Ferromagnetism and Kondo insulator behavior in the disordered periodic Anderson model

The effect of binary alloy disorder on the ferromagnetic phases of f-electron materials is studied within the periodic Anderson model. We find that disorder in the conduction band can drastically enhance the Curie temperature T{c} due to an increase of the local f moment. The effect may be explained qualitatively and even quantitatively by a simple theoretical ansatz. The emergence of an alloy Kondo insulator at noninteger filling is also pointed out.     

Competing isomerizations of [12]annulenes: Diels-Alder reaction versus electrocyclization

Experimentally, tri-trans-[12]annulene and tris(cyclohexeno)[12]annulene exhibit differing reactivities. Whereas the former, after isomerizing to its di-trans isomer, undergoes sequential electrocyclizations, the latter follows a Diels-Alder pathway after initial electrocyclization. B3PW91/6-31+G*//B3LYP/6-31G* calculations indicate that cyclohexenofusion simultaneously hinders the second electrocyclization and facilitates Diels-Alder reaction, primarily by inducing greater puckering in the intermediate eight-membered ring.     

Simplified method of the quantum chemical analysis for determination of thermodynamic parameters of 2D cluster formation of amphiphilic compounds at the air/water interface

A simplified method is proposed to estimate the thermodynamic parameters of clusterization at the air/water interface for various classes of amphiphilic compounds with a single alkyl chain. The method is based on the calculation of thermodynamic characteristics only for one of the homologous series of dimers (n=6-16) governing the formation of infinite clusters. The method is used to calculate the thermodynamic parameters of clusterization for alcohols, thioalcohols, carboxylic acids and amines, and the dependencies of the Gibbs energy of clusterization on the alkyl chain length are evaluated. It is shown that the alkyl chain length, at which the spontaneous clusterization begins, as calculated using the proposed simplified method, is in fact the same as that calculated using the additive scheme developed earlier. The simplified method proposed was verified using alkylnitriles as example. In contrast to alcohols, thioalcohols and amines, infinite 'rhombic' clusters are formed rather than 'rectangular' clusters for this class of compounds. Spontaneous clusterization of nitriles is shown to start for alkyl chains containing 18-19 carbon atoms. This value agrees with that obtained from experimental data with 17-18 carbon atoms. The proposed simplified method introduces an exact and suitable tool for the estimation of thermodynamic parameters of the clusterization of amphiphilic compounds.     

Chiral discrimination in stearoyl amine glycerol monolayers

Chiral discrimination in Langmuir monolayers of amphiphilic 1-stearylamine-glycerol is studied using the hybrid quantum mechanical/molecular mechanical method. Using the experimental information about the lattice structure [D. Vollhardt, U. Gehlert, J. Phys. Chem. B. 2002, 106, 4419], the intermolecular interaction profiles for enantiomeric and racemic pair are studied as a function of mutual tilt and azimuth for different values of intermolecular separation. The present study reveals that, at shorter separation, the interaction profile of the racemic pair has deeper minima than the enantiomeric pair, whereas at larger separation the minimum of the enantiomeric pair is deeper. Thus, the theoretical studies reveal an interesting crossover from heterochiral preference to homochiral preference in 1-stearylamine-glycerol monolayers, with the increase in the intermolecular separation corresponding to a larger area per molecule in the monolayer. This predicts that, with gradual compression, the interactions between racemic pair dominate the experimental features, whereas, under nonequilibrium conditions at the beginning of the formation of the condensed phase, the experimental characteristics of homochirality are observable. The study conclusively shows that the chiral structure of the molecule and the lattice packing drive the chiral preference at the mesoscopic level.     

