One-dimensional exchange

The one-dimensional exchange experiment ODESSA (see Section 2.4) was used in a study of the effects of hydration in the protein barstar and the polypeptide polyglycine.116 For the experiments on barstar, a uniformly labelled 15N sample was used, and natural-abundance 13C was used for polyglycine. Only the wet barstar sample showed any signs of molecular reorientation in this case, with correlation times between 50 and 100 ms. 

CRAMPS NMR [138]. The differences in the intermolecular distances of the carboxylic acid groups involved in different types of hydrogen bonding have been visualised using ODESSA (one-dimensional exchange spectroscopy by sideband alteration) and 2D EXSY (exchange spectroscopy). The ODESSA technique [139] can measure internuclear distances (up to 9 A) between chemically equivalent nuclei with the same isotropic chemical shift. Potential applications of this approach are widespread. 

The pulse sequence used by these authors is ODESSA (one-dimensional exchange spectroscopy by sideband alternation) [48], The experiment is based on the fact that intermolecular distances between chemically equivalent nuclei differ significantly in racemates and enantiomers according to slight but substantial... 

For an excellent review of all the one-dimensional exchange methods, their relative efficacy and the analysis of their results, the reader is referred to ref. 56. An experimental comparison of CODEX and trODESSA is undertaken in ref. 60. 

For low-frequency motions (10 -10 s), the 2D exchange experiments are useful techniques (19). Other exchange experiments are also informative, e.g., the one-dimensional exchange spectroscopy by sideband alternation (ODESSA) (151), time-reversed ODESSA (152), and centerband-only detection of exchange (CODEX) (153). Many advanced techniques are given in a recent, excellent review by Brown and Spiess (14). 

The ultraslow motions can be observed by exchange NMR experiments. There are a number of techniques, performed in ID and 2D modes which are based on the manipulation of spinning sidebands under MAS. The first of them, applied to study of solid peptides was ODESSA (one-dimensional exchange spectroscopy by sideband-alternation) [76] and next the time-reversed ODESSA [77] (tr-ODESSA). The interesting modification, which represents this group of sequences is CODEX (centreband-only detection of exchange) introduced by Schmidt-Rohr and coworkers [78]. CODEX can be executed at any MAS speed, while ODESSA and tr-ODESSA... 

Figure 5.60. Representation of one-dimensional exchange interactions of Cu (S=l/2) spins (circles with arrows) with neighbors (7a d) and with carriers (7 d) in Cu. ...

Entrance andExit SpanXireas. The thermal design methods presented assume that the temperature of the sheUside fluid at the entrance end of aU tubes is uniform and the same as the inlet temperature, except for cross-flow heat exchangers. This phenomenon results from the one-dimensional analysis method used in the development of the design equations. In reaUty, the temperature of the sheUside fluid away from the bundle entrance is different from the inlet temperature because heat transfer takes place between the sheUside and tubeside fluids, as the sheUside fluid flows over the tubes to reach the region away from the bundle entrance in the entrance span of the tube bundle. A similar effect takes place in the exit span of the tube bundle (12). 

Aside from merely calculational difficulties, the existence of a low-temperature rate-constant limit poses a conceptual problem. In fact, one may question the actual meaning of the rate constant at r = 0, when the TST conditions listed above are not fulfilled. If the potential has a double-well shape, then quantum mechanics predicts coherent oscillations of probability between the wells, rather than the exponential decay towards equilibrium. These oscillations are associated with tunneling splitting measured spectroscopically, not with a chemical conversion. Therefore, a simple one-dimensional system has no rate constant at T = 0, unless it is a metastable potential without a bound final state. In practice, however, there are exchange chemical reactions, characterized by symmetric, or nearly symmetric double-well potentials, in which the rate constant is measured. To account for this, one has to admit the existence of some external mechanism whose role is to destroy the phase coherence. It is here that the need to introduce a heat bath arises. 

Several simplifying assumptions are adopted (1) the flow in the tube is considered to be one dimensional without turbulence or mixing (2) the processes in the tube are adiabatic. There is no heat conduction in the tube and no heat exchange between the gas and the tube walls (3) each heat exchanger is isothermal. 

An NMR investigation of water exchange at [Pt(H20)2(oxalate)2] is relevant to the mechanism of formation of one-dimensional mixed valence oxalatoplatinum polymers. In fact the rate constant for this presumably dissociative (AS = + 42 JK mol-1) reaction is considerably too low for water loss to be, as recently proposed, the first step in formation of these polymers. The mechanism of trans to cis isomerization for this oxalate complex, and for its (2 -methyl)malonate analogues, is intramolecular (Bailar or Ray-Dutt twist), since there is no concurrent incorporation of labeled solvent (177). 

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