Biogeochemistry, a valuable tool in mineral exploration and environmental assessment

Biogeochemistry enables us to assess the quality of an environment by determining whether the trace elements in that environment are normal or anomalous. Moreover, a biogeochemical study can be undertaken without harming that environment and, at the same time, may lead to the discovery of ore bodies.     

Acylation of extended enolate ions from α-phenylthio (PhS-)crotonate esters

C-Acylation, like alkylation, occurs at the α opposition of the title compounds to give unsaturated keto-esters from which the PhS group may be removed     

Jahn-Teller activity in eight-coordinate Oh fx systems

The angular overlap model is used for the calculation of the Jahn-Teller coupling constants for O_h, MX₈, f^x systems in the (LSJM_J) basis. For the 5f¹ anion, [UF₈]^3?, the Γ₈ ground state is predicted to show substantial Jahn-Teller activity for coupling to the τ_2g bending mode, with E_JT comparable to ω, a result consistent with the available magnetic susceptibility data.     

Synthesis of crosslinked poly(styrene-co-sodium styrenesulfonate) latexes

Latexes of 100 nm diameter were synthesized by emulsifier-free copolymerization of styrene, sodium styrenesulfonate, and 1–5 wt% divinylbenzene using persulfate initiator at 91°C. A shot growth method was used to incorporate a high density of sulfonate groups. Coefficients of variation of particle size were 0.04–0.08 without resort to seed growth polymerization. Redox initiation at 40–50°C produced larger, more polydisperse, and less colloidally stable crosslinked latexes. © 1994 John Wiley & Sons, Inc.     

Nodal Nomenclature–General Principles

A new, comprehensive graph-based nomenclature system for chemical substances is proposed. Substances are named by using nodal nomenclature to name graphs derived from their structures by ignoring chemical details such as atom identities, bonding, charges, etc. After the fundamental graph is named, these chemical details are introduced by modifications and additions to the name of the graph to yield the name of the actual substance. Although development of nodal nomenclature originated with problems in naming some organic systems, it is not limited to organic substances. In this paper, only general principles for naming the graph are described; applications to specific substances will be discussed in future papers.     

Low frequency double electron muon resonance (Demur) in fused quartz

Double electron muon resonance was studied with comparable amplitudes of the static and rf magnetic fields. The experiments were carried out in fused quartz and employed a resonant linearly-polarized rf field and a small static field. For small amplitudes of the rf field a three-line frequency spectrum was obtained consisting of a central line at the rf frequency and two weaker lines of equal amplitude symmetrically positioned about the central line. A symmetrical three line spectrum is predicted for muonium using the rotating wave approximation (RWA). For large rf fields the amplitudes of the three lines were not symmetric and depended on whether positrons were observed parallel or perpendicular to the beam. For very large rf fields the total amplitude of all the lines was less than for lower rf fields. These results do not agree with the theory using the RWA and no satisfactory theory currently exists.     

EXCITON TRANSFER OUT OF OPEN PHOTOSYSTEM II REACTION CENTERS

Abstract A simple photochemical model of photosystem II consisting of antenna chlorophyll and a reaction center was used to examine the phenomenon of exciton detrapping, i.e. the transfer of excitation energy from open reaction centers back to the antenna. η, the ratio of the probability of detrapping when the reaction centers are all open, Ψ_t(o) to the probability when the centers are closed, Ψ_t(x) was used as a variable parameter to examine the various pathways of energy dissipation in a system in which P, the yield of photochemistry, and R, the ratio of the maximum to the minimum yields of fluorescence, were assumed to be known (e.g. R= 4.0 and P= 0.90). It is shown that η must fall within a range of values between 0 and R (1 –P) and that, for given values of R and P, Ψ_t(o) and the ratio of the rate constant for photochemistry at the reaction center, k_p, to the rate constant for energy transfer back to the antenna, k_t, can be determined for any assumed value of η. Even though detrapping occurs at open reaction centers, it is the magnitude of the yield of nonradiative decay at closed reaction centers, Ψ_a(x) which sets the upper limit on η. Equations for the overall yields of fluorescence and nonradiative decay in the antenna chlorophyll and of nonradiative decay at the reaction center chlorophyll, under conditions of both open and closed reaction centers, were derived in conventional probability terms and in terms of R, P and η. As η increases within its range of permissible values, energy dissipation in the antenna decreases and nonradiative decay at the reaction center increases. The determination of a specific value of η or of the ratio k_pk_t would require additional information such as the value of the maximum yield of fluorescence and the ratio of the rate constants for fluorescence and nonradiative decay in the antenna chlorophyll. The characteristics of a system in which there is no nonradiative decay in the reaction center (i.e. k_d= 0), in which case R (1 –P) = 1.0, were also examined. In this case the yield of detrapping has no influence on energy dissipation in the system. Finally, the question of heterogeneity in PSII was considered. It is suggested that Ψ_d(x) may be greater in PSII_β than in PSII_α so that the probability of detrapping could be greater in the PSII_α fraction.     

