Perpendicular Geometry Research Articles

Carbonyl⋯carbonyl interactions in first-row transition metal complexes

Carbonyl⋯carbonyl interactions involving Tr–CO moieties (Tr = first-row transition element) have been studied using crystal structure data retrieved from the Cambridge Structural Database and by use of DFT calculations. By comparison with organic ketones, Tr–CO systems show an increased tendency to form close CO⋯CO interactions, with 45% of these forming pairwise interactions in a sheared antiparallel dimer motif and 55% having a perpendicular (single interaction) geometry. The bulky Tr and steric hindrance arising from other ligands at Tr play a significant role in the formation and geometry of the interactions. DFT calculations for the antiparallel dimer indicate that interaction energies for Tr–CO systems are slightly stronger than for organic ketones, and there is evidence of a stronger CO bond dipole in Tr–CO systems. With interaction energies comparable to those for medium strength hydrogen bonds, we conclude that CO⋯CO interactions in Tr–CO species may have a role to play in the design of novel carbonyl-containing inorganic and metal–organic structures.

Photophysics of intramolecularly hydrogen-bonded aromatic systems: ab initio exploration of the excited-state deactivation mechanisms of salicylic acid

Excited state reaction paths and the corresponding energy profiles of salicylic acid have been determined with the CC2 method, which is a simplified version of singles-and-doubles coupled cluster theory. At crucial points of the potential energy hypersurfaces, single-point energy calculations have been performed with the CASPT2 method (second-order perturbation theory based on the complete active space self-consistent field reference). Hydrogen transfer along the intramolecular hydrogen bond as well as torsion and pyramidization of the carboxy group have been identified as the most relevant photochemical reaction coordinates. The keto-type planar S(1) state reached by barrierless intramolecular hydrogen transfer represents a local minimum of the S(1) energy surface, which is separated by a very low barrier from a reaction path leading to a low-lying S(1)-S(0) conical intersection via torsion and pyramidization of the carboxy group. The S(1)-S(0) conical intersection, which occurs for perpendicular geometry of the carboxy group, is a pure biradical. From the conical intersection, a barrierless reaction path steers the system back to the two known minima of the S(0) potential energy surface (rotamer I, rotamer II). A novel structure, 7-oxa-bicyclo[4.2.0]octa-1(6),2,4-triene-8,8-diol, has been identified as a possible transient intermediate in the photophysics of salicylic acid.

Femtosecond Dynamics in Ferromagnetic Metals Investigated with Soft X-Ray Resonant Emission

We present measurements of the magnetic circular dichroism in x-ray resonant emission in the perpendicular geometry (circularly polarized x rays at normal incidence to the magnetization) for L(2,3) the absorption region in Fe, Co, and Ni metal. The results show that spin-dependent screening of the core hole takes place within the scattering time scale, which is supported by the absence of the effect in ionic systems. This allows an assessment of the time scale for the screening process (up to a few femtoseconds). The process is almost complete within the scattering time for Fe and Co, but this is not the case for the narrow band metal Ni which shows a much slower dynamics.

Ab Initio Study of Hydrogen Bonding and Proton Transfer in 3:1 FH:NH3 and FH:Collidine Complexes: Structures and One- and Two-Bond Coupling Constants across Hydrogen Bonds