Role of dipolar interaction in the mesoscopic domains of phospholipid monolayers: dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylethanolamine

The role of dipolar interactions in determining the lipid domain shapes at the air-water interface with a change in the chemical structure of the head groups of lipids is theoretically studied. The phospholipids considered are dipalmitoylphosphatidylcholine (D,L-DPPC) and dipalmitoylphosphatidylethanolamine (DPPE). Despite closely similar chemical structures, the domains of the two lipids are strikingly different. The DPPC domains exhibit elongated arms, while the DPPE domains are nearly round-shaped. To compare the dipolar repulsions in the domains of the two phospholipids, different energy-minimized conformers of DPPC and DPPE are studied using the semiempirical quantum chemical method (PM3). It is found that the dipole moment of DPPC is significantly larger than that of DPPE. The in-plane and out-of-plane components of the dipole moments are calculated using grazing incidence X-ray diffraction data at different surface pressure values, as used in the experiment. The result indicates that the magnitude of the dipolar interaction is significantly larger in DPPC than that in DPPE over the surface pressure range considered. The enhanced dipolar repulsion corroborates well with the difference in the domain shapes in the two phospholipid monolayers. The larger dipolar repulsion in DPPC leads to development of elongated domain arms, while relatively less dipolar repulsion allows a closed shape of the condensed-phase DPPE domains.     

Recognition and dissociation kinetics in the interfacial molecular recognition of barbituric acid by amphiphilic melamine-type monolayers

Progress in the understanding of interfacial molecular recognition kinetics is obtained by use of the sweeping technique for experimental studies of the reaction kinetics between a host monolayer and a non-surface-active species dissolved in the aqueous subphase. The experimental results show that the interfacial recognition reaction between a 2C(11)H(23)-melamine (2,4-di(n-undecylamino)-6-amino-1,3,5-triazine) monolayer and dissolved barbituric acid is reversible when the 2C(11)H(23)-melamine/barbituric acid monolayer is transferred back onto a pure water subphase. The kinetics of the recognition and dissociation reaction is experimentally and theoretically investigated. The approximate additive theoretical model developed recently is extended to consider the dissociation kinetics of the interfacial supramolecular complex. The kinetic constants for the recognition and dissociation reactions in the mixed monolayer consisting of 2C(11)H(23)-melamine and 2C(11)H(23)-melamine/barbituric acid complex are determined. It is shown that the kinetic constant of the recognition reaction is nearly independent of temperature, whereas that of the dissociation reaction increases with increasing temperature.     

Correlated electrons in the presence of disorder

Several new aspects of the subtle interplay between electronic correlations and disorder are reviewed. First, the dynamical mean-field theory (DMFT) together with the geometrically averaged (“typical”) local density of states is employed to compute the ground state phase diagram of the Anderson-Hubbard model at half-filling. This non-perturbative approach is sensitive to Anderson localization on the one-particle level and hence can detect correlated metallic, Mott insulating and Anderson insulating phases and can also describe the competition between Anderson localization and antiferromagnetism. Second, we investigate the effect of binary alloy disorder on ferromagnetism in materials with f-electrons described by the periodic Anderson model. A drastic enhancement of the Curie temperature T_c caused by an increase of the local f-moments in the presence of disordered conduction electrons is discovered and explained.     

Cobalt-mediated cyclic and linear 2:1 cooligomerization of alkynes with alkenes: a DFT study

The mechanism of the cobalt-mediated [2 + 2 + 2] cycloaddition of two alkynes to one alkene to give CpCo-complexed 1,3-cyclohexadienes (cyclic oligomerization) has been studied by means of DFT computations. In contrast to the mechanism of alkyne cyclotrimerization, in which final alkyne inclusion into the common cobaltacyclopentadiene features a direct "collapse" pathway to the complexed arene, alkene incorporation proceeds via insertion into a Co-C sigma-bond rather than inter- or intramolecular [4 + 2] cycloaddition. The resulting seven-membered metallacycle 7 is a key intermediate which leads to either CpCo-complexed cyclohexadiene 5 or hexatriene 13. The latter transformation, particularly favorable for ethene, accounts, in part, for the linear oligomerization observed occasionally in these reactions. With aromatic double bonds, a C-H activation mechanism by the cobaltacyclopentadiene seems more advantageous in hexatriene product formation. Detailed investigations of high- and low-spin potential energy surfaces are presented. The reactivity of triplet cobalt species was found kinetically disfavored over that of their singlet counterparts. Moreover, it could not account for the formation of CpCo-complexed hexatrienes. However, triplet cobalt complexes cannot be ruled out since all unsaturated species appearing in this study were found to exhibit triplet ground states. Consequently, a reaction pathway that involves a mixing of both spin-state energy surfaces is also described (two-state reactivity). Support for such a pathway comes from the location of several low-lying minimum-energy crossing points (MECPs) of the two surfaces.