Dynamical hindrance

In the production of heavy transuranium nuclei with heavy ions, additional factors that enhance or decrease the yields of transuranium products should be considered. They are fusion enhancement at sub-barrier energy and dynamical hindrance to fusion. 

To explain the small cross sections for SHE synthesis and in particular the fast decrease of the cold fusion cross sections, the concept of extra push has been developed. In this model, the barrier for complete fusion is shifted dynamically above the Coulomb barrier until the experimental cross section is reproduced (Hessberger et al. 1985). The barrier shift is explained (in terms of a macroscopic approach) as a result of dynamical hindrance of the nuclei on their way to fusion (Swiatecki 1982 Armbruster 1989). However, for the transactinide elements known to exist only due to shell effects, the application of a macroscopic model to explain all structure effects related to their production is questionable if not wrong (Miinzenberg 1988). In contrast to the extra push concept, experiment shows that the maximum of the excitation function shifts toward a smaller excitation energy for heavier systems, with Cn being the coldest compound nucleus ever produced at an excitation energy of only 10 MeV (Hofmann et al. 1996). 

In the example above in which isotopes of nobelium are produced by hot and cold fusion, the difference between the observed cross sections and the geometrical cross sections derive from two different effects. In the hot-fusion reaction, the compound nucleus is unlikely to survive the competition between fission and each of the four neutron-evaporation steps, leading to a small cross section. In the coldfusion reaction, the probability that the compound nucleus avoids the fission process is orders of magnitude higher than in the hot-fiision reaction, but the dynamical hindrance to complete fusion results in a lower probability for formation of that compound nucleus [227, 235-237]. It is a matter of some serendipity that the nobelium evaporation-residue cross sections for the two reaction types are approximately the same. 

As mentioned above, there is an exponential downward trend in cold-fusion evaporation-residue cross sections with increasing atomic number of the product, attributed more to an increase in dynamical hindrance to fusion than to a decrease in < rjFf> [105, 108, 227, 232, 235, 236, 296]. For the transactinides, representative cross sections for In-channel cold-fiision reactions [198, 201, 246] are plotted in Fig. 2. In going from Z = 105 to Z = 113, the cross section decreases... 

Besides the effect of shell stabilization in the entrance channel on the of the compound nucleus, the mechanism of " Ca-induced hot-fusion reactions shares another aspect of the character of cold-fusion reactions. While deexcitation of the hot compound nuclei is dominated by the competition between fission and neutron emission, attempts to reproduce the evaporation-residue cross sections by a simple r /ry treatment results in values that are much higher than those that are observed experimentally [300-302]. It is necessary to invoke a significant dynamical hindrance to fusion and a two-step mechanism [303, 304] to reproduce the cross sections for " Ca-induced reactions that result in transactinide nuclides [305, 306], which increases as the atomic number of the target nuclide increases. Like the cold-fusion reaction intermediate, the reaction trajectory from nuclei in contact to a compound nucleus can be diverted into a more probable path leading to quasifission, even though the potential energy of the compound nucleus is lower than or approximately equal to that of the reacting nuclei in contact [8, 105,123,174,220, 301, 307-312]. Only a small number of dinuclear intermediates reach the compact shape associated with the compound nucleus. 

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. 

Evaporation residues arising in complete-fusion reactions between actinide targets and radioactive-beam particles are controlled by the same < r /Ff > and dynamical hindrance effects as are the reaction products from stable-ion beam irradiations. It has been observed that fusion cross sections for reactions with neutron-rich radioactive beam particles can be enhanced over those with stable-isotope beams at the same Z, possibly due to an effective lowering of the fusion barrier with the increasing neutron number of the projectile facilitated by neutron flow in the dinuclear reaction intermediate [226, 454, 458]. It is unclear how dynamical hindrance effects and a reduced resistance to deexcitation by fission at high excitation energies in heavier systems will influence the formation of evaporation residues. It has been suggested that the formation of products at the... 

In generalized Rouse models, the effect of topological hindrance is described by a memory function. In the border line case of long chains the dynamic structure factor can be explicitly calculated in the time domain of the NSE experiment. A simple analytic expression for the case of local confinement evolves from a treatment of Ronca [63]. In the transition regime from unrestricted Rouse motion to confinement effects he finds ... 

For 340a and 341a, another dynamic process was also frozen out. This process must be due to nitrogen inversion in the imino function, which results in E/Z isomerization (93MI2). This inversion could occur via amino-imino tautomerism (91JOC3194). The E and Z isomers are practically equally stable since there is no noteworthy steric hindrance in either form. 

Another effect of the high steric hindrance in dithiin 25 due to the 3,3 -dibornane skeleton is the fact that this dithiin can be oxidized readily (with 1 mol of t-chloroperbenzoic acid (MCPBA) at 0°C) to sulfoxide 26 <1995T13247, 1994TL1973> (Scheme 30), a stmcture which is remarkably stable in contrast with the usual characteristics of sulfoxides in the dithiin series. The six-membered ring interconversion of the sulfoxide 26, slowed obviously by the steric bulk hindrance, was investigated by dynamic NMR (AG ca. 10-11 kcalmoP ). With an excess of MCPBA at elevated temperature, the sulfoxide 26 is oxidized to the sulfone 27 parallel treatment with oxygen, however, failed to yield the same compound. 

Differing from Bitsanis et al. who neglected the intramolecular hydro-dynamic interaction and equated DL0 to D[0, Doi et al. incorporated this interaction and set D,0 equal to D 0/2. The increase of the transverse diffusivity should enhance the disengagement of the rotation of the test rod from hindrances and tend to diminish the entanglement effect. 

The 2,2,6,6-tetramethylpiperidinoxyl radical (TEMPO) was first prepared in 1960 by Lebedev and Kazarnovskii by oxidation of its piperidine precursor.18 The steric hindrance of the NO bond in TEMPO makes it a highly stable radical species, resistant to air and moisture. Paramagnetic TEMPO radicals can be employed as powerful spin probes for elucidating the structure and dynamics of both synthetic and biopolymers (e.g., proteins and DNA) by ESR spectroscopy.19 Unlike solid-phase 1H-NMR where magic angle spinning is required in order to reduce the anisotropic effects in the solid-phase environment, solid-phase ESR spectroscopy can be conducted without specialized equipment. Thus, we conducted comparative ESR studies of various polymers with persistent radical labels, and we also determined rotational correlation times as a function of... 

Since the main degradation product of BC6 was assumed to be the mononitro derivative (BC6-NO2), nitro compounds have been synthesized and the distribution ratios measured. Extraction results with 1 mol L 1 nitric acid showed that the presence of nitro groups reduced the extraction of cesium DCs were 19.5, 8.5, and 6 x 10 3 for solutions 10-2 mol L 1 of BC6, BC6-N02, and BC6-4N02, respectively (68), whereas the extraction of Na+ was slightly affected. Theoretical approaches by molecular dynamics simulations indicated that the nitro group was not ideally located to efficiently participate in the complexing of Cs+ or Na+, and therefore the loss of efficiency with nitro compounds arose from steric hindrance around the complexing site. 

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