Ab initio EOM-CCSD calculations have been performed on 3:1 FH:NH3 complexes at their own optimized MP2/6-31+G(d,p) geometries and at the optimized geometries in the hydrogen-bonding regions of corresponding 3:1 FH:collidine complexes. The isolated gas-phase equilibrium 3:1 FH:NH3 complex has an open structure with a proton-shared Fa-Ha-N hydrogen bond, while the isolated equilibrium 3:1 FH:collidine complex has a perpendicular structure with an Fa-Ha-N hydrogen bond that is on the ion-pair side of proton-shared. The Fa-N coupling constant ((2h)J(Fa-N)) for the equilibrium 3:1 FH:NH3 complex is large and negative, consistent with a proton-shared Fa-Ha-N hydrogen bond; (2h)JFb-Fa is positive, reflecting a short Fb-Fa distance and partial proton transfer from Fb to Fa across the Fb-Hb-Fa hydrogen bond. In contrast, (2h)JFa-N has a smaller absolute value and (2h)JFb-Fa is greater for the 3:1 FH:NH3 complex at the equilibrium 3:1 FH:collidine geometry, consistent with the structural characteristics of the Fa-Ha-N and Fb-Hb-Fa hydrogen bonds. Coupling constants computed at proton-transferred 3:1 FH:collidine perpendicular geometries are consistent with experimental coupling constants for the 3:1 FH:collidine complex in solution and indicate that the role of the solvent is to promote further proton transfer from Fa to N across the Fa-Ha-N hydrogen bond, and from Fb to Fa across the two equivalent Fb-Hb-Fa hydrogen bonds. The best correlations between experimental and computed coupling constants are found for complexes with perpendicular proton-transferred structures, one having the optimized geometry of a 3:1 FH:collidine complex at an Fa-Ha distance of 1.80 A, and the other at the optimized 3:1 FH:collidine geometry with distances derived from the experimental coupling constants. These calculations provide support for the proposed perpendicular structure of the 3:1 FH:collidine complex as the structure which exists in solution.

Determination of microfibril angle distribution by X-ray diffraction

X-ray diffraction is a well-established method for the determination of the mean microfibril angle (MFA). When the sample is a slice of wood variations in the fibre orientation, the shape of the cells, and the measurement geometry affect the intensity curve. A general form for diffraction conditions in terms of angles describing the fibre orientation and the shape of the cell was derived. Intensity curves were calculated by using Monte Carlo method and compared with experimental ones. Both peak fitting and variance methods were used for determining the mean MFA from the intensity curves. Norway spruce was used as an example. Results indicate that deviations in the fibre orientation, the spiral grain, do not affect the mean MFA considerably when using the symmetrical transmission geometry. When using the perpendicular transmission geometry large deviations in spiral grain or tips tend to increase the MFA determined with the variance method and decrease the MFA determined with the fitting method. The shape of the cell should be considered when using the reflection 200 and the fitting method. The variance method is insensitive to the shape of the cell.

Thermal Isomerization at a C=N Double Bond: How Does the Mechanism Vary with the Substituent?

Abstract Thermal isomerizations at the C=N double bonds in R1R2C=NC6H4X proceed via an inversion mechanism, in which the C=NC6H4X bond angle is near 180° at the transition state (TS). Available experimental data on the reactions of (CF3)2C=NC6H4X and YC6H4CH=NC6H4X have suggested that the reaction mechanism changes with substituent X. In the present study, HF, B3LYP, and MP2 computations were carried out to analyze the modes of mechanistic change with the substituent for the reactions of (CF3)2C=NC6H4X, YC6H4CH=NC6H4X, and other systems. There are two possible pathways within the inversion mechanistic framework: a planar pathway with the R1R2C=N double-bond plane and the aromatic ring being coplanar at the TS and a perpendicular one with the two planes being perpendicular to each other at the TS. It was found that the mechanistic change is not due to a change in the relative importance of two independent and competitive reaction pathways, but arises from a change of the character of a single TS from planar to perpendicular geometry when the substituent becomes more electron-withdrawing.

Alignment effects in charge transfer and excitation forH+andHe2+collisions withH2+

