Joseph B. Lambert

Tetrahedron report number 273 : The interaction of silicon with positively charged carbon

Electron deficient species have long intrigued organic chemists. More than half a century ago, Whitmore developed the chemistry of carbocations here at Northwestern University and later at The Pennsylvania State University. The interaction of positively charged carbon with directly attached substituents that possess n or n electrons led to better understanding of the concept of conjugation, as in the familiar carbocations CH-CH-CH:, C6H5-CH:, and CH,O-CH: . The interaction with more remotely substituted groups, also possessing K or n electrons, as in PhCH,CH: or Me2NCH2CH:, led to the concepts of neighboring group participation and homoconjugation, In one of his last studies, Whitmore described the remarkable manner in which silyl substituents interact with positive charge on carbon. ’ In basic hydrolyses and ethanolyses, Whitmore and his co-workers found that a silyl group beta to the leaving group enormously enhances the rate of solvolysis, that a gamma silyl group also has an enhancing effect, but that an alpha silyl group actually decreases reactivity in comparison with hydrocarbon models. Sommer, a principal coworker with Whitmore on this study, went on to become one of the leading silicon chemists of his generation. The chart below summarizes possible structural relationships between a silicon-containing sub-

Crystal Structure of a Silyl Cation with No Coordination to Anion and Distant Coordination to Solvent

The crystal structure of a stable silyl cation, triethylsilylium, in the form of its tetrakis (pentafluorophenyl)borate salt [Et3Si+ (C6F5)4B-] (Et, ethyl) shows no coordination between cation and anion. The closest silicon-fluorine distance is greater than 4 angstroms. A toluene solvent molecule is close enough to cause some deviations from planarity at the silicon, but the silicon-toluene distance is well beyond the sum of the silicon and carbon covalent radii. The toluene molecule is essentially planar and undistorted, as expected if little or no positive charge has been transferred from silicon to toluene.

The Silicate-Mediated Formose Reaction: Bottom-Up Synthesis of Sugar Silicates

Silicate in the Primordial Soup Direct evidence for how prebiotic synthesis of complex organic molecules paved the way for the origin of life is extremely scarce. Thus, studies are mainly limited to controlled simulations of likely reactions in early Earth conditions. Similarly, chemical reactions in the laboratory may generate the products necessary for biosynthesis, but may nevertheless be geochemically irrelevant. Lambert et al. (p. 984) show that silicate ions, present in Earths surface waters at relatively high concentrations, catalyze the formation of four- and six-carbon sugars from simple sugars via the formose reaction. The resulting complexes stabilize the sugar molecules, allowing sugars to accumulate in greater abundance. Silicate stabilization also circumvents the need for the formose reaction to proceed at high temperatures, thus extending the range of possible environments in which life could have originated. Sugar growth and stabilization catalyzed by silicate ions is a possible prebiotic synthetic pathway. Understanding the mechanism of sugar formation and stabilization is important for constraining theories on the abiotic origin of complex biomolecules. Although previous studies have produced sugars from small molecules through the formose and related reactions, the product mixtures are complex and unstable. We have demonstrated that simple two- and three-carbon molecules (glycolaldehyde and glyceraldehyde), in the presence of aqueous sodium silicate, spontaneously form silicate complexes of four- and six-carbon sugars, respectively. Silicate selects for sugars with a specific stereochemistry and sequesters them from rapid decomposition. Given the abundance of silicate minerals, these observations suggest that formose-like reactions may provide a feasible pathway for the abiotic formation of biologically important sugars, such as ribose.

Carbon functionalities in amber.

High-resolution nuclear magnetic resonance spectra of the carbon nuclei in powdered amber, obtained by using the techniques of magic angle spinning and cross polarization, provide detailed information about the types of carbon functionalities. The entire spectrum of Baltic amber (succinite) is identical for several samples. Baltic amber shows minor differences from Sicilian amber and drastic differences from Burmese, Romanian, and Bohemian ambers.

