cm−1 higher in energy and is more intense by two orders of magnitude; in contrast, the first absorption maximum of Ru(Ind)2 is 6600 cm−1 lower in energy than observed for ruthenocene. The characteristics and relaxation dynamics of the lowest energy excited states are discussed. Key words: absorption spectroscopy, indenyl, nickel, ruthenium, EHMO analysis.
Variable-energy photoelectron spectroscopy of CpM(CO)3 (M = Mn, Re) and CpFe(CO)2I
Authors: Hu Yong-Feng , Bancroft G. Michael , Tan Kim H. , Tse John S. , Yang Dong-Sheng
Published: May 2006

Photoelectron spectra of the valence, inner-valence, and core levels of CpM(CO)3 (M = Mn, Re) and CpFe(CO)2I were obtained using He I radiation and synchrotron radiation between 21 eV and 150 eV photon energies. The high-resolution Mn 3p, Fe 3p, and I 4d core level spectra are reported. Broadening of the I 4d spectrum is due to vibrational and ligand field splittings. Observation of the 3p spectra is critical to fully interpreting the np → nd resonance effect in the valence band spectra. This resonance effect is very useful for assigning the metal d orbital bands in the valence spectra. A Xα-SW calculation of CpMn(CO)3 was used to confirm the assignment of the valence and inner-valence orbitals. The experimental and theoretical branching ratios are in good agreement. The general trend of the metal d orbital ionization potentials for CpML3 (M = transition metal) complexes is also discussed. Key words: photoelectron, synchrotron radiation, CpMn(CO)3, CpRe(CO)3, CpFe(CO)2I, np → nd resonance, Xα-SW calculation.
In situ FTIR studies of 4-cyanopyridine adsorption at the Au(111) electrode
Authors: Chen A.C. , Yang D.F. , Lipkowski J. , Sun S.G. , Pettinger B.
Published: May 2006

In situ electromodulated reflectance Fourier transform infrared (FTIR) spectroscopy has been employed to study the adsorption of 4-cyanopyridine (4-CNPy) at an Au(111) electrode surface. The vibrational spectra have been used to study (i) the dependence of the band intensity on the surface coverage, (ii) the character of surface coordination, and (iii) the stability of adsorbed 4-CNPy molecules. It has been observed that the vibrational bands in the spectra acquired in the electroreflection experiment are significantly broader than the corresponding spectra acquired in a transmission cell. Some weaker bands seen in the spectra recorded in transmission were not observed in the electroreflectance experiment. The electroreflectance spectra were dominated by the two ring deformation bands observed at 1416 cm−1 and 1554 cm−1. The intensities of these bands correlated well with the surface concentrations of 4-CNPy molecules determined from independent electrochemical studies. The integrated absorption intensities of the bands recorded in the electroreflection experiments were larger by a factor of five than the absorption intensities measured in the transmission cell. This indicates that the electric field of the photon acting on a molecule, present in front of the gold electrode, is significantly enhanced by reflection from the electrode. The infrared experiments suggest that at positive potentials the 4-CNPy molecules are coordinated to the metal surface through the nitrogen atom of the aromatic ring. The 4-CNPy molecules are oxidized at the Au electrode at potentials higher than 0.6 V (SCE) and are reduced to form (4-CNPy)−• ion at potentials lower than −1.1 V (SCE). Key words: adsorption, infrared spectroscopy, 4-cyanopyridine, gold electrode.
Solvent and substrate isotope effects on the enolization and carbon-acid ionization of isobutyrophenone
Authors: Keeffe J.R. , Kresge A.J.
Published: May 2006

Bromine scavenging was used to measure rates of acid-catalyzed enolization of isobutyrophenone in H2O and in D2O solution and of isobutyrophenone-α-d in D2O solution. The results provide the solvent isotope effect kH +/kD + = 0.56 and the substrate isotope effect kH/kD = 6.2 on the enolization reaction, both of which are consistent with the generally accepted mechanism for this process. The present results in combination with literature information also provide the solvent isotope effect on the enolization equilibrium, KE(H2O)/KE(D2O) = 0.92, and the solvent isotope effect on the ionization of isobutyrophenone as a carbon acid, kaK(H2O)/kaK(D2O) = 5.4, as well as the product of isotopic fractionation factor and medium effect, [Formula: see text], for isobutyrophenone enol and the medium effect, Φ = 0.47, for its enolate ion. The isotope effect on KE is the first ever determined for the keto–enol equilibrium of a simple aldehyde or ketone; its near-unit value is consistent with expectation on the basis of fractionation factors for the species involved. Key words: isobutyrophenone, keto–enol equilibrium, carbon-acid ionization, solvent isotope effects, isotopic fractionation factors.
Photolithography of self-assembled monolayers: optimization of protecting groups by an electroanalytical method
Authors: Jennane Jamila , Boutros Tanya , Giasson Richard
Published: May 2006

