Shell-closure reactions

Cram reported the preparation of the first soluble carceplex, carceplex 2 guest, from the shell closure reaction between two bowl-shaped tetrol molecules la, base, and four molecules of bromochloromethane under conditions of high dilution in dipolar, aprotic solvents (Scheme 4-1) [12, 13],... 

Carceplexes have also been synthesized using templates from cavitands with functionalities other than phenols. Cram synthesized benzylthia-bridged carceplex 13 guest through the shell-closure reaction between tetra(benzyl chloride) cavitand 11 and tetra-benzylthiol cavitand 12 in the solvents 2-butanone, 3-pentanone, ethanol/benzene (1 2), dimethylformamide (DMF), methanol/benzene (2 1), and acetonitrile/benzene (2 1), yielding carceplexes 13-2-butanone, 13-3-pentanone, 13-ethanol, 13-DMF, 13-2 methanol, and 13-2 MeCN respectively (Scheme 4-6) [22]. Technically, 13-2 MeCN is a hemicarceplex, as one acetonitrile molecule was found to escape the interior of the host via a billiard-ball effect, leaving 13-MeCN when heated in toluene [22]. The forma-... 

Because it was assumed that the next protonic magic number was 126 (by analogy with the neutrons), early studies of possible superheavy elements did not receive much attention [12—15), since the predicted region was too far away to be reached with the nuclear reactions available at that time. Moreover, the existence of such nuclei in nature was not then considered possible. The situation changed in 1966 when Meldner and Roper [16, 17) predicted that the next proton shell closure would occur at atomic number 114, and when Myers and Swiatecki [18) estimated that the stability fission of a superheavy nucleus with closed proton and neutron shells might be comparable to or even higher than that of many heavy nuclei. 

The nuclear models that resulted in the prediction of an island of superheavy nuclei have evolved in response to experimental measurements of the decay properties of the heaviest elements. While the prediction of a spherical magic N = 184 is robust and persists across the models [8], the shell closure associated with Z — 114 is weaker, and different models place it at higher atomic numbers, from Z = 120 to 126 [60-69] or even higher [70] (see Nuclear Structure of Superheavy Elements ). Interpretation of the decay properties of the heaviest elements may support this [71, 72], but the most part decay and reaction data do not conclusively establish the location of the closed proton shell. Because of this, the domain of the superheavy elements can be considered to start at approximately Z = 106 (seaborgium), the point at which the liquid-drop fission barrier has vanished [9]. For our purposes, the transactinide elements (Z > 103) will be considered to be superheavy (see Nuclear Structure of Superheavy Elements ). 

In spite of the lower excitation energies obtained in cold-fusion reactions, hot-fusion reactions produce evaporation residues that are more neutron rich, a consequence of the bend of the line of fi stability toward neutron excess. For the purposes of studying nuclei whose stability is more strongly influenced by the spherical 184-neutron shell clostrre, hot fusion is the more viable path. If nuclei were constrained to be spherical, or deformed into simple quadrupole shapes like those that influence the properties of the actinide isotopes with N — 152, one would expect cold-fusion reactions to quickly veer into ZJ space where nuclides would be characterized by very short partial half-lives for decay by spontaneous fission. In fact, there is a region of nuclear stability centered at Z = 108 and N — 162 [12, 19-21], removed from the line of fi stability toward proton excess, where the nuclei derive a resistance to spontaneous fission from a minor shell closure associated with complicated nuclear shapes, making a emission their most probable decay mode [133, 240]. 

Rutherfordium (Z = 104), first synthesized by hot fusion (see Sect. 2.2), has also been produced in cold-fusion reactions. Early cold-fusion work was focused on demonstrating the disappearance of the effect of the TV = 152 shell closure on the spontaneous fission half-life systematics of the heavy elements [198]. Rutherfordium isotopes are produced in reactions between Pb targets and Ti projectiles. The most neutron-rich cold-fusion Rf isotope is Rf (J a — 4.7 s), produced in the ° Pb( °Ti,n) reaction with a cross section of 5 nb [210, 246, 247]. 

Superficial examination of Fig. 3 cross sections would lead one to believe that Z — 114 is the closed proton shell since that is where the maximum Ca-induced transactinide evaporation cross sections are observed, followed by a decline at higher atomic numbers. Actually, as discussed above, the dynamical hindrance of fusion adversely affects " Ca-induced reaction cross sections, which probably increases with increasing atomic number. It is quite possible that both dynamic hindrance and the reduced survival probability of the compound nucleus at high excitation energies contribute to the fall-off of cross sections beyond N — 174, regardless of the location of the proton shell closure. More high-statistics excitation-function information will be required to sort this out. 

