Chemical shift vs. concentration

These theoretical treatments show, as Lussan points out, that the shape of the curves of chemical shift vs. concentration will not be affected by the model chosen for the dimer. 

Space models of these alcohols shows immediately how important must be the steric influences on the association processes. One can see how difficult it is for molecules such as pentamethylethanol to form linear hydro-gen-bonded chains of the Zachariasen type. The alkyl groups of alternate molecules can be seen to be close together if no bending occurs in the hydrogen bond chain, and when alkyl substitution occurs in the a position, there is certain to be some hindrance to free rotation around the —C—O— bond. Accordingly, from the lack of a multimer band in the infrared spectrum and the linearity of the curve of NMR chemical shift vs. concentration out to 0.35M we can presume that only dimers are formed initially in pentamethylethanol, with possibly cyclic multimers at higher concentrations. 

Because of this behavior we have been able to obtain values for the enthalpy of dimerization from infrared data by the method of Liddel and Becker (18) and from NMR data by the method of Davis, Pitzer, and Rao ( ) without having to resort to such low concentrations as usual. Extrapolated values of the infrared molar extinction coefficients, em, of the monomer OH band at zero concentration were obtained from curves of em vs. concentration at 25° and 50°C., and from these a value of 5.4 kcal. per mole was obtained for the enthalpy of dimerization. The limiting slopes of the curves of NMR chemical shift vs. concentration at 0° and 35°C. gave a value of 6.1 kcal. per mole. These values are in reasonable agreement with each other and significantly lower than the values of 8 kcal. for 1-heptanol and 7.9 kcal. for 2,4-dimethyl-3-pentanol, obtained from NMR data only. 

Figure 6, Plots of chemical shifts vs. concentration for ring methyl and NH protons from 60 MHz proton magnetic resonance spectra of 2,4-dipropionyldeuteroporphyrin IX dimethyl ester in CDCI3 at 35° C...

Solutions in acetone-dg of known concentration close to 3 x 10 mol L in choline or acetylcholine perchlorates were prepared and used as solvents for calixarene salts so as to obtain series of solutions in which the host guest (calixarene quaternary ammonium group) ratio varied between 0 and 2.00. Proton chemical shifts were measured for the trimethylammonium resonances of all solutions using a Bmker WP80 instmment. Inclusion equilibrium constants were determined from nonlinear least-squares fitting of the chemical shift vs. host guest ratio curves. 

Table 1 Concentration of the sulfanes H2Sn and of Sg in two crude sulfane oils A and B (S chemical shift in ppm vs TMS after [19])...

FIGURE 6. 29Si chemical shifts in 2,3,4-tri(0-trimethylsilyl)-l,6-anhydro-/i-D-glucopyranose (2) vs. molar concentration of chloroform in a ternary mixture with ()< , )< ,. (P is the molar ratio of chloroform to the (7,1)6). Reproduced with permission of Collection of Czechoslovak Chemical Communications from Reference 47... 

The observation that the chemical shift of added methyl chloride in Al2Me6-cyclopentane solutions is very close to the chemical shift of the same concentration of methyl chloride in cyclopentane solutions (Figure 3, Tables I and II) indicates that there is little if any Al2Me6 MeCl complex formed in solutions in cyclopentane. Therefore, the chemical shift of MeCl in Al2Me6-cyclopentane solutions is a convenient measure of MeCl concentration, and the plot of the W /2 of AlMe3 vs. the chemical shift of MeCl in cyclopentane solutions should be linear. This is observed and is illustrated in Figure 4. 

Figure 5 shows the chemical shift of the methylene protons of Al-i-Bu3 as a function of the ratios of concentrations of [Al-i-Bu3]/[MeCl] in cyclopentane solution at ambient temperature. The sharp break in the plot at 1.7 [Al-i-Bu3]/[MeCl] suggests complex formation between these compounds. Similarly, the plot of the corrected chemical shift of methyl chloride—i.e., the chemical shift of MeCl in Al-i-Bu3-cyclopen-tane solutions minus the chemical shift of MeCl in the same concentration in cyclopentane solutions vs. [Al-i-Bu3]/[MeCl] (Figure 6), indicate complex formation. Obtaining more chemical shift measurements on... 

Fig. 7—Plot of the Chemical Shifts of the Proton Resonances of D-Arabinose Diethyl Dithioacetal (197) vs. the Relative Concentration of PiCl3 6D20 added. (Values for the unperturbed chemical-shifts were estimated by extrapolation back to zero concentration of Pr3+.)...

