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Hydrogen Atoms (hydrogen + atom)
Terms modified by Hydrogen Atoms Selected AbstractsChemInform Abstract: A Straightforward Synthesis of Some Fused Aza-Arenes via Nucleophilic Displacement of a Ring Hydrogen Atom in Nitroarenes by Aromatic Hydrazone Anions.CHEMINFORM, Issue 30 2002Koji Uehata Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] A Stacking Interaction between a Bridging Hydrogen Atom and Aromatic , Density in the n -B18H22,Benzene SystemCHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2006Ewan J. M. Hamilton Prof. Abstract The structures of n -B18H22 and of n -B18H22,C6H6 were determined by single-crystal X-ray analysis at ,60,°C. The geometry of the boron cluster itself does not seem to be appreciably affected by solvation. There does, however, appear to be an unusual interaction of a polyborane bridging hydrogen atom with the benzene , system, giving rise to an extended stacked structure. The 1H{11B} spectrum of n -B18H22 in [D6]benzene differs from that in [D12]cyclohexane most noticeably in the bridging proton region. Upon moving from the aliphatic to the aromatic solvent, the greatest increase in shielding was for the signal corresponding to the bridge hydrogen atom that interacts with the , system of benzene; the signal was shifted upfield by 0.49 ppm. Density functional theory calculations were performed on 1:1 and 2:1 complexes of the n -B18H22 unit with benzene. [source] Zinc Chloride Enhanced Arylations of Secondary Benzyl Trifluoroacetates in the Presence of ,-Hydrogen Atoms,ANGEWANDTE CHEMIE, Issue 36 2010Hui Duan Zweimal Zink: Mit der Titelreaktion unter Anwendung von Arylzinkreagentien konnten elektronenreiche und elektronenarme Arensubstrate under milden Bedingungen aryliert werden. Dieses Verfahren bietet einen potenziellen Syntheseweg für diverse Diarylalkanmotive. TFA = Trifluoracetyl. [source] Nanotexture Switching of Single-Layer Hexagonal Boron Nitride on Rhodium by Intercalation of Hydrogen Atoms,ANGEWANDTE CHEMIE, Issue 35 2010Thomas Brugger Dr. Mit H eben, ohne H gewellt: Bei der Behandlung einer Monoschicht aus hexagonalem Bornitrid auf Rhodium mit Wasserstoffatomen verschwindet die h -BN-Oberflächenwellung (siehe Bild: blauer Bereich gewellt, oranger Bereich flach). Diese Veränderung der Oberflächentextur, eine Folge der Interkalation von Wasserstoffatomen, lässt sich rückgängig machen, indem die Wasserstoffatome durch Erhitzen auf etwa 600,K entfernt werden. [source] The Reaction of Hydrogen Atoms with Methionine Residues: A Model of Reductive Radical Stress Causing Tandem Protein,Lipid DamageCHEMBIOCHEM, Issue 11 2006Carla Ferreri Dr. Abstract The occurrence of tandem damage, due to reductive radical stress involving proteins and lipids, is shown by using a biomimetic model. It is made of unsaturated lipid vesicle suspensions in phosphate buffer in the presence of methionine, either as a single amino acid or as part of a protein such as RNase A, which contains four methionine residues. The radical process starts with the formation of H. atoms by reaction of solvated electrons with dihydrogen phosphate anions, which selectively attack the thioether function of methionine. The modification of methionine to ,-aminobutyric acid is accompanied by the formation of thiyl radicals, which in turn cause the isomerization of the cis fatty acid residues to the trans isomers. The relationship between methionine modification and lipid damage and some details of the reductive radical stress obtained by proteomic analysis of irradiated RNase A are presented. [source] ChemInform Abstract: Hydrated Electron Production by Reaction of Hydrogen Atoms with Hydroxide Ions: A First-Principles Molecular Dynamics Study.CHEMINFORM, Issue 44 2008Jean Philippe Renault Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] How Many Hydrogen Atoms Can Be Bound to a Metal?CHEMINFORM, Issue 4 2005Predicted MH12 Species Abstract For Abstract see ChemInform Abstract in Full Text. [source] Reactions of Hydrogen Atoms with Met-Enkephalin and Related PeptidesCHEMISTRY - A EUROPEAN JOURNAL, Issue 7 2007Olivier Mozziconacci Abstract The reactions of hydrogen atoms with enkephalins and related peptides have been investigated by radiolytic methods in aqueous solutions and lipid vesicle suspensions. Pulse radiolysis experiments indicate that methionine residue (Met) is the main target. In Met-enkephalin (Tyr-Gly-Gly-Phe-Met) the attack of the hydrogen atom occurs to about 50,% on Met with formation of methanethiyl radical. The remaining percentage is divided roughly evenly between Tyr and Phe. With Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu) the site of attack is limited to Tyr and Phe. Using a peptide,liposome (that is, 1-palmitoyl-2-oleoyl phosphatidylcholine