Chemical Physics Letters (v.316, #5-6)
Photoelectron spectromicroscopy of electrochemically induced oxygen spillover at the Pt/YSZ interface by B. Luerßen; S. Günther; H. Marbach; M. Kiskinova; J. Janek; R. Imbihl (331-335).
Scanning photoelectron microscopy (SPEM) has been applied to study the processes at the interface between the oxygen ion conducting solid electrolyte YSZ (yttrium stabilized zirconia) and a microstructured 500 Å thick Pt film on top of the YSZ when electrical potentials are applied. An electrochemically induced oxygen spillover onto the Pt surface has been observed upon electrochemical pumping with a positive potential applied to the Pt film. The spillover species was characterized in X-ray photoelectron spectroscopy by an O 1s binding energy of 530.4 eV which is identical to that of chemisorbed oxygen from the gas phase.
Evidence for an adsorbate-dependent mechanism for surface resistivity: formic acid, oxygen and CO on Cu(100) by Ching-Ling Hsu; E.F McCullen; R.G Tobin (336-342).
When epitaxial Cu(100) films are exposed to formic acid, oxygen or CO gas at low temperature, both the dc electrical resistance and the infrared reflectance of the film change. The ratio of the resistance change to the reflectance change, however, is approximately twice as large for formic acid and CO as for oxygen, on identically prepared samples. This difference in slope is inconsistent with models in which the increase in film resistance arises solely from scattering of conduction electrons by the adsorbates. Changes in conduction electron density may also be important for some systems, especially when the chemisorption bond involves large charge transfer.
Electron transfer between adjacent layers in self-assembled films by Vitor B.P. Leite; Marysilvia Ferreira; Osvaldo N. Oliveira Jr (343-348).
A simple model of electron transfer is adapted to explain fluorescence quenching in self-assembled films of poly(p-phenylene vinylene) (PPV) alternating with poly(thiophene acetic acid) (PTAA). Quenching is caused by a photo-induced electron transfer between the excited PPV (donor, D) and the PTAA (acceptor, A). The electron-transfer process can be mediated by insertion of electronically inert spacing bilayers between the D and A layers. As the number of bilayers is increased, the fluorescence is gradually recovered which is explained theoretically by assuming that the electron-transfer rate can be described as k=Z exp(−βr) where r is the distance between D and A.
Structure and growth of aligned carbon nanotube films by pyrolysis by De-Chang Li; Liming Dai; Shaoming Huang; Albert W.H. Mau; Zhong L. Wang (349-355).
Transmission electron microscopic study on the aligned carbon nanotubes has demonstrated a growth mechanism which involves two sizes of iron nanoparticles. While the small particle is catalytically active for the nucleation of the nanotube, the large particle produces the carbon atomistic species required for the growth of the nanotubes. The aligned nanotubes are believed to be the result of a competition growth process along the normal direction of the substrate. The surface diffusion of carbon atoms on the large iron particle leads to the formation of the observed bamboo-like structure.
Ultrafast photoinduced charge generation in conjugated polymers by Daniel Moses; Arthur Dogariu; Alan J Heeger (356-360).
Transient excited-state absorption measurements in the spectral region spanning the infrared active vibrational active (IRAV) modes in prototypical luminescent polymers, poly(phenylene vinylene), PPV, and poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)], MEH–PPV, reveal charge carrier generation within 100 fs after photoexcitation. The photocarrier quantum efficiency in MEH–PPV is φ o≈0.1 in zero applied electric field. There is no correlation between the temporal behavior of the photoinduced IRAV signals and the exciton lifetime. Thus, carriers are photoexcited directly and not generated via a secondary process from exciton annihilation. Our data indicate that the carrier recombination rate is sensitive to the strength of the inter-chain interaction.
A new and simple method for thin graphitic coating of magnetic-metal nanoparticles by Satoshi Tomita; Masahiro Hikita; Minoru Fujii; Shinji Hayashi; Keiichi Yamamoto (361-364).
