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Ab Initio Molecular Orbital Calculations (ab + initio_molecular_orbital_calculation)
Selected AbstractsSodiated ,-Diphosphonate Carbanions: Characterization of the Tetrameric Cubane and the Hexameric Ladder Complexes [{(iPrO)2P(O)}2CHNa]4 and [{(EtO)2P(O)}2CHNa]6EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 18 2003Roger Nassar Abstract Reaction of the methylene diphosphonates [(RO)2P(O)]2CH2, where R = iPr or Et, with the metalating reagents NaH, nBuNa or the superbasic mixture tBuONa/nBuLi leads to monodeprotonation, and formation of the complexes [{(iPrO)2P(O)}2CHNa]4 (1) and [{(EtO)2P(O)}2CHNa]6 (2). Single-crystal X-ray diffraction studies revealed that 1 is tetrameric in the solid state, with a central Na4O4 cubane core. Each metal in 1 is rendered pentacoordinate by binding to three ,3 -O phosphoryl bridges within the cubane, a chelating phosphoryl unit, and also a single iPrO group. In contrast, the structure of 2 is hexameric, and is composed of an unusual six-rung Na6O6 ladder core. Three distinct sodium environments as well as ,1 -, ,2 - and ,3 -O phosphoryl units are present within the ladder framework. No Na,C interactions are present in either 1 or 2. All of the six-membered NaOPCPO chelate rings display localized P=O double bonds and partial multiple bonding on the P,C,P linkages (mean P,O: 1.49 Å, and mean P,C: 1.69 Å). Ab initio molecular orbital calculations (HF/6,31G*) on model complexes indicate that the hexameric arrangement found for 2 is thermodynamically more stable than the cubane structure of 1, in the absence of steric effects. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [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] Amino and cyano N atoms in competitive situations: which is the best hydrogen-bond acceptor?ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2001A crystallographic database investigation The relative hydrogen-bond acceptor abilities of amino and cyano N atoms have been investigated using data retrieved from the Cambridge Structural Database and via ab initio molecular orbital calculations. Surveys of the CSD for hydrogen bonds between HX (X = N, O) donors, N,T,C,N (push,pull nitriles) and N,(Csp3)n,C,N molecular fragments show that the hydrogen bonds are more abundant on the nitrile than on the amino nitrogen. In the push,pull family, in which T is a transmitter of resonance effects, the hydrogen-bonding ability of the cyano nitrogen is increased by conjugative interactions between the lone pair of the amino substituent and the C,N group: a clear example of resonance-assisted hydrogen bonding. The strength of the hydrogen-bonds on the cyano nitrogen in this family follows the experimental order of hydrogen-bond basicity, as observed in solution through the pKHB scale. The number of hydrogen bonds established on the amino nitrogen is greater for aliphatic aminonitriles N,(Csp3)n,C,N, but remains low. This behaviour reflects the greater sensitivity of the amino nitrogen to steric hindrance and the electron-withdrawing inductive effect compared with the cyano nitrogen. Ab initio molecular orbital calculations (B3LYP/6-31+G** level) of electrostatic potentials on the molecular surface around each nitrogen confirm the experimental observations. [source] Theoretical study of acetonitrile-exchange reactions on hexasolvated divalent cations in the first transition series elementsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2009Hiroaki Wasada Abstract Penta-, hexa-, and heptaacetonitrile complexes of divalent cations of the first transition series are studied by ab initio molecular orbital calculations. The factors that determine the structural stability and the reaction mechanism of solvent-exchange reactions are discussed. All the penta- and hexaacetonitrile species are at local minima, whereas the geometrical stability of the heptacoordinated species depends on the 3d electron configurations. The structural stability of heptaacetonitrile species is intermediate between those of hydrogen cyanide complexes and hydrates. Acetonitrile exchange reactions have more dissociative character than hydrogen cyanide exchange reactions because the inductive effect of the methyl group in CH3CN destabilizes the heptacoordinated structures. The successive binding energies show that associative mechanisms are favorable for acetonitrile exchange with earlier members of the first transition series, whereas dissociative mechanisms become favorable for later members. