Influence of bond fixation in benzo-annulated N-salicylideneanilines and their ortho-C(=O)X derivatives (X = CH3, NH2, OCH3) on tautomeric equilibria in solution

1H, 13C, and 15N NMR spectra show that an ortho-C(=O)X group present in the molecules of N-salicylideneanthranilamide (X = NH2), methyl N-salicylideneanthranilate (X = OCH3), N-salicylidene-o-aminoacetophenone (X = CH3), and their benzo analogues have only a minor effect on the tautomeric OH/NH-equilibrium in solution. Only two of three possible tautomers were detected. Lability of the absent form was proved by theoretical calculations. Calculated energies show that the enolimino form (OH) is less stable than the enaminone (NH) form only for dibenzo-annulated N-salicylideneanilines. The population of each species in the tautomeric mixture was found to be inversely proportional to its energy. Application of the geometry-based aromaticity index HOMA shows that the effectiveness of the pi-electron delocalization in different rings in the molecule depends mostly on the position of benzo-annulation. Both the NH...O and N...HO hydrogen bonds present in the NH and OH tautomers, respectively, increase the aromaticity of the quasirings H-O-C=C-C=N and O=C-C=C-N-H and decrease the aromatic character of the fused benzene ring. These results seem to be reliable when N-salicylideneanilines studied are compared with naphthalene and their benzo-annulated derivatives, i.e., phenanthrene, anthracene, and triphenylene. An analysis of the effectiveness of pi-electron delocalization confirms that in all cases studied, the OH form is more stable. Although the HOMA values and calculated energies are not a criterion that allows determination of the dominating tautomer, both of these parameters correctly show the effect of changes in the molecular topology on tautomeric preferences.     

Combined Raman spectroscopy and transmission electron microscopy studies of a NanoBud structure

We succeeded in providing for the first time the evidence of the NanoBud structure (fullerenes attached to the surface of SWCNT) by Raman spectroscopy in combination with TEM. This work was performed on the individual freestanding NanoBud structure. The Raman features corresponding to the typical vibrational modes of fullerenes were observed in the range of 1440-1480 cm(-1). The simultaneous presence of SWCNT and fullerene features in the Raman spectrum is in good agreement with the TEM and ED investigations on the same NanoBud structure, which confirmed the Raman measurement interpretation of the SWCNT chirality assignment (16,11) and the presence of fullerenes on the surface of the SWCNT.     

Lennard-Jones parameters for small diameter carbon nanotubes and water for molecular mechanics simulations from van der Waals density functional calculations

Lennard-Jones (LJ) parameters are derived for classical nonpolarizable force fields for carbon nanotubes (CNTs) and for CNT-water interaction from van der Waals (vdW) enhanced density functional calculations. The new LJ parameters for carbon-carbon interactions are of the same order as those previously used in the literature but differ significantly for CNT-water interactions. This may partially originate from the fact that in addition to pure vdW interactions the polarization and other quantum mechanics effects are embedded into the LJ-potential.     

The use of NH3 to promote the production of large-diameter single-walled carbon nanotubes with a narrow (n,m) distribution

We demonstrate here a simple and effective (n,m)-selective growth of single-walled carbon nanotubes (SWCNTs) in an aerosol floating catalyst chemical vapor deposition (CVD) process by introducing a certain amount of ammonia (NH(3)). Chiralities of carbon nanotubes produced in the presence of 500 ppm NH(3) at 880 °C are narrowly distributed around the major semiconducting (13,12) nanotube with over 90% of SWCNTs having large chiral angles in the range 20°-30°, and nearly 50% in the range 27°-29°. The developed synthesis process enables chiral-selective growth at high temperature for structurally stable carbon nanotubes with large diameters.     

Selective growth of SWNTs on partially reduced monometallic cobalt catalyst

SiO(2) supported cobalt (Co) catalyst could be partially reduced and anchored by unreduced Co ions during a carbon monoxide (CO) chemical vapor deposition (CVD) process. This resulted in the formation of sub-nanometre metallic Co clusters catalyzing the growth of single-walled carbon nanotubes (SWNTs) with a narrow diameter distribution.     