The study of ion-atom collisions has a long history for both experimental and theoretical techniques. There has been much less effort for ion-molecule collisions. Computationally, the ion-molecule collisions are much more difficult than ion-atom collisions. This is partly due to the multicenter nature of the problem instead of the two centers for ion-atom collisions. Perhaps more importantly, the cross sections may depend on several other coordinates. Thus, there is a dramatic increase in the number of calculations needed to explore the physically relevant parameters. There is also the possibility that interference due to the extended size of the electron orbitals will cause the results to depend sensitively on the accuracy of the calculation. Finally, molecules have a larger number of low-lying excited states compared to similar atoms which may sensitively affect the outcome of collisions. To see where some of the difficulties lie, we examine what is needed for ions scattering from a diatomic molecule that has no initial angular momentum about the internuclear axis. We simplify the picture further by assuming the ion may be treated as a classical particle traveling in a straight line and the nuclei in the molecule may be treated as fixed in space during the collision. In the comparable case for ionatom collisions, one only needs to calculate the probability for a process as a function of the impact parameter. For the ion-molecule collision, the probability for a process needs to be calculated as a function of the impact parameter in a plane i.e., two dimensions, as a function of the separation of the nuclei, and as a function of the angle between the ion’s velocity and the internuclear axis. Thus, we have gone from one parameter to four; assuming we need 5–10 examples of each parameter for an adequate exploration of ion-molecule collision we find there are approximately 100–1000 times more trajectories that are needed for ion-molecule calculations than for a comparable ion-atom calculation. The simplest ion-molecule system is when there is only one electron present. References 1,2 studied the collision of He 2+ with the D 2 + molecular ion. Reference 1 measured the total charge transfer cross section and compared the results to calculations using a model for the electronic orbital for the molecule. The model used a linear combination of atomic orbitals and the transition amplitudes are approximated as a linear superposition of the amplitudes for each atomic orbital 3. The calculated and measured total charge transfer cross sections were in good agreement. Reference 2 measured the dependence of the total charge transfer cross section on the angle between the velocity of the atomic ion and the internuclear axis and compared the measurement to the results of a model calculation again based on Ref. 3. Although the agreement was not as good, the general trends were the same: the cross section is much larger for perpendicular geometry than when the velocity is parallel to the internuclear axis. At velocity 0.4 a.u., the measurement gives a cross section 3–4 times larger at 90° than at 0° while the calculation gives a factor of 7–8. At velocity 0.5 a.u., the measurement gives a cross section 3–6 times larger at 90° compared to 1.5–2 for the calculation. Reference 4 presents calculations for total charge transfer for this system using both classical methods and a 2+1 -center close-coupling approximation. The total charge transfer cross section was 20–30 % lower than the previous measurement and calculation. In this paper, we present the results of calculations for charge transfer from the H 2 + molecular ion to He 2+ ions and to H + ions. Because the protons of the molecular ion are fixed during the collision, our results apply equally well to D2 + . The focus of this paper is the dependence of the cross section on the angle between the ion’s velocity vector and the internuclear axis and the dependence on the internuclear separation. Our method of calculation consists of direct numerical solution of Schrodinger’s equation on a grid of points; grid methods can be thought of as a basis set method with the number of basis functions equal to the number of grid points 10 7 in our case but with a momentum cutoff roughly equal to the inverse of the separation of grid points.

Theoretical Studies of Inorganic Compounds. 341) Energy Decomposition Analysis of E–E Bonding in Planar and Perpendicular X2E−EX2 (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I)

AbstractThe nature of E–E bonding in group 13 compounds X2E–EX2 (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I) has been investigated by means of an energy decomposition analysis (EDA) at the BP86/TZ2P level of theory. The calculated equilibrium geometries of all molecules B2X4−Tl2X4 have a perpendicular (D2d) geometry. The largest energy barriers for rotation about the E‐E bond are predicted for the hydrogen species B2H4−Tl2H4. The EDA shows that the rotational barriers of B2X4−Tl2X4 may not be used for an estimate of the hyperconjugative strength in the D2d structures except for the tetrahydrides. The values for the planar (D2h) transition states reveals that π conjugation of the halogen lone‐pair electrons stabilizes the transition states. The bonding analysis shows that hyperconjugation in B2I4 is stronger than in B2H4 although the latter compound has a higher rotational barrier than the former. In B2F4, hyperconjugative stabilization of the perpendicular structure and conjugative stabilization of the planar structure nearly cancel each other yielding a nearly vanishing rotational barrier. The heavier analogues Al2X4−Tl2X4 have low rotational barriers and rather weak hyperconjugative interactions. The larger rotational barriers of the hydrogen systems Al2H4−Tl2H4 compared with the tetrahalogen compounds is explained with the cooperation of the relatively large hyperconjugation in the perpendicular form and the relatively weak conjugation in the planar transition structures. The EDA also indicates that the electrostatic (ΔEelstat) and molecular orbital (ΔEorb) components of the E–E bonding are similar in magnitude.Thecalculated B‐B bond dissociation energies of B2X4 (De = 93.0–108.4 kcal/mol) show that the bonds are rather strong. The heavier analogues Al2X4−Tl2X4 have weaker bonds (De = 16.6–61.7 kcal/mol). In general, the X2E‐EX2 bond dissociation energies follow the trend for atoms E: B ≫ Al > Ga > In > Tl and for atoms X: H > F > Cl > I.