vesicles) model, the cis,trans isomerization of phospholipids could be detected due to the catalytic action of thiyl radicals. The radiation chemical yields of the H. and, consequently of CH3S. radical, was modulated by the experimental conditions and the nature of peptide. Large amounts of trans lipids observed in phosphate buffer vesicle suspensions indicated the efficient role of double-bond isomerization as marker of Met-containing peptide damage. [source] The hydrogen atom, revisited: Parallel-field magnetic resonance,CONCEPTS IN MAGNETIC RESONANCE, Issue 5 2006John A. Weil Abstract Consideration of the magnetic resonance spectroscopy of the free hydrogen atom (1H0) reveals that, in addition to the well-known transitions (EPR and NMR) occurring between the four spin states when the excitation magnetic field B1 is perpendicular to the static external field B, there exists a combination line (simultaneous electronic and nuclear spin flips) when B1 , B, which becomes strong under some circumstances. The latter phenomenon is focused on and discussed herein, in some detail. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part A 28A: 331,336, 2006 [source] Dependency of Tritium Retention in Graphite on Temperature Control of Molecular DynamicsCONTRIBUTIONS TO PLASMA PHYSICS, Issue 3-5 2010A. Ito Abstract We have investigated the carbon plasma facing material and hydrogen atom interaction by the use of molecular dynamics simulation to clarify chemical erosion processes on divertor plate. The present paper is our first try at elucidation of temperature dependence by the molecular dynamics. Temperature was controlled by using Langevin thermostat method. As a result, the retention of hydrogen atom achieve steady state, and the CH4 was generated, which was not found MD simulations without a temperature control method. About 30 percent of injected hydrogen atoms are retained. CH4 yields has a peak at 600 K, which accords with experimental results. A dominant path of CH4 generation found by the present molecular dynamics simulation is as follows: a CH is detached from eroded surface and then it grows into CH4 adsorbing hydrogen atoms via CH2 and CH3. In addition, we propose the problem that the hydrogen atom retention and CH,, yields depend on the thermal relaxation time in MD simulation using temperature control methods (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photochemistry of Salicylaldoxime in Solid Argon: An Experimental and Theoretical StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 27 2010Joanna Grzegorzek Abstract The photochemistry of salicylaldoxime in solid argon has been investigated by FTIR spectroscopy and DFT calculations. The salicylaldoxime molecule trapped in the matrix from the vapor above the solid sample has the most stable syn1 conformation with an intramolecular hydrogen bond. Irradiation (, > 320 nm) leads to conversion of the syn1 conformer into the syn3 one, in which the C(H)NOH and (C)OH groups are rotated around the C,C and C,O bonds, respectively, and the intramolecular hydrogen bond is broken. The photochemistry of syn3 involves three possible routes: (i) conversion of syn3 into anti2 conformer, this process requires rearrangement of the NOH group with respect to the C=N bond; (ii) photodissociation of salicylaldoxime into 2-cyanophenol and water, which form a hydrogen-bonded complex; and (iii) regeneration of the syn1 conformer. The third route is a very small contribution to the overall process. The study performed with [D2]salicylaldoxime indicates that the dehydration reaction of salicylaldoxime involves cleavage of the N,O bond and formation of OH and Ph(OH)C(H)N radicals in the first step. Then, the OH radical abstracts a hydrogen atom from the CH group to form 2-cyanophenol and water molecules. When the sample is exposed to the full output of the mercury lamp the 2-cyanophenol complex with water becomes the dominating product. [source] Rate constant estimation for C1 to C4 alkyl and alkoxyl radical decompositionINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2006H. J. Curran Rate coefficients for alkyl and alkoxy radical decomposition are important in combustion, biological, and atmospheric processes. In this paper, rate constant expressions for C1C4 alkyl and alkoxy radicals decomposition via ,-scission are recommended based on the reverse, exothermic reaction, the addition of a hydrogen atom or an alkyl radical to an olefin or carbonyl species with the decomposition reaction calculated using microscopic reversibility. The rate expressions have been estimated based on a wide-range study of available experimental data. Rate coefficients for hydrogen atom and alkyl radical addition to an olefin show a strong temperature curvature. In addition, it is found that there is a correlation between the activation energy for addition and (i) the type of atom undergoing addition and (ii) whether this radical adds to the internal or terminal carbon atom of the olefin. Rate coefficients for alkoxy radical decomposition show a strong correlation to the ionization potential of the alkyl radical leaving group and on the enthalpy of reaction. It is shown that the activation energy for alkyl radical addition to a carbonyl species can be estimated as a function of the alkyl radical ionization potential and enthalpy of reaction. