Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

A high resolution visible spectrum of iridium monophosphide

Laser induced fluorescence spectra of iridium monophosphide, IrP, have been obtained at low and high resolution in the blue region of the visible spectrum. Two electronic transitions were observed with origins near 459.6 and 471.9 nm. Three vibronic bands in each of these transitions have been observed at high resolution allowing for full characterization of the states. A J-independent doubling of the rotational lines has been ascribed to nuclear electric quadrupole coupling in the ground state. Multireference configuration interaction (MRCI) calculations have been performed in order to confirm the nature of the ground state and aid in the assignment of the excited states. The two observed transitions have been assigned as the [21.7]¹Σ⁺-X¹Σ⁺ and the [21.2] ³Σ⁺-X¹Σ⁺ electronic systems based on comparison with the theoretical calculations. The v + 2 level of each of these electronic transitions was found to be heavily perturbed and a successful deperturbation analysis was performed allowing for a complete global fit of the data.     

Long holographic lifetimes in bacteriorhodopsin films

The D85N genetic variant of bacteriorhodopsin (BR) displays a nearly permanent lifetime of the photochromic P(490) state. We present pump-probe measurements that demonstrate this behavior. However, diffraction efficiency measurements made from holograms recorded in a hydrated D85N film show markedly different decay behavior, suggesting that a molecular diffusion process is occurring in the film. Holograms recorded with different grating frequencies display correspondingly different decay times, thus supporting this hypothesis. A thin D85N film was fabricated that was chemically cross linked, resulting in the elimination of diffusion of BR molecules within the polymer matrix. This film exhibits a grating lifetime of the order of weeks or more, thus permitting the long-term holographic storage of information.     

Counterion Effect in Cobaltate-Catalyzed Alkene Hydrogenation

We show that countercations exert a remarkable influence on the ability of anionic cobaltate salts to catalyze challenging alkene hydrogenations. An evaluation of the catalytic properties of [Cat][Co(η⁴-cod)₂] (Cat=K ( 1 ), Na ( 2 ), Li ( 3 ), (^Depnacnac)Mg ( 4 ), and N(ⁿBu)₄ ( 5 ); cod=1,5-cyclooctadiene, ^Depnacnac={2,6-Et₂C₆H₃NC(CH₃)}₂CH)]) demonstrated that the lithium salt 3 and magnesium salt 4 drastically outperform the other catalysts. Complex 4 was the most active catalyst, which readily promotes the hydrogenation of highly congested alkenes under mild conditions. A plausible catalytic mechanism is proposed based on density functional theory (DFT) investigations. Furthermore, combined molecular dynamics (MD) simulation and DFT studies were used to examine the turnover-limiting migratory insertion step. The results of these studies suggest an active co-catalytic role of the counterion in the hydrogenation reaction through the coordination to cobalt hydride intermediates.     

A mndo study of 3-, 5-, 7- and 9-membered carbocyclic,completely conjugated,planar carbenes and their nonplanar isomers

Molecular orbital calculations have been conducted on a series of planar, cyclic, completely conjugated carbenes and their nonplanar isomers. Energy surfaces have been probed using reaction coordinate diagrams and vibrational force constant calculations. The importance of the nonplanar allenic and carbenic isomers is emphasized. In most cases, planar, cyclic, completely conjugated carbenes are not found to be energy minima.     

Modeling of a type II photofrin-mediated photodynamic therapy process in a heterogeneous tissue phantom

We present a quantitative framework to model a Type II photodynamic therapy (PDT) process in the time domain in which a set of rate equations are solved to describe molecular reactions. Calculation of steady-state light distributions using a Monte Carlo method in a heterogeneous tissue phantom model demonstrates that the photon density differs significantly in a superficial tumor of only 3 mm thickness. The time dependences of the photosensitizer, oxygen and intracellular unoxidized receptor concentrations were obtained and monotonic decreases in the concentrations of the ground-state photosensitizer and receptor were observed. By defining respective decay times, we quantitatively studied the effects of photon density, drug dose and oxygen concentration on photobleaching and cytotoxicity of a photofrin-mediated PDT process. Comparison of the dependences of the receptor decay time on photon density and drug dose at different concentrations of oxygen clearly shows an oxygen threshold under which the receptor concentration remains constant or PDT exhibits no cytotoxicity. Furthermore, the dependence of the photosensitizer and receptor decay times on the drug dose and photon density suggests the possibility of PDT improvement by maximizing cytotoxicity in a tumor with optimized light and drug doses. We also discuss the utility of this model toward the understanding of clinical PDT treatment of chest wall recurrence of breast carcinoma.     

Synthesis and characterization of hydrogen-bonded assemblies of W6S8L6 clusters

Ligand-exchange reactions involving octahedral W6S8 clusters and a family of pyridine-based ligands (isonicotinic acid, isonicotinamide, 4-hydroxypyridine, 4-aminopyridine, 4-pyridineacetamide) have been explored with the goal of preparing compounds that crystallize in hydrogen-bonded arrays. Two new compounds, W6S8(4-pyridineacetamide)6.DMF.4-pyridineacetamide (1) and W6S8(4-aminopyridine)6.4DMF (2), were isolated and characterized by single-crystal X-ray diffraction. Both compounds crystallize in the P2(1)/c space group with a = 16.461(1), b = 33.08(2), c = 13.165(10) A, beta = 103.270(15) degrees for 1 and a = 13.8988(5), b = 13.2791(5), c = 15.6293(6) A, beta = 108.5410(10) degrees for 2. Each compound was further characterized by 1H NMR spectroscopy, elemental (CHN) analysis, and thermogravimetric analysis. Examination of the structures shows that 1 forms a three-dimensional hydrogen-bonded network in which each 4-pyridineacetamide ligand interacts with ligands on neighboring clusters or with the free ligand of crystallization. This is the first hydrogen-bonded network formed from W6S8 clusters. In 2, the amino groups act as hydrogen-bond donors toward DMF molecules of crystallization, but an extended array is not formed. In addition, the binding strengths of these pyridine-based ligands to the W6S8 cluster were studied through quantitative 1H NMR studies of ligand-exchange reactions. A qualitative relationship was found between ligand binding strengths and Hammett substituent constants for this group of ligands.     

The effect of detuning upon resonance fluorescence

The excitation spectrum of a two-level atom interacting with a strong laser field is considered as a function of the detuning. The spectral function of the system is calculated by making use of the Green's function formalism and includes a physical process which is neglected in many earlier treatments of the one photon problem. It is found that for zero and small detunings, the spectral function describes the well known three peak spectrum which evolves into a four line spectrum as the detuning is increased. Results of numerical calculations for a wide range of detuning are presented and analytic expressions are derived to describe the small and large detuning limits.     

Numerical solution of the Zakharov equations

A new algorithm for the numerical solution of the Zakharov equations is presented. This algorithm is explicit and it is second-order accurate. The convergence of the algorithm is demonstrated by numerically solving the Zakharov equations for several test cases.