Electromagnetic interactions with rough metal surfaces

When surfaces are structured on the scale of the wavelength, we can expect incident light to be strongly modified by the surface. This is especially the case when the surface is metallic. We have developed a formalism for computing these modifications, closely analogous to electron scattering theory, which we briefly review and present some results for optical properties of, and electron energy loss in, colloids. Our main theme is another effect associate with rough or structured metallic surfaces: Surface Enhanced Raman Scattering, or SERS. We model the rough surface by a periodic array of spheres and obtain the correct magnitude for the enhancement and for the frequency shifts observed.     

Fluorine absorption spectra and some fluorides in condensed state

The absorption spectra of liquid F₂, NF₃, N₂F₄, CF₄, BF₃, NF₃, SF₆ have been obtained at diminished temperatures in the near ultra-violet region of the spectrum. It is shown that the absorption spectrum does not differ from the spectra in the gaseous phase, therefore the elementary absorption act is characterized by the cross section of photon absorption by an individual molecule. The absorption cross sections of the above mentioned molecules are represented in the liquid phase, which do not differ strongly from absorption cross sections of these molecules in the gaseous phase. The dependence of the absorption cross sections of liquid fluorine on its concentrations in solutions with N₂, Ar, NF₃, O₂ at - 196°C has been studied. The cross sections of photon absorption by the fluoride molecule in different liquid media with small fluorine concentrations have been obtained.     

Structural studies on layered alkylpyridinium iodopalladate networks

The reaction of alkylpyridinium (CnH2n + 1NC5H5, hereafter Cn-Py) iodide salts in aqueous acetonitrile with a preformed palladium iodide precursor afforded two different types of organic-inorganic phases depending on the molar ratio. A 2:1 ratio yielded the phase [Cn-Py]2[PdI4] (3, n = 14, 16), which crystallized in the triclinic crystal system. The X-ray crystal structure of 3, (n = 14), refined in the space group P1 (a = 8.918(3) A, b = 9.894(3) A, c = 29.062(12) A, alpha = 93.51(3) degrees, beta = 94.17(3) degrees, gamma = 115.60(3) degrees, and Z = 2), consists of interdigitated bilayers with a basal spacing of 29.0 A. The aliphatic chains of the cations, which run almost parallel to the stacking direction, are fully stretched between polar planes built on isolated [PdI4]2- anions and cation headgroups. Changing the organic cation to palladium ratio to 1:1 led to a new phase [Cn-Py]2[Pd2I6] (4, n = 14, 16), which crystallizes in the triclinic P1 space group (a = 9.399(4) A, b = 14.264(6) A, c = 29.415(13) A, alpha = 92.11(4) degrees, beta = 90.07(4) degrees, gamma = 104.53(3) degrees, Z = 3 for 4(n = 14); a = 9.417(2) A, b = 14.215(3) A, c = 31.552(6) A, alpha = 87.96(3) degrees, beta = 87.63(3) degrees, gamma = 75.67(3) degrees, Z = 3 for 4(n = 16)). The layered structure is basically of a continuously interdigitated single-layer type, with a bilayer sublattice superimposed. Isolated [Pd2I6]2- anions contribute to the inorganic planes. A high degree of interdigitation and tilting of the aliphatic chains lead to basal spacings of 29.4 and 31.5 A for 4(n = 14) and 4(n = 16), respectively. The [Cn-Py]2[PdI4] and [Cn-Py]2[Pd2I6] phases were characterized by thermal analysis. Mesomorphic behavior was observed only for 3(n = 16), which was confirmed by variable-temperature powder XRD and optical microscopy.     

Cubic phases as structures of disclinations

We look for possible ordered geometrical configurations with bicontinuous or cellular topologies, optimizing the frustration of a periodic system of frustrated fluid films. The solutions found have the same symmetries as those observed for cubic phases of amphiphilic molecules. This agreement leads to consider cubic structures as structures of defects of rotation, or disclinations.This paper was presented at the workshop Ringing gels and Cubic phases, University of Bayreuth, October 25–26th, 1988.     

The synthesis of cis- and trans-1,2-dibromocyclopropane

A preparative route to cis- and trans-1,2-dibromocyclopropane ($\text{1}$) was developed via the Hunsdiecker reaction of silver cyclopropane-1,2-dicarboxylate ($\text{2}$). Cis- and trans-$\text{2}$ gave the same ratio of cis- and trans-$\text{1}$ (1:3.2). The mechanism of this reaction is briefly discussed.     

Nitration of 5-formyl-substituted 2-cyclopropylfurans and 2-methylfurans and the corresponding thiophenes

In the nitration of 5-formyl-substituted 2-cyclopropylfurans and the corresponding thiophenes, in addition to the formation of the corresponding 3-nitro derivative the replacement of the formyl group by a nitro group takes place. For a thiophene derivative the latter direction of the reaction is observed to a substantially smaller degree. Under nitration conditions, 5-formylsylvane is converted only into 5-nitrosylvane, while the corresponding formylmethylthiophene is nitrated exclusively in position 3. The difference observed in the behavior on nitration of the furans and thiophenes studied is explained by the different degrees of participation of the heteroatom in the delocalization of the charges in the heterocyclic ipso-ions formed as intermediates.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 455–460, April, 1980.     