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 250,275, 2006 [source] Gas-phase radical,radical recombination reactions of nitroxides with substituted phenyl radicalsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2004J. L. Heidbrink Fourier-transform ion cyclotron resonance mass spectrometry has been used to examine gas-phase reactions of four different nitroxide free radicals with eight positively charged pyridyl and phenyl radicals (some containing a Cl, F, or CF3 substituent). All the radicals reacted rapidly (near collision rate) with nitroxides by radical,radical recombination. However, some of the radicals were also able to abstract a hydrogen atom from the nitroxide. The results establish that the efficiency (kreaction/kcollision) of hydrogen atom abstraction varies with the electrophilicity of the radical, and hence is attributable to polar effects (a lowering of the transition-state energy by an increase in its polar character). The efficiency of the recombination reaction is not sensitive to substituents, presumably due to a very low reaction barrier. Even so, after radical,radical recombination has occurred, the nitroxide adduct was found to fragment in different ways depending on the structure of the radical. For example, a cationic fragment was eliminated from the adducts of the more electrophilic radicals via oxygen anion abstraction by the radical (i.e., the nitroxide adduct cleaves heterolytically), whereas adducts of the less electrophilic radicals predominantly fragmented via homolytic cleavage (oxygen atom abstraction). Therefore, differences in the product branching ratios were found to be attributable to polar factors. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 216,229 2004 [source] Reaction of H + ketene to formyl methyl and acetyl radicals and reverse dissociationsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 1 2003Jongwoo Lee Thermochemical properties for reactants, intermediates, products, and transition states important in the ketene (CH2CO) + H reaction system and unimolecular reactions of the stabilized formyl methyl (C·H2CHO) and the acetyl radicals (CH3C·O) were analyzed with density functional and ab initio calculations. Enthalpies of formation (,Hf°298) were determined using isodesmic reaction analysis at the CBS-QCI/APNO and the CBSQ levels. Entropies (S°298) and heat capacities (Cp°(T)) were determined using geometric parameters and vibrational frequencies obtained at the HF/6-311G(d,p) level of theory. Internal rotor contributions were included in the S and Cp(T) values. A hydrogen atom can add to the CH2 -group of the ketene to form the acetyl radical, CH3C·O (Ea = 2.49 in CBS-QCI/APNO, units: kcal/mol). The acetyl radical can undergo ,-scission back to reactants, CH2CO + H (Ea = 45.97), isomerize via hydrogen shift (Ea = 46.35) to form the slight higher energy, formyl methyl radical, C·H2CHO, or decompose to CH3 + CO (Ea = 17.33). The hydrogen atom also can add to the carbonyl group to form C·H2CHO (Ea = 6.72). This formyl methyl radical can undergo , scission back to reactants, CH2CO + H (Ea = 43.85), or isomerize via hydrogen shift (Ea = 40.00) to form the acetyl radical isomer, CH3C·O, which can decompose to CH3 + CO. Rate constants are estimated as function of pressure and temperature, using quantum Rice,Ramsperger,Kassel analysis for k(E) and the master equation for falloff. Important reaction products are CH3 + CO via decomposition at both high and low temperatures. A transition state for direct abstraction of hydrogen atom on CH2CO by H to form, ketenyl radical plus H2 is identified with a barrier of 12.27, at the CBS-QCI/APNO level. ,Hf°298 values are estimated for the following compounds at the CBS-QCI/APNO level: CH3C·O (,3.27), C·H2CHO (3.08), CH2CO (,11.89), HC·CO (41.98) (kcal/mol). © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 20,44, 2003 [source] Highly accurate solutions for the confined hydrogen atomINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 7 2007N. Aquino Abstract The model of the confined hydrogen atom (CHA) was developed by Michels et al. 1 in the mid-1930s to study matter subject to extreme pressure. However, since the eigenvalues cannot be obtained analytically, even