Self-chemical ionization of diethylzinc

The self-chemical ionization of diethylzinc is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and semiempirical molecular orbital calculations. Electron impact of diethylzinc neutral produces the radical cation, C(4)H(15)Zn(+) (m/z x 122), which reacts further with the neutral (C(2)H(5))(2)Zn to give the following product ions: Zn(+) (m/z x 64), C(2)H(5)Zn(+) (m/z x 93), C(4)H(9)Zn(+) (m/z x 121), C(4)H(11)Zn(2)(+) (m/z x 187), and C(6)H(15)Zn(2)(+) (m/z x 215). To determine the structure and pathways for production of these ions, monoisotopic (12)C(4)H(15)(64)Zn(+), (64)Zn(+) and (12)C(2)H(5)(64)Zn(+) were individually isolated and reacted with the neutral background. We also performed semiempirical molecular orbital calculations (ZINDO/1). The molecular orbital calculations and experimental data are consistent in predicting that the ethyl group on the diethylzinc cation carries the positive charge. Copyright 1999 John Wiley & Sons, Ltd.     

DOCK 4.0: Search strategies for automated molecular docking of flexible molecule databases

In this paper we describe the search strategies developed for docking flexible molecules to macomolecular sites that are incorporated into the widely distributed DOCK software, version 4.0. The search strategies include incremental construction and random conformation search and utilize the existing Coulombic and Lennard-Jones grid-based scoring function. The incremental construction strategy is tested with a panel of 15 crystallographic testcases, created from 12 unique complexes whose ligands vary in size and flexibility. For all testcases, at least one docked position is generated within 2 Å of the crystallographic position. For 7 of 15 testcases, the top scoring position is also within 2 Å of the crystallographic position. The algorithm is fast enough to successfully dock a few testcases within seconds and most within 100 s. The incremental construction and the random search strategy are evaluated as database docking techniques with a database of 51 molecules docked to two of the crystallographic testcases. Incremental construction outperforms random search and is fast enough to reliably rank the database of compounds within 15 s per molecule on an SGI R10000 cpu.     

A modular model of the crystal structure of the pyrochlore-murataite polysomatic series

This paper reports on the results of a structural investigation of an important group of synthetic compounds with derivative structures of the mineral murataite, a potential matrix for immobilization of radioactive wastes. A model describing the structure of the entire group of compounds is proposed on the basis of analyzing the results obtained using chemical (electron microprobe) analysis methods, high-resolution transmission electron microscopy (high-resolution electron images and microdiffraction patterns), and X-ray diffractometry of a large array of artificial murataite ceramics. In essence, the proposed structural model is as follows: in the structure, layers of two (pyrochlore and murataite) packing types alternate along the [111] direction of the cubic supercell, so that the resulting structures can undergo substitutional and displacive modulations. Experiments on irradiation of different murataite modifications with heavy ions demonstrate that these modifications have close values of the radiation resistance, which is higher than that of the pyrochlore coexisting with them.     

Eulerian–Lagrangian localized adjoint methods for reactive transport with biodegradation

The microbial degradation of organic contaminants in the subsurface holds significant potential as a mechanism for in-situ remediation strategies. The mathematical models that describe contaminant transport with biodegradation involve a set of advective–diffusive–reactive transport equations. These equations are coupled through the nonlinear reaction terms, which may involve reactions with all of the species and are themselves coupled to growth equations for the subsurface bacterial populations. In this article, we develop Eulerian–Lagrangian localized adjoint methods (ELLAM) to solve these transport equations. ELLAM are formulated to systematically adapt to the changing features of governing partial differential equations. The relative importance of retardation, advection, diffusion, and reaction is directly incorporated into the numerical method by judicious choice of the test functions that appear in the weak form of the governing equation. Different ELLAM schemes for linear variable–coefficient advective–diffusive–reactive transport equations are developed based on different operator splittings. Specific linearization techniques are discussed and are combined with the ELLAM schemes to solve the nonlinear, multispecies transport equations. © 1995 John Wiley & Sons, Inc.     

Spontaneous desorption of vibrationally excited molecules physically adsorbed on surfaces

The energy within a vibrationally excited physisorbed molecule often exceeds that needed to break its bond to the surface. Energy transfer from the vibrating chemical bond to the surface bond causes the surface bond to rupture and the vibrationally relaxed adsorbate is released from the surface. We present a theoretical model which allows an estimation of the residence time of a vibrationally excited adsorbate on a surface. Because of uncertainties in the nature of the surface bond, the lifetimes obtained from the analytical expressions presented have only qualitative significance. The results are interpreted in terms of Franck-Condon overlaps between the wavefunctions which describe the adsorbate-substrate complex and the released adsorbate. Lifetimes are calculated for hydrogen isotopes adsorbed on sapphire surfaces. Guide-lines are given for estimating lifetimes of other systems in terms of a few easily calculated parameters.Let us summarize this guide to spontaneous desorption of physically adsorbed vibrationally excited molecules. The most efficient desorption processes will occur for adsorbates with a small number of bound states (d₀ small) and when released the adsorbate has small translational momentum (small q_m). This momentum gap correlation is most succinctly revealed by fig. 3. Smaller translational momentum will be achieved if the adsorbate can take up energy into its internal motions. Absorption of energy into lattice modes of the substrate will also serve to reduce the translational momentum and provide for more efficient desorption. However, if the vibrational frequency of the adsorbate is in near resonance with surface polarons or plasmons of the substrate, energy transfer to the solid will be so efficient that desorption will be quenched.A test of these possible relaxation channels awaits the first experimental measurements of desorption of vibrationally excited molecules.     

Calculation of spin orbit relaxation rates by near resonant E → V energy transfer

This paper calculates the cross sections and their temperature dependences of the collisional relaxation of Pb 6p²³P₂ by D₂ and Te 5p⁴³P_{1, 0} by H₂. Relaxation of these electronic metastable states can take place by a near resonant E → V energy transfer. The theory considers coupling of initial and final states by long range attractive forces. A calculation based on quadrupole-quadrupole coupling gives a reasonably close estimate of observed rates. The selection rules and limitations of the theory are briefly discussed.     

The conformation of meso and racemic 2,3,4-trichloropentanes—model compounds for polymers derived from 1,2-dichloroethylenes

On the basis of coupling constants and chemical shifts the conformation of meso and racemic 2,3,4-trichloropentanes have been determined. The meso isomer exists as an equilibrium between the expected rotameric forms but the racemic isomer is anomalous and exists almost entirely as a distorted form of a rotamer which has parallel 1:3 interactions. The significance of these conformations is discussed in relation to the likely conformation and stability of the corresponding stereoregular polymers, which may be derived from cis- and trans-1, 2-dichloroethylene.