Description: This article discusses Public-Access Computer Systems (PACS) and the Internet. Recursive Reviews is a new column that will identify and briefly describe articles that deal with PACS and related topics in both library and computer science literature. The "recursive" in the name of the column emphasizes the idea that the discussion of information technology in libraries changes the underlying precepts of the discussion. The dialogue concerning uses of library technology redefines itself in this way, and can therefore be seen as recursive. All followers of the PACS-L forum are aware by now that a great many library catalog systems are accessible via the Internet. The availability of these resources raises a great many questions and possibilities in the library and network user communities. What can be accomplished with this new communications channel? Exactly what is the Internet? What is its extent, and how does it differ from other computer networks? The articles and books reviewed in this column will be of use to anyone having questions about library systems and the Internet, from those unfamiliar with networking technology to those very conversant with it.

Description: This article discusses principal resonance contributors to high-valent, transition-metal alkylidene complexes. The results of ab initio calculations are reported for prototypical high-valent, alkylidene complexes. Stationary points on each potential energy surface are characterized and compared to experimental information where available; as long as a suitably flexible valence basis set is used, good agreement between theoretically calculated and experimentally determined geometries is obtained. The complexes of interest include group IVB (Ti, Zr and Hf) and group VB (Nb and Ta) alkylidenes with hydride ligands as well as models for the four-coordinate, olefin metathesis catalysts (Mo-, W-, and Re-alkylidenes) which have been recently synthesized and characterized. In light of the fact that much of the discussion concerning the reactivity of transition-metal carbene complexes has been presented in terms of the resonance contributors derived from rearranging the electrons in the M-C σ and π orbitals, the minima obtained from the portion of the study are then subjected to a further procedure to calculate these contributions. Resonance structures in which the carbon is the negative end of the M-C bond (i.e., nucleophilic resonance structures) contribute 50% to the ground-state wave function of these complexes. Those in which the carbon is formally neutral account for ...

Description: This article discusses K-shell x-ray-production cross sections. Abstract: K-shell x-ray-production cross sections are reported for elements with K-shell x-ray energies between 277 eV (C) and 1487 eV (Al). The x-ray measurements were made with a windowless Si(Li) detector that was calibrated for efficiency by comparing bremsstrahlung spectra from electron bombardment of thin foils of aluminum, silver, and gold with theoretically determined bremsstrahlung spectral distributions. The x-ray-production cross-section measurements are compared to first-order Born and perturbed-stationary-state with energy-loss, Coulomb deflection, and relativistic corrections (ECPSSR) ionization theories using single-hole fluorescence yields. The ECPSSR and first-order Born theoretical predictions are, in general, in close agreement with each other and both generally fit the data quite well.

Description: This article discusses high-valent transition-metal alkylidene complexes. Abstract: An ab initio investigation into the effects of ligand and substituent modification on the metal-carbon double bond is reported. Prototypical group IVB (Ti, Zr, Hf) and Group VB (Nb, Ta) alkylidenes are chosen for this study. The MC/LMO/CI (multiconfiguration/localized molecular orbital/configuration interaction) procedure is used to examine the electronic structures of these complexes in terms of the prime resonance contributors to the ground-state wave function. The main conclusion drawn from this work is that the intrinsic nature of the metal-carbon double bond can typically be changed only within certain limits by modification of the electronegativity of the ligands (L) and substituents (Z). In other words, the Ta=C bond in H₃TaCCl₂ and Cl₃TaCH₂ and presumably in experimentally characterized analogues with larger ligands and substituents, e.g., Cp and neopentyl. Significant changes in the electronic structure are effected in three ways: The first way is through the introduction of a highly electropositive substituent, e.g., Li. This makes the metal-carbon bond closer to a triple bond for the Ta-alkylidenes. The second way to change the electronic structure of the alkylidenes significantly is to change the central metal atom. The heaviest members of groups IVB (Hf) and ...

Description: This article discusses direct evidence for the amorphous silicon phase in visible photoluminescent porous silicon. Abstract: We report on micro-Raman spectroscopy studies of porous silicon which show an amorphous silicon Raman line at 480 R cm-1 from regions that emit visible photoluminescence. A Raman line corresponding to microcrystalline silicon at 510 R cm-1 is also observed. X-ray photoelectron spectroscopy data is presented which shows a high silicon-dioxide content in porous silicon consistent with an amorphous silicon phase.