Heteroatom Polysilylenes

Heteroatom polysilylenes with Cl, OR and NR₂ substituents attached to the silicon chain through the heteroatom are described. The relationship between the single crystal X-ray structures of octachlorocyclotetrasilane Si₄Cl₈ and perchloropolysilylene, (SiCl₂)_n, is discussed with reference to possible mechanisms for its phototopochemical synthesis from Si₄Cl₈. Substitution with alcohols yields the bis(alkoxy)polysilylenes, and the origin of the red-shifted UV spectra are discussed in terms of the electronic interaction between oxygen n and silicon orbitals. Substitution with secondary amines yields soluble, yellow polymers with even longer wavelength UV absorption maxima. Effects of the alkyl chain length on hydrolytic stability are considered.     

Fermi surface of 3d1 perovskite CaVO3 near the Mott transition

We present a detailed de Haas-van Alphen effect study of the perovskite CaVO3, offering an unprecedented test of electronic structure calculations in a 3d transition metal oxide. Our experimental and calculated Fermi surfaces are in good agreement, but only if we ignore large orthorhombic distortions of the cubic perovskite structure. Subtle discrepancies may shed light on an apparent conflict between the low energy properties of CaVO3, which are those of a simple metal, and high energy probes which reveal strong correlations that place CaVO3 on the verge of a metal-insulator transition.     

Casimir effect in source theory III

A proper-energy method is used to find the energy of attraction between two dielectric slabs. Independent derivations of the attractive force are also presented.     

Photoprotection of Mammalian Acid-Soluble Collagen by Cuttlefish Sepia Melanin In Vitro

Several important clinical conditions can result in close association between the pigment melanin and dermal collagen. Because melanin and its precursors can be chemically reactive in ground and excited states, it is important to know whether the resulting melanin-collagen interaction results in photoprotection or photoaggression. Acidic and neutral air-saturated collagen suspensions (0.033%) were irradiated with0–2.6 times 104 J/m2 UVC or with0–83 times 104 J/m2 solar-simulating UV radiation (SSR). Photochemical destruction of a photolabile collagen fluorophore (δ_em 360 nm) and collagen chain degradation were monitored as functions of irradiation time in the presence and absence of added (0–100μg) sepia eumelanin. Melanin retarded collagen photodamage but did not qualitatively alter the fluorescence fading kinetics. Both H202 and 02 can be produced by UV irradiation of eumelanin. Added H202 and K02 destroyed collagen fluorescence and caused 50% chain degradation at ca10–20-fold molar excess. Previous studies have demonstrated that eumelanins efficiently scavenge 02 . We demonstrated that eumelanin also efficiently scavenges H202 as evidenced by its ability to (a) compete with scopoletin for peroxide uptake and (b) directly take up H202 through a dialysis bag. The latter observation suggests that peroxide scavenging could occur in vivo by melanin sequestered in melanophages. Thus, neither UV-generated 02 nor H202 are likely to be present in concentrations high enough to cause measurable collagen damage. Absorption and/or scattering of excitation radiation away from the target chromophore appears to be the primary photoprotection mechanism, although scavenging of active 02 intermediates may play an important, if subtle role.     

Temperature dependence of the reactions of HO2 with NO and NO2

Mixtures of N₂O, H₂, O₂, and trace amounts of NO and NO₂ were photolyzed at 213.9 nm, at 245°–328°K, and at about 1 atm total pressure (mostly H₂). HO₂ radicals are produced from the photolysis and they react as follows: Reaction (1b) is unimportant under all of our reaction conditions. Reaction (1a) was studied in competition with reaction (3) from which it was found that k_1a/k₃^1/2 = 6.4 × 10^?6 exp { z?(1400 ± 500)/RT} cm^3/2/sec^1/2. If k₃ is taken to be 3.3 × 10^?12 cm³/sec independent of temperature, k_1a = 1.2 × 10^?11 exp {?(1400 ± 500)/RT} cm³/sec. Reaction (2a) is negligible compared to reaction (2b) under all of our reaction conditions. The ratio k_2b/k₁ = 0.61 ± 0.15 at 245°K. Using the Arrhenius expression for k_1a given above leads to k_2b = 4.2 × 10^?13 cm³/sec, which is assumed to be independent of temperature. The intermediate HO₂NO₂ is unstable and induces the dark oxidation of NO through reaction (?2b), which was found to have a rate coefficient k_?2b = 6 × 10¹⁷ exp {?26,000/RT} sec^?1 based on the value of k_1a given above. The intermediate can also decompose via Reaction (10b) is at least partially heterogeneous.     