A new and simple method for synthesizing graphitic carbon-coated magnetic-metal (Co, Ni and Fe) nanoparticles has been reported. The mixture of metal and diamond nanoparticles was just annealed at 1700°C in vacuum. The annealed samples were studied by high-resolution transmission electron microscopy, selected-area electron diffraction and X-ray diffraction. These analyses revealed that magnetic-metal particles are coated with uniform graphitic layers several nanometers in thickness.
Radially modulated nitrogen distribution in CNx nanotubular structures prepared by CVD using Ni phthalocyanine by K. Suenaga; M. Yudasaka; C. Colliex; S. Iijima (365-372).
Chemical mapping with sub-nanometer-scale resolution across the CN x tubular nanostructures synthesized by chemical vapor deposition (CVD) has revealed nitrogen enrichment at the periphery and at the inner hollow while these tubules do not contain much nitrogen in the graphitic layers. This result indicates that the solubility of nitrogen in the graphitic layers is slight, at least under the catalytic CVD condition. It is supposed that the nitrogen atoms are bonded to the edges of layers, otherwise they form amorphous carbon nitride in the nanotube hollow.
Modelling the 13 C NMR chemical shifts of C84 fullerenes by T. Heine; M. Bühl; P.W. Fowler; G. Seifert (373-380).
Simulations of 13 C NMR shielding constants for fullerenes, based on the density-functional based tight-binding (DFTB) and individual gauge for local orbitals (IGLO) methods, are presented. This IGLO–DFTB model incorporates a correction scheme for the energies of the virtual states together with one empirical scaling factor. The IGLO–DFTB chemical shifts for a large test set of fullerenes are well correlated with those from ab initio calculations. Chemical shifts of the 24 isolated-pentagon isomers C84 are evaluated and compared with recent experimental work.
Electronic structure of Eu@C60 studied by XANES and UV–VIS absorption spectra by T. Inoue; Y. Kubozono; S. Kashino; Y. Takabayashi; K. Fujitaka; M. Hida; M. Inoue; T. Kanbara; S. Emura; T. Uruga (381-386).
Eu endohedral C60, Eu@C60, has been extracted with aniline from soot prepared by arc-heating of a graphite/Eu2O3 composite rod and obtained at high concentration by combining sublimation and repeated high-performance liquid chromatography. The laser desorption time-of-flight mass spectrum showed a pronounced peak of Eu@C60 +. The UV–VIS absorption spectrum of this sample has a red-shift in the onset (>900 nm) in comparison with those for C60 and C70, as expected from electron transfer from the Eu atom to the C60 cage. The valence state of the Eu atom in Eu@C60 has been determined to be +2 by Eu LIII-edge XANES.
Surface-induced reactions of C n +, 50⩽n⩽60 by T Fiegele; O Echt; F Biasioli; C Mair; T.D Märk (387-394).
Singly charged fullerene ions C n +, where 50⩽n⩽60, have been collided with a hydrocarbon-covered stainless steel surface. Fragmentation and pickup reactions are measured as a function of the collision energy. Compared to C60 +, ions with n⩽58 fragment much more strongly. This is attributed to the fact that the initial excitation energy in these projectiles, which are formed from C60 by electron impact ionization, is higher. However, identical values of 6.8%±0.5% are derived for the conversion efficiency of translational into internal energy.
Time-resolved resonance Raman and density functional study of the radical cations of chlorobiphenyls by Duohai Pan; Lian C.T. Shoute; David Lee Phillips (395-403).
We have obtained time-resolved resonance Raman spectra of the radical cations of 4-chlorobiphenyl, 3-chlorobiphenyl, and 2-chlorobiphenyl. We have performed density functional theory B3LYP/6-31G∗∗ calculations to find the optimized structures and predicted vibrational spectra for the ground states of the chlorobiphenyl radical cations. The computational and experimental results suggest that all of these chlorobiphenyl radical cations are nonplanar. The 2-chlorobiphenyl radical cation is noticeably more nonplanar than the other two radical cations and this appears to be due mainly to steric interactions. The radical cations are more noticeably nonplanar than the corresponding T1 states of the neutral molecules.
Observation of two paramagnetic species in electron transfer reactions within cesium modified X and Y zeolites by Rimma I. Samoilova; Alexander A. Shubin; Michael K. Bowman; Jürgen Hüttermann; Sergei A. Dikanov (404-410).
