Photochemistry of α-oxo-oximes—VI: Irradiation of 3-ethoxyimino-1,7,7-trimethylbicyclo[2.2.1]heptan-2-one

The photolysis of 3-ethoxyimino-1,7,7-trimethylbicyclo[2.2.1]heptan-2-one 1 with λ 254 nm has been investigated in acetonitrile as solvent. Photoexcited 1 undergoes N-O bond homolysis with formation of a cyclic α-oxo-iminyl and an ethoxy radical. Cage recombination of these radicals affords the ethyl cyanocyclopentanecarboxylate 9. The formation of the products 6–8 is explained by decarbonylation of the α-oxo-iminyl radical, followed by disproportionation and H-abstraction. The occurrence of the α-oxo-iminyl radical and (by loss of CO) the tertiary cyanocyclopentyl radical is substantiated by an ESR photoexperiment in the presence of 2-methyl-2-nitrosopropane.     

Conformational study on some β-phenyl-α,β-unsaturated ketones

The conformation of three (E)-β-phenyl-α,β-unsaturated ketones and their corresponding (Z)-isomers 1, 2, and 3 was established by IR and ASIS. (Z)-1 and (E)-1 have exclusively the s-trans conformation. The two isomers of 2 occur in both conformations but there is a higher s-cis to s -trans ratio with the (Z) than with the (E)-isomer. (Z)-3 appears to exist exclusively in the s-cis conformation, the (E)-isomer has a small content of the s-trans conformation. It was concluded that the ASIS for H_b, is a measure of the s-cis content of the conformational equilibrium.     

Photochemistry of dienones—X: Photochemistry of (e)-β-ionone oxime ethyl ether

(E)-β-ionone oximc ethyl ether [(E, E)-4] upon direct irradiation with λ either254or 313 nm yields the geometrical isomer (E, Z)-4 and (Z)-retro-γ-ionone oxime ethyl ether (Z,E)-5 as the sole primary products, illustrating (E)-(Z) isomerization (φ₃₁₃ =0.49) and a 1, 5-hydrogen shift (φ313 =0.15) respectively. From studies with triplet photosensitizers and with ethyl iodide (to enhance the singlet-triplet intersystem crossing) it is concluded that these two products in the direct irradiation result only from the singlet excited state, and that the inter-system crossing quantum yield is relatively low. Upon prolonged irradiation of (E,E)-4 with λ 313 nm the eventual products are (Z,E)-5 and (Z,Z)-5, whereas with λ 254 nm they are (E,E)-5 and [(Z,E)-5 and/or (E,Z)-5]. Upon triplet photosensitization (E,E)-4 undergoes only (E)-(Z) isomerization, leading to a mixture of all the four geometrical isomers of4. From the dependence of the geometrical isomer distribution in the photostationary state on the triplet energy of the sensitizer the triplet energies of (E,E)-4, (E, Z)-4, (Z, E)-4, and (Z, Z)-4 have been determined to be ca 55, < 55,57, and 57 $\text{kcal}\text{mol}$ respectively.     

Parametric polariton amplification in semiconductor microcavities

We present novel experimental results demonstrating the coherence properties of the nonlinear emission from semiconductor microcavities in the strong coupling regime, recently interpreted by parametric polariton four-wave mixing. We use a geometry corresponding to degenerate four-wave mixing. In addition to the predicted threshold dependence of the emission on the pump power and spectral blueshift, we observe a phase dependence of the amplification which is a signature of a coherent polariton wave mixing process.     

Spin-singlet clusters in the ladder compound NaV2O5

The space group of alpha(')-NaV2O5 turns below T(c) = 34 K from Pmmn with all V sites equivalent, into Fmm2 with three independent vanadium sites per layer. This is incompatible with models of charge ordering into V4+ and V5+. Our structure determination indicates that the phase transition consists of a charge ordering with three distinct valence states, formally V4+, V4.5+, and V5+. The singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, we ascribe the quadrupling of the c axis to the large polarizability of the V2O5 skeleton.     

An Earth-based equivalent low stretch apparatus for material flammability assessment in microgravity and extraterrestrial environments

The standard oxygen consumption (cone) calorimeter (described in ASTM E 1354 and NASA STD 6001 Test 2) is modified to provide a bench-scale test environment that simulates the low velocity buoyant or ventilation flow generated by or around a burning surface in a spacecraft or extraterrestrial gravity level. The equivalent low stretch apparatus (ELSA) uses an inverted cone geometry with the sample burning in a ceiling fire (stagnation flow) configuration. For a fixed radiant flux, ignition delay times for characterization material PMMA are shown to decrease by a factor of 3 at low stretch, demonstrating that ignition delay times determined from normal cone tests significantly underestimate the risk in microgravity. The critical heat flux for ignition is found to be lowered at low stretch as the convective cooling is reduced. At the limit of no stretch, any heat flux that exceeds the surface radiative loss at the surface ignition temperature is sufficient for ignition. Regression rates for PMMA increase with heat flux and stretch rate, but regression rates are much more sensitive to heat flux at the low stretch rates, where a modest increase in heat flux of 25 kW/m² increases the burning rates by an order of magnitude. The global equivalence ratio of these flames is very fuel rich, and the quantity of CO produced in this configuration is significantly higher than standard cone tests. These results demonstrate that the ELSA apparatus allows us to conduct normal gravity experiments that accurately and quantifiably evaluate a material’s flammability characteristics in the real-use environment of spacecraft or extraterrestrial gravitational acceleration. These results also demonstrate that current NASA STD 6001 Test 2 (standard cone) is not conservative since it evaluates a material’s flammability with a much higher inherent buoyant convective flow.     

Substituent effects in IR-(tricarbonylchromium)arenes: II. An IR study of the CO stretching vibrations in alkylsubstituted 7r-(tricarbonylchromium)benzenes

the IR wavenumbers and half-band width's of the carbonyl stretching vibrations of several substituted 7r-(tricarbonylchromium)benzenes in isooctane solution are reported. only small perturbations of C3,.symmetry are observed. the shape of the out-of-phase band is influenced by electronic effects, conformer distribution and solute-solvent interaction. the asymmetry of the E-band in (1'-t-butyl-2',2'-dimetliylpropyl)-7r-(tricarbonylchromium)benzene is ascribe to a direct steric effect. ,me correlation between the CO force constants and electronic substituent parameters is satisfactory for the monosubstituted complexes. the substituent effects are approximately additive for the di- and tri-alkylsubstituted benzene complexes.     

Measurements of depolarization ratios and polarizability anisotropies of gaseous molecules

Values for the depolarization ratio (?^T_V) of Rayleigh scattered light are reported for a large number of gas molecules. When possible, the results are combined with refractive index data to yield the anisotropy in the optical polarizability.