Nonstatistical distribution

A kinetic isotope effect observed by a single reactant, having isotopic atoms at equivalent reactive positions, which reacts to produce isotopomeric products with a nonstatistical distribution. The pathway favored will be the one having lower force constants for the displacement of the isotopic nuclei in the transition state. 

Mixture B K[SiPh(3-fcat)2 and K[SiPh(dbcat)2] (3-fcat 2,3-dihydroxybenzaldehyde, dbcat 3,5-di-f-butylcatechol) contained two complexes with asymmetric catechols. Each complex showed the presence of two resonances due to the isomerism described above. The equilibrated mixtures showed the presence of two further species (Figure 9). These are attributed to isomers of the [SiPh(3-fcat) (dbcat)]- anion. Equilibrium was not established even after 8 weeks, whereupon decomposition prevented a more quantitative kinetic analysis. Flowever, it is apparent from the two experiments described that the kinetics of redistribution of ligands between complexes varies dramatically according to the cate-cholate involved. It is reasonable to conclude that the rate of redistribution decreases as the strength of the catecholate derivative increases. The nonstatistical distribution of complexes in a mixture indicates a thermodynamic stability of the complexes in Me2SO. The likely explanation lies in the electronic rather than the steric effects in the complex, since the live-coordination imposes little steric constraint. 

A strong motivation for the Rice et al. [99] simulations was to try to interpret the Zhao et al. [33] observations that the HONO elimination channel dominates while the N-N bond rupture reaction does not occur. A possible explanation is that the nascent CH2NN02 product of the RDX ring fission reaction is highly excited and has a nonstatistical distribution of energy. Sewell and Thompson [35] estimated that it may be formed with 55 to 65 kcal/mol of energy, which is well in excess of the predicted energy... 

However, their nonstatistical distribution as well as retention of stereochemistry if alkenyl organometallics were used (see Scheme 10.14),12 suggested that free radicals were not intermediates in such reactions. Such Wurtz-type couplings therefore probably proceeded by transmetallation and then recombination (Scheme 10.1). 

Besides classical resin beads, other polymeric carriers were also used for the synthesis ofpeptide libraries in various formats. Poly aery late-grafted polypropylene pins were used for the synthesis of the first combinatorial chemical library [1,2], This type of support continues to be heavily used in multiple peptide [27] and non-peptide [28] library synthesis. Cellulose paper, originally used by Frank et al. as a solid-phase support for oligodeoxy-ribonucleotide synthesis [29], has also been used as the support for multiple SPOT synthesis of peptide libraries [30,31], Polystyrene-grafted polyethylene film (PS-PE) may also be used in combinatorial peptide library synthesis [32], The specific feature of the membrane type of carrier is its dividability. This feature has been used for the synthesis of libraries with a nonstatistical distribution of library members, where no compound is missing and none is represented more than once [33],... 

With alloys and substitutional solid solutions, it is possible that a mixture of atoms (of similar size, valence, etc.) may reside at a general or special position and all its equivalent coordinates. The fraction of atoms of one type residing at that position is given by the site occupancy, or site occupation factor. The sum of the site occupation factors for that site must equal unity. The distribution of two or more types of atoms over a single site is completely random. Where two atoms are distributed over all the equivalent coordinates of different sites with similar local coordination environments (but not identical site symmetry), electronic, or other, effects can result in partial site preferences. That is, there can be a nonstatistical distribution over the two sites. 

Most reactions on surfaces are complicated by variations in mass transfer and adsorption equilibrium [70], It is precisely these complexities, however, that afford an additional means of control in electrochemical or photoelectrochemical transformations. Not only does the surface assemble a nonstatistical distribution of reagents compared with the solution composition, but it also generally influences both the rates and course of chemical reactions [71-73]. These effects are particularly evident with photoactivated surfaces the intrinsic lifetimes of both excited states and photogenerated transients and the rates of bimolecular diffusion are particularly sensitive to the special environment afforded by a solid surface. Consequently, the understanding of surface effects is very important for applications that depend on chemical selectivity in photoelectrochemical transformation. 

Isotope effects like the above, involving a direct or indirect comparison of the rates of reaction of iso-TOPOLOGUEs, are called intermolecular, in contrast to intramolecular isotope effects, in which a single substrate reacts to produce a nonstatistical distribution of isotopologue product molecules. 

E. Tenailleau, P.Lancelin, R.J. Robins, S. Akoka (2004 a) NMR approach to the quantification of nonstatistical distribution in natural products vaniUin. Anal. Chem. 72, 3818-3825... 

In conclusion, our results show unequivocably that the Na -(-CO reaction is not a resonant E V transfer process and also that the reaction, presumably taking place via an ionic Na (CO) intermediate, produces CO with a nonstatistical distribution. This is in sharp contrast with the statistical distribution found in the 0( Z)) + C0 reaction, in which nearly the same amount of electronic energy is involved. This implies that the transition of the assumed Na" (CO) intermediate to Na( S) + CO(o) occurs approximately in the period of a collision. More detailed discussion of the relaxation of electronically excited Na atoms has recently been made by Hertel." ... 

We previously mentioned that warm irradiation compared to cold irradiation results in higher ductility and lower strength. In addition, the crosslink density of warm irradiated UHMWPE is lower than that of cold irradiated UHMWPE at the same dose level. This could be due to the biphasic morphology developed in UHMWPE after warm irradiation, which is evidenced by the dual peak melting peaks. We speculate that the biphasic morphology is a result of the nonstatistical distribution of crosslinks that is known to occur at high irradiation temperatures with poly-ethylenes [12, 16, 17]. One of the phases, possibly the continuous one, may be lightly crosslinked and maintain the mechanical properties of the uncrosslinked UHMWPE. The... 

The obvious nonstatistical distributions of the residuals in Figs. 10b and 10c allow the rejection of the representations (b) and (c). Unfortunately, this necessary data analysis is missing in some publications on this subject ... 

Nonstatistical unimolecular product energy distributions have been either measured or deduced from infrared multiphoton absorption, molecular beam, arrested infrared chemiluminescence, and gas bulb chemical activation experiments. Except for a few anomalous cases, these nonstatistical distributions probably result from exit-channel coupling effects. These effects are due to potential energy characteristics in the exit-channel and at the present time very little is known about them. It may prove to be possible to classify unimolecular potential energy surfaces by their exit-channel properties. However, the classifications will most likely be considerably more detailed than what has been found for A -f BC AB + C potential energy surfaces,17-19,1315132... 

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