Ovatifolin,a sesquiterpene lactone

Ovatifolin, C₁₇H₂₂O₅, is a sesquiterpene lactone isolated from the aerial parts ofPodanthus mitiqui Lindl. (Compositae) collected near Pichidangui, Chile. Extracts of the plant exhibit cytotoxic activity against KB cell cultures. The pale yellow crystals belong to the orthorhombic space groupP2₁2₁2₁ with cell dimensionsa = 8.159(1),b = 6.640(1), andc = 29.314(4) Å. Data were collected on an automatic diffractometer and the structure was solved by direct methods. The model was refined anisotropically to anR value of 0.047 using all 1742 measured intensities. The 10-memberedtrans-trans-germacradiene ring adopts a chair-chair conformation with C(4) and C(10) methyl groups orientedsyn on the face of the molecule. The conformation leads to a crossed double-bond system in which transannular interactions shift the ultraviolet absorption to 210 nm. The five-membered lactone ring adopts a distorted chair conformation and istrans-fused to the 10-membered ring.FASTBIOS contribution number 36.     

Note On the equality of star products

We study the problem of finding conditions for two star products of n vectors to be equal when the vectors in each star product are not linearly independent, and the problem of finding conditions for one star product to be zero. We reprove results of M. Marcus and J. Chollet in a very simple way. We present also some open problems.     

Spectral density of surface states: Clean W(001) surface

The transfer matrix approach is applied to calculate the spectral density of electronic states for the W(001) surface. We use a tight-binding hamiltonian with a nine function basis: 6s, 6p and 5d. Results for $\text{k}$ points along the Σ and ? symmetry lines are presented for occupied states. Both surface and bulk features of the spectral density are in good agreement with angularly resolved photoemission spectra.     

Hyperfine spectroscopic study of the metal-insulator transition in V2O3

The metal-insulator (M-I) transition in vanadium sesquioxide V₂O₃ has been investigated by time differential perturbed angular correlation measurements of the electric fieldgradient (EFG) and the magnetic hyperfine field at dilute¹¹¹Cd impurities. The EFG undergoes a first-order change at the M-I transition at T_t=160 K, but does not reflect the high temperature resistivity anomaly. The increase of the EFG with temperature in the metallic phase can be attributed to thermal variations of the oxygen sublattice. The temperature dependence of the magnetic hyperfine field in the insulating phase follows a Brioullin function with a saturation value of H_hf(O)=15 KOe and an extrapolated Neel temperature, which, depending on the impurity concentration, varies between 188 and 230 K.     

Flexural-flexural oscillations of Beck's column subjected to a planar harmonic excitation

The non-linearly coupled flexural-flexural oscillations of a non-conservative column with two independent load parameters are investigated by analytical methods with the objective of determining the quantitative and the qualitative behavior of the response. The column is subjected to a planar periodic distributed load whose frequency is near the eigenfrequency of its free bending oscillations. The analytical results of the analysis are compared with those obtained by numerical integration of a set of non-linear differential equations obtained by the application of Galerkin's method to the original equations.     

An accurate relativistic universal Gaussian basis set for hydrogen through Nobelium without variational prolapse and to be used with both uniform sphere and Gaussian nucleus models

An accurate relativistic universal Gaussian basis set (RUGBS) from H through No without variational prolapse has been developed by employing the Generator Coordinate Dirac-Fock (GCDF) method. The behavior of our RUGBS was tested with two nuclear models: (1) the finite nucleus of uniform proton-charge distribution, and (2) the finite nucleus with a Gaussian proton-charge distribution. The largest error between our Dirac-Fock-Coulomb total energy values and those calculated numerically is 8.8 mHartree for the No atom.     

Mixed effect of the supporting electrolyte and the zinc anode in the electrochemical homocoupling of 2-bromopyridines catalyzed by nickel complexes in an undivided cell

[reactions: see text] Nickel-catalyzed electroreductive homocoupling of 2-bromomethylpyridines and 2-bromopyridine has been investigated in an undivided cell in the presence of a zinc sacrificial anode. A series of reactions were performed with various types and concentrations of supporting electrolyte. It was observed that a key step in this process is the formation of an arylzinc through a nickel-zinc transmetalation. This intermediate can be transformed back to the reactive arylnickel species to afford the homocoupling as the final product. The back process from the arylzinc intermediate is, however, suppressed in the presence of high concentration (0.2 M) of tetraalkylammonium salts. On the contrary, with NaI, the formation of the dimer is not prevented, whatever the NaI concentration.     

Charge-transfer complexation as a general phenomenon in the copigmentation of anthocyanins

Color intensification of anthocyanin solutions in the presence of natural polyphenols (copigmentation) is re-interpreted in terms of charge transfer from the copigment to the anthocyanin. Flavylium cations are shown to be excellent electron acceptors (E(red) approximately -0.3 V vs SCE). It is also demonstrated, for a large series of anthocyanin-copigment pairs, that the standard Gibbs free energy of complex formation decreases linearly with EA(Anthoc) - IP(Cop), the difference between the electron affinity of the anthocyanin, EA(Anthoc), and the ionization potential of the copigment, IP(Cop). Based on this correlation, copigmentation strengths of potential candidates for copigments can be predicted.