Structure,rearrangements and evaporation of rotating atomic clusters

Eigenvector-following techniques are used to explore the classical effective potential energy surface of rotating clusters composed of identical rare gas atoms. Precise determination of transition states reveals the mechanisms for atomic rearrangements and allows us to calculate rate constants for the evaporation of an atom from the cluster surface using RRK theory. We examine the variation in the effective potential energy surface with the magnitude of the angular momentum, with particular reference to centrifugal distortion and the resulting spectroscopic constants, the Hessian index of the stationary points, and the centrifugal barriers to evaporation. Most of the calculations are performed using a pairwise additive Lennard-Jones potential, but comparisons are made with the accurate Aziz potential augmented by a three-body Axilrod-Teller term.     

Polarization effects in liquid metals and charge stabilized colloidal dispersions

The induced polarization contributions to the effective interionic potentials are derived for liquid metals, liquid alloys and charge stabilized colloidal dispersions. These contributions have been obtained within a perturbative scheme that includes up to three-body terms. The formalism is illustrated by calculating the static structure factor S(k) of liquid Ga near melting. It is found that the main effects due to the induced polarization in S(k) are first lowering the height of the principal diffraction peak, and second sharpening the subsidiary peak present in liquid Ga.     

Application of the elongation method to nonlinear optical properties: finite field approach for calculating static electric (hyper)polarizabilities

A novel finite-field approach for calculating electric (hyper)polarizabilities based on the elongation method is developed. The method was tested at the semi-empirical PM3 level by using three model systems: the hydrogen chain, the water chain and polyacetylene. The results satisfactorily reproduce the ‘exact’ MOPAC values. The most important advantage of this approach is the large saving of computer time since the dimension of the SCF equation remains the same regardless of the number of atoms in the system. Thus, it is a very useful tool to treat large systems. The method can also be applied to building up a chain containing an arbitrary sequence of monomers.     

Molecular dynamics study of the mobility in poly(butyl methacrylate) isomers

Molecular mobility is studied in poly(butyl methacrylate) isomers with molecular dynamics simulations in order to understand the role of the side chain steric hindrance on the β and δ relaxation mechanisms. The simulations suggest that in the temperature range studied the δ process is weakly influenced by the α process. Conversely, the β process seems to be significantly changed as compared with the low frequency regime. The results exposed also show evidence that the intramolecular cooperativity is higher in PnBMA than in PtBMA, which, could be related to the distance to the α-β crossover.     

1H NMR investigation of the hetero-association of aromatic molecules in aqueous solution: factors involved in the stabilization of complexes of daunomycin and acridine drugs

Hetero-association of the anthracycline drug, daunomycin (DAU), with typical mutagens, the acridine dyes proflavine (PF) and acridine orange (AO), has been studied by 500 MHz ¹H NMR spectroscopy as a function of concentration and temperature in 0.1 mol dm^?3 phosphate buffered aqueous solutions at pD = 7.1. The results have been analysed in terms of a statistical-thermodynamical model of hetero-association of aromatic molecules, described previously [Davies, D. B., Veselkov, D. A., and Veselkov, A. N., 1999, Molec. Phys., 97, 439], but generalized in this work, so that there is no limitation on the magnitudes of the self-association constants of the interacting molecules. Expressions suitable for the analysis of NMR parameters of both components in the mixed solution have been developed enabling both the structural and thermodynamic properties of hetero-association to be determined. The magnitude of the equilibrium constant for hetero-association of PF + DAU is found to be substantially higher than the self-association constants of these molecules, whereas that for hetero-association of AO + DAU is intermediate between the equilibrium constants of self-association of AO and DAU. Intermolecular cross-peaks observed in 2D-ROESY spectra of PF + DAU mixed solutions are consistent with formation of a hetero-association complex in which an intermolecular hydrogen bond can form between either of the 3,6-diamino groups of the PF chromophore and the 9-MeCO group of DAU, which is in contrast to AO + DAU hetero-association, where such hydrogen bonds are unable to form. Quantitative structural and thermodynamical analysis of PF + DAU complexation is consistent with an intermolecular hydrogen bond contributing to the stability of the hetero-complex in aqueous solution. The NMR results show that hydrophobic interactions play a substantial role in the stabilization of the AO-DAU complex, characterized by a relatively small entropy change on complexation, compared to the PF-DAU hetero-complex, which is mainly stabilized by hydrogen bond and dispersive van der Waals interactions.     