Comportement thermique des propionates hydrates de calcium,strontium et baryum

Résumé Les propionates monohydratés de calcium, strontium et baryum ont été étudiés par radiocristallographie (sur monocristal et sur poudre) et par TG et ATD (à l'air et en milieu inerte). Ils cristallisent dans les systèmes monoclinique (Ca) ou orthorhombique (Sr, Ba). Ils perdent leur eau en une fois (Ca, Sr) ou deux fois (Ba) avec formation d'un hémihydrate intermédiaire. Tous les sels anhydres, d'abord amorphes aux rayons X, cristallisent vers 200°. Le propionate de baryum anhydre cristallisé subit une transformation polymorphique à 240°. Après 300° à l'air et 350° sous atmosphère inerte, la fusion et la décomposition se produisent simultanément: il y a dégagement de diéthylcétone et le résidu solide est du carbonate métallique bien cristallisé.

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The monohydrates of calcium, strontium and barium propionates have been investigated by radiocrystallography (using monocrystals and the powder method) and by TG and DTA (in air and in an inert atmosphere). These compounds crystallize in the monoclinic system (Ca) or the orthorhombic system (Sr, Ba). Dehydration occurs in a single stage for calcium and strontium propionates and in two stages for barium propionate. The compound Ba(C₂H₅CO₂) 0.5 H₂O is well-defined. All the anhydrous salts, first amorphous to X-rays, crystallize at about 200°. Barium propionate undergoes another phase transition at 240°. Melting and simultaneous decomposition take place after 300° in air and after 350° in an inert atmosphere, leading to evolution of diethylketone and to a solid residue of well-crystallized metal carbonate.

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Zusammenfassung Die Propionatmonohydrate von Calcium, Strontium und Barium wurden mittels Radiokristallographie (an Einkristallen und an Pulver), sowie mittels TG und DTA (in Luft und in inertem Medium) untersucht. Sie kristallisieren in monoklinen (Ca) oder orthorhombischen (Sr, Ba) Systemen. Sie verlieren ihr Kristallwasser auf einmal (Ca, Sr) oder zweimal (Ba) unter Bildung eines intermediären Hemihydrats. Sämtliche Anhydrosalze, welche sich zuerst bei Röntgenbestrahlung amorph verhalten, kristallisieren in der Nähe von 200°. Das kristalline wasserfreie Bariumpropionat erfährt eine polymorphe Änderung bei 240°. Nach 300° in Luft und 350° in inerter Atmosphäre treten Schmelzen und Zersetzung gleichzeitig auf: Diäthylaceton wird freigesetzt und der feste Rückstand besteht aus kristallinem Metallcarbonat.

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, , , . - () — Sr, Ba. , — . (₂₅)₂. 0,5₂. 200°. 240° . 300° 350° , .

     

Skeletal rearrangements in mass spectra. Part I. Bis-aryl compounds

Examination of a large number of spectra indicates that bis-unsaturated compounds commonly undergo a skeletal rearrangement in which part or all or the bridging moiety is eliminated, often with concomitant loss of hydrogen atoms. The spectra of labeled azobenzene, diphenyl sulfide, and their p,p′-dimethyl derivatives show that scrambling of hydrogen atoms precedes or accompanies such rearrangements, in contrast to the loss of a p-methyl group from the latter derivatives. These results are rationalized in terms of the radical site formed on one unsaturated functional group attacking the polarizable π-electrons of the other unsaturated group.     

Correlating second harmonic optical responses of single Ag nanoparticles with morphology

Femtosecond laser excited second harmonic (SH) activity from single Ag nanoparticles is reported. A correlation of SH single-particle measurements with high-resolution imaging of particle morphology by TEM was achieved by creating position markers on an optical and electron transparent substrate (Si3N4 thin film, approximately 100 nm). We compared the SH activity of single Ag nanoparticles (nanospheres versus nanorods) and cluster structures (composed of two or multiple particles, e.g., dimers and trimers). The direct correlation of single-particle structures and SH activity, spectral and power dependence, strongly suggests one-photon resonant driven nonlinear oscillator response mechanism.     

How homogeneous are the trehalose, maltose, and sucrose water solutions? An insight from molecular dynamics simulations

The structural properties resulting from the reciprocal influence between water and three well-known homologous disaccharides, namely, trehalose, maltose, and sucrose, in aqueous solutions have been investigated in the 4-66 wt % concentration range by means of molecular dynamics computer simulations. Hydration numbers clearly show that trehalose binds to a larger number of water molecules than do maltose or sucrose, thus affecting the water structure to a deeper extent. Two-dimensional radial distribution functions of trehalose solutions definitely reveal that water is preferentially localized at the hydration sites found in the trehalose dihydrate crystal, this tendency being enhanced when increasing trehalose concentration. Over a rather wide concentration range (4-49 wt %), the fluctuations of the radius of gyration and of the glycosidic dihedral angles of trehalose indicate a higher flexibility with respect to maltose and sucrose. At sugar concentrations between 33 and 66 wt %, the mean sugar cluster size and the number of sugar-sugar hydrogen bonds formed within sugar clusters reveal that trehalose is able to form larger clusters than sucrose but smaller than maltose. These features suggest that trehalose-water mixtures would be more homogeneous than the two others, thus reducing both desiccation stresses and ice formation.