[18]Annulene

The geometry of [18]annulene has been fully optimized within the D_6h point group (la) utilizing ab initio MO theory at the STO-3G level to give carbon-carbon bond lengths of 1.391 and 1.400 Å. This structure is found to be 35.7 kcal/mol less stable than a D_3h structure (1b) with alternating bond lengths of 1.330 and 1.479 Å. The ionization potentials are calculated to be in the ranges. 6.2–7.1 (1a) and 7.4–8.1 eV (1b)     

Rehybridization and the Chemical Reaction: POAV and 3D-HMO Analysis

In this study chemical reactions are examined from a new standpoint — rehybridization. With the possible exception of oxidation-reduction processes, all chemical reactions involve rehybridization. Bondmaking and bond-breaking processes are nothing more than changes in hybridization. ln the present study two categories of pericyclic reactions are examined with the π-orbital axis vector (POAV) analysis and-the three-dimensional (3D) HMO theory. The first makes use of a series of related molecular structures derived from X-ray crystallography which provide information on the reaction pathway between the valence tautomers 1,6-methano[10]annulene (1a) and the bisnorcaradiene (1b). The second category involves structures calculated by Dewar and coworkers along the minimum energy reaction pathway of the Cope rearrangement of 1,5-hexadiene (2). The analysis provides a clear delineation of the structural and electronic changes which accompany chemical reaction. The sense of the π-orbital axis vectors at the bridgehead atoms is shown to be the determining feature in dividing the potential surface of 1 between bridged-[10]annulene (1a) and bisnorcaradiene (1b).     

Large-scale fabrication of aligned single-walled carbon nanotube array and hierarchical single-walled carbon nanotube assembly

Aligned single-walled carbon nanotubes (SWNTs) and hierarchical SWNT assembly were fabricated by electrospinning. The high fiber elongation and high DC electric field applied during the electrospinning process result in the orientation of the SWNTs along the axial direction of the fiber. The alignment of the electropsun composite fiber transfers this local SWNT orientation to macroscopically aligned SWNTs. After removing the polymer component from the aligned composite fiber, we produced large area aligned SWNTs. The results show that the directional control of SWNT alignment and debundling of SWNTs into individual tubes can be simultaneously realized.     

Sidewall functionalization of single-walled carbon nanotubes by addition of dichlorocarbene

We report the sidewall functionalization of soluble HiPco single-walled carbon nanotubes (SWNTs) by addition of dichlorocarbene. The dichlorocarbene-functionalized SWNTs [(s-SWNT)CCl(2)] retain their solubility in organic solvents such as tetrahydrofuran and dichlorobenzene. The degree of dichlorocarbene functionalization was varied between 12% and 23% by using different amounts of the dichlorocarbene precursor. Because the addition of dichlorocarbene saturates the carbon atoms on the sidewall of the SWNTs and effectively replaces the delocalized partial double bonds with a cyclopropane functionality, the optical spectra of the SWNTs change dramatically. We estimate that the saturation of 25% of the pi-network electronic structure of the SWNTs is sufficient to remove all vestiges of the interband transitions in the infrared spectrum. The transitions at the Fermi level in the metallic SWNTs that appear in the far-infrared (FIR) region of the spectrum show a dramatic decrease of intensity on dichlorocarbene functionalization. The FIR region of the spectrum allows a clear differentiation between the covalent and the ionic chemistry of SWNTs. In contrast with covalent functionalization, we show that reaction of the SWNTs with bromine vapor leads to a strong increase in absorptions at the Fermi level that is observable in the FIR due to hole doping of the semiconducting SWNTs. Thermal treatment of the (s-SWNT)CCl(2) above 300 degrees C resulted in the breakage of C-Cl bonds, but did not restore the original electronic structure of the SWNTs.     

High resolution capillary electrophoresis of carbon nanotubes

Purification of single-walled carbon nanotubes by capillary electrophoresis (CE) is demonstrated. Real-time Raman spectroscopy of the separation process and single-wavelength UV/vis detection show the ability of CE to provide high-resolution separations of nanotube fractions with baseline separation. AFM images of collected fractions demonstrate that separations are based on tube length. The separation method is suggested to be based on alignment of the nanotubes along the separation field.     

The effects of sample preparation methods on the variability of the electrospray ionization response for model drug compounds

A post-column infusion system was developed in order to analyze suppression of electrospray ionization (ESI) tandem mass spectrometry response in the presence of endogenous plasma interferences. By enabling direct detection of these interfering components, this experimental system was used to analyze the ability of several common extraction procedures to remove endogenous plasma components that cause changes in the ESI response of model drug substances. Methyl-t-butyl ether (MTBE) liquid-liquid, Oasis and Empore solid-phase, and acetonitrile (ACN) protein precipitation sample preparation methods were tested using the post-column infusion system. In all cases, ACN protein precipitation samples showed the greatest amount of ESI response suppression while liquid-liquid extracts demonstrated the least. In addition, the three test compounds, phenacetin, caffeine, and a representative Merck compound, demonstrated that ESI response suppression is compound dependent. Suppression was greatest with caffeine, the most polar analyte, and the smallest for the Merck compound, the least polar analyte. Copyright 1999 John Wiley & Sons, Ltd.