Showing papers by "City University London published in 1973"

Equivalence between AND/OR graphs and context-free grammars

Abstract: Recent research in artificial intelligence has led to AND/OR graphs as a model of problem decomposition (Nilsson [3]; Simon and Lee [4]). However, AND/OR graphs of a restricted type are equivalent to context-free grammars. This can be set-up formally (the beginnings of a formalism of AND/OR graphs is contained in [4]), but the formalism is so obvious that a brief discussion and example suffice.

Studies of the Reactions of Ethynyl Radicals with Hydrocarbons

Abstract: It has been shown that ethynyl radicals may be satisfactorily generated by the photolysis, at 253.7 nm, of bromoacetylene in the presence of nitric oxide. Acetylene and butadiyne are primary products, being formed exclusively by the reactions C$\_{2}$H$^{\cdot}$ + C$\_{2}$HBr $\rightarrow $ C$\_{2}$H$\_{2}$ + C$\_{2}$Br$^{\cdot}$, (4) C$\_{2}$H$^{\cdot}$ + C$\_{2}$HBr $\rightarrow $ C$\_{4}$H$\_{2}$ + Br$^{\cdot}$. (2) Nitric oxide decreases the rates of formation of both products, indicating the effective scavenging of ethynyl radicals by this compound. Addition of an inert gas (nitrogen or carbon dioxide) increases the ratio $[$$\text{C}\_{4}$H$\_{2}$$]$/$[$$\text{C}\_{2}$H$\_{2}$$]$ from 3.5 (no inert gas) to 7 (total pressure 80 kPa (1 Pa = n N m$^{-2}$)), the ratio thereafter remaining constant. The most obvious explanation for this behaviour is that, during photolysis, ethynyl radicals produced in the absence of inert gas have excess translational energy and, probably, enhanced reactivity. With increasing inert gas pressure, fewer 'hot' radicals react and the change in the ratio $[$$\text{C}\_{4}$H$\_{2}$$]$/$[$$\text{C}\_{2}$H$\_{2}$$]$ reflects the change in selectivity of 'thermalized' ethynyl radicals. On account of this, investigations of the reactions of C$\_{2}$H$^{\cdot}$ with added hydrocarbons were carried out with a standard 1:1:100 bromoacetylene-nitric oxide-nitrogen mixture. Results obtained with added alkanes (methane, ethane, 2,2-dimethylpropane) showed that ethynyl radicals abstract hydrogen atoms to form acetylene: C$\_{2}$H$^{\cdot}$ + RH $\rightarrow $ C$\_{2}$H$\_{2}$ + R$^{\cdot}$. (1) The relative importance of reactions (1) and (2) has been estimated and values for $k$$\_{1}$/$k$$\_{2}$ of 0.016 $\pm $ 0.005, 0.54 $\pm $ 0.04 and 0.91 $\pm $ 0.04 have been obtained for methane, and ethane 2,2-dimethylpropane respectively. The ratio $k$$\_{1}$/$k$$\_{2}$ did not vary over the temperature range 298 to 478 K in the case of 2,2-dimethylpropane but with methane, values for $E$$\_{1}$ - $E$$\_{2}$ and $A$$\_{2}$ / $A$$\_{1}$ of 12.54 $\pm $ 1.27 kJ mol$^{-1}$ and 0.54 $\pm $ 0.25, respectively, were obtained. Studies of the reactions of ethynyl radicals with alkynes (acetylene, butadiyne and propyne) have shown that the radicals abstract hydrogen atoms (to form acetylene), displace hydrogen atoms (to form a di- or triyne) and, in the case of propyne, displace a methyl radical. For propyne, the relevant reactions are C$\_{2}$H$^{\cdot}$ + C$\_{3}$H$\_{4}$ $\rightarrow $ C$\_{2}$H$\_{2}$ + C$\_{3}$H$\_{3}^{\cdot}$, (7) C$\_{2}$H$^{\cdot}$ + C$\_{3}$H$\_{4}$ $\rightarrow $ C$\_{4}$H$\_{2}$ + CH$\_{3}^{\cdot}$, (8) and C$\_{2}$H$^{\cdot}$ + C$\_{3}$H$\_{4}$ $\rightarrow $ C$\_{5}$H$\_{4}$ + H$^{\cdot}$. (9) Values of 25 $\pm $ 3, 5 $\pm $ 2, 9.9 $\pm $ 1 and 23 $\pm $ 3 at 298 K have been obtained for $k$$\_{7}$/$k$$\_{9}$, $k$$\_{4}$/$k$$\_{9}$, $k$$\_{8}$/$k$$\_{9}$ and $k$$\_{2}$/$k$$\_{9}$ respectively. In the presence of butadiyne, acetylene and hexatriyne are formed as primary products. Acetylene is formed by reactions (4) and (13), C$\_{2}$H$^{\cdot}$ + C$\_{4}$H$\_{2}$ $\rightarrow $ C$\_{2}$H$\_{2}$ + C$\_{4}$H$^{\cdot}$, (13) whilst hexatriyne is formed by the displacement reaction (14) C$\_{2}$H$^{\cdot}$ + C$\_{4}$H$\_{2}$ $\rightarrow $ C$\_{6}$H$\_{2}$ + H$^{\cdot}$. (14) Kinetic measurements have shown that at 298 K $k$$\_{4}$/$k$$\_{14}$ = 0.6 $\pm $ 0.1 and $k$$\_{13}$/$k$$\_{14}$ = 1.1 $\pm $ 0.2. Addition of acetylene-d$\_{2}$ to bromoacetylene-nitrogen mixtures yields acetylene-d$\_{1}$ and butadiyne-d$\_{1}$ C$\_{2}$H$^{\cdot}$ + C$\_{2}$D$\_{2}$ $\rightarrow $ C$\_{2}$HD + C$\_{2}$D$^{\cdot}$, (11) C$\_{2}$H$^{\cdot}$ + C$\_{2}$D$\_{2}$ $\rightarrow $ C$\_{4}$HD + D$^{\cdot}$. (12) The rate-constant ratios $k$$\_{12}$/$k$$\_{11}$ and $k$$\_{2}$/$k$$\_{12}$ are 2.8 $\pm $ 2.5 and 1.5 $\pm $ 0.3 respectively. This work thus indicates that ethynyl radical addition-elimination reactions, leading to polyalkynes, occur to a comparable extent to hydrogen-abstraction reactions in acetylene-containing systems. These results are shown to be of significance in regard to the formation and subsequent reactions of polyalkynes in both the pyrolysis and flames of acetylene and other hydrocarbons.