THE FATE OF PYRIMIDINE DIMERS IN ULTRAVIOLET-IRRADIATED CHLAMYDOMONAS

Abstract— We have developed a chromatographic technique for the separation of ³²P-labeled pyrimidine nucleotide dimers of the form Py_pPy from ³²P-phosphate in enzymatic hydroly sates of ultraviolet-irradiated DNA. Application of this technique to ³²P-labeled Chlamydomonas reinhardii shows that ultraviolet irradiation of this organism induces pyrimidine dimers in both nuclear and chloroplast DNA. We have found no evidence that these dimers are excised from either DNA species after several hours incubation under non-photoreactivating conditions. A function has been derived to permit the pyrimidine-dimer content determined from radioactive-thymine-labeled cells to be conveniently compared to that obtained from ³²P-phosphate-labeled cells.     

STUDIES ON SOME INTERMEDIATES AND PRODUCTS OF THE PHOTOREDUCTION OF 9,10-ANTHRAQUINONE *,†

Abstract— The mechanism of the photoreduction of 9,10-anthraquinone (AQ) in alcohol and hexane has been studied by flash photolysis. The fluorescence spectrum of the photoproduct, 9,10-dihydroxy anthracene shows a large shift between hexane and ethanol. The quantum yields of photoreduction for AQ are solvent-dependent, the reaction between the solvent radical and AQ determining the quantum yield.
The absorption spectrum of the 9,10-anthrasemiquinone (AQH.) has a long-wavelength absorption band with peaks at 631 and 678 nm. The second-order decay constants for AQH. were estimated to be 1.3 × 10⁹, 6.7 × 10⁸ and 2.0 × 10⁸ M ^-1 sec^-1 in ethanol, 2-propanol and ethylene glycol, respectively.
A long-wavelength absorption band was observed for 9,10-anthrasemiquinone radical anion, having peaks at 776 and 860 nm; epsi;_max= 1900 at 776 nm. This spectrum is compared with the spectra of 9,10-dihydroxy anthracene mono- and di-anions. The 9,10-anthrasemiquinone radical anion was found to photoreduce quantitatively to 9,10-dihydroxy anthracene mono-anion with a quantum yield of 0.1.     

PHOTOPHYSICAL AND PHOTOBIOLOGICAL PROPERTIES OF DIPORPHYRIN ETHERS

Spectral properties of several diporphyrin ethers were assessed in different solvents and after accumulation by leukemia L1210 cells in vitro. To facilitate studies in a variety of solvents, both tetramethylesters of the diporphyrin ethers and free acids were employed. For comparison, studies on the corresponding porphyrin monomers were also carried out. The joining of two porphyrins by an ether linkage had several consequences. We observed a blue shift in the Soret band of the ethers, but not of the corresponding simple porphyrins, in protic solvents. This phenomenon is likely related to ether aggregation under conditions which promote H-bonding. The presence of an ether linkage was associated with enhanced fluorescence at 630-640 nm and decreased fluorescence lifetimes and yields, especially in protic solvents. The ether linkage was unaffected by intracellular enzymes, but porphyrin esters were readily hydrolyzed upon accumulation by L1210 cells. The joining of two hematoporphyrin molecules by an ether linkage promoted dye accumulation by L1210 cells. In contrast, accumulation of mesoporphyrin and protoporphyrin was thereby retarded.     

CARTAN DECOMPOSITION FOR COMPLEX LOOP GROUPS

In this note we formulate and prove a version of Cartan decomposition for holomorphic loop groups, similar to Cartan decomposition for p-adic loop groups, discussed in [3], [6]. The main technical tool that we use is the (well-known) interpretation of twisted conjugacy classes in the holomorphic loop group in terms of principal holomorphic bundles on an elliptic curve.