Electronic Structure and Size of TiO2 Nanoparticles of Controlled Size Prepared by Aerosol Methods

Summary.  A complete characterization of nanostructures has to deal both with electronic structure and dimensions. Here we present the characterization of TiO₂ nanoparticles of controlled size prepared by aerosol methods. The electronic structure of these nanoparticles was probed by X-ray absorption spectroscopy (XAS), the particle size by atomic force microscopy (AFM). XAS spectra show that the particles crystallize in the anatase phase upon heating at 500°C, whereas further annealing at 700°C give crystallites of 70% anatase and 30% rutile phases. Raising the temperature to 900°C results in a complete transformation of the particles to rutile. AFM images reveal that the mean size of the anatase particles formed upon heating at 500°C is 30 nm, whereas for the rutile particles formed upon annealing at 900°C 90 nm were found. The results obtained by these techniques agree with XRD data. Received October 5, 2001. Accepted (revised) December 6, 2001     

Structural studies on lithocholyl-N-(2-aminoethyl)amide in the solid state

The synthetic procedure of lithocholyl-N-(2-aminoethyl)amide yielded a mixture of several forms detected by solid state ¹³C CP/MAS NMR although the solution state NMR unambiguously ascertained that the compound was pure. By recrystallization from various solvents one pure polymorph alongside with four solvates were isolated. The structures of the pure polymorph and the solvates were characterized by ¹³C and ¹⁵N CP/MAS NMR and powder X-ray diffraction (PXRD) methods. Variable contact time and dipolar dephasing experiments were employed to obtain optimized CP parameters and to distinguish various CH _n (n = 0–3) resonances. CSA analyses of spinning side bands at different spinning rates showed small variations in the shielding tensor values of the carbonyl group between the pure polymorph (recrystallized from acetonitrile, tetrahydrofuran and 1,4-dioxane) and p-xylene solvate.     

Cantilever-based photoacoustic detection of carbon dioxide using a fiber-amplified diode laser

A compact and sensitive photoacoustic setup has been developed based on a recently demonstrated cantilever technique. A micromechanical cantilever transducer is attached to a cylindrical photoacoustic cell and the cantilever’s deflection is monitored with a compact Michelson interferometer. A commercial 1-Watt optical fiber amplifier was used to enhance the performance of the system. A normalized sensitivity of 1.4×10^-10 cm^-1 W Hz^-1/2 was achieved in the detection of carbon dioxide at 1572 nm wavelength. Using 34 mW optical power from a DFB diode laser, the noise-equivalent detection limit for carbon dioxide at this wavelength is 4.0 ppm. Employing the fiber amplifier, we improved the sensitivity to yield measurement of sub-ppm concentrations. PACS 42.62.Fi; 42.55.Px; 82.80.Ch     

Structural, thermoanalytical and molecular modeling studies on N-(3-hydroxypropyl) 3 alpha,12 alpha-dihydroxy-5 beta-cholan-24-amide and its monohydrates

The synthetic method for preparing N-(3-hydroxypropyl) 3 alpha,12 alpha-dihydroxy-5 beta-cholan-24-amide can lead to formation of at least three different crystal forms - an anhydrous compound and two monohydrates. The structural and thermal properties of these forms have been characterized by 13C-CP/MAS-NMR and IR spectroscopy, thermo- gravimetry, differential scanning calorimetry and by powder and single crystal x-ray crystallography. In addition, theoretical 13C-NMR chemical shift calculations were also performed for the anhydrous compound and for the first monohydrate, starting from single crystal structures and the structures of these species have now been verified. The first monohydrate, C27H47NO4 x H2O, crystallizes in orthorhombic space group P2(1)2(1)2(1) with cell parameters: a = 7.1148(2), b = 18.1775(5), c = 20.1813(6), Z = 4.     

NMR and quantum chemical studies on association of 2,6-bis(acylamino)pyridines with selected imides and 2,2′-dipyridylamine

Abstract  

Association constants of 2,6-bis(alkylcarbonylamino)pyridines (alkyl = methyl or ethyl) and their perfluoroalkyl analogues with succin- and maleimide as well as with 2,2′-dipyridylamine (complementary DAD and ADA hydrogen bonding motifs are responsible for formation of the associates) have been determined by NMR titrations and quantum chemical calculations. Interactions of 2,6-bis(alkylcarbonylamino)pyridines with imides differ by character from these of perfluoroalkyl analogues. Such large difference was not observed for the 2,2′-dipyridylamine associates. Since fluorine atoms cause carbonylamino groups to be stronger hydrogen bond donors, perfluorinated species of this type were found to be more stable. Single crystal X-ray structures of 2,6-bis(trifluoromethylcarbonylamino)pyridine and 2,6-bis(pentafluoroethylcarbonylamino)pyridine have been also determined.