Random fragmentation model

Brush-heap model, see Random-fragmentation model... 

Ramsey theory, 22 201-204 Random-fragmentation model, Szilard-Chalmers reaction and, 1 270 Random-walk process, correlated pair recombination, post-recoil annealing effects and, 1 288-290 Rare-earth carbides, neutron diffraction studies on, 8 234-236 Rare-earth ions energy transfer, 35 383 hydration shell, 34 212-213 Rare gases... 

The hot-zone model differs from the random-fragmentation model in an important respect. The random-fragmentation model is based upon free radical recombinations. The authors model suggests that in view of the relatively low concentration of free radicals present in the hot zone, chemical reactions with the parent species may also play an important part. The picture may be further complicated by ion-molecule reactions (71). 

Lounkine, F., Batista, J. and Bajorafh, J. (2007) Mapping of activity-specific fragment pathways isolated from random fragment populations reveals the formation of coherent molecular cores. J. Chem. Irf. Model., 47, 2133—2139. 

For example, in the case of H tunneling in an asymmetric 0i-H - 02 fragment the O1-O2 vibrations reduce the tunneling distance from 0.8-1.2 A to 0.4-0.7 A, and the tunneling probability increases by several orders. The expression (2.77a) is equally valid for the displacement of a harmonic oscillator and for an arbitrary Gaussian random value q. In a solid the intermolecular displacement may be contributed by various lattice motions, and the above two-mode model may not work, but once q is Gaussian, eq. (2.77a) will still hold, however complex the intermolecular motion be. 

Values for kj and kjj are assumed and the above equations are integrated subject to the initial conditions that a = 2, b = 0 at t = 0. The integration gives the model predictions amodel(j) and bmodel(j). The random search technique is used to determine optimal values for the rate constants based on minimization of and S. The following program fragment shows the method used to adjust kj and kjj during the random search. The specific version shown is used to adjust kj based on the minimization of S, and those instructions concerned with the minimization of S appear as comments. 

We have designed PBUILD, a new CHEMLAB module, for easy construction of random copolymers. A library of monomers has been developed from which the chemists can select a particular sequence to generate a polymeric model. PBUILD takes care of all the atom numbering, three dimensional coordinates, and knows about stereochemistry (tacticity) as well as positional isomerism (head to tail versus head to head attachment). The result is a model of the selected polymer (or more likely a polymer fragment) in an all trans conformation, inserted into the CHEMLAB molecular workspace in literally a few minutes. 

We have added a companion option to PBUILD, PRANDOM which eases considerably the problem of finding good conformations of a polymer segment. PRANDOM automatically selects all of the polymer backbone and/or side chain bonds and will randomly select rotations for each bond. In a few minutes, one can not only build a polymer fragment, but also set up a Monte-Carlo search of its conformational space. However, even this cannot solve the problems for large models (pentamer or larger), again due to the number of bonds to be rotated. 

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