J. Bretón

Endohedral and exohedral adsorption in C60: An analytical model

A simple description of the interaction potential between a small molecule and a fullerene 60 cage is discussed on the basis of the continuum approximation. General conditions are obtained for the validity of this continuum approximation and for the confinement mechanism of the molecule inside (or outside) the cage. Application to the insertion of alkali metal ions and rare gas atoms shows that these conditions can adequately interpret the equilibrium positions of inside (outside) adducts, the adsorption energy and the collision experiments between the ions and atoms and the C60 cage.

Five new labdane diterpene oxides from Eupatorium jhanii

The authors would like to thank Dr. A. Morales, University of Merida (Venezuela) for gathering the plant and for the sustained interest he has shown in the progress of this work.

Biotransformation of 6β-Eudesmanolides Functionalized at C-3 with Curvularia lunata and Rhizopus nigricans Cultures

Abstract A series of biotransformations of 6β-santonin and its derivatives with functions at C-3, were carried out with Curvularia lunata and Rhizopus nigricans cultures. Rhizepus nigricans was more active in the biotransformation process against these substrates. The biotransformation of 6β-santonin yielded its 2α-hydroxy-1,2-dihydro derivative. The biotransformation of ketones at C-3 obtained by partial or total hydrogenation of double bonds in ring A led to 3S alcohols. Incubation of the 3S-hydroxyl-4S-13S- 6α-eudesmanolide with Rhizopus nigricans produced epimerization at C-4 and hydroxylation at C-8, C-1 or C-4, in decreasing order. This epimerization is probably produced with the participation of the hydraxyl goup at C-3. Microbial functionalization at C-8 can provide access to the synthesis of 8,12-eudesmanolides.

Interaction potential and chiral discrimination between an alanine molecule and a quartz surface

The calculation of the interaction energy between a quartz crystal surface and a chiral molecule is considered. The various contributions (electrostatic, dispersion, hydrogen bond, and repulsion) to this energy are separately treated and computed using atom–atom summation or Fourier expansion techniques. The discussion of the resulting potential surface allows us to exhibit an influence of the chiral character of the adsorbed molecule; this may amount to a few percent of the total interaction energy for closest approaches. A possible connection with the experimental results is suggested.

Chlorohyssopifolin C, D, E and vahlenin, four new sesquiterpene lactones from Centaurea hyssopifolia☆

Abstract On the basis of chemical and spectroscopic evidence, structures are assigned to four new sesquiterpene lactones: chlorohyssopifolin C, D and E (guaiano

Electronic structure and polarizabilities of icosahedral fullerenes: A Pariser–Parr–Pople approach

A Pariser–Parr–Pople approach, complemented with physical consistency criteria based on the expected molecular response to a weak electric field, has been used to predict the electronic level structure and polarizabilities of five icosahedral fullerenes in the range C60–C720. The behavior of the polarizability α as a function of the fullerene size is given by the expression α=0.75Re3, where Re is an effective molecular radius. It is argued that fullerenes would present the maximum polarizability values allowed for carbon shells, as if they were made of graphene.

Modeling water clusters on cationic carbonaceous seeds.

The Dang-Chang many-body polarizable potential has been used to model the interaction between water molecules and a cationic carbonaceous molecule X(+), with X = C(60) (buckminsterfullerene), C(24)H(12) (coronene), or C(20)H(10) (corannulene). The most stable structures of (H(2)O)(n)X(+), located with the basin-hopping method, consist of a water cluster next to the carbon cation but often deviate from those obtained for pure water clusters. The accuracy of the intermolecular potential is checked by performing dedicated high-level electronic structure calculations using the B97-1 density functional. Finally, some thermodynamical and dynamical manifestations of the nonwetting behavior are discussed.

Endohedral adsorption in graphitic nanotubules

Calculations based on simple interaction potentials are performed to define the adsorption characteristics of molecules encapsulated in carbon nanotubules. The continuum approximation used to describe the cylindrical sheets is shown to work fairly well within a large range of tubule diameters. Criteria for molecule confinement are given which include the influence of the number of graphitic shells describing the whole tubule and they are compared to similar results obtained for carbon Buckyballs. Application to the most common encaged species, i.e., rare gas atoms and alkali‐metal ions, confirms these general rules.

Lowest-energy structures of (C60)nX (X=Li+,Na+,K+,Cl-) and (C60)nYCl (Y=Li,Na,K) clusters for n</=13.

Basin-hopping global optimization is used to find likely candidates for the lowest minima on the potential energy surface of (C(60))(n)X (X=Li(+),Na(+),K(+),Cl(-)) and (C(60))(n)YCl (Y=Li,Na,K) clusters with n</=13. The energy is evaluated using the Girifalco form for the C(60) intermolecular potential along with a polarization potential, which depends on the first few nonvanishing C(60) multipole polarizabilities. We find that the ions occupy interstitial sites of a (C(60))(n) cluster, the coordination shell being triangular for Li(+), tetrahedral for Na(+) and K(+), and octahedral for Cl(-). When the required coordination site does not exist in the corresponding (C(60))(n) global minimum, the lowest minimum of the doped system may be based on an alternative geometry. This situation is particularly common in the Cl(-) complexes, where the (C(60))(n) global minima with icosahedral packing change into decahedral or closed-packed forms for the ions. In all the ions we find a significant binding energy for the doped cluster. In the alkali chloride complexes the preferred coordination for the diatomic moiety is octahedral and is basically determined by the Cl(-) ion. However, the smaller polarization energies in this case mean that a change in structure from the (C(60))(n) global minimum does not necessarily occur if there is no octahedral site.

Rotational spectra for off‐center endohedral atoms at C60 fullerene

Rotational spectra for endohedral Li+@C60 and Na+@C60 are calculated at different temperatures. Most of the features in these spectra are related with the degree of anisotropy in the atom–cage interaction. While the low anisotropy for Na+@C60 results in rather simple spectra with the 2B oscillation typical of a diatomic molecule, the more eccentric and anisotropic Li+@C60 produces complex spectra with rotational and librational bands. Some interesting effects are induced by the cage rotation, which has been incorporated through a semiclassical formalism.