Analysis and Experiments for a Computational Model of a Heat Bath

A question of some interest in computational statistical mechanics is whether macroscopic quantities can be accurately computed without detailed resolution of the fastest scales in the problem. To address this question a simple model for a distinguished particle immersed in a heat bath is studied (due to Ford and Kac). The model yields a Hamiltonian system of dimension 2N+2 for the distinguished particle and the degrees of freedom describing the bath. It is proven that, in the limit of an infinite number of particles in the heat bath (N), the motion of the distinguished particle is governed by a stochastic differential equation (SDE) of dimension 2. Numerical experiments are then conducted on the Hamiltonian system of dimension 2N+2 (N1) to investigate whether the motion of the distinguished particle is accurately computed (i.e., whether it is close to the solution of the SDE) when the time step is small relative to the natural time scale of the distinguished particle, but the product of the fastest frequency in the heat bath and the time step is not small—the underresolved regime in which many computations are performed. It is shown that certain methods accurately compute the limiting behavior of the distinguished particle, while others do not. Those that do not are shown to compute a different, incorrect, macroscopic limit.     

Dynamic Hierarchical Packing of Wireless Switches Using a Seed, Repair and Replace Genetic Algorithm

Suppose that items of equipment are to be added to a supply station (e.g., new switch modules are to be added to a telecommunications switch) over time to meet growing demand requirements. Both supply and demand have multiple components: an item of equipment supplies different amounts of several resources, and demand may be expressed in terms of the vector of resources required. There are several different types of equipment to choose among, each type supplying known amounts of each resource per unit of equipment. The supply station is organized into bays, shelves, or other capacitated containers so that when the cumulative amount of equipment added exceeds the holding capacity of the installed containers, new containers must be added, creating a relatively large jump in cumulative costs. Thus, it is desirable to sequentially pack items of equipment into the available containers, by choosing which types of equipment to install when, so as to minimize the total cost of covering demand in each period. We discuss an instance of this problem arising from wireless telephony and describe the performance of a conventional branch-and-bound optimization algorithm for solving it. The branch-and-bound approach works well on small instances of the problem, and has been used successfully in practical planning. However, it can take CPU-days to run, thus preventing development of a useful interactive planning tool. Therefore, we introduce a novel seed, repair, and replace genetic algorithm (SRR-GA) for solving dynamic packing problems of this type. We contrast its performance with the branch-and-bound algorithm's on both hand-generated and randomly-created dynamic packing problems, finding that the SRR-GA is two to three orders of magnitude faster and produces solutions of equal or better quality on practical problems. Variations of the dynamic packing problem and of the SRR-GA for solving it are mentioned, and the paper concludes by suggesting other potential applications of the SRR-GA to hard combinatorial optimization problems.     

Perspectives of approximate dynamic programming

Approximate dynamic programming has evolved, initially independently, within operations research, computer science and the engineering controls community, all searching for practical tools for solving sequential stochastic optimization problems. More so than other communities, operations research continued to develop the theory behind the basic model introduced by Bellman with discrete states and actions, even while authors as early as Bellman himself recognized its limits due to the “curse of dimensionality” inherent in discrete state spaces. In response to these limitations, subcommunities in computer science, control theory and operations research have developed a variety of methods for solving different classes of stochastic, dynamic optimization problems, creating the appearance of a jungle of competing approaches. In this article, we show that there is actually a common theme to these strategies, and underpinning the entire field remains the fundamental algorithmic strategies of value and policy iteration that were first introduced in the 1950’s and 60’s.