Conflict Resolution and Controlled Communication: Some Further Comments:

Abstract: As someone who has been associated with the Centre for the Analysis of Conflict in London, and with John Burton's development of the mediatory technique of 'controlled communication', I read Robert Yalem's recent evaluation of the technique (Yalem 1971) with interest, and experienced a sense of regret that Yalem had not been able to find the time to visit London for discussions with those who had direct experience of controlled communication in operation. Yalem's article raises very relevant points about the weakntesses of the technique, many of which have occasioned considerable debate and much disagreement within the Centre. However, there are a number of places where it is possible to take issue with his conclusions, particularly with regard to the assumptions underlying the use of controlled communication, and also its efficacy in helping to resolve international disputes. This is hardly surprising. Controlled communication and its allied experiments in the use of problem-solving settings for the analysis of ongoing conflicts is a new and (I believe) original development in third party interventions, so that it is not unusual that all its implications are not immediately apparent. I would, therefore, like to raise a few points in response to Yalem's criticisms, and make some brief comments upon his strictures, and some of the general problems raised by the use of the technique. As a preliminary, however, I think that it is worth stressing something that is often obscured in any discussion of the efficacy of controlled communication, even by its originator. This is that evaluation of the technique involves two quite separate sets of issues: (1) problems arising from the technique of controlled communication itself, whether it works effectively in changing attitudes and perceptions of those individuals who take part in the exercise, and of the factors which make for success (or failure) in bringing about such changes; and (2) problems about the actual impact of a controlled communication exercise on the substantive dispute and whether (and how) a successful exercise can contribute significantly (or even insignificantly) to bringing about a major change in the pattern of conflict interaction and the relations between the parties to that conflict. These two issues often seem to be confused in any discussion, and I think it is important to have them quite clearly separate before embarking on any critique of the method.

