H-transfer rates in ion-neutral complexes versus those in intact aliphatic ions

The relative rates of H-transfer between partners in ion-neutral complexes were compared with those in intramolecular rearrangements using results of first differential photoionization mass spectrometry measurements. Complex-mediated H-transfers are inferred to have rates of the same order as those for intramolecular hydrogen rearrangements, suggesting a similar range of motion of the reactive sites in both types of reactions. It is also concluded that at their fastest H-transfers take place between the partners in ion-neutral complexes within at most the time of several rotations of the partners in the complexes. Copyright 1999 John Wiley & Sons, Ltd.     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

Study of molecular motion in polysilastyrene by pulsed 1H nuclear magnetic resonance

Pulsed NMR spectra of protons in polysilastyrene, $ \rlap{--} [{\rm Si(CH}_{\rm 3} {\rm )}_{\rm 2} {\rm  Si(CH}_{\rm 3} )({\rm C}_6 {\rm H}_5 )\rlap{--} ]_n $, with n ≈ 60, have been measured in the temperature range 80–450 K. The linewidth is constant at 7.4 G up to 200 K and narrows considerably above 250 K to a constant value of 0.3 G above 360 K. The motion responsible for this effect has an activation energy of 43.7 kJ/mol and is identified with the large-scale motion occurring in the vicinity of the glass transition temperature. The spin-lattice relaxation time T₁ was measured by the π-t-½π pulse sequence as a function of temperature. Two motional minima in T₁ were observed. The low-temperature motion has an activation energy of 3.7 kJ/mol and is identified with methyl group reorientation. The high-temperature motion has an activation energy of 29.1 kJ/mol and might be due to segmental motion.     

QUANTUM YIELD VALUES FOR THE DECOMPOSITION OF SEVERAL SURFACE-ACTIVE PHOTOREACTIVE AMPHIPHILES

The quantum yield values for the decomposition of 12-azido-1-diazo-2-dodecanone, 1-diazo-11-dodecanone, 1-diazo-17-octadecen-2-one, stearoyl azide and 10-undecenyl azidoformate in hexane were determined. It was found that the compounds decomposed with relatively high quantum yields, suggesting their use in biological applications, especially for binding phospholipids to membrane proteins.     

Total flowtime and makespan for a no-wait <Emphasis Type="Italic">m</Emphasis>-machine flowshop with set-up times separated

This paper addresses the m-machine no-wait flowshop problem where the set-up time of a job is separated from its processing time. The performance measures considered are the total flowtime and makespan. The scheduling problem for makespan reduces to the travelling salesman problem (TSP), and the scheduling problem for total flowtime reduces to the time-dependent travelling salesman problem (TDTSP). Non-polynomial time solution methods are presented, along with a polynomial heuristic.     

First direct observation of reversible oxygen addition to a carotenoid-derived carbon-centered neutral radical

Direct observation of reversible oxygen addition to a carotenoid-derived carbon-centered neutral radical is reported for the first time. The influence of temperature on the observed reaction kinetics has been used to obtain kinetic and thermodynamic parameters relating to the reversible addition of oxygen to the carotenoid radical obtained from reaction of 7,7'-dihydro-beta-carotene (77DH) with phenylthiyl radical (PhS.) in benzene. In addition, the rate constant for oxygen addition to the equivalent beta-carotene (beta-CAR) derived radical is also reported. [reaction: see text]     

Photoactivity tests of TiO2-based inorganic sunscreens. Part 1: Non-aqueous dispersions

A new method of measuring photoactivity of a major group of inorganic sunscreens (coated titanium dioxide) is presented based on the photobleaching of the radical 1,1-diphenyl-2-picrylhydrazyl.