Tetrachloro-1,2-benzoquinone (o-chloranyl) has been used as a probe molecule to examine its interaction with catalytically active basic sites of cesium modified X and Y zeolites. Applying CW and pulsed EPR we were able to detect for the first time two paramagnetic species formed in the electron transfer reaction of active sites with o-chloranyl. One species is an anion-radical of o-chloranyl trapped in zeolite cage in the surrounding of alkaline ions. The other one is located on the zeolite lattice and comprises a center with a hole localized on oxygen(s) adjacent to the aluminum carrying about 50% of unpaired spin density.
Enhancement of magnetic field effect in Ru(bpy)3 2+/MV2+ system by Ru(bpy)3 2+-Ag+ exciplex formation by Lajos Fodor; Attila Horváth; Karsten A. Hötzer; Stefan Walbert; Ulrich E. Steiner (411-418).
The influence of ionic strength variation and of exciplex formation between silver ions and triplet excited RuL3 2+ (L=bipyridine, phenanthroline) on the photo electron transfer kinetics of the complex with methylviologen and the magnetic field dependence of free radical formation efficiency has been studied by laser flash spectroscopy. The magnetic field effect consisting in a decrease of the efficiency of cage escape η ce in the Ru(bpy)3 2+/MV2+ system is enhanced by 40% when exciplex formation between Ag+ ions and triplet excited RuL3 2+ complexes takes place. The relevant kinetic parameters have been determined by magnetokinetic model calculations.
Stimulated 7 Li echo NMR spectroscopy of slow ionic motions in a solid electrolyte by R. Böhmer; T. Jörg; F. Qi; A. Titze (419-424).
Lithium spin-alignment spectroscopy is presented as an NMR technique for studying slow translational motions in solid and solid-like ionic conductors. We employ phase cycling that allows to measure two-time translational correlation functions via the generation of a pure quadrupolar ordered state. Correlation functions of the crystalline electrolyte Li3Sc2(PO4)3 were recorded for times ranging from about 0.1 ms to more than 10 s, implying that translational diffusion coefficients smaller than 10−20 m2/s become accessible.
Isotope effects in the deactivation of O( 1 D) atoms by XCl and XF (X=H,D) by A.I. Chichinin (425-432).
The method of time-resolved laser magnetic resonance (LMR) was used to study the deactivation of O( 1 D) by HCl, DCl, HF, and DF at room temperature. For O( 1 D)+XF(X=H, D), the effect of deuteration on the rates of physical quenching and chemical reaction was negligible. For O( 1 D)+XCl, no essential isotope effect on the deactivation rate, on the Cl+OX reaction channel probability, and on the relative quantum yields of spin–orbit excited Cl( 2 P1/2) atoms was found. While the formation of H+ClO is found to account for 0.18±0.04 of the deactivation rate, in the deuterated case this channel was negligible (<0.03).
IVR in the S1 state of jet-cooled cis- and trans-p-dimethoxybenzene by G.N. Patwari; S. Doraiswamy; S. Wategaonkar (433-441).
Differences in the vibronic spectroscopy and IVR characteristics in the S1 state of cis and trans isomers of p-dimethoxybenzene (PDMB) were studied using a laser-induced fluorescence technique in supersonic jet expansion. Hole-burning spectroscopy was used to separate the transitions belonging to the individual isomers. In a few select modes the Franck–Condon activity in excitation and emission was different for the two isomers. The onset of IVR inferred from the dispersed fluorescence spectra was found at much lower energy for the cis isomer than that for the trans isomer. This was attributed to the difference in the electronic structure of the two isomers.
Photoionization and density functional theory study of clusters of acetone containing an alkali metal atom, M((CH3)2CO) n (M=Li, Na): intracluster electron transfer from metal to acetone in 1:1 complexes by Hironori Tsunoyama; Keijiro Ohshimo; Yoshihiro Yamakita; Fuminori Misaizu; Koichi Ohno (442-448).