Electron spin injection into GaAs from ferromagnetic contacts in remanence

We demonstrate electrical spin injection into a (GaIn)As∕GaAs light-emitting diode from the remanent state of ferromagnetic contacts in perpendicular geometry. Using a Fe∕Tb multilayer structure with perpendicular magnetic anisotropy and a reverse-biased Schottky contact, we achieve a circular polarization degree of the emitted light of 0.75% at 90K.

Magnetic properties of Co–Pt alloy nanowire arrays in anodic alumina templates

Microstructure and magnetic properties of ferromagnetic Co 9 Pt nanowire arrays electrodeposited into self-assembled anodic alumina templates have been investigated. X-ray diffraction shows that as-prepared Co–Pt nanowires are of FCC polycrystalline structure. The nanowires with diameter d < 80 nm are mainly of (1 1 1) preferred orientation along the wire axis. For a specific diameter, the coercivity and the remanent ratio with the external magnetic field parallel to the nanowire axis, i.e., the parallel geometry, are larger than those of the perpendicular geometry. With temperature increasing from 90 to 600 K , above two physical quantities in the perpendicular geometry decrease monotonically, while those of the parallel geometry change little. The dependence of the coercivity and the remanent ratio on temperature and the nanowire diameter can be explained after the shape and the magnetocrystalline anisotropies, and the dipolar magnetic interaction are considered. For the parallel and perpendicular geometries, the coercivity decreases with increasing diameter d as a linear scale of 1 / d 2 and approaches each other in the two geometries for large d. The degradation of coercivity in Co–Pt nanowires with increasing diameter is suggested to come from the curling mode of magnetization reversal process.

New Trends in Magnetic Exchange Bias

New Trends in Magnetic Exchange Bias

The AGASA and SUGAR Anisotropies and TeV Gamma Rays from the Galactic Center: A Possible Signature of Extremely High Energy Neutrons

Recent analysis of data sets from two extensive air shower cosmic-ray detectors shows tantalizing evidence of an anisotropic overabundance of cosmic rays toward the Galactic center region that turns on around 1018 eV. We demonstrate that the anisotropy could be due to neutrons created in the Galactic center region through charge exchange in proton-proton collisions, where the incident, high-energy protons obey a ~E-2 power law associated with acceleration at a strong shock. We show that the normalization supplied by the gamma-ray signal from EGRET GC source 3EG J1746-2851 (ascribed to p-p-induced neutral pion decay at GeV energies), together with a very reasonable spectral index of 2.2, predicts a neutron flux at ~1018 eV fully consistent with the extremely high energy cosmic-ray data. Likewise, the normalization supplied by the very recent GC data from the HESS air Cerenkov telescope at ~TeV energies is almost equally compatible with the ~1018 eV cosmic-ray data. Interestingly, however, the EGRET and HESS data appear to be themselves incompatible. We find a plausible resolution of this discrepancy in an effective two-source model. Finally, we argue that the shock acceleration is probably occurring in the shell of Sagittarius A East, an unusual supernova remnant located very close to the Galactic center. In support of this contention we note that (1) the extended shell of this object could provide both of the sources suggested by the gamma-ray data and (2) the unusually strong magnetic field at this remnant, together with a perpendicular shock geometry, allow for acceleration of protons up to the extreme energies required to explain the cosmic-ray anisotropy. If the connection between the anisotropy and Sagittarius A East could be firmly established, it would be the first direct evidence for a particular Galactic source of cosmic rays up to energies near 1019 eV.