Attempts have been made to use the a-decay properties of the " Ca-induced reaction products to benchmark the nuclear mass evaluations arising from the various model calculations and to determine the location of the closed proton shell in transhassium space. Globally, the decay properties most closely match the predictions of the macroscopic-microscopic model, which predicts a spherical shell closure at Z = 114 [52, 58, 371-373]. However, the resiliency of theory is such that the a-decay Q values are also adequately reproduced by other models that predict a higher Z closed shell. Discrimination among the model calculations will only come about through the measurements of the decay properties of nuclides with higher Z and/or N than are currently known [8, 66]. 

The known superheavy element isotopes are plotted in Fig. 5 as a section of the Chart of the Nuclides, overlaying the microscopic shell corrections from Fig. 1. Even with the relative neutron excess afforded by " Ca-induced reactions, the known nuclei are located far from the Af = 184 shell closure. The trend in increasing half-life with increasing neutron number is expected to continue out to the vicinity of the shell. The superheavy nuclei with half-lives sufficiently long to be of interest to the radiochemist are listed in Table 1. When no long-lived isotope of an element is known, the longest-lived nuclide is given. 

The line of beta stability passes close to doubly magic 114 (Fig. 1), which is much more neutron rich (iV/Z = 1.614) than are nuclides that exist in Natore (e.g., Ca, MZ= 1.40 Pb, MZ = 1.54 N Z= 1.59). Therefore, it is not possible to reach the neutron shell closure at = 184 for compound nuclei near Z — 114 in fusion reactions with off-the-shelf projectiles and targets. The most asymmetric reactions that could hypothetically produce a compound nucleus beyond A = 184 are Pu - - Ni and - - Zn, both of which are associated with a compound nucleus with Z— 122. Examination of Figs. 2 and 3 would not lead one to believe that either reaction would have a signihcant probability of resulting in an evaporation residue. 

Cycloaddition reactions of dienophiles to extremely strained cyclopropanes, such as bicyclobutanes or bicyclopentanes (Table 1), have been reviewed by Gassman. The rate-determining step of these reactions, most probably, is the formation of a diradical intermediate for which mainly two pathways are open for recombination to a closed-shell product a) ring closure to give bicyclic structure 34 (formal [27r- -2ff] cycloaddition), and b) hydrogen shift to give the homo-ene-type product 35 (formal [27 -l-2(r- -27t] addition). 

The three-fragment MOVB theory described and applied above to the case of the allyl cation electrocyclization can be easily applied to any electrocyclic ring closure of a closed shell system with the conclusion always being that the stereoselectivity of the reaction is dictated by the symmetry labels of the pi HOMO and pi LUMO of the core pi system in the following way ... 

Probing superheavy element space by " Ca-induced hot-fusion reactions is characterized by advancing beyond the = 162 deformed subshell closure toward nuclei that are spherical and tightly bound. The macroscopic-microscopic model characterizes the ground-states of nuclei with > 175 as having a prolate deformation parameter 62 < 0.1, making them nearly spherical [8, 58, 60]. At neutron numbers below N — 175, any cross section benefit of the " Ca-induced hot-fusion approach to the Island of Stability is expected to decrease, as the shell stabilization of the ground state of the compound nucleus decreases. If a coldfusion path to a particular superheavy nuclide is available, it is expected to be the better one however, very little experimental evidence of this is available. As an example, the attempt to produce Cn (Z — 112) in the UC Ca,4n) reaction was unsuccessful (cross section limit <0.6 pb) [8, 316], in contrast to its production in the ° Pb( °Zn,n) reaction (cross section 0.5 pb) [263, 271]. 

The reaction of hex-3-ene-1,5-diyne derivatives (enediynes) to p-benzyne, the Bergman cyclization, is related to the electrocyclic ring closure of hexatriene. Following the discovery of a new class of potent antitumor antibiotics that possess an enediyne unit, this reaction has received considerable attention in recent years. Although the reaction has been investigated using semiempirical methods, the use of more advanced methods for the study of a closed-shell species reacting to a diradical is clearly desirable. 

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