Due to their unique architectural features, the cycloparaphenylenes (1, Figure 1) have attracted attention from many vantage points. 1 Their strained and distorted aromatic systems and radially oriented p orbitals (2) have intrigued synthetic chemists, theoreticians, supramolecular chemists, and materials scientists alike. Despite this widespread interest, the cycloparaphenylenes remain an unsolved synthetic challenge. 2-4 Here we describe the first synthesis of [9]-, [12]-, and [18]cycloparaphenylene. We also utilize computational methods to understand the structural and optical properties of these new macrocycles. As these structures correspond to the shortest-possible segment of an armchair carbon nanotube, 5 we refer to them as "carbon nanohoops."The heart of the synthetic challenge for the cycloparaphenylenes lies in the strain energy of the bent aromatic system. 6 Our strategy was to build up strain sequentially during the synthesis, using a 3,6-syn-dimethoxy-cyclohexa-1,4-diene moiety as a masked aromatic ring in a macrocyclic intermediate (see structure 4, Scheme 1). We reasoned that the cyclohexadiene unit would provide the curvature and rigidity necessary for macrocyclization to afford a marginally strained intermediate. Subsequent aromatization would then provide the driving force necessary to achieve the strain energies of target compounds, calculated to be 47, 28, and 5 kcal/ mol for [9]-, [12]-, and [18]cycloparaphenylenes, respectively (Vide infra).

No abstract

No abstract

Crystalline technetium dioxide was prepared and for the first time its crystal structure determined by neutron powder diffraction. In addition, electronic structure calculations using density functional theory were performed to further elucidate the bonding mechanisms in TcO2. The crystal structure determined by Rietveld analysis with the NPD data is of a distorted rutile type, similar to that of MoO2; space group P21/c, a = 5.6891(1), b = 4.7546(1), c = 5.5195(1) A, and beta = 121.453(1) degrees . The NPD analysis also establishes a new neutron scattering length of 6.00(3) fm for 99Tc. Our results clearly show metal-metal bonding between Tc pairs along the edge-sharing chains of TcO6 octahedra. The Tc-Tc bond was found to be 2.622(1) A from NPD profile analysis and 2.59 A from first-principles DFT calculations. The bond is somewhat longer than expected from earlier predictions, suggesting that the nature of the Tc-Tc interaction is weaker than anticipated for the Tc(IV) cation with three outer electrons. The NPD results supported by the DFT calculations suggest that the filling of antibonding orbitals and the influence of the crystal field stabilization of the d3 Tc cations lead to more regular TcO6 octahedra and diminish the metal-metal bond strength compared with closely related oxides such MoO2 and WO2.