Proton concentration versus temperature

The second in situ technique is NMR. An autoclave fitting with the NMR cavity was designed by Gerardin et al. [59] and allows to follow the evolution of many parameters of the synthesis via the NMR characteristics of the different nuclei versus temperature and reaction time. The first measurement that can be reached now is the absolute value of the pH in hydrothermal conditions and the quantitative evolution of the concentration of protons in the bomb with the parameters of the synthesis [60], They proved that 14N NMR chemical shifts of well chosen amine compounds (imidazole and DABCO which possess complementary pKas) are precise pH indicators in aqueous solutions from room temperature to 475 K. Use of both amines permit to cover a wide range of about 9 pH units, with a precision of 0.1 pH unit. 

The diffusive contributions AHq and AHo are frequency-dependent and are proportional to l/ /wD. From the data o AH versus temperature, the temperature dependence of the localized site concentration cl(T) can be extracted. This requires that D(T) be known. In the work of Ref. 56, D T) was obtained from data of the proton relaxation rate as a function of temperature and frequency. There are also two contributions to the nuclear relaxation from diffusive spins and from localized spins. However, due to the local hyperfme field, the protons located around the localized electronic spins are not seen by NMR. Consequently, only the diffusive contribution [ (1 - c l)] is effective for T]. ... 

An example of a potentiometric measurement is one in which the pH (or number of protons bound versus those bound at some reference condition) is measured as a function of the denaturing condition. Such an approach would require a difference in the pKa of one or more amino acid side chains in the native and unfolded state. Usually, several such amino acid side chains are in a protein. However, the potentiometric approach requires technical skill, and it is difficult to use in combination with high concentrations of chemical denaturants or temperatures far from ambient. 

A connnon approach has been to measure the equilibrium constant, K, for these reactions as a fiinction of temperature with the use of a variable temperature high pressure ion source (see section (Bl.7.2)1. The ion concentrations are approximated by their abundance in the mass spectrum, while the neutral concentrations are known from the sample mlet pressure. A van t Hoff plot of In K versus /T should yield a straight Ime with slope equal to the reaction enthalpy (figure B1.7.11). Combining the PA with a value for basicityG at one temperature yields a value for A.S for the half-reaction involving addition of a proton to a species. While quadnipoles have been tire instruments of choice for many of these studies, other mass spectrometers can act as suitable detectors [19, 20]. 

HOptner et al.118 have carried out 1FI and 19F SLR time as a function of temperature. Fluorines are known to be relaxed mainly by the reorientational motion of the anions and by the interaction with fixed paramagnetic impurities, the protons are relaxed additionally above 150 K predominantly by highly mobile paramagnetic species, whose concentration could be determined directly via the NMR signal amplitude. Korringa relation observed for proton relaxation shows that it is metallic above 183 K. Further, 1/Ti versus (vL) 1/2 dependence of the proton relaxation supports the ID spin transport and also confirms that only protons of the cation stacks are relaxed by the highly mobile paramagnetic species. 

Figure 9 shows the temperature (T) dependence of the susceptibility in the case of the solution with 10 wt% polyanilme in sulf uric acid. The temperature dependence of die peak-to-peak linewidth (AHpp) of the ESR absorption line is presented in Figure 10. All the solutions which were studied (with different polymer concentrations) exhibited the same dependences of X and AHpp versus T as those shown in Figure 9 and Figure 10. The magnetic susceptibility of the [B-NH-B-NH-]+ n(X )n salt obtained by precipitation from sulfuric acid (and subsequently treated with HCl to ensure complete protonation) is also shown for comparison in Figure 9.

R= Me, Et, Pr , or Pr ) mixtures, and the stability constants (Xml) decreased with increasing alcohol concentration. Xml values were found to correlate with the protonation constants of the various anilines. The formation rate constants decrease with increasing concentrations of MeOH or EtOH, and the lowest rates were observed in propan-2-ol. In aqueous solution (7=0.25 mol dm [NaC104]), a plot of ATT versus SS is linear with an isokinetic temperature of 437 K. ... 

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