Fission path

Reactions with Iodine and Bromine. All the reactions discussed above proceed at limiting rates that are independent of the concentration and nature of the reactant. However, reactions of iodine with Mn2(CO)j0 (6) and Re2(C0)1g (30) proceed by paths that are first order in [I2] as well as by the [I2]-independent homolytic fission paths. P-donor substituents increase the rates of reaction with I2 by several orders of magnitude (31) so that they proceed rapidly even at room temperature. Thus reaction of Mn2(C0)g P(C6Hu53)2 is estimated to occur over 108 times faster than Mn2(C0)jQ by a path first order in [I2] at 25°C in cyclohexane. In all cases the reactions proceed with fission of the metal-metal bonds to form the mononuclear iodo complexes. 

The S—C bond fission path has been exploited as a method for photodeprotection of alcohols. The earliest examples of this were reported by Pete and his group226-228, who demonstrated that the free alcohol could be obtained in reasonable yields on irradiation of tosyloxy derivatives. Scheme 19 shows the proposed mechanism for the process where irradiation brings about S—C bond fission affording a biradical. Loss of S02 affords the alkoxy radical and ultimately the alcohol. Many examples of this deprotection path have been reported over the years. This area of study has been reviewed by Binkley in a number of articles229-231 with particular reference to deprotection of carbohydrate derivatives. Thus the compounds shown in Scheme 20 are converted into the free alcohols on irradiation in yields up to 87%232,233. Even higher yields can be obtained as with the irradiation of 261 which affords 100% detosylation affording 262234. 

FIGURE 8.TI The upper part of the figure shows the collective potential energy surface for with the ground-state position and various fission paths through the barrier. The middle part shows various collective masses, all calculated in the TCSM. In the lower part the calculated fission half lives are depicted. 

The focus of this chapter is to review the latest results obtained for the excited states in the second and third minima of the potential barrier. In addition to these exotic shapes, it is also an interesting and longstanding question, at which points of the fission path the mass and energy distributions of the fission fragments are determined. Can one get different mass distributions after the fission of the super- and hyperdeformed states as suggested by Cwiok et al. (1994) What kind of clusterization is expected in these exotic nuclear states Does the predicted cold valley exist in the fission barrier These are very interesting questions but the presently available experimental information is still not sufficient to answer them. 

The HD states lying in the third well of the fission barrier may play the role of a doorwaylike state before fission, from which the fission can only occur through a limited number of fission paths, resulting in a sharper mass distribution (Krasznahorkay et al. 2000, 2001a, b, 2003, 2004). 

Track-etched membranes are made by exposing thin films (mica, polycarbonate, etc) to fission fragments from a radiation source. The high energy particles chemically alter material in their path. The material is then dissolved by suitable reagents, leaving nearly cylindrical holes (19). 

In the hydration of compounds 2f and 2g, besides the expected ester, three other products (acetic acid, an alkene, and alcohol) were observed. These products were postulated to arise via a fragmentation of the intermediate vinyl cation, 6, as shown in Scheme II. The importance of the fragmentation path is presumably determined by the stability of the alkyl cation formed by the alkyl oxygen fission. 

Aeberhardt, A. (1961). Contribution to the Study of the Metabolism of Fission Products. Research on the Physico-Chemical State and the Metabolic Path of Radiocerium Solutions, French Atomic Energy Commission Report No. CEA-1856, English translation Report No. AEC-tr-5242 (National Technical Information Service, Springfield, Virginia). 

The r-process path is terminated by (neutron-induced or yd-delayed) fission near A max = 270, feeding matter back into the process at around Amax/2, followed by recycling as long as the neutron supply lasts, assuming sufficient seed nuclei to start the process off. The number of heavy nuclei is thus doubled at each cycle, which could take place in a period of a few seconds, yd-delayed fission also occurs after freeze-out, when the yd-decay leaves nuclei with A > 256 or so with an excessive positive charge (see Eq. 2.90). 

Fig. 5.6. Path of s and r processes across the Z, N) plane. Everything begins with iron. The s process follows roughly along the valley of statrility, flowing like a river along the banks it defines. It ends with the a decay of bismuth-209. The r process takes matter far out of the valley on the neutron-rich side, whilst the weak interaction brings it back to the fold. In this case neutron capture continues until the nucleus undergoes fission. The climb to neutron-rich summits is indeed vertiginous.

In order to determine the maximum atomic mass produced in the r process, we must find the point when induced (destructive) fission enters into competition with (constructive) neutron capture on the path followed by the process across the (N, Z) map of the isotopes. This question requires calculation of the fission barrier far from the region of known nuclei, which is no simple matter. The possibility of producing mythical, superheavy, transuranium nuclei (around Z = 114 and = 184) has not yet been demonstrated. 

The unimolecular reaction of the ion aggregate follows a similar course and the intermediate faces the same three possibilities for reaction. The rate of bond fission will not necessarily be the same as that of the free ion because the solvation environment has changed. We see this effect in the ion pair-catalyzed solvolytic reactions (7). In addition, since the reagent Y is in position before the five-coordinate intermediate is formed, the path by which X re-enters the coordination shell becomes less probable as a result of more effective competition by Y, and the rate is increased. 

Portion of the Chart of the Nuclides showing s-process and r-process pathways. The s-process pathway, shown by the dark line in the center of the valley of p-stability, shows how a nuclide that successively captures individual neutrons would evolve. Each added neutron moves the nuclide to the right on the diagram, until it reaches an unstable nuclide, in which case it will p-decay to the stable nuclide with a higher Z. In contrast, in situations where nuclides capture neutrons very rapidly ( -process), they will be driven far to the right of the valley of p-stability until the timescale for neutron capture matches that for p-decay. They will then move to higher Z and capture more neutrons until they either reach a size that causes them to fission (break) into smaller nuclei (which can then capture more neutrons) or until the neutrons disappear, in which case they will p-decay back to the first stable isotope along paths of constant A (arrows). 

Isomerization of 2,3-dipbenyM-indahone follows still a third type of reaction path (Eq. 476), yielding a lactone by Ting fission and iecycliaation.M-81l, IM-1B 1 and 2,3,.r>,6-tetraphenyl-l -indanonn wW-goes a similar transformation,818 in contrast with a previous report.-1... 

The first two equations represent the fact that the D-D reaction can follow either of two paths, producing tritium and one proton or hehum-3 and one neutron, with equal probability. The products of the first two reactions form the fuel for the third and fourth reactions and are burned with additional deuterium. The net reaction consists of the conversion of six deuterium nuclei lnlo two helium nuclei, two hydrogen nuclei, and two neutrons along with a net energy release of 43.1 MeV. The reaction products—helium, hydrogen, and neutrons—are harmless as contrasted with the myriad fission products obtained in a fission reactor. The neutrons produced may be absorbed in sodium to produce an additional 0.25 MeV per cycle. Therefore, the D-D reaction produces at least 7 MeV per deuterium atom (deuteron) and, with absorption in sodium, more than 10 MeV per fuel atom. 

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