Practical and dosimetric implications of a new type of packaging for radiographic film

Recently, Kodak introduced new light-tight packages (vacuum packaging, aluminium layer under black polyethylene and different paper) for their oncology films (EDR-2, X-Omat V and PPL-2). In order to avoid additional uncertainty and to ensure transferability of previously published results, we assessed in this study the effect of the old and new packages on the dosimetric response of EDR-2 radiographic film. Therefore, sensitometric measurements were performed for different film assemblies (new envelope + new paper, old envelope + old paper, new envelope without paper and old envelope without paper). In addition, to assess possible effects of the package on the film depth–dose response, packaged films were irradiated in parallel geometry, and central depth–dose curves were retrieved. For the perpendicular geometry, on the other hand, the effect of the package was assessed at large depth for a high intensity-modulated inverse-pyramid beam. The results of the sensitometric measurements reveal no difference between the packages. However, the white colour of the paper in both the packages induces a dose-dependent increase in optical density (0–0.12) of the film. The depth–dose curves show better reproducibility for the new package and the new paper improves the accuracy of film dosimetry, but despite the company's effort to evacuate the air out of the new envelope, it remains necessary to clamp the films in the phantom for the parallel irradiation geometry. At 5 cm depth, the films irradiated in parallel geometry show an under-response of 3–5% compared to films irradiated perpendicularly. Finally, even at locations of large photon scatter, no filtration effect from the aluminium layer incorporated in the new envelope has been observed for perpendicular irradiation geometry.

Density functional studies of the COCu18 system†

Potential energy functions for the adsorption process CO(g) → CO(ads)/Cu(100) are investigated for the two perpendicular CO-substrate geometries. A Cu18 cluster model is used to represent the metal surface. Results from density functional theory using several density functionals are discussed and compared to results from traditional ab initio calculations. The TH3 functional developed by Handy and Tozer is found to be particularly well adapted for the derivation of qualitatively correct potential energy curves.

Theory of electric transport through Fe/V/Fe trilayers including the effect of impurities

AbstractThe influence of Al and Si impurity layers on the giant magnetoresistance (GMR) and the magnetic properties of Fe/V/Fe(110) trilayers is investigated. The calculations are performed by employing the spin‐polarized Kubo–Greenwood approach and the screened Korringa–Kohn–Rostoker method for layered systems. All calculations are carried out with a fully‐relativistic version. Therefore, we are able to consider also anisotropic magnetoresistance effects, which are common in Fe/V systems. We find that the AMR always makes a tiny contribution to the resistivity in alike multilayers so that the magnetoresistance is entirely due to the GMR. A reduction of the GMR due to the Al and Si impurities is observed for current in‐plane (CIP) and perpendicular (CPP) geometry. However, in the case of CIP geometry the influence of the impurities decreases with increasing V layer thickness, whereas in the CPP case the difference alternates between 0 and 7%. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Global 1 1A″ potential energy surface of CH2and quantum dynamics of a sideways insertion mechanism for the C(1D) + H2→ CH(2Π) + H reaction

A global adiabatic potential energy surface (PES) corresponding to the second singlet state 1 1A" (1 1B1) of CH2 has been computed in a similar way as the first singlet state 1 1A' in our previous work [B. Bussery-Honvault et al., J. Chem. Phys., 2001, 115, 10 701]. This PES has a calculated well depth of 79.9 kcal mol(-1) relative to the C(1D) + H2 asymptote and correlates to CH(2pi) + H. It presents large barriers in the C(1D) + H2 arrangement for both collinear and perpendicular geometries but no barrier for geometries about 60 degrees and leads to a sideways insertion mechanism for the reaction C(1D) + H2 --> CH(2pi) + H. The ab initio calculations were carried out for 4644 geometries and the resulting energies were fitted to a many-body expansion. Accurate three-dimensional quantum mechanical scattering calculations have been performed for the C(1D) + H2(v = 0, j = 0) reaction on this ab initio 1 1A" PES in the collision energy range [0-11.5 kcal mol(-1)]. The J = 0 reaction probabilities show dense resonance structures as those obtained with the 1 1A' PES. However some different dynamical features have been found.