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Assignment of the Raman-active modes of the antitumor agent azathioprine by normal-mode calculationsJOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2001Alberto Vivoni Azathioprine is a slow-release prodrug of 6-mercaptopurine and an established clinical agent for the treatment of human leukemias and other immunologically mediated diseases. The Raman spectra (1600,600 cm,1) of solid and solution azathiopurine were recorded and are presented along with results from normal-mode calculations. The band assignments were derived from semi-empirical and ab initio molecular orbital calculations. The ab initio calculations were performed with the restricted Hartree,Fock method and the semi-empirical methods utilized the AM1, PM3 and MNDO-d methods. Copyright © 2001 John Wiley & Sons, Ltd. [source] Is the Addition-Fragmentation Step of the RAFT Polymerisation Process Chain Length Dependent?,MACROMOLECULAR THEORY AND SIMULATIONS, Issue 5 2006Ekaterina I. Izgorodina Abstract Summary: The chain length dependence of the addition-fragmentation equilibrium constant (K) for cumyl dithiobenzoate (CDB) mediated polymerisation of styrene has been studied via high level ab initio molecular orbital calculations. The results indicate that chain length and penultimate unit effects are extremely important during the early stages of the polymerisation process. In the case of the attacking radical (i.e., R, in: R,,+,SC(Z)SR,,,,RSC,(Z)SR,), the equilibrium constant varies by over three orders of magnitude on extending R, from the styryl unimer to the trimer species and actually increases with chain length, further confirming that K is high in this system. When the reactions of the cumyl leaving group and cyanoisopropyl initiating species, which are also present in CDB-mediated polymerisation of styrene in the presence of the initiator 2,2,-azoisobutyronitrile, are also included, the variation in K extends over five orders of magnitude. Although less significant, the influence of the R, group should also be taken into account in a complete kinetic model of the RAFT process. However, for most practical purposes, its chain length effects beyond the unimer stage may be ignored. These results indicate that current simplified models of the RAFT process, which typically ignore all chain length effects in the R and R, positions, and all substituent effects in the R, position, may be inadequate, particularly in modelling the initial stages of the process. [source] Chemopreventive Action of Xanthone Derivatives on Photosensitized DNA Damage,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2005Kazutaka Hirakawa ABSTRACT Photosensitized DNA damage participates in solar-UV carcinogenesis, photogenotoxicity and phototoxicity. A chemoprevention of photosensitized DNA damage is one of the most important methods for the above phototoxic effects. In this study, the chemopreventive action of xanthone (XAN) derivatives (bellidifolin [BEL], gentiacaulein [GEN], norswertianin [NOR] and swerchirin [SWE]) on DNA damage photosensitized by riboflavin was demonstrated using [32P]-5,-end-labeled DNA fragments obtained from genes relevant to human cancer. GEN and NOR effectively inhibited the formation of piperidine-labile products at consecutive G residues by photoexcited riboflavin, whereas BEL and SWE did not show significant inhibition of DNA damage. The four XAN derivatives decrease the formation of 8-oxo-7,8-dihydro-2,-deoxyguanosine (8-oxodGuo), an oxidative product of G, by photoexcited riboflavin. The preventive action for the 8-oxodGuo formation of these XAN derivatives increased in the following order: GEN > NOR , BEL > SWE. A fluorescence spectroscopic study and ab initio molecular orbital calculations suggested that the prevention of DNA photodamage is because of the quenching of the triplet excited state of riboflavin by XAN derivatives through electron transfer. This chemoprevention is based on neither antioxidation nor a physical sunscreen effect; rather, it is based on the quenching of a photosensitizer. In conclusion, XAN derivatives, especially GEN, may act as novel chemopreventive agents by the quenching mechanism of an excited photosensitizer. [source] Amino and cyano N atoms in competitive situations: which is the best hydrogen-bond acceptor?ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2001A crystallographic database investigation The relative hydrogen-bond acceptor abilities of amino and cyano N atoms have been investigated using data retrieved from the Cambridge Structural Database and via ab initio molecular orbital calculations. Surveys of the CSD for hydrogen bonds between HX (X = N, O) donors, N,T,C,N (push,pull nitriles) and N,(Csp3)n,C,N molecular fragments show that the hydrogen bonds are more abundant on the nitrile than on the amino nitrogen. In the push,pull family, in which T is a transmitter of resonance effects, the hydrogen-bonding ability of the cyano nitrogen is increased by conjugative interactions between the lone pair of the amino substituent and the C,N group: a clear example of resonance-assisted hydrogen bonding. The strength of the hydrogen-bonds on the cyano nitrogen in this family follows the experimental order of hydrogen-bond basicity, as observed in solution through the pKHB scale. The number of hydrogen bonds established on the amino nitrogen is greater for aliphatic aminonitriles N,(Csp3)n,C,N, but remains low. This behaviour reflects the greater sensitivity of the amino nitrogen to steric hindrance and the electron-withdrawing inductive effect compared with the cyano nitrogen. Ab initio molecular orbital calculations (B3LYP/6-31+G** level) of electrostatic potentials on the molecular surface around each nitrogen confirm the experimental observations. [source] | ||||||||||