the most accurate calculations have yielded little more than 10 figure accuracy. In this work, we show that it is possible to obtain the CHA eigenvalues with extremely high accuracy (up to 100 decimal digits) and we do that using two completely different methods. The first is based on formal solution of the confluent hypergeometric function while the second uses a series method. We also compare radial expectation values obtained by both methods and conclude that the wave functions obtained by these two different approaches are of high quality. In addition, we compute the hyperfine splitting constant, magnetic screening constant, polarizability in the Kirkwood approximation, and pressure as a function of the box radius. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Common generating function for two-dimensional hydrogen atom complete wave functionsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2007L. Chaos-Cador Abstract The Schrödinger equation for the two-dimensional hydrogen atom is known to be separable and integrable in circular, parabolic, and elliptical coordinates. This makes it possible to construct a common generating function for the complete wave functions of the atom in the respective coordinates. The connections with the corresponding generating function and wave functions for the harmonic oscillator are recognized and applied in this work. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Two-dimensional hydrogen atom confined in circles, angles, and circular sectorsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2005L. Chaos-Cador Abstract The Schrödinger equation for the 2D hydrogen atom is separable in circular coordinates (, = , , = tan,1y/x). In this article the energy eigenvalues and eigenfunctions of such an atom in three different situations of confinement inside (a) a circle (0 , , , ,0, 0 , , , 2,), (b) an angle (0 , , , ,, 0 , , , ,0), and (c) a circular sector (0 , , , ,0, 0 , , , ,0) are explicitly constructed. Characteristic properties of the atom in its ground state for each situation of confinement such as the polarizability for (a) and electric dipole moment for (b) and (c) are also evaluated. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Continuum eigen-functions of 1-D time-independent Schrödinger equation solved by symplectic algorithmINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2005Yue-Ying Qi Abstract We present the symplectic algorithm for solving the continuum eigen-functions of 1-dimensional (1D) time-independent Schrödinger equation, which keeps Wronskian conservation of linearly independent continuum eigen-functions. This symplectic algorithm has been applied to the computation of the linearly independent continuum eigen-functions of both 1D soft-coulomb potential and Pöshl,Teller short-range potential as well as the radial continuum eigen-functions of hydrogen atom. The numerical results using the symplectic algorithm are in agreement with the existing theories. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Designer polynomials, discrete variable representations, and the Schrödinger equationINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2002Charles A. Weatherford Abstract The general procedure for constructing a set of orthonormal polynomials is given for an arbitrary positive definite weight function, w(x), in the interval [a, b]. The Lanczos method is used to generate the three-term recursion relation, which is then used to produce the polynomial coefficients. A discrete variable representation (DVR) is constructed from Gaussian nodes and weights that result from the three-term recursion relation. These are termed "designer polynomials" and the associated "designer DVRs." It will be shown by construction that every such set of "synthetic polynomials" carries an associated DVR. The term "designer" derives from the fact that the interval [a, b] and the weight function w(x) are arbitrary (except that w(x) must be positive definite on [a, b] and must have continuous derivatives except at a finite number of isolated discontinuities) and may be adapted to the physical problem of interest. The difficulties of applying a DVR to a "bare" Coulomb problem will be illustrated on a "toy" model in one dimension (1-D hydrogen atom). A solution for the 1-D Coulomb problem will be given, thereby motivating the need for designer DVRs. In doing so, a new set of polynomials is defined with a weight function w(x) = |x|kexp(,,|x|), (such that k = ,1, 0, +1, +2, ,) between the symmetrical limits [,,, +,]. These are called "synthetic Cartesian exponential polynomials (SCEP)." These polynomials are then used in a spectral and pseudospectral (DVR) representation to solve the 1-D hydrogen atom problem. