Chemical exchange temperature dependence

Figure B2.4.3. Proton NMR spectrum of the aldehyde proton in N-labelled fonnainide. This proton has couplings of 1.76 Hz and 13.55 Hz to the two amino protons, and a couplmg of 15.0 Hz to the nucleus. The outer lines in die spectrum remain sharp, since they represent the sum of the couplings, which is unaffected by the exchange. The iimer lines of the multiplet broaden and coalesce, as in figure B2.4.1. The other peaks in the 303 K spectrum are due to the NH2 protons, whose chemical shifts are even more temperature dependent than that of the aldehyde proton.

To avoid the high cost of chemical reflux the dual temperature H2S/H20 exchange was independently suggested by Geib (1946) and Spevack (1957) (GS). The method exploits the fact that the equilibrium constant for isotope exchange is temperature dependent. The scheme is illustrated in Fig. 8.13. To carry out the exchange... 

The chemical shift dispersion (Table 1) and the temperature dependence of the resonance hne shape provides a qualitative measure of whether the structure is well ordered [2]. However, NMR spectroscopy also provides information relevant to the problem of protein folding in the study of the molten globule states. NMR spectroscopic investigations of molten globules may be more demanding than those of ordered proteins due to spectral overlap arising from poor shift dispersion and to short relaxation times that are due to conformational exchange at intermediate rates on the NMR time scale. 

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]. 

The dependence of chemical exchange rate constant k on temperature T can be expressed by the Arrhenius equation ... 

Most chemical processing plants include pure or multicomponent streams which change phase or which have strongly temperature-dependent heat capacities. Under these conditions the minimum approach temperature in a network can occur anywhere inside an exchanger. Therefore integral or differential equations are generally required to locate A Tm. 

---