Bone diagenesis and dietary analysis

Alteration of buried bone through natural diagenesis can vitiate any analysis of ancient diet based on concentrations of inorganic elements. Consequently, several methods have been developed to assess the presence and extent of diagenetic effects. These include comparison of modern with excavated bone, comparison of different skeletal components such as rib with femur, examination of elemental content as a function of the age of the individual at death, elemental distribution across the bone cross section, analysis opf archeological soils, comparison of herbivores and carnivores, and analysis of isotope ratios. Strontium and zinc appear to be the least sensitive elements to diagenesis. Calcium and sodium may be lost through leaching but may still be useful in the dietary context. Magnesium and lead give mixed results by these tests and might be useful under certain circumstances. Iron, manganese, aluminium, potassium, copper, barium, vanadium, and uranium are particularly sensitive to diagnetic effects.

Chemical signatures of fossilized resins and recent plant exudates.

Amber is one of the few gemstones based on an organic structure. Found over most of the world, it is the fossil form of sticky plant exudates called resins. Investigation of amber by modern analytical techniques provides structural information and insight into the identity of the ancient plants that produced the source resin. Mass spectrometric analysis of materials separated by gas chromatography has identified specific compounds that are the basis of a reliable classification of the different types of amber. NMR spectroscopy of bulk, solid amber provides a complementary classification. NMR spectroscopy also can be used to characterize modern resins as well as other types of plant exudates such as gums, gum resins, and kinos, which strongly resemble resins in appearance but have very different molecular constitutions.

Induced metal-ion exchange in excavated human bone

Excavated human bone was exposed to aqueous solutions containing high concentrations of a single added metal ion in order to examine the extent of introduction of contaminating materials during burial. Variables included pH, temperature, ion concentration, state of bone (whole or crushed), structure of buffer, and counterion. Calcium and sodium showed little increase, and even a decrease in some cases. Strontium, zinc, lead, and magnesium showed large increases probably through heteroionic replacement of calcium. Manganese, aluminum, and potassium showed increases, particularly under neutral conditions, probably through infiltration into voids and defects.

Nuclear magnetic resonance in archaeology

Nuclear magnetic resonance spectroscopy is a useful analytical tool for the examination of archaeological artifacts. Both organic and inorganic materials have been examined in solution and in the solid. NMR can identify sources of raw materials, verify artifact authenticity, delineate ancient technology, and specify ancient diet.

Strong conductance variation in conformationally constrained oligosilane tunnel junctions

The effects of molecular conformation on conductance in oligosilane-bridged metal-molecule-metal junctions are studied theoretically using density functional theory combined with a nonequilibrium Greens function approach. Varying the internal SiSiSiSi dihedral angles in hexasilane diamine chains changes the conductance by up to 3 orders of magnitude. This conformational dependence is due to the effects of sigma-delocalization on the positions of the highest occupied molecular orbital (HOMO) energies. The conductance values for the different conformations are related to electron transfer rates in donor-bridge-acceptor systems, and the effect of shifting the injection energy is examined. The transport properties are found to be extremely sensitive to the alignment between the HOMO energies and Fermi level of the gold electrodes.

Inorganic analysis of excavated human bone after surface removal

Abstract In order to decrease the effects of contamination during burial, we have removed about 1 mm from the surfaces of 50 human femurs from the Gibson (Middle Woodland), Ledders (Late Woodland), and Helton (Emergent Mississippian) sites. Elemental analysis of these abraded samples was compared with analysis of unabraded samples from the same femurs. Surface removal reduced levels of contaminative elements such as Fe, Al, and Mn but had little or no effect on Ca, Sr, and Ba. Analysis of variance was altered by cleaning. Many apparent differences between the sexes disappeared and must have been due to differential contamination. Other differences based on sex, age, site, or time are retained, improved, or initially uncovered by surface cleaning.