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] Adsorption of water in mordenite,An ab initio studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2001Th. Demuth Abstract The adsorption of water in mordenite has been investigated using density functional theory using gradient corrections to the exchange,correlation functional. In the neutral complex water is strongly physisorbed through two different hydrogen bonds, the stronger between the acid site and the water oxygen atom, the weaker between a hydrogen atom of the adsorbed molecule and a framework oxygen atom. Strong polarizations and structural distortions of both the acid site and the molecule have been observed. To elucidate the question if a chemisorbed complex (creation of a hydroxonium ion) is possible, ab initio molecular dynamics have been performed, indicating that a protonation of water occurs even for the low coverage of one water molecule per unit cell. However, this ionic configuration cannot be stabilized by the electrostatic field of the zeolite framework and is therefore not a minimum of the potential energy surface. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 84: 110,116, 2001 [source] Highly Enantioselective Construction of the ,-Chiral Center of Amides via Iridium-Catalyzed Hydrogenation of ,,,-Unsaturated AmidesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2009Wei-Jing Lu Abstract The chiral center at the ,-position of amides is installed in excellent enantioselectivity via the iridium-catalyzed asymmetric hydrogenation of ,,,-unsaturated amides under mild conditions. Even aliphatic amides are suitable substrates. The presence of a hydrogen atom on the nitrogen of the amide is important for the enantioselectivity of the reaction. [source] Mechanisms of cytochrome P450 substrate oxidation: MiniReviewJOURNAL OF BIOCHEMICAL AND MOLECULAR TOXICOLOGY, Issue 4 2007F. Peter Guengerich Abstract Cytochrome P450 (P450) enzymes catalyze a variety of oxidation and some reduction reactions, collectively involving thousands of substrates. A general chemical mechanism can be used to rationalize most of the oxidations and involves a perfenyl intermediate (FeO3+) and odd-electron chemistry, i.e. abstraction of a hydrogen atom or electron followed by oxygen rebound and sometimes rearrangement. This general mechanism can explain carbon hydroxylation, heteroatom oxygenation and dealkylation, epoxidation, desaturation, heme destruction, and other reactions. Another approach to understanding catalysis involves analysis of the more general catalytic cycle, including substrate specificity, because complex patterns of cooperativity are observed with several P450s. Some of the complexity is due to slow conformational changes in the proteins that occur on the same timescale as other steps. © 2007 Wiley Periodicals, Inc. J Biochem Mol Toxicol 21:163,168, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jbt.20174 [source] Transition metals as electron traps.JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 10 2009Abstract Transition metal cations Co2+, Ni2+ and Zn2+ form 1 : 1 : 1 ternary complexes with 2,2,-bipyridine (bpy) and peptides in aqueous methanol solutions that have been studied for tripeptides GGG and GGL. Electrospray ionization of these solutions produced singly charged [Metal(bpy)(peptide , H)]+ and doubly charged [Metal(bpy)(peptide)]2+ ions (Metal = metal ion) that underwent charge reduction by glancing collisions with Cs atoms at 50 and 100 keV collision energies. Electron transfer to [Metal(bpy)(peptide)]2+ ions was less than 4.2 eV exoergic and formed abundant fractions of non-dissociated charge-reduced intermediates. Charge-reduced [Metal(bpy)(peptide)]+ ions dissociated by the loss of a hydrogen atom, ammonia, water and ligands that depended on the metal ion. The Ni and Co complexes mainly dissociated by the elimination of ammonia, water, and the peptide ligand. The Zn complex dissociated by the elimination of ammonia and bpy. A sequence-specific fragment was observed only for the Co complex. Electron transfer to [Metal(bpy)(peptide , H)]+ was 0.6,1.6 eV exoergic and formed intermediate radicals that were detected as stable anions after a second electron transfer from Cs. [Metal(bpy)(peptide , H)] neutrals and their anions dissociated by the loss of bpy and peptide ligands with branching ratios that depended on the metal ion. Optimized structures for several spin states, electron transfer and dissociation energies were addressed by combined density functional theory and Møller,Plesset perturbational calculations to aid interpretation of experimental data. The experimentally observed ligand loss and backbone cleavage in charge-reduced [Metal(bpy)(peptide)]+ complexes correlated with the dissociation energies at the present level of theory. The ligand loss in +CR, spectra showed overlap