Optimal strong Mal’cev conditions for congruence meet-semidistributivity in locally finite varieties

Locally finite, congruence meet-semidistributive varieties have been characterized by numerous Mal’cev conditions and, recently, by two strong Mal’cev conditions. We provide three new strong Mal’cev characterizations and a new Mal’cev characterization each of which improves the known ones in some way.     

Strongly etale extensions and weak henseliziations

Abstract

Since the circulation, in 1974, of the first draft of “The construction D + XD _S [X], J. Algebra 53 (1978), 423–439” a number of variations of this construction have appeared. Some of these are: The generalized D + M construction, the A + (X)B[X] construction, with X a single variable or a set of variables, and the D + I construction (with I not necessarily prime). These constructions have proved their worth not only in providing numerous examples and counter examples in commutative ring theory, but also in providing statements that often turn out to be forerunners of results on general pullbacks. The aim of this paper will be to discuss these constructions and the remarkable uses they have been put to. I will concentrate more on the A + XB[X] construction, its basic properties and examples arising from it.     

A variational approach for compact homogeneous Einstein manifolds

((no abstract given))     

Conformational analysis: crystallographic,NMR, and molecular mechanics studies of flexible sulfonic esters

Two novel X-ray structures of the sulfonic ester derivatives 2-(6-iodo-1,3-benzodioxol-5-yl)ethyl 4-nitrobenzenesulfonate, 3, and 2-(6-iodo-1,3-benzodioxol-5-yl)ethyl 4-methylbenzenesulfonate, 4, have been obtained in a study aimed at analyzing the structures and conformations of sulfonic ester derivatives that are routinely used in alkaloid syntheses. The crystal structure of 4 is highly unusual, containing four independent molecules that belong to two distinct conformational types: (1) a hairpin conformation (stabilized mainly by intramolecular pi-stacking) and (2) a stepped conformation (stabilized mainly by intermolecular pi-stacking). Compound 3, on the other hand, crystallizes exclusively as the hairpin conformer. New MM+ force field parameters for sulfonic esters have been developed using the X-ray data, empirical rules, and DFT calculations to estimate the bond dipole parameters. Grid searches of conformational space for 3 and 4 using MM methods show that there are several gas-phase conformations within 5 kcal/mol of the global minimum and that the lowest energy conformations (by approximately 4.6 kcal/mol) are of the hairpin type. Analysis of the MM conformational energies suggests that the dominant intramolecular interaction stabilizing the hairpin conformations of 3 and 4 is van der Waals attraction. Moreover, the lattice energies for packing the hairpin conformations of 3 and 4 are approximately 4 kcal/mol more favorable than for the stepped conformations. Various intermolecular interactions contribute to the complexity of the observed crystal structures of 3 and 4, including electrostatic attraction between O and I atoms in neighboring molecules. Langevin dynamics (LD) simulations at several temperatures (6.0 ns, friction coefficient = 2.5 ps(-1)) indicate that the conformational exchange rates are approximately 10(10)-10(11) s(-1) over the temperature range 213-400 K, accounting for the temperature-independent (1)H NMR spectra of 3 and 4.     

Para primal varieties: A study of finite axiomatizability and definable principal congruences in locally finite varieties

We introduce new sufficient conditions for a finite algebraU to possess a finite basis of identities. The conditions are that the variety generated byU possess essentially only finitely many subdirectly irreducible algebras, and have definable principal congruences. Both conditions are satisfied if this variety is directly representable by a finite set of finite algebras. One task of the paper is to show that virtually no lattice varieties possess definable principal congruences. However, the main purpose of the paper is to apply the new criterion in proving that every para primal variety (congruence permutable variety generated by finitely many para primal algebras) is finitely axiomatizable. The paper also contains a completely new approach to the structure theory of para primal varieties which complements and extends somewhat the recent work of Clark and Krauss.     

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.     

Defect-induced dimer pinning on the Si(0 0 1) surface

Room-temperature STM images frequently show regions of antisymmetric dimer ordering surrounding certain types of defect on the Si(0 0 1) surface. While it has been generally believed that any defect asymmetric with respect to the dimer row would induce this dimer pinning effect, recent experimental results have shown that this is not the case. We present a model, based on a nearest-neighbour Ising treatment of the surface, which allows the extent of pinning caused by a dimer to be predicted from ab initio calculations. We use this model to predict the pinning extent for three phosphorus-containing structures important in a proposed silicon-based quantum computer fabrication scheme, and identify one of these asymmetric features as causing no appreciable pinning. In addition, we use ab initio calculations to identify the effects governing the interaction between neighbouring dimers.