Patterned surfaces presenting a high density of chemically reactive functional groups can be prepared through photolithography of self-assembled monolayers (SAM). In this paper, we report the synthesis and the evaluation of three reagents that can be used in SAM-photolithographic applications. These reagents are made up of a triethoxysilylpropylamine moiety in which the amine is temporarily blocked by photolabile protecting groups: NVOC (o-nitroveratryloxycarbonyl), ONB (o-nitrobenzyl), or DDZ (α,α-dimethyl-3,5-dimethoxybenzyloxycarbonyl). The presence of the triethoxysilyl group allows self-assembled monolayer formation. Release of chemically reactive amino groups is achieved by irradiation of the surface. An electroanalytical method was developed and used to monitor and optimize the three steps of the methodology occurring on surfaces: monolayer formation, photodeprotection, and subsequent functionalization of the released amino groups. Quantitative information on the efficiency of the photodeprotection step was obtained by this method. It was found that the DDZ group is superior to the ONB and NVOC photolabile protecting groups for SAM-photolithographic applications. Percentages of liberated amino groups are generally above 50% with the DDZ group, while they are generally inferior to 25% and 2% for NVOC and ONB, respectively. These differences are attributed to the destruction of some of the released amino groups through a subsequent reaction with the photoproduct, o-nitrosobenzaldehyde for ONB and 3,4-dimethoxy-6-nitrosobenzaldehyde for NVOC, and to partial loss or destruction of the monolayer during prolonged irradiations. Key words: self-assembled monolayers, surface modification, photolithography, photolabile protecting groups, cyclic voltammetry.
A photochemical retro-Friedel–Crafts alkylation. Rapid rearrangement of cyclohexadienyl cations
Authors: Macknight Earl , McClelland Robert A.
Published: May 2006

This paper reports the use of laser flash photolysis (LFP) techniques to show that cyclohexadienyl cations (σ complexes) of the Friedel–Crafts reaction of 1,3-dimethoxybenzene and the diphenylmethyl cation rearrange on the ns time scale without separating the aromatic compound and the electrophile. This is demonstrated through a study of the photochemical behaviour of 2-diphenylmethyl-1,3-dimethoxybenzene (4) in 1,1,1,3,3,3-hexafIuoroisopropyl alcohol (HFIP). Derivatives of 1,3-dimethoxybenzene have previously been found to selectively protonate at C2 upon excitation in HFIP, and indeed the principal products with 4 are 1,3-dimethoxybenzene (6) and Ph2CHOCH(CF3)2 (7), the species expected if the cyclohexadienyl cation formed in the C2 protonation cleaved Ph2CH+. These products are, however, accompanied by 4-diphenylmethyl- 1,3-dimethoxybenzene (8), a rearranged isomer of 4. A portion of this product is explained by the combination of Ph2CH+ and 1,3-dimethoxybenzene as the latter accumulates during the irradiation. However, 11.5% of 8 is also seen upon extrapolation to zero time. LFP experiments on the ps time scale reveal that the C2 protonated cation, the 1-diphenylmethyl-2,6-dimethoxybenzenium ion (5), is formed within 100–200 ps, and reacts with k = 9 × 108 s−1, with absorbance for Ph2CH+ growing in as 5 decays. LFP studies on the ns time scale reveal that there is a second quantity of Ph2CH+ that grows in, with k = 5.0 × 105 s−1. The precursor for this has been identified as the 1-diphenylmethyl-2,4-dimethoxybenzenium ion (10), the thermodynamically more stable isomer of 5. A mechanistic model is proposed in which excited 4 is C2 protonated in HFIP with k ≥ 1 × 1010 s−1 to form 5, which loses Ph2CH+ with k = 3 × 108 s−1 in competition with rearrangement to 10 with k = 6 x 108 s−1. The cation 10 serves as the second source of Ph2CH+, losing Ph2CH+ with k = 4 × 105 s−1; in competition 10 is deprotonated by HFIP to give 8 with k = 8 × 104 s−1. The 11.5 % of the rearranged 8 that is observed at zero conversion is thus shown to come from an intramolecular pathway in which the key step is the migration of a diphenylmethyl group without separation: 4 → 4* → 5 → 10 → 8. Key words: cyclohexadienyl, photoprotonation, Friedel–Crafts, diphenylmethyl, 1,3-dimethoxybenzene.
Solvent effects on nucleophilic substitution reactions. III. The effect of adding an inert salt on the structure of the SN2 transition state
Authors: Pham T.V. , Westaway K.C.
Published: May 2006

The nitrogen and secondary α-hydrogen–deuterium kinetic isotope effects found for the SN2 reaction between thiophenoxide ion and benzyldimethylphenylammonium ion at different ionic strengths in DMF at 0 °C indicate that the structure of the transition state changes markedly with the ionic strength of the reaction mixture. In fact, a more reactant-like, more ionic, transition state is found at the higher ionic strength. This presumably occurs because a more ionic transition state is more stable in the more ionic solvent. Key words: transition state, ionic strength, secondary α deuterium kinetic isotope effects, nitrogen isotope effects, SN2.
1996 Polanyi Award Lecture Proton reactions in clusters
Authors: Kapral Raymond , Consta Styliani , Laria Daniel
Published: May 2006

Reactions in mesoscopic, molecular clusters may proceed by mechanisms and with rates that differ from those in bulk solvents. Two examples of reactions in large, liquid-state, molecular clusters are described to illustrate the distinctive features of these reactions: acid dissociation and proton transfer in aprotic, polar solvents. Both of these reactions involve proton dynamics so methods for dealing with mixed quantum–classical systems must be utilized to investigate the reaction dynamics. Surface versus bulk solvation effects play an important role in determining the reaction mechanisms as do the strong cluster fluctuations. Mechanisms for proton transfer within clusters that have no bulk analogs will be described. Keywords: proton reactions, mesoscopic clusters.