of dissociations in charge-reduced [Metal(bpy)(peptide , H)] complexes and their anionic counterparts which complicated spectra interpretation and correlation with calculated dissociation energies. Copyright © 2009 John Wiley & Sons, Ltd. [source] Proton affinity ladder for uridine and analogs: influence of the hydroxyl group on the sugar ring conformationJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 6 2005S. Mezzache Abstract A ladder of relative proton affinities (PA) for a series of modified uridines (e.g. araU, ddU, 5BrU, 5BrdU and 5IU) was established from competitive dissociations of proton-bound heterodimers using Cooks and co-workers' kinetic method. The studied heterodimers are constituted of a modified nucleoside and either an amino acid or a nucleoside with known PA value. These non-covalent heterodimers were prepared under electrospray conditions to be selected and dissociated into the ion-trap analyzer. These results allowed our PA ladder of uridine and deoxyuridine analogs substituted at the C-5 position in the uracil ring to be extended. From this scale, it was showed that the substitution of hydrogen atom at the C-2, position in the sugar ring by a hydroxyl group involves a decrease of about 7 kJ mol,1. The experimental values for U, 5MeU, dU, 5MedU, ddU and araU are consistent with those obtained by DFT calculations (B3P86/6,31+G*//B3LYP/6,31G*). Several neutral and protonated conformations of these compounds were considered, in particular the ring conformation of furanose and the orientation of the base with respect to the sugar ring. These calculated results showed the influence of sugar substituent on the conformation of the neutral form of theses nucleosides. However, the most stable protonated structure is the same for all the studied nucleosides except for araU, where the position of the anti 2,-OH group imposes a specific conformation. Copyright © 2005 John Wiley & Sons, Ltd. [source] Hydrogen storage properties of B- and N-doped microporous carbonAICHE JOURNAL, Issue 7 2009Lifeng Wang Abstract A B- and N-doped microporous carbon has been synthesized via a substitution reaction. The obtained carbon exhibited much higher surface area than the previously reported B- and N-doped carbon. The hydrogen storage measurements indicated that the B- and N-doped microporous carbon had a 53% higher storage capacity than the carbon materials with similar surface areas. Furthermore, hydrogen storage via spillover was studied on Ru-supported B- and N-doped microporous carbon and a storage capacity of 1.2 wt % at 298 K and 10 MPa was obtained, showing an enhancement factor of 2.2. Ab initio molecular orbital calculations were also performed for the binding energies between the spiltover hydrogen atom and various sites on the doped carbon. The theoretical calculations can explain the experimental results well, which also shed light on the most favorable and possible sites with which the spiltover hydrogen atoms bind. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] The Use of Atomic Charges and Orbital Energies as Hydrogen-bonding-donor Parameters for QSAR Studies: Comparison of MNDO, AM1 and PM3 MethodsJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 6 2000TARAVAT GHAFOURIAN Hydrogen-bonding, important in drug-receptor interactions, also determines the solubility and partitioning of drugs between phases. It is, therefore, important to incorporate the effects of hydrogen-bonding in studies of quantitative structure-activity relationships (QSAR). In this study the atomic charge on the most positively charged hydrogen atom in a molecule and the energy of the lowest unoccupied molecular orbital (LUMO) have been used as a measure of hydrogen-bond-donor capacity. For several hydrogen-bonding acids the Mulliken atomic charges and the energy of the LUMO produced by use of three semi-empirical methods, AM1, PM3 and MNDO, and MNDO electrostatic-potential-derived atomic charges, have been compared in correlations with solvatochromic hydrogen-bonding acidity (,,2H). Atomic charges and LUMO energies, particularly those calculated by use of the AM1 and MNDO methods, were found to correlate well with ,,2H. They were also found to be good models of hydrogen-bonding in QSAR correlations. [source] The role of mid-chain radicals in acrylate free radical polymerization: Branching and scissionJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2008Thomas Junkers Abstract The past 5 years have seen a significant increase in the understanding of the fate of so-called mid-chain radicals (MCR), which are formed during the free radical polymerization of monomers that form highly reactive propagating radicals and contain an easily abstractable hydrogen atom. Among these monomers, acrylates are, beside ethylene, among the most