Applications of electron spin resonance to environmental problems

A selective review is given of some e.s.r investigations related to problems of environmental concern including ozone depletion, photochemical smog formation, and acid deposition.     

Mécanisme de l'hydrolyse acide des carbamates

N, N-dialkyl carbamates decompose in strongly acidic media to carbon dioxide, olefin, alkyl halide and alcohol, the rate of reaction of the secondary esters closely following the acidity function. This fact, together with the variation in rate of hydrolysis of carbamates of cyclic alcohols with the ring size, shows that, unlike the solvolyses of the corresponding chloroformates which lead to carboxonium ions (\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm R - O -}\mathop {\rm C}\limits^{{\rm (+)}} = {\rm O} $\end{document}), these reactions involve the intermediate formation of carbonium ions.     

Polyelectronic perturbation treatment of chemical reactivity

The usual practice of relating reactivity to a particular MO index does not allow for changes in the relative reactivity of various positions in a conjugated electron donor with the nature of the electrophilic reagent. By regarding the formation of the transition state as a mutual perturbation of the Molecular Orbitals of both reagents, the relative reactivity of various reacting centers is shown to vary with the magnitude of the perturbation. This treatment determines the factors responsible for these changes in reactivity. It outlines the conditions under which the frontier orbitals may determine the course of a reaction and also draws attention to the importance of electrostatic interaction.

---

Zusammenfassung Im Rahmen der MO-Theorie wird die Reaktivität gewöhnlich nur zu gewissen Indizes in Verbindung gesetzt. Damit ist die Reaktivität an den verschiedenen Zentren eines konjugierten Donators aber unabhängig von der Art des elektrophilen Agens. Wenn man den Übergangszustand als eine wechselseitige Störung der Molekülorbitale beider Agentien auffaßt, ändert sich die relative Reaktivität verschiedener Zentren mit der Größe dieser Störung. Dieses Verfahren bestimmt verschiedene Faktoren, die für den Wechsel in der Reaktivität verantwortlich sind. Es erklärt, unter welchen Bedingungen die Grenzorbitale den Verlauf einer Reaktion bestimmen und verweist auf die Bedeutung von elektrostatischen Wechselwirkungen.

---

Résumé La pratique habituelle, qui consiste à corréler la réaotivité et un indice particulier de la méthode des orbitales moléculaires, ne tient pas compte des variations relatives de réactivité des différentes positions d'un donneur d'électrons conjugué avec la nature du réactif électrophile. En considérant la formation de l'état de transition comme une perturbation mutuelle des orbitales moléculaires des deux réactifs, la réactivité relative des différents centres réactifs varie avec l'importance de la perturbation. Notre travail détermine les facteurs responsables de ces variations de réactivité. Il définit les conditions ou les orbitales frontiéres peuvent déterminer le cours d'une réaction, et attire l'attention sur l'importance de l'interaction électrostatique.

     

The interaction of an O2 molecular beam with an Fe(110) surface

The initial interaction between an O₂ molecular beam and a cleaned Fe(110) surface has been studied by a combination of Auger electron spectrometric (AES) and mass spectrometric techniques. The incident molecular beam intensity was calibrated using a stagnation detector, and the initial sticking coefficient for chemisorption was determined by mass spectrometric measurement of the transient in molecular scattering behavior observed when the cleaned surface was exposed to the molecular beam. This permitted an absolute calibration of the AES system for oxygen, and allowed comparison of the kinetic measurements of the oxygen adsorption process by the two techniques. Results indicate that the initial sticking coefficient is 0.2 ± 0.01. Oxygen is initially chemisorbed up to a coverage of 1.6 ± 0.16 × 10¹⁵ cm^?2, by a process following Langmuir kinetics. Beyond this point AES studies indicate a slower rate of oxygen uptake which is independent of gas-phase oxygen pressure. The mass spectrometric studies further indicate that for a cleaned, annealed surface those oxygen molecules which are not chemisorbed are scattered in a non-diffuse manner.     

Nuclear structure of light nuclei using the selectivity of high energy transfer reactions with heavy ions

We present data obtained by bombarding light nuclei with beams of ¹¹B, ¹²C and ¹⁴N at energies of about 10 MeV/nucleon. Reactions involving transfers of various numbers of nucleons were investigated and the spectra of the detected ions correspond to a wide range of excitation energies in the residual nuclei. The results characteristically show only a few strong peaks in each spectrum and the cross sections exhibit many of the features of direct reactions. We explain the apparent high degree of selectivity in terms of the interplay of reaction mechanisms and the structure of the strongly populated states. Firstly, it is shown that a simple semiclassical model of the reaction process indicates that states of the final nucleus with high spin are greatly favoured. On the other hand, it is demonstrated that the shell-model gives a reasonably complete explanation for the occurence of such high spin states at the appropriate excitation energies. These shell-model states are found to be dominated by configuration with the structure of a multi-nucleon cluster, in the ground state of its internal motion, orbiting the core nucleus so that the centre of mass motion of the cluster carries all the excitation quanta.     

Effect of mode-locking on the multiple-photon reaction of SF6

The effect of mode locking on the CO₂ laser-induced reaction of SF₆ is reported. A mode-locked laser pulse enhances the SF₆ reaction yield by a small (8%) but significant amount. The mode-locked pulse also decreases the isotopic selectivity of the reaction. This small effect is greater for enrichment of ³³SF₆ than for ³⁴SF₆. These results suggest that absorption mechanisms that have high power dependence play a minor role in the multiple-photon reaction.     

ORCADIAN OSCILLATORS AND PHOTORECEPTORS IN THE GASTROPOD,APLYSIA*

Abstract— The sea slug, Aplysia, is a useful model system for research on the neurophysiology of circadian integration. The animal contains several circadian oscillators and several photoreceptors. Each eye contains a circadian oscillator as well as photoreceptors. The ocular oscillators can be entrained by extraocular photoreceptors as well as their own ocular photoreceptors. The abdominal ganglion probably contains another oscillator but it has been much more difficult to manipulate in the laboratory than have the oscillators in the eyes. There is also a circadian rhythm in overt behavioral activity. This rhythm is controlled in part by extraocular oscillators and extraocular photoreceptors and in part by the eyes. In exerting their influence on the behavioral rhythm, the eyes appear to act in the capacity of oscillators and not merely as photoreceptors. Although neurons in the retina have neurosecretory morphology, the entire influence of the eyes on the behavioral rhythm appears to be mediated by nerve signals which travel in the optic nerve. As yet there is no evidence to suggest that any two oscillators in Aplysia are internally coupled. There is also no evidence yet for hormonal coupling between photoreceptors and oscillators or between oscillators and rhythmic outputs.     

Electron transfer and catalytic control by the iron-sulfur clusters in a respiratory enzyme, E. coli fumarate reductase

Factors governing the efficacy of long-range electron relays in enzymes have been examined using protein film voltammetry in conjunction with site-directed mutagenesis. Investigations of the fumarate reductase from Escherichia coli, in which three Fe-S clusters relay electrons over more than 30 A, lead to the conclusion that varying the medial [4Fe-4S] cluster potential over a 100 mV range does not have a significant effect on the inherent kinetics of electron transfer to and from the active-site flavin. The results support a proposal that the reduction potential of an individual electron relay site in a multicentered enzyme is not a strong determinant of activity; instead, as deduced from the potential dependence of catalytic electron transfer, electron flow through the intramolecular relay is rapid and reversible, and even uphill steps do not limit the catalytic rate.