Ionization threshold energies of clusters of Li and Na atoms solvated by acetone have been determined by laser photoionization. The thresholds for 1:1 complexes agree well with calculated adiabatic ionization potentials based on density functional theory. The structures and charge distributions obtained from the calculation show that electron transfer from the alkali atom to acetone occurs more efficiently in Li((CH3)2CO) than in Na((CH3)2CO). This difference in the extent of electron transfer can be understood by a consideration of the orbital overlap between the metal np and O2p orbitals and the sp hybridization on the alkali atom.
Induced HSiCl emission in the UV photodissociation of 2-chloroethenylsilane by Rebeca de Nalda; Andonis Mavromanolakis; Stelios Couris; Marta Castillejo (449-454).
The laser induced fluorescence of the HSiCl radical, detected in the 525–600 nm range, has been observed in the photolysis of 2-chloroethenylsilane at 193 and 212 nm. The results indicate that this radical is produced with considerable rotational and vibrational excitation in the electronic ground state. Quenching with Argon of the excited electronic state A 1A″ is very slow, on the order of 8×10−13 cm3 molecule−1 s−1. The implications of these observations for the primary photofragmentation paths of the 2-chloroethenylsilane molecule are discussed.
Vibrational spectroscopy of the F−·H2O complex via argon predissociation: photoinduced, intracluster proton transfer? by Patrick Ayotte; Jude A. Kelley; Steen B. Nielsen; Mark A. Johnson (455-459).
The mid-IR vibrational spectrum of the strongly bound F−·H2O complex is reported via predissociation of the size-selected F−·H2O·Ar m (m=1–3) clusters. A weak, sharp band at 3690 cm−1 confirms that this species adopts the asymmetric arrangement typical of the heavier halides, while the band arising from the ionic H-bond (IHB) is shifted very far to the red of the free H2O bands (shift ∼800 cm−1). The observed band position is actually found in the region of the predicted `overtone' of the H-bonded oscillator, where the OH stretching vibration occurs in a very strongly anharmonic O–H⋯F− potential surface in which the first excited vibrational level samples the HF–OH− proton transfer configuration.
Ar-induced pressure effects in the ν 1+3ν 3 absorption band in 12 C 2 H2 by F Herregodts; D Hurtmans; J.Vander Auwera; M Herman (460-464).
We have used a Ti:sapphire autoscan laser spectrometer to measure Ar-induced pressure-broadening, narrowing and shift effects for 24 individual vibration–rotation lines in the ν 1+3ν 3 band of 12 C 2 H2. The acetylene and total pressures ranged between 10 and 20 Torr, and between 30 and 310 Torr, respectively. The Voigt profile accounted for the higher pressure measurements while the soft and hard collision profiles were required to fit the lower pressure measurements. Lineshift parameters that are significantly different from the overall behaviour in the band were observed for the R(17) and P(19) lines and are qualitatively attributed to the influence of an intramolecular Coriolis-type coupling on the interparticle interaction potential. Those results confirm all trends of self-collisional measurements recently reported for this band.
Non-linear response and hydrogen bond dynamics for electron solvation in methanol by László Turi; Péter Mináry; Peter J. Rossky (465-470).
Non-equilibrium and equilibrium adiabatic mixed quantum-classical molecular dynamics computer simulations of the solvation dynamics of an excess electron in methanol are reported. We develop the connection between the multiple time scales reflected in solvent response and individual physical phenomena, such as the radial collapse of the electron and structural relaxation of the hydrogen-bonding network of the solvent. The significant role of the latter aspect appears responsible for the breakdown of the linear response approximation for the relaxation of the adiabatic ground state energy of the excess electron.
Rotational effects in the unimolecular dissociation of the acetyl radical by Emilio Martı́nez-Núñez; Saulo A. Vázquez (471-476).
The dissociation dynamics of rotating acetyl radical at energies near the threshold (19–23 kcal/mol) has been investigated by classical dynamics. Under microcanonical initial conditions and with zero total angular momentum, the dissociation of CH3CO is intrinsically RRKM. When the rotational degrees of freedom are taken into account the phase space becomes non-ergodic. At 23 kcal/mol and for 6.7 ps, which is the average lifetime at this energy, between 15% and 40% of K a space is mixed, suggesting that rotation–vibration coupling is modest, in agreement with experimental findings. Finally, excitation of the torsional mode decreases the rate by a factor of two.