Figure 8. Room temperature chemical shift and line width changes vs. water concentration. The reference for the chemical shifts measured at 60 MHz is a mixture of 95% D,Oand5% H,0.

The C isotropic chemical shifts of adsorbates in zeolites are typically within 3 ppm of their corresponding values in solution. Small downfield shifts attributable to hydrogen bonding are often seen, mirroring the familiar solvent effects ob.served with changes in concentration or solvent. For example. Fig. 7 shows that gas-phase methanol has a chemical shift of 48.0 ppm vs 53.1 ppm in zeolite HZSM-5 at the same temperature 31. Thus, in many cases, one can use C shifts from solution data to help assign MAS spectra of products formed during in situ experiments. Because of the small shifts mentioned above, however, it is... 

Tin oxide-based materials are potent oxidation and isomerization catalysts. Their bulk and surface properties, as well as their presumed mechanism in oxidation catalysis, have been reviewed (53j. Considerable uncertainty remains concerning the phase compositions, solid-solution range, and the redox behavior (Sn / Sn" vs. Sb WSb ) of these materials. Structural investigations have so far concentrated on the use of " Sn and Sb Mossbauer spectroscopy. Surprisingly, no " Sn solid-state NMR studies have appeared to date on this system, although it was recently demonstrated that isotropic " Sn chemical shifts and chemical shift anisotropies give characteristic fingerprints of the various tin coordination environments in Sn(IV) oxide compounds [54]. In situ C NMR has been used to study the double bond shift of 1-butene to t /.s-2-butene, and the subsequent cis-trans isomerization over tin antimony oxide catalysts [55 j. 

Figure 16. Change in the chemical shifts for the 13C nmr signals of sodium octanoate vs the reciprocal octanoate concentration. (Taken from Drakenberg and Lindman, 1973.)...

In all cases the addition of the chymotrypsin reversible inhibitor, indole, caused an upfield shift of the native line, i.e. in the direction of the denatured enzyme towards the bulk solution environment. Since we found that all of these spectra met fast chemical exchange conditions an indole titration was possible, as shown in Figure 2, where we have shown the upfield shift VS total indole concentration and the theoretical hyperbolic fit to these data for the o-CF labeled enzyme at pH 4.2. The dissociation constants for the three derivatives were 5.2, 3.2, and 3.7 mM for the o-CF f and p-CF labeled chymotrypsins, respective-... 

Fig. 11.4 (a) UV-vis transmission spectra of 10-cycle ZlF-8 film grown on glass substrate after exposure to propane of various concentrations from 0 to 1(X) % and (b) corresponding interference peak (originally at 612 nm) shift vs. propane concentration. The propane concentration is expressed as a percentage of the total gas flow where nitrogen is used as diluents (Reprinted with permission from Lu and Hupp 2010, Copyright 2010 American Chemical Society)... 

Fig. 1 line width at halfsignal height (A) and chemical shift (B) vs. number (n) of formamide (FA) or dimethylformamide (DMFA) molecules per lecithin molecule. Soybean lecithin concentration in n-decane was 70 mg/ml. Temperature 22 °C... 

Combining Equations [3] and [4], a linear plot of Avi/2 vs the experimentally measurable value vlT is observed. The Avi/2 values, unlike As chemical shifts, are generally strongly dependent on the type of solvent employed, partly surely owing to ion pairing in fact, the line widths for (AsEt4) Br are reported to be 168, 455, 670 and 700 Hz in water, acetonitrile, dimethylformamide and dimethyl sulfoxide, respectively. Finally, the concentration of the sample influences the arsenic line width, whereas the counterion seems not to play a significant role in temperature variation. 

Figure 18. Chemical shift of l Xe adsorbed in polyphenylene oxides at room temperature vs. Xe concentration in the polymer. The lines are labeled as in Figure 11...

One should note overall, that while some of these suggested mechanisms may in the future prove to have a role in the control of smooth muscle contraction, in chemically skinned preparations maximum force development follows activation by the MLCK active subunit in extremely low Ca " ion concentrations. The conclusion can hardly be avoided that phosphorylation alone is sufficient for activation, and if another mechanism is involved, it is not necessary for the initial genesis of force. If such mechanisms are operative, then they might be expected to run in parallel or consequent to myosin phosphorylation. A possible example of this category of effect is that a GTP-dependent process (G-protein) shifts the force vs. Ca ion concentration relationship to lower Ca ion concentrations. This kind of mechanism calls attention to the divergence of signals along the intracellular control pathways. 

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