The proceedings of the Eleventh International Congress of Pure and Applied Chemistry, held in London from July 17 to 24, covered a wide variety of topics of medical interest. Joint meetings between sections primarily concerned with human beings, animals, and plants proved of special value. Sir Henry Dale's congress lecture on "The Part of Chemistry in the New Therapeutics " was published in our last issue (Aug. 2, p. 161). The report which follows excludes also discussions on bacterial chemotherapy, the principal points from which were summarized last week in a leading article (p. 179), and the reported discovery of a lactation vitamin which was the subject of an annotation (p. 181).

Pure and Applied Chemistry

Stereoelectronic Switching in Single-Molecule Junctions

A convenient experimental system is described, with which electron transport through structurally well-defined, 2–5 nm-thick, organic films can be examined. Two types of junction J have been studied in which self-assembled monolayers (SAMs, for example, SAM(1) formed on Ag from aliphatic and aromatic thiols, and SAM(2), formed on Hg from hexadecanethiol) are in contact through either van der Waals interactions (see picture) or through covalent, hydrogen, or ionic bonds.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/1521-3757%2820020201%29114%3A3%3C394%3A%3AAID-ANGE1111394%3E3.0.CO%3B2-2

Correlating Electron Transport and Molecular Structure in Organic Thin Films

There are six categories of calculated favored dihedral angles in linear MnX2n+2 chains. The Prelog−Klyne nomenclature is not helpful for classifying them, and we propose the following labels and symbols:  A, anti, reserved for torsional angles within a few degrees of 180°; T, transoid, ω ≃ 165°; D, deviant, ω ≃ 150°; O, ortho, ω ≃ 90°; G, gauche, ω ≃ 60°; C, cisoid, ω ≃ 40°. With the exception of C, all of these categories have been observed in alkanes, perfluoroalkanes, or oligosilanes.

Conformations of Linear Chains. Systematics and Suggestions for Nomenclature

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Molecular Main Group Metal Hydrides

Conformational effects in UV absorption spectra of tetrasilanes

The making and breaking of σ bonds is an integral part of almost all photochemi- cal reactions. Yet, the electronic states of σ electrons are not nearly as well understood as the states of π-electron systems. Efforts in our laboratory to enhance the current state of their un- derstanding are described, using the specific example of oligosilanes. We address the intrin- sically cyclic nature of σ delocalization and its dependence on chain length and conforma- tion, both in terms of theory and spectroscopic experiments, from the simplest disilane chromophore to the spectral properties of the individual conformers of permethylated hep- tasilane. We also describe a new low-energy luminescence from certain conformers of per- methylated oligosilanes.

/pdf/s-bonds-electronic-structure-photophysics-and-photochemistry-2dmxq5d88c.pdf

σ Bonds: Electronic structure, photophysics, and photochemistry of oligosilanes

Four cyclic carbosilanes [(SiMe2)4(CH2)n], n = 1−4 (1−4), have been synthesized and used as models of conformationally constrained tetrasilane. Results of matrix isolation IR spectroscopy, annealing, and photodestruction, combined with HF/3-21G* calculations on low-energy conformers, suggest that in each compound the SiSiSiSi dihedral angle ω is constrained within relatively narrow limits:  1, 0°; 2, 35−55°; 3, 45−65° (30° for a higher-energy conformer); and 4, 60−80°. It is argued that other structural differences between 1−4 and the linear chain analogue, decamethyltetrasilane Si4Me10 (5), are of secondary importance, that Koopmans' theorem can be used within a family of closely related structures, and that measurements of photoelectron spectra of 1−5 permit the construction of an experimental counterpart to the orbital energy correlation diagram for the syn−anti conformational transformation in tetrasilane. The trends found for the first three ionization potentials, assigned to electron removal from th...

Conformational Effects in Photoelectron Spectra of Tetrasilanes

Magnetic circular dichroism (MCD) of five peralkylated tetrasilanes (1–5) conformationally constrained to angles ranging from nearly 0° to 180° and of the open chain tetrasilane Si4Me10 (6) shows a clear conformational dependence and permits the detection of previously hidden transitions. In the tetrasilane CH2Si4Me8 (1), with the smallest dihedral angle, comparison of MCD with absorption spectra reveals four low-energy electronic transitions. In the tetrasilanes 2–4, three distinct transitions are apparent. In tetrasilanes 5 and 6, MCD reveals the very weak transition that has been predicted to be buried under the first intense peak and to which the anomalous thermochromism of 6 and other short-chain oligosilanes has been attributed.

Roman Imhof

Papers

Conformational effects in UV absorption spectra of tetrasilanes

σ Bonds: Electronic structure, photophysics, and photochemistry of oligosilanes

Conformational Effects in Photoelectron Spectra of Tetrasilanes

Magnetic circular dichroism of peralkylated tetrasilane conformers