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Joseph B Lambert

Publications and source records attributed to Joseph B Lambert.

9 recordsLinked to original sources
PubMed2003

The C5SiMe7+ cation: pyramidal, bicyclic, or cyclohexadienyl?

The monosila analogue of the C6Me7+ cation can be accessed by hydride abstraction from (pentamethylcyclopentadienyl)dimethylsilane, Cp*SiMe2H. Treatment of this material with triphenylmethylium tetrakis(pentafluorophenyl)borate in dichloromethane at -50 degrees C produced a single cationic species stable for a day or longer. The presence of a single resonance for the ring methyls and a single resonance for the silyl methyls in both the 1H and the 13C spectra indicated either a static structure with fivefold symmetry or a dynamic structure in which the dimethylsilyl group equilibrates rapidly among positions. Density function theory calculations found a minimum whose structure had the unsymmetrical silabicyclo[3.1.0]hexenyl structure. Calculated 1H, 13C, and 29Si chemical shifts for this structure were in good agreement with the observed values for this structure. Another calculated minimum was the silacyclohexadienyl structure, but its calculated chemical shifts were very different from those observed. We conclude that the cation has the bicyclic structure but rapidly interconverts the ring positions to produce the very simple NMR spectra.

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PubMed2003

A novel family of ordered, mesoporous inorganic/organic hybrid polymers containing covalently and multiply bound microporous organic hosts.

We have prepared a new family of periodic hybrid polymers containing microporous cavities provided by covalently bound organic hosts. Cyclodextrin (CD) or calixarene (CX) hosts are attached to four or more trialkoxysilyl groups, which are polymerized to form a polysilsesquioxane matrix. Structural integrity is provided by copolymerization with tetraethoxysilane, which produces a polysilicate co-matrix. Periodic order is created by carrying out the polymerization in the presence of a surfactant, cetyltrimethylammonium bromide. The resulting as-synthesized polymers from these three starting materials were characterized by solid-state nuclear magnetic resonance spectroscopy. The (13)C and (29)Si spectra provided evidence for intact polysilsesquioxane, polysilicate, organic host, and surfactant. Removal of the surfactant by washing produced a polymer containing cavities of mesoporous dimensions, in addition to the microporous host cavities. The purpose of introducing mesoporosity is to allow enhanced access of guests to the microporous hosts. Transmission electron microscopy demonstrated that both as-synthesized and solvent-extracted polymers have a periodic structure. All polymers are completely insoluble in water. The as-synthesized CD-containing polymers extracted up to >99% of 4-nitrophenol from aqueous solution, and the solvent-extracted CX-containing polymers extracted up to 67% of Fe(3+) and lesser amounts of other metal cations from aqueous solution, with interesting selectivity patterns. Simple filtration then removes the polymer containing the extracted organic molecule or metal cation. These extraction abilities are superior to previous materials.

Journal Article↗
PubMed2003

A free, tricoordinate stannylium cation.

Tris(2,4,6-triisopropylphenyl)stannylium tetrakis(pentafluorophenyl)borate constitutes a free, tricoordinate tin cation according to its X-ray structure. There is no coordination between the cation and either solvent or anion, and there are no atoms at apical positions. DFT calculations confirm the structure and indicate that there is no agostic bonding between the ortho isopropyl methinyl hydrogens and the Sn atom. Calculation of the 119Sn chemical shift is in good agreement with the observed value.

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PubMed2002

Crystallographic evidence for a free silylium ion.

Evidence for a three-coordinate silyl cation is provided by the crystal structure of [(Mes)3Si][H-CB11Me5Br6].C6H6 (where Mes is 2,4,6-trimethylphenyl). Free (Mes)3Si+ cations are well separated from the carborane anions and benzene solvate molecules. Ortho-methyl groups of the mesityl substituents shield the silicon atom from the close approach of nucleophiles, while remaining innocent as significant ligands themselves. The silicon center is three-coordinate and planar. The downfield 29Si nuclear magnetic resonance chemical shift in the solid state (226.7 parts per million) is almost identical to that in benzene solution and in "gas phase" calculations, indicating that three-coordination can be preserved in all phases.

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PubMed2002

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Journal Article↗
PubMed2002

Vertical and nonvertical participation by sulfur, selenium, and tellurium.

The mechanism of stabilization of positive charge on carbon by sulfur, selenium, or tellurium at the beta-position has been investigated kinetically, by measurement of rate enhancements, and structurally, by variation of the bond strength to the leaving group. Stabilization can occur either nonvertically with formation of a bridged intermediate or vertically through hyperconjugation within an open carbocation. We observed large rate enhancements (10(5) for S, 10(6) for Se) in 97% aqueous trifluoroethanol with trifluoroacetate as the leaving group. These enhancements are consistent with either mechanism. Product structures also are consistent with either mechanism. Nine crystal structures revealed that the bond to the leaving group (C-O) is lengthened by the presence of S or Se at the beta-position, in proportion to the basicity of the leaving group. This lengthening is not accompanied by angle distortions expected for the bridging mechanism. The crystallographic data support vertical (hyperconjugative) character along the reaction coordinate, more so for selenium than sulfur.

Journal Article↗
PubMed2002

Amber: the organic gemstone.

Resins are produced by woody plants on a worldwide basis. We have found several distinct classes of modern diterpenoid resins based phenomenologically on the solid-state (13)C NMR spectra of the bulk material. Resin fossilizes over millions of years into a robust material sometimes called amber. We have characterized several hundred samples of fossil resin by solid-state (13)C NMR spectroscopy. We can relate one globe-spanning group of fossil resins to the modern genus Agathis, based on spectral evolution over time. A second large group has not been related with certainty to specific modern plants. Fossil resins from Europe fall into two categories, the famous Baltic ambers and another that resembles the Agathis group. Fossil resins from the Americas and Africa are closely related to the modern genus Hymenaea. Based on spectral distinctions, fossil resin found in an archaeological context sometimes can be assigned to a specific geographical origin on the basis of its (13)C NMR spectrum.

Amber↗