Chemical shifts, temperature variation

The rehability of these analytical methods may be questionable when chemical shift differences of derivatives are of the same magnitude as variations encountered from solvent, concentration, and temperature influences. Reported fluorine chemical shift ranges for tnfluoroacetylated alcohols (1 ppm), p-fluorobenzoylated sterols (1 ppm), and p-fluorobenzoylated ammo acids (0.5 ppm) are quite narrow, and correct interpretation of the fluonne NMR spectra of these denvatized mixmres requires strict adherence to standardized sampling procedure and NMR parameters. 

Lead-proton and lead-carbon coupling constant values have structural uses, as with tin. Lead chemical shifts are quite sensitive to temperature variations. 

As a result, the relative position of L1 or L2 in relation to the aromatic ring in the sp conformation can be established from the sign of the variations in the chemical shifts of substituents L1 and L2 with temperature (positive or negative A(5tit2) Assigning the configuration of the chiral center is then straightforward. 

The chemical shift variations, the linewidths and the longitudinal relaxation times are all in agreement in favor of two different adsorbed species, at high and low temperature, respectively. At high temperature the 1- and 2-butene molecules are more stabilized as 7r-complexes on the NaGeX zeolite surface, while the cyclic type adsorbed species is preferred at low temperature. 

The chemical structures of five dextrans were partially determined by methylation, and found to be branched molecules having the following types of substitution (a) 6-0 and 3,6-di-O, (b) 6-0, 3-0, and 3,6-di-O, (c) 6-0,3,6-di-O, and 2,3-di-O, (d) 6-0, 4-0, and 3,4-di-O, and (e) 6-0 and 2,3-di-O. At 27° and pH 7 (external, Me4Si standard), the 13C shifts ofO-substituted, non-anomeric carbon atoms were C-2 (76.5), C-3 (81.6), and C-4 (79.5). The C-l resonances were also recorded, and may be used for reference purposes. Some variation of chemical shifts, relative to each other, was observed with changing temperature. (The work serves to emphasize the importance of accurately measuring the temperature of the solution when determining chemical shifts.102)... 

Fig. 6. Temperature variations of observed 0 transverse relaxation rates (a), reduced transverse relaxation rates (b) and reduced chemical shifts (c) calculated from Swift and Connick equations for Bq = 14.1 T (largest effect), 4.7 T, and 1.4 T (smallest effect).

Variations in the absolute concentration of the carbocation solutions and temperature had minor effects on chemical shifts. The counter ion effect and the equilibrium could be minimized by going to higher superacidity systems with lower nucleophilicity counter ions. Resonances due to the PAH itself were considerably shielded. Solvation by FSO3H and the formation of ion pair-molecule clusters were suggested as possible reasons. 

Static forms of cations such as 7 and 8 have been observed by NMR in solid SbFs matrices, with chemical shifts for the static ions matching computed values. The signals for the static ions could be observed even at temperatures above the solution-phase limits, which suggests there is a variation in the lattice sites in the solid, and the cations find themselves in different environments. A broad distribution of rearrangement barriers results. 

Low temperature XH NMR of [Nb2(OMe)10] in a variety of solvents has revealed significant variations of the chemical shifts, different for the various non-equivalent methoxo sites, particularly where bulky polar solvents are concerned these were interpreted in terms of preferential solvation.17 ... 

We have prepared a number of acylium ions on metal halide powders and measured the principal components of their chemical shift tensors (43-45). Most of these cations have isotropic l3C shifts of 154 1 ppm. Often insensitivity to substituents results from opposite and offsetting variations in the principal components. The acetylium ion has an axially symmetric chemical shift tensor because of its C3 rotation axis. When the symmetry is reduced from C3v to C2v or lower, a nonzero 27 value may be observed. The sensitivity of chemical shift tensors to symmetry is a powerful means of probing molecular structure and temperature-dependent molecular dynamics. Multiple orders of spinning sidebands may offend those who seek solution-like NMR spectra of solids, but discarding most of the information inherent in the chemical shift is a considerable concession to aesthetics. 

The solid-state hexamers (2)—(4) at first appeared to dissolve intact in benzene (94). Cryoscopic rmm measurements over a range of concentrations (0.03-0.09 M, molarity expressed relative to the empirical formula mass) implied n values of 5.9-6.1. Furthermore, their room-temperature 7Li NMR spectra in c/8-toluene each consisted of broad singlets within the narrow chemical shift (6) range of + 0.6 to -0.2 ppm (relative to external phenyllithium in the same solvent). However, variations in temperature and concentration affected the 7Li NMR spectra of (2) and, in particular, of (4) (95). Figure 18a shows these spectra for three d8-toluene solutions of (4) at -100°C. The most concentrated solution has a dominant signal at 8 -+0.7, though five or six other signals (indicated by asterisks) are apparent. On dilution,... 

It is the purpose of the present paper to study the effect of pore size on the adsorption of xenon on mesoporous MCM-41 molecular sieves. In particular, much attention will be focused on the temperature variation of l2,Xe NMR chemical shifts at low Xe loading to realize the characteristics of the Xe-wall interactions. 

Figure 2. Variations of l29Xe NMR chemical shift with apparent Xe loading for an MCM-41 sample (pore diameter 2.54 nm) at various temperatures.

Typical proton chemical shifts relative to TMS are given in Table 9-4.13 The values quoted for each type of proton may, in practice, show variations of 0.1-0.3 ppm. This is not unreasonable, because the chemical shift of a given proton is expected to depend somewhat on the nature of the particular molecule involved, and also on the solvent, temperature, and concentration. 

When a proton is directly bonded to a strongly electronegative atom such as oxygen or nitrogen its chemical shift is critically dependent on the nature of the solvent, temperature, concentration, and whether acidic or basic impurities are present. The usual variations in chemical shift for such protons are so large (up to 5 ppm for alcohols) that no very useful correlations exist. 

The proton and carbon chemical shifts of twenty-one and twenty different anthocyanins are presented in Table FI. 4.4 and Table FI. 4.5, respectively. These anthocyanins are chosen to illustrate the chemical shifts of the majority of anthocyanin building blocks reported. The linkage positions of the various anthocyanin building blocks may be conspicuous through shift comparison. However, be aware of shift effects caused by variation in solvent, pigment concentration and temperature. Table FI.4.6 contains typical H- H coupling constants of the most common anthocyanidins. 

The acoustics of a gas in which occur reversible chemical reactions whose equilibrium shifts with pressure and temperature variations in the acoustic wave was studied by Albert Einstein [1]. His results were later applied to the study of the very fast processes encountered at the boundary between physics and chemistry—the transition of molecules from one vibrational state to another [2]. 

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