The reactions of O(3P) with ozone and carbonyl sulfide

The competitive reactions between 2-trifluoromethylpropene (TMP) and OCS for O(³P) atoms were studied between 300° and 523°K, using the mercury-senstitized photolysis of N₂O as a source of O(³P). From the known value for the rate constant of the O(³P) + TMP reaction, k₃ was found to be 1.6 × 10^?11 exp (?4500/RT) cm³/particle-sec, where reaction (3) is Mixtures of O₃ and OCS were photolyzed at 197°, 228°, 273°, and 299°K with radiation above 4300 Å to produce O(³P) from the photolysis of O₃, and thus study the competition between reaction (3) and From the above value of k₃, k₁ could be computed. When combined with all the previous data, the best espression for k₁ is k₁ = 1.2 × 10^?11 exp (?4300/RT) cm³/particle-sec.     

Semigroups satisfying variable identities

The purpose of this note is to generalize a theorem of Tamura’s [3] providing a self-contained and, we think, more elementary proof than Tamura’s in that it avoids using the theory of contents. Tamura’s result states that a semigroup S satisfies an identify xy=f(x,y) with f(x,y) a word of length greater than 2 which starts with y and ends in x if and only if S is an inflation of a semilattice of groups satisfying the same identity. We investigate semigroups as in Tamura’s Theorem, except that we permit f(x,y) to vary with x and y.     

Kinetics of particle growth VII. NH4NO3from the NH3-O3 reaction in the presence of air and water vapor

Our earlier work on the formation of particulate NH₄NO₃ in the NH₃? O₃ reaction at 25°C is extended to include air as a diluent and H₂O vapor as an additive. More extensive data at different values of [NH₃]/[O₃]₀ were obtained also, where [O₃]₀ is the initial O₃ concentration. In our earlier work we concluded that NH₄NO₃ vapor was dissociated to NH₃ + HNO₃ and that the HNO₃ was removed by diffusion to the walls with a rate coefficient k_diff = 0.4 min^?1 or by condensation on the suspended particles. Particles were nucleated by 8 NH₃? HNO₃ pairs when their concentration product reached 5.8 × 10²⁷ molec²/cm⁶ with a rate coefficient k_nucl of 6.2 × 10^?224 cm⁴⁵/min and removed by coagulation with a rate coefficient k_coag of 1.3 × 10^?7 cm³/min. A corrected calculation modifies the number of pairs required to 6–7 with a correspondingly changed value of k_nucl. With the more extensive data of the present study the indications are that the vapor-phase NH₄NO₃ monomer is not dissociated and that its diffusion constant for loss to the walls varies between 0.3 and 0.9 min^?1 for different reaction conditions. Nucleation occurs when the NH₄NO₃ vapor concentration reaches 1.0 × 10¹² molec/cm³ via. where r is 9 and the nucleation rate coefficient k_nucl is 3 × 10^?108 cm²⁴/min. With 5.0 or 9.5 torr of H₂O vapor present, there is an excess of particles produced over that expected from this rate coefficient, indicating an additional nucleation step in which H₂O vapor participates directly to produce a hydrated salt. The coagulation coefficient of (1.87 ± 0.14) × 10^?7 cm³/min found here is in good agreement with that found previously.     

New method for estimating survival curves based upon subpopulations

The overall Kaplan-Meier estimate of the survival curve underlying a study population does not individually account for the ages at which patients were first seen and thence followed up in the study. Since the number of patients in these age-specific subgroups varies quite arbitrarily for a given study, the overall Kaplan-Meier curve which assumes that all patients were seen from the beginning of the study usually either overestimates or underestimates the survival probability, depending on the varying sample sizes. In this paper, we therefore propose an estimate of the overall survival curve underlying a study population based on the individual age-specific Kaplan-Meier estimates and the probability distribution of the age at which patients were first seen in the study. In the particular case when there is no censoring, it is shown that the usual overall Kaplan-Meier survival curve which assumes that all patients irrespective of their age at entry into the study were followed from the beginning of the study is identical with the proposed estimate. This result does not seem to necessarily hold good in the presence of arbitrary censoring. Further, the estimate is unbiased and consistent, a property which the Kaplan-Meier estimate also enjoys. The efficiency of the proposed estimate relative to the Kaplan-Meier estimate is being investigated and will be reported along with results of its application in a separate communication.