A density functional theory study of the interaction of oxygen with a reduced SnO2 (110) surface by Yoichi Yamaguchi; Yosuke Nagasawa; Satoshi Shimomura; Kenji Tabata; Eiji Suzuki (477-482).
Ab initio calculations using density functional theory within the generalized gradient approximation have been performed for the interaction of oxygen with a reduced SnO2 (110) surface. We found two species of chemisorbed O2 that can be described as a peroxo (O2 2−) species for side-on type adsorption and a superoxo (O2 −) species for end-on type adsorption on the basis of their geometries and vibrational frequencies. We also found a low dissociation barrier of 2.0 kcal/mol for the peroxo species and high migration barriers of ∼13–18 kcal/mol for an oxygen atom produced by dissociation of O2 on the reduced SnO2 surface. An analysis of the electronic density of states indicates that an adsorbed oxygen atom coupled with the nearest-neighboring bridging oxygen vacant site is an O− species that displays catalytic activity on the surface.
The intermolecular interaction in the charge-transfer complex between NH3 and F2. A subtle case by Alfred Karpfen (483-488).
The intermolecular interaction between NH3 and F2 was investigated systematically with the aid of several electron correlation methods ranging from MP2 to the CCSD(T) level applying extended basis sets. Despite the weak charge transfer character of the NH3–F2 complex, electron correlation contributions beyond MP2 do have a significant influence on the properties of the complex. Moreover, structure optimization with explicit inclusion of the counterpoise correction leads to a substantial lengthening of the computed intermolecular distance. Convergence with respect to basis set extension is considerably slower and sensitivity to the correlation method applied is much greater than in related hydrogen-bonded complexes.
Bivalent cation binding effect on formation of the peptide bond by Milan Remko; Bernd Michael Rode (489-494).
The reactions between formic acid (or glycine) and ammonia, without and with Mg2+, Ni2+ and Cu2+ cations as catalysts, have been studied as model reactions for peptide bond formation using the Becke3LYP functional and 6-311+G(d,p) basis set of DFT theory. Enthalpies and free energies for the stationary points of each reaction have been calculated to determine the thermodynamics of reactions investigated. A substantial decrease in reaction enthalpies and free energies was found for formic acid-ammonia and glycine-ammonia reactions catalysed by Mg2+, Ni2+ and Cu2+ ions compared with those of the uncatalysed amide bond formation. The catalytic effect of the transition metal ions Ni2+ and Cu2+ is of similar strength and more pronounced than that of the Mg2+ cation.
Hybridization peaks in Pt–Cl XANES 1 Supported in part by the US DOE and by UOP LLC. 1 by A.L. Ankudinov; J.J. Rehr; Simon R. Bare (495-500).
Ab initio self-consistent X-ray absorption near-edge structure (XANES) calculations for the Pt L3 edge of (NH4)2PtCl4 and K2PtCl6 are in good agreement with experiment. These results allow us to assign a notable postedge XANES feature as a `hybridization peak', i.e., a mixture between the photoelectron state and unoccupied, atomic Cl 3d-states, mediated by multiple-scattering. An analogous interpretation applies to certain `pre-edge' peaks. A characteristic of such peaks is their approximate energy independence, in contrast to X-ray absorption fine-structure peaks. For Pt–Cl compounds, this suggests a way of monitoring the number of neighboring atoms in chemical processes using XANES.
The coupled-cluster correction to multi-reference configuration interaction method: application to excited states by Leszek Meissner; Marcel Nooijen (501-509).
The multi-reference configuration interaction method applies equally well to ground and excited states. However, a disadvantage of the MR-CI scheme is that its accuracy can be seriously affected by inadequate description of dynamical electron correlation effects. This problem can be alleviated to a large extent through an a posteriori correction based on the coupled-cluster (CC) expansion. Since our correction is derived within the single-reference CC formulation, its application to excited states is less obvious although possible and fully justified. In this Letter, we explore this possibility and study the performance of the correction for excited states.
Liquid tin tetrachloride: a Monte Carlo simulation study by Willian R. Rocha; Kátia J. De Almeida; Wagner B. De Almeida (510-516).