prominent. Typically, a secondary propagating acrylate-type macroradical (SPR) can easily transfer its radical functionality via a six-membered transition state to a position within the polymer chain (in a so-called backbiting reaction), creating a tertiary MCR. Alternatively, the radical function can be transferred intramolecularly to any position within the chain (also forming an MCR) or intermolecularly to another polymer strand. This article aims at providing a comprehensive overview of the up-to-date knowledge about the rates at which MCRs are formed, their secondary reactions as well as the consequences of their occurrence under variable reaction conditions. We explore the latest aspects of their detection (via electron spin resonance spectroscopy) as well as the characterization of the polymer structures to which they lead (via high resolution mass spectrometry). The presence of MCRs leads to the formation of branched polymers and the partial formation of polymer networks. They also limit the measurement of kinetic parameters (such as the SPR propagation rate coefficient) with conventional methods. However, their occurrence can also be used as a synthetic handle, for example, the high-temperature preparation of macromonomers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7585,7605, 2008 [source] Hyperfine structure of hydrogen and geoniumLASER PHYSICS LETTERS, Issue 2 2004A.V. Andreev Abstract The self-consistent theory of hyperfine atomic structure is developed. The theory is based on Lorentz and gauge invariant equation for action of spin 1/2 particle. The specific feature of proposed equation for action (or Lagrangian) is that it is enable to introduce the three material constants: mass m0, charge q, and magneton (i.e. magnitude of magnetic moment) ,. The analytically tractable solutions of the wave equation for the electron motion in Coulomb field and electron motion in uniform magnetic field are found. In both cases the calculated spectra include the hyperfine splitting that is agreed well with the experimentally observed spectra. The calculated frequencies of 8(12)d3/2 , 8(12)d5/2 transitions in hydrogen atom are compared with the results of experimental measurements by the highprecision spectroscopy methods. It is shown that the results of calculations are in good agreement with the experimentally measured data. (© 2004 by HMS Consultants. Inc. Published exclusively by WILEY-VCH Verlag GmbH & Co.KGaA) [source] Ab Initio Quantum Chemical Investigation of the First Steps of the Photocycle of Phototropin: A Model Study,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 1 2003Christian Neiß ABSTRACT Phototropin is a blue light,activated photoreceptor that plays a dominant role in the phototropism of plants. The protein contains two subunits that bind flavin mononucleotide (FMN), which are responsible for the initial steps of the light-induced reaction. It has been proposed that the photoexcited flavin molecule adds a cysteine residue of the protein backbone, thus activating autophosphorylation of the enzyme. In this study, the electronic properties of several FMN-related compounds in different charge and spin states are characterized by means of ab initio quantum mechanical calculations. The model compounds serve as idealized model chromophores for phototropism. Reaction energies are estimated for simple model reactions, roughly representing the addition of a cysteine residue to the flavin molecule. Excitation energies were calculated with the help of time-dependent density functional theory. On the basis of these calculations we propose the following mechanism for the addition reaction: (1) after photoexcitation of FMN out of the singlet ground state S0, excited singlet state(s) are populated; these relax to the lowest excited singlet state S1, and subsequently by intersystem crossing FMN in the lowest triplet state, T1 is formed; (2) the triplet easily removes the neutral hydrogen atom from the H,S group of the cysteine residue; and (3) the resulting thio radical is added. [source] Electron spin resonance characterization of phosphorus-doped CVD diamond filmsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2004M. Katagiri Abstract Electron spin resonance technique has been applied to characterize defects and to determine local structure of phosphorus in phosphorus-doped (111)-homoepitaxially grown CVD diamond films. Dominant defects have g -value similar to that of the H1 center in polycrystalline CVD diamond, however, are not accompanied with a unique nearby hydrogen atom. These defects are likely to be located near substrate/ epilayer interface. New phosphorus related center with a tetragonal symmetry has been found. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||||||||||||||||||||||||||||||||