Chemical exchange process

A variant of the H2/NH2 chemical exchange process uses alkyl amines in place of ammonia. Hydrogen exchange catalyzed by NH2 is generaHy faster using alkyl amines than ammonia, and a dual-temperature flow sheet for a H2/CH2NH2 process has been developed (69). 

Molecular Motions and Dynamic Structures. Molecular motions are of quite general occurrence in the solid state for molecules of high symmetry (22,23). If the motion does not introduce disorder into the crystal lattice (as, for example, the in-plane reorientation of benzene which occurs by 60° jumps between equivalent sites) it is not detected by diffraction measurements which will find a seemingly static lattice. Such molecular motions may be detected by wide-line proton NMR spectroscopy and quantified by relaxation-time measurements which yield activation barriers for the reorientation process. In addition, in some cases, the molecular reorientation may be coupled with a chemical exchange process as, for example, in the case of many fluxional organometallic molecules. ... 

In some cases, the NMR signals of a small-molecule analyte might be broad and poorly resolved because of a chemical exchange process, for example,... 

A chemical exchange process (18) such as spins move from site A to site B and conversely from site B to site A (of course, we are only concerned here about two nuclear spins). If we denote by x the residence time in each site (this implies identical concentrations for A and B, thus in accord, precisely with the situation considered for the two spins 7 and S), then a = —1/x = —k. Moreover the exchange rate k must be added to each specific rate 7 ( and 7 f. 

No such chemical exchange is observed for NP2—ImH or NP3—ImH, for which WEFT-NOESY spectra have cross peaks due only to through-space nuclear Overhauser effect interactions (91). Interestingly, these two proteins have only one proline in the A-B loop (Fig. 4). However, considerable additional NMR investigation needs to be carried out in order to determine whether the chemical exchange process shown in the NOESY spectra of NPl-ImH is indeed due to the dynamics of the A-Bloop. 

An even more useful property of supercritical fluids involves the near temperature-independence of the solvent viscosity and, consequently, of the line-widths of quadrupolar nuclei. In conventional solvents the line-widths of e. g. Co decrease with increasing temperature, due to the strong temperature-dependence of the viscosity of the liquid. These line-width variations often obscure chemical exchange processes. In supercritical fluids, chemical exchange processes are easily identified and measured [249]. As an example. Figure 1.45 shows Co line-widths of Co2(CO)g in SCCO2 for different temperatures. Above 160 °C, the line-broadening due to the dissociation of Co2(CO)g to Co(CO)4 can be easily discerned [249]. 

A further complication associated with N—H and H—C—N resonances is their variable chemical shift and line width in the presence of acidic substances because of a chemical exchange process of the type illustrated in Equation 23-l ... 

Calculation of DNMR spectra is based on the construction of operators of interactions of the system and solving their eigenvalue problem. The most complicated interactions are the perturbation effects of the chemical shielding and the scalar coupling on the resonance frequency. The descriptions for these interactions are the same in both dynamic and static (free from chemical exchange processes) systems. Let n be the number of nuclei in a spin system, fi and /( the indices of the nuclei in it, v/( the frequency (in Hz) corresponding to the chemical shift of fi nuclei and the coupling... 

Fractional distillation of NO is an effective method for concentrating N. Chemical exchange processes for nitrogen isotope separation include the following ... 

The utility and simplicity of a laboratory scale chemical exchange process with chemical reflux is illustrated by the Nitrox process for the production of (23). The exchange reaction is... 

The only chemical exchange processes which have been developed for large scale production are either parasitic processes or ones for which thermal reflux has been adapted to the chemical exchange process. Two... 

For quadrupolar nuclei with dominant quadrupolar relaxation, the relaxation rates for longitudinal and transverse magnetization, Riq and R2q> are identical in the motional narrowing limit. This situation is met in many cases for Li, while for Li the relation R2>R will generally be found. For both nuclei, however, chemical exchange processes are quite common and the strongest contribution to Ri will then originate from this source. 

Secondly, NMR averages over chemical exchange processes that are faster than the... 

Chemical exchange processes are characterized by large a, high recovery, and low energy requirement. 

Fig. 3 Chemical exchange processes (A) monothermal, (B) bithermal, and (C) bithermal process with stripping sections.

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