A simple five-site intermolecular potential function has been derived to reproduce the results from high-level ab initio calculations of the gas-phase interaction energy for SnCl4. This adjusted intermolecular potential was used in a Monte Carlo statistical mechanical simulation of the liquid at a temperature of 25°C and a pressure of 1 atm. The structural results, presented as a radial distribution function, reveal that in the first solvation shell each SnCl4 molecule is surrounded by twelve nearest neighbours. These results, as well as thermodynamical properties, are in good agreement with experimental findings.
On the relaxation of electron energy distribution function in LIBS plasmas by M. Capitelli; G. Colonna; M. Catella; F. Capitelli; A. Eletskii (517-523).
The relaxation of electron energy distribution function (eedf) in excited atomic nitrogen plasma (N, N+, e) is numerically studied by solving an appropriate time-dependent Boltzmann equation including elastic, inelastic, superelastic and Coulomb collisions. Conditions typically met under laser breakdown plasma spectroscopy (LIBS) and characterized by large ionization degrees are considered. The results show that the relaxation of eedf after an abrupt decrease of temperature goes through non-Maxwell distribution functions to the final Maxwell distribution function. It is shown that in conditions characteristic for LIBS plasma the deviation of the eedf from the maxwellian one is mainly caused by superelastic collisions. For atmospheric plasmas characteristic equilibration times span μs and sub-μs regions.
Density functional theory and resonance Raman investigation of the ultraviolet electronic excited states of CF2I2 by Xuming Zheng; David Lee Phillips (524-530).
We report density functional theory calculation results that examine the ultraviolet electronic transitions of CF2I2 and CH2I2. We make preliminary assignments of several transitions to the ultraviolet absorption spectra of CF2I2 and CH2I2. We compare our present results to previous experimental and computational work. We also examine the molecular orbitals involved in the electronic transitions assigned to the absorption spectra.
An ab initio and density functional study on the ring-chain tautomerism of (Z)-3-formyl-acrylic acid by Zuzana Škrinárová; Keith Bowden; Walter M.F. Fabian (531-535).
Ab initio (HF and MP2) and density functional (B3LYP) calculations with different basis sets (6-31G∗, 6-311+G∗∗, 6-311+G(2df,p)) on the various conformers of (Z)-3-formyl-acrylic acid and the corresponding ring-closed tautomer 5-hydroxy-furan-2(5H)-one are performed. At all levels of theory the ring tautomer is predicted to be the most stable one. Inclusion of bulk solvent effects (H2O) by the self-consistent isodensity surface polarized continuum model (SCIPCM) further shifts the ring-chain tautomeric equilibria towards the furanone form. Among the open chain tautomers, the s-cis–s-trans conformation is the most stable.
The concerted trimerization of ethyne to benzene revisited by Jerzy Cioslowski; Guanghua Liu; David Moncrieff (536-540).
CCSD(T)/6-311G∗∗//QCISD/6-311G∗∗ calculations on the concerted [2+2+2] trimerization of ethyne to benzene yield ΔH trim o (HCCH)=−140.2 kcal/mol and ΔH act o(HCCH)=53.1 kcal/mol. The corresponding transition state (TS) possesses C2 symmetry, although both the planar D3h and nonplanar D3 structures are negligibly higher in energy, indicating extreme flatness of the potential energy hypersurface along the distortion paths. The analogous trimerizations of HCCCl and ClCCCl are predicted to be considerably more exothermic. As the respective TSs cannot be located and the planar pseudo-TSs that possess several imaginary vibrational frequencies are associated with high reaction barriers, the concerted mechanism can be ruled out for these reactions.
Vibrational corrections to static electric properties of diatomics by Numerov–Cooley integration by Victoria E. Ingamells; Manthos G. Papadopoulos; Andrzej J. Sadlej (541-550).
We present a semi-numerical method based on Numerov–Cooley (NC) integration for calculating vibrational corrections to static electric properties of diatomic molecules. The approach exploits the Born–Oppenheimer energy and property functions, utilizing such information in the NC integration of the field-dependent vibrational equation, in a way which exposes separate contributions to the total electric property. The scheme is illustrated by computation of electronic and vibrational contributions to the dipole polarizability and first hyperpolarizability of the KLi dimer. A comparison with results obtained from finite-order perturbation theory reveals various sources of anharmonicity.
