Studies on the mechanism of action of prekinamycin, a member of the diazoparaquinone family of natural products: evidence for both sp2 radical and orthoquinonemethide intermediates

The putative reductive activation chemistry of the diazoparaquinone antibiotics was modeled with Bu(3)Sn-H and prekinamycin dimethyl ether along with prekinamycin itself. Reaction in various combinations of aromatic solvents, with and without the nucleophile benzylmercaptan present, led to isolation of both radical-trapping arene adducts and nucleophilic capture benzyl thioether products. On the basis of these product distribution studies, the intermediacies of, first, a cyclopentenyl radical and, next, an orthoquinonemethide electrophile are postulated.     

Waiting and sojourn times in a multi-server queue with mixed priorities

We consider a multi-server queue with K priority classes. In this system, customers of the P highest priorities (P<K) can preempt customers with lower priorities, ejecting them from service and sending them back into the queue. Service times are assumed exponential with the same mean for all classes. The Laplace–Stieltjes transforms of waiting times are calculated explicitly and the Laplace–Stieltjes transforms of sojourn times are provided in an implicit form via a system of functional equations. In both cases, moments of any order can be easily calculated. Specifically, we provide formulae for the steady state means and the second moments of waiting times for all priority classes. We also study some approximations of sojourn-time distributions via their moments. In a practical part of our paper, we discuss the use of mixed priorities for different types of Service Level Agreements, including an example based on a real scheduling problem of IT support teams.      

IR Spectroscopic Study of Chemical Transformations upon Irradiation of the Poly(vinyl chloride)–Triallyl Cyanurate System

Chemical transformations resulting from irradiation of the poly(vinyl chloride) (PVC)–triallyl cyanurate (TAC) system were studied by IR spectroscopy. It was shown that crosslinking was accompanied by scission of the network structure formed in the initial dose range, and the scission process occurred even at small irradiation doses. The scission is assumed to be localized at the interface boundary between PVC and poly(iso-TAC) or poly(TAC).     

Positive Hole Transfer Upon Irradiation of Frozen Alkylbenzene Solutions in Freonic Matrices at 77 K

The process of positive hole transfer between alkylbenzene molecules of various structures occurring upon irradiation in electron-scavenging (freonic) matrices at 77 K was studied by ESR. The factors controlling the efficiency and direction of this process were analyzed. In the case of the ethylbenzene–toluene pair, it was shown that the direction of hole transfer depends on the conformation of the ethylbenzene radical cation. If the conformation effects are absent, the hole transfer occurs in accordance with the gas-phase ionization potentials (IP) and appears to be efficient, provided that the difference in the IP values is greater than 0.3 eV.     

Dielectric properties and Fourier transform IR analysis of MCM-48, Al-MCM-48 and Ti-MCM-48 mesoporous materials

 Broad-band dielectric spectroscopy was used to investigate the dielectric properties of the meso- porous materials MCM-48, Al-MCM-48 and Ti-MCM-48. The samples were examined in the frequency range from 20 Hz to 1 MHz and in the temperature range from −100 to 250 °C. The dielectric relaxation of the materials has a complex nonexponential behavior with some common features for all the samples. The dielectric spectroscopy and Fourier transform IR measurements identified the relaxation process related to percolation of H⁺ ions associated with silanol groups and water adsorbed in the materials. The non-Debye behavior of the macroscopic dipole correlation functions related to the percolation process allowed us to extract the fractal dimensions of the paths of excitation transfer within the porous medium, and the porosity of each sample was estimated. Received: 7 September 1999 Accepted: 10 December 1999     

Variational Evolution Problems and Nonlocal Geometric Motion

We consider two variational evolution problems related to Monge-Kantorovich mass transfer. These problems provide models for collapsing sandpiles and for compression molding. We prove the following connection between these problems and nonlocal geometric curvature motion: The distance functions to surfaces moving according to certain nonlocal geometric laws are solutions of the variational evolution problems. Thus we do the first step of the proof of heuristics developed in earlier works. The main techniques we use are differential‐equation methods in the Monge‐Kantorovich theory. (Accepted May 19, 1998)     

Quasi-long-range order in the random anisotropy Heisenberg model

The random field and random anisotropy N-vector models are studied with the functional renormalization group in 4−ε dimensions. The random anisotropy Heisenberg (N=3) model has a phase with an infinite correlation length at low temperatures and weak disorder. The correlation function of the magnetization obeys a power law 〈m(r ₁)m(r ₂)〉∼|r ₁−r ₂|^{− 0.62ε}. The magnetic susceptibility diverges at low fields as χ∼H ^−1+0.15ε. In the random field N-vector model the correlation length is finite at arbitrarily weak disorder for any N>3. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 130–135 (25 July 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.