Dynamic light scattering by optically anisotropic colloidal particles in polyacrylamide gels

The translational and rotational motions of optically anisotropic spherical particles embedded in cross-linked polyacrylamide gels is studied by dynamic light scattering. The particles are liquid crystal droplets solidified in the nematic phase. The amount of cross linkers is varied to cross the sol-gel transition where the system becomes nonergodic for both translational and rotational diffusion modes of the probes. The translational and rotational dynamic correlation functions are obtained by measuring the intensity correlation function between crossed polarizers in the parallel and perpendicular geometries. Data from nonergodic systems is analyzed using an extension, to include rotations, of the method of Pusey and van Megen [Physica A 157, 705 (1989)]. Both diffusion modes are observed to be arrested as the rigidity of the gel increases.

Magnetic properties and interlayer coupling of sputtered Ni/V multilayers

The magnetic properties of sputtered Ni/V multilayers have been studied in a vibrating sample magnetometer, torque magnetometer and by ferromagnetic resonance (FMR). The magnetization decreases with decreasing Ni layer thickness, which is an indication of the structural imperfections at interface. The interface contribution to the magnetic anisotropy is practically negligible. The spin-waves resonance modes were observed for perpendicular geometry, which implied that spin waves were sustained by the whole film and propagated through V layers in some Ni/V multilayers. The relation of the resonance field H n with the mode number n obeys the so-called n 2 law and the interlayer exchange constants were determined.

VLBA polarization observations of BL Lac objects and passive elliptical galaxies

We present Very Long Baseline Array (VLBA) 5-GHz polarimetric observations of 23 BL Lacertae (BL Lac) objects and radio galaxies selected from the 200-mJy sample. BL Lac objects have core polarization values lower than those found in previous samples. The magnetic field geometry in the jets is not unique: both parallel and perpendicular geometries are observed, even in the same sources. The parsec-scale morphology of radio galaxies is clearly divided in two classes: one-sided core-jet sources, which show polarized emission in the core and/or jet, and two-sided symmetric objects which are not polarized. We discuss and compare the parsec-scale polarization properties of the radio cores and jets for the BL Lac objects and the radio galaxies in relation to their parsec-scale morphology and high-frequency integrated spectral index.

Electron transfer from optically prepared states: He+ + Na(32P) → He(2 1,3P) + Na+ angular differential scattering

This paper reports experimental and theoretical scattering angle-dependent differential cross sections (DCS) for the electron transfer processes He+ + Na(3s or 3p) → He(2s or 2p) + Na+ at an impact energy of 1 keV. The He(2p) channels are the dominant contributors to electron transfer from the Na(3p) states. Beam experiments with optically prepared Na(3p) states are analysed using time-of-flight spectroscopy without resolution of the final He 21P and 23P states. The theoretical approach is based on a one-electron two-centre atomic orbital expansion of the electronic singlet and triplet scattering states, using the coupled-channel impact parameter method and the eikonal Bessel transformation to obtain quantal DCS predictions. The theoretical results show distinct differences between singlet and triplet channels. In spite of the smaller statistical weight, the singlet contribution about equals the triplet contribution due to the smaller singlet energy defect. For Na(3s) → He(2s) transfer DCS the theoretical results predict a Fraunhofer-type diffraction pattern as seen previously in Na(3s) → Li(2s) transfer, however, the rings are too narrow to be resolved in the present experiment. The spatial pattern of the scattered neutrals is strongly forwardly peaked with typical scattering angles of less than 0.05°. The results reveal pronounced spatial anisotropies, strongly dependent on the initial target preparation. In particular, when a Na(3p) orbital is prepared aligned along the direction of the initial ion beam, electron transfer is much more efficient than for the perpendicular orbital geometry. Furthermore, if the orbital is tilted by 45° with respect to the incident beam direction a strong left–right transfer scattering asymmetry is observed, with more neutrals being scattered to the side to which the p orbital is pointing than to the opposite side. The experimental observations compare well with the theoretical predictions for the spin-averaged electron transfer. Similarities and differences with earlier electron transfer studies for H+ and Li+ on Na(3p) at the same impact energy are pointed out.