One approach to the control of intramolecular hydrogen transfer by Y. Ohta; T. Yoshimoto; K. Nishikawa (551-557).
One approach using the counterintuitive pulse sequence (CPS) is presented to control the intramolecular hydrogen transfer. We have applied the asymmetric double-well model to the substituted malonaldehyde and have shown the complete population transfer from one local minimum state to another one through the barrier using CPS. We also have found in our simulation that the high product yield is robust with respect to the changes of the laser parameters. The mechanism of the population transfer is elucidated with our approach with the Hückel theory in a systematic and pictorial way.
Anomalous migration of DNA by Jayanta F Mohanty (558-562).
A quantitative model for the electrophoretic mobility of very small fragments of A-DNA and Z-DNA in gel is described. The model is a synthesis of counterion condensation features of ionic oligomers, screening of the hydrodynamic interactions of an oligomer, and a suitable extension of the Ogstron pore-size distribution model of gel migration. Anomalous migration of very small fragments of A- and Z-DNA is predicted. The electrophoretic mobility is an increasing function of its length for small fragments, reaches a maximum and then starts to slow down for larger fragments. For very small sequence lengths studied here, A-DNA migrates faster than Z-DNA of the same length in polyacrylamide gel.
Theoretical C–H bond dissociation enthalpies for CH3OCl and CH3OBr by J Espinosa-Garcı́a (563-568).
Standard enthalpies of formation for the free radicals CH2OCl and CH2OBr were theoretically estimated using two hydrogenation and two isodesmic reactions as working chemical reactions. Energy differences were computed at two high ab initio levels of calculation, using gaussian 2 (G2) theory (Level I) and Truhlar's basis set limit method (Level II). The recommended standard enthalpies of formation (at 298 K and 1 atm) are the unweighted averages of the results obtained at these two levels with the four working chemical reactions, namely: CH2OCl, 32.0±3.5; and CH2OBr, 36.1±3.9 kcal mol−1. These enthalpies of formation, correspond to bond dissociation enthalpies of DH298° (H-CH2OCl)=97.3±5.3 and DH298° (H-CH2OBr)=96.9±6.3 kcal mol−1.
The combination of density functional theory with multi-configuration methods – CAS-DFT by Jürgen Gräfenstein; Dieter Cremer (569-577).
CAS-DFT is presented as a method that allows an economical simultaneous treatment of static and dynamic correlation effects in molecules with multi-reference character. Central problems of CAS-DFT concern the double counting of dynamic correlation effects and the choice of the proper input quantities for the DFT functional. Also, the question of treating both active and inactive orbitals in a consistent way is discussed. Test calculations with CAS-DFT for the ring opening of dioxirane and the excitation energies of methylene prove that the method works reasonably.
Quantifying theory of optical orientation processes in spectrally distinguishable photoisomers: application to a spiropyran-type chromophore by Hidekazu Ishitobi; Zouheir Sekkat; Satoshi Kawata (578-584).
We present the theoretical treatment which allows for the quantification of optical orientation processes in individualizable photoisomers, including the measurement of the relative orientation of molecular optical transitions and the photochemical quantum yields in ordered systems. The theory is confirmed experimentally for a spiropyran-type molecule introduced into a poly-methyl-methacrylate film. In particular we found that the transitions at 365 and 633 nm of the B isomer are oriented at an angle of 71.25° to each other, and that the isomerization quantum yields are φ AB 365=0.053, φ BA 365=0.030 and φ BA 546=0.003.
Theoretical analysis of femtosecond excitation and fragmentation dynamics of Fe(CO)5 by O. Rubner; T. Baumert; M. Bergt; B. Kiefer; G. Gerber; V. Engel (585-592).
We present the application of a time-dependent model to Fe(CO)5 dissociation after femtosecond excitation to understand transient signals obtained in recent pump/probe experiments. The calculation of time-dependent energy distributions allows us to interpret the decay behaviour of the various fragments. The results indicate that at least two dissociation pathways have to be considered to explain the transient signals obtained for the different fragments. Furthermore, it is shown that, within each of the pathways discussed, concerted and sequential fragmentation compete.