The mechanistic origin of regiochemical changes in the nitrosative N-dealkylation of N,N-dialkyl aromatic amines

The regioselectivity of the nitrous acid mediated dealkylation of 4-substituted-N-ethyl-N-methylanilines is a function of the acidity of the reaction mixture. At high acidity deethylation predominates, whereas demethylation is the predominant reaction in nitrosamine formation at pH 2 and above. In some cases the regioselectivity of nitrosative dealkylation changes as the run proceeds. Through the use of the corresponding 4-nitroaniline as the primary substrate, CIDNP, kinetics, kinetic deuterium isotope effects and other transformations involving nitrosations with NO2 or NOBF4 in aprotic solvents, a new mechanism of tertiary amine nitrosation has been deduced and proposed to explain regioselective deethylation. The mechanism involves the oxidation of the substrate to the amine radical cation by NO+. This is followed by the abstraction of a hydrogen atom from the carbon adjacent to the amine nitrogen by NO2 to produce an iminium ion which reacts further to produce the corresponding aldehyde and the nitrosamine. Depending upon the acidity, this process competes with three other mechanistic pathways, two of which give the nitrosamine through the iminium ion, and one leads to the formation of C-nitro compounds. The competing pathways to nitrosamine formation involve NOH elimination from a nitrosammonium ion and deprotonation of the radical cation to give an alpha-amino radical which rapidly oxidized to the iminium ion. Predominant, but not highly regioselective demethylation occurs by these pathways. Nitro compound formation principally arises from the reaction of NO2 with the radical cation followed by deprotonation, but also occurs by para C-nitrosation followed by oxidation.     

Validity of wave-front reconstruction and propagation of ultrabroadband pulses measured with a Hartmann-Shack sensor

Wave-front reconstruction for ultrabroadband laser pulses is verified by use of a Hartmann-Shack sensor. We estimate the accuracy of numerical wave-front propagation by comparing numerical with experimental results and verify that wave fronts of ultrabroadband laser pulses from a hollow fiber can be propagated correctly by a single polychromatic wave-front measurement to a place where detection is not practicable, e.g., inside a vacuum chamber or laser focus.     

Hydrogenation of single-walled carbon nanotubes

Towards the development of a useful mechanism for hydrogen storage, we have studied the hydrogenation of single-walled carbon nanotubes with atomic hydrogen using core-level photoelectron spectroscopy and x-ray absorption spectroscopy. We find that atomic hydrogen creates C-H bonds with the carbon atoms in the nanotube walls, and such C-H bonds can be completely broken by heating to 600 degrees C. We demonstrate approximately 65 +/- 15 at % hydrogenation of carbon atoms in the single-walled carbon nanotubes, which is equivalent to 5.1 +/- 1.2 wt % hydrogen capacity. We also show that the hydrogenation is a reversible process.     

Negative differential conductance and hot phonons in suspended nanotube molecular wires

Freely suspended metallic single-walled carbon nanotubes (SWNTs) exhibit reduced current carrying ability compared to those lying on substrates, and striking negative differential conductance at low electric fields. Theoretical analysis reveals significant self-heating effects including electron scattering by hot nonequilibrium optical phonons. Electron transport characteristics under strong self-heating are exploited for the first time to probe the thermal conductivity of individual SWNTs (approximately 3600 W m-1 K-1 at T=300 K) up to approximately 700 K, and reveal a 1/T dependence expected for umklapp phonon scattering at high temperatures.     

Langmuir monolayers of bent-core molecules

A systematic study of five different, symmetric bent-core liquid crystals in Langmuir thin films at the air/water interface is presented. Both the end chains (siloxane vs hydrocarbon) and the core (more or less amphiphilic) are varied, to allow an exploration of different possible layer structures at the interface. The characterization includes systematic surface pressure isotherms, Brewster angle microscopy, and surface potential measurements. The properties of these layers are strongly dependent on the individual type of molecule: the molecules with amphiphilic end chains lie quite flat on the surface, while the molecules with hydrophobic end chains construct multilayer structures. In both cases, the three-dimensional collapse structure is reversible.     

Synthesis and characterisation of cis-dioxomolybdenum(VI) complexes of N-substituted 3-hydroxy-2-pyridinones

The butoxy- and methoxy-derivatives (HLc, and HLb, respectively) of 1-[(ethoxycarbonyl)methyl]-3-hydroxy-2-(1H)-pyridinone, HLa, were synthesized by transesterification and the mononuclear complexes [MoO2(L(a-c))2] were obtained by the reaction of the ligands with molybdenyl acetylacetonate. The three complexes were fully characterized by (1)H and (13)C NMR spectroscopy, mass spectrometry and elemental analyses. A variable temperature NMR study showed that the N-substituents do not have a significant influence on the racemisation process of the complexes. The comparison of the crystal structure of [MoO2(Lb)2] with [MoO2(La)2] confirmed that an additional methylene group does not affect the immediate coordination environment of the molybdenum centre. The methoxy-derivative HLb was also found to produce a dinuclear complex of composition [(MoO2(Lb)O)2], the structure of which was determined by X-ray diffraction.     

Hydrogen-bonded cubanes in the crystal structure of 2,6-di(Pr(i))aniline hydrochloride and their inorganic analogues [M(2+)(2,6-di(Pr(i))C6H5N(2-))]4 (M = Sn, Pb)

The crystal structure of 2,6-di(Pr(i))aniline hydrochloride contains hydrogen-bonded cubanes analogous to those in the complexes [M(2+)(2,6-di(Pr(i))C6H5N(2-))]4, M = Sn, Pb.