An enantiomerically pure adamantylimido molybdenum alkylidene complex. An effective new catalyst for enantioselective olefin metathesis

An enantiomerically pure Mo-based complex that bears an alkylimido ligand is prepared and characterized through NMR spectroscopy and X-ray analysis. Mo complex 4 is the only reported chiral alkylimido catalyst; all previous chiral complexes are arylimido systems. These studies show that the chiral Mo catalyst exists exclusively as the syn isomer and that it offers unique reactivity and selectivity profiles in asymmetric olefin metathesis.     

Shape resonances in slow electron scattering by aromatic molecules. I. Anthraquinone derivatives

A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd.     

TEMPORAL CHANGES IN SURFACE ULTRAVIOLET RADIATION: A STUDY OF THE ROBERTSON-BERGER METER AND DOBSON DATA RECORDS

Abstract— Ultraviolet radiation data sets obtained by Robertson-Berger meters located at Bismarck, ND and Tallahassee, FL show variations over the time period 1974–1985 which we interpret in terms of clouds and ozone. Cloudiness is a major source of variance in the irradiance measurements. When this variance is minimized, the monthly mean Robertson-Berger meter record contains trends which are in good agreement with irradiance calculations based on the Dobson ozone measurements in spring, summer and early autumn. Despite the agreement among trends, predictions based on the ozone data explain 40% or less of the variance in the monthly mean radiation values over the 11-year period. The radiation measurements contain negative trends in winter which are contrary to expectations based on the behavior of ozone alone. These trends remain when we minimize the effects of cloudiness. Based on the information available in this study, it is not possible to determine whether the wintertime trends have an instrumental or environmental origin.     

A finite element formulation for global linear stability analysis of a nominally two‐dimensional base flow

A stabilized finite element method, to carry out the linear stability analysis of a two‐dimensional base flow to three‐dimensional perturbations that are periodic along span, is presented. The resulting equations for the time evolution of the disturbance requires a solution to the generalized eigenvalue problem. The analysis is global in nature and is also applicable to non‐parallel flows. Equal‐order‐interpolation functions for velocity and pressure are utilized. Stabilization terms are added to the Galerkin formulation to admit the use of equal‐order‐interpolation functions and to eliminate node‐to‐node oscillations that might arise in advection‐dominated flows. The proposed formulation is tested on two flow problems. First, the mode transitions in the circular Couette flow are investigated. Two scenarios are considered. In the first one, the outer cylinder is at rest, while the inner one spins. Two linearly unstable modes are identified. The primary mode is real and represents the axisymmetric Taylor vortices. The second mode is complex and consists of spiral vortices. For the counter‐rotating cylinders, the primary transition is via the appearance of spiral vortices. Excellent agreement with results from earlier studies is observed. The formulation is also utilized to investigate the parallel and oblique modes of vortex shedding past a cylinder for the Re = 100 flow. It is found that the flow is associated with a large number of unstable oblique shedding modes. The parallel mode of vortex shedding is a special case of this family of modes and is associated with the largest growth rate. Copyright © 2014 John Wiley & Sons, Ltd.     

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LONG-TERM VARIATIONS IN ULTRAVIOLET SUNLIGHT REACHING THE BIOSPHERE: . CALCULATIONS FOR THE PAST THREE DECADES

The long-term data base on atmospheric ozone combined with a set of radiative transfer calculations provides estimates of the variability in ultraviolet (UV) sunlight that should have occurred over the period1957–1988 under clear, pollution-free skies. Results refer to the earth's surface at specific locations in the Northern Hemisphere and to averages over collections of sites located in three latitude bands from 30 to 64°N. For any one year the annually integrated solar irradiance, weighted by the action spectrum for erythema, typically lies within3–4% of the 32-year mean. No statistically significant trends span the entire 32-year time frame. However, over the shorter time period1970–1988 the annually integrated erythemal irradiance shows an upward trend of +2.1 ± 1.2% per decade based on all ozone data at latitudes from 40 to 52°N. No trends exist in lower (30-39°N) and higher (53-64°N) latitude bands. We caution that a trend line provides a very simple index of the variability in UV sunlight, and these results should not necessarily be extrapolated into the future.     

Oscillator models for multiphoton optical bistability and phase conjugacy

Nonlinear oscillator models are constructed to treat the bistability in situations involving elementary excitations in solids. Such models are shown to be useful not only in describing single photon but also multiphoton bistability. The resulting bistability both with and without cavity is considered. The two-photon excitonic bistability in CuCl is in detail. The effect of local field corrections can also be incorporated, in a simple manner, in such models.     

Mass spectral studies of unsaturated carboxylic acids. II—Some acids containing two double bonds

The mass spectra of some doubly unsaturated carboxylic acids are reported. Where C-2 is quaternary, the fragmentation pathway is similar to that previously demonstrated for βγ-unsaturated acids. The unbranched acids show different behaviour, similar to their thermal pyrolysis.