Molecular redistribution

Molecular exchange between the crystallites and the intercrystalline space may, however, be controlled by processes other than ordinary diffusion. A substantial retardation of molecular exchange may be caused by transport resistances on the external surface of the crystallites. It has been shown in PFG NMR studies that such surface barriers may be brought about during the process of zeolite manufacturing (e.g. by hydrothermal treatment) [1,6] and by coke depositions [1,7]. In this case, irrespective of possibly large rates of molecular redistribution within the crystallites, the rate of molecular escape out of the crystallites may be slowed down dramatically. In effect, in this case, the product molecules should be distributed essentially homogeneously over the whole space of the individual crystallites. 

PFG NMR is a method to study the rate of molecular redistribution under equilibrium conditions. For this purpose, prior to the measurements the samples are subjected to a reproducible pretreatment consisting of sample activation under vacuum (ca. 10 Pa) at elevated temperatures (ca. 400°C) and a subse-... 

R.L. Burnett, T.R. Hughes - Mechanism and Poisoning of the Molecular Redistribution Reaction of Alkanes with a Dual-Functional Catalyst System,... 

Figure B2.5.18 compares this inter molecular selectivity with intra molecular or mode selectivity. In an IR plus UV, two-photon process, it is possible to break either of the two bonds selectively in the same ITOD molecule. Depending on whether the OFI or the OD stretching vibration is excited, the products are either IT -t OD or FIO + D [24]- hr large molecules, mirmnolecular selectivity competes with fast miramolecular (i.e. unimolecular) vibrational energy redistribution (IVR) processes, which destroy the selectivity. In laser experiments with D-difluorobutane [82], it was estimated that, in spite of frequency selective excitation of the...

At any interface between two different phases there will be a redistribution of charge in each phase at the interface with a consequent loss of its electroneutrality, although the interface as a whole remains electrically neutral. (Bockris considers an interface to be sharp and definite to within an atomic layer, whereas an interphase is less sharply defined and may extend from at least two molecular diameters to tens of thousands of nanometres the interphase may be regarded as the region between the two phases in which the properties have not yet reached those of the bulk of either phase .) In the simplest case the interface between a metal and a solution could be visualised as a line of excess electrons at the surface of the metal and an equal number of positive charges in the solution that are in contact with the metal (Fig. 20.2). Thus although each phase has an excess charge the interface as a whole is electrically neutral. 

Thus, mixing in liquids is achieved by several mechanisms which gradually reduce the size or scale of the fluid elements and then redistribute them in the bulk. If there are initially differences in concentration of a soluble material, uniformity is gradually achieved, and molecular diffusion becomes progressively more important as the element... 

In studies of molecular dynamics, lasers of very short pulse lengths allow investigation by laser-induced fluorescence of chemical processes that occur in a picosecond time frame. This time period is much less than the lifetimes of any transient species that could last long enough to yield a measurable vibrational spectrum. Such measurements go beyond simple detection and characterization of transient species. They yield details never before available of the time behavior of species in fast reactions, such as temporal and spatial redistribution of initially localized energy in excited molecules. Laser-induced fluorescence characterizes the molecular species that have formed, their internal energy distributions, and their lifetimes. 

Figure 5. Redistribution of higher molecular weight milk proteins on the SW 2000 column. Whole milk (100 yL) was fractionated under the conditions iruiicated in figure 4. A 100-yL aliquot of eluant collected from the shaded portion of the profile on the left was rechromatographed as shown on the right.

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