Corporate financial simulation models for top management

Abstract: Use of corporate financial simulation models for strategic decision taking is increasing rapidly. They are needed because conventional approaches have severe limitations. Corporate models represent the enterprise by means of mathematical and logical expressions. They may be optimizing or purely descriptive; assume a certain world or explicitly recognize variation and risk; be flexible or inflexible; use highly aggregated or disaggregated data; be batch run or interactive. Characteristics chosen should depend on the specific situation of the firm. A series of questions to focus attention on issues which should influence choice are posed. These are followed by brief reviews of the kind of models used by firms in the U.S.A. and U.K., of conditions that contribute to success, and of the range of likely costs.

Retrieval characteristics of the index to metals abstracts

Abstract: A small‐scale test of Metals abstracts index, with ten profiles and five sections of the abstracts as data base, was carried out with a methodology which endeavoured to simulate real life use of this Index by an information scientist for current awareness searching. The performance results were 67 per cent recall, 40 per cent precision and 2 per cent fallout. An error analysis showed that 65 per cent of the errors were attibutable to human (searcher) factors. Some of the factors that must inevitably affect the assessment of printed indexes are discussed, together with the observation of two unexpected phenomena, namely an apparent relationship between precision and searching effort, and relationships between generality and both recall and fallout.

Studies of the spontaneous ignition of hydrocarbonsand the application of computer techniques

Abstract: Measurements have been made of the ignition limits of n -decane-air mixtures, both in empty spherical vessels and in spherical vessels with heated tubes of different diameter passing through the center. It has been shown that thermal-ignition theory gives a good explanation of the variation with mixture composition at a fixed pressure of the minimum temperature required for spontaneous ignition. Furthermore, it provides a good description of the way in which the boundary separating slow combustion from cool flames and two-stage ignition varies with initial temperature and pressure. A computer program has been devised on the basis of the above application of thermal ignition theory to the experimental results. This makes it possible to predict the spontaneous-ignition limits in a spherical vessel of a given alkane, the only information required being the radius of the vessel, the molecular structure of the alkane, and the nature of the oxidant. By use of this program, the temperature limits for ignition in a spherical vessel containing heated tubes of known diameter may be estimated for given alkanes.

Configuration and Conformation of Bicuculline, a GABA Antagonist (Reply)

Abstract: DR STEWARD replies: Reporting a crystal structure determination of bicuculline, Gilardi1 points out that in discussions of the conformation of the molecule in relation to its action as a specific antagonist to γ-aminobutyric acid (GABA), an incorrect configuration has been used2,3. The configuration was, in fact, consistent with that given in Chemical Abstracts at that time. Subsequently, we suspected that the wrong diastereoisomer was portrayed. Confirmed by the full structure determination (D. Moss, personal communication), this did not affect the content of our discussion3 which drew attention to the possible congruence of N and O = C–O in both GABA and bicuculline, compared with Curtis's suggested2 correspondence of N and O = C–O in GABA to N and C–C=O in bicuculline.

A limit state anchorage bond test for reinforcement in concrete

Abstract: A new standard anchorage bond test for steel reinforcement in concrete has been developed in accordance with limit state principles. The basis of the test is non-failure of the anchorage.

The effects of training in written communication on verbal skills

Abstract: Summary. This was a pilot study to test the effectiveness of a training procedure which it was hoped would increase students' awareness of the reader's requirements when writing descriptions of technological equipment. Pairs of students took turns at commenting on each other's descriptions. Some improvement occurred in performance on verbal items on the AH5 and in quality of written descriptions made by trained experimental subjects (N=26) compared with untrained controls (N=19).

The student use of satellites for third world development

Abstract: In the past few years the technical problems associated with geostationary satellites broadcasting directly to television receivers on the ground have to a large extent been solved. One use of these will be in establishing an educational service in the developing countries with widely dispersed populations.