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Evaluation of Chemical Shifts

Unfortunately, not all the chemical shift values in the literature have been obtained in the same manner and conversions are required to make them compatible. [Pg.531]

We shall take the standard definition of chemical shift as 5=10. (v —v,)/v, (ppm), where s and r refer to the sample and reference resonances, respectively. However, many of the earlier measurements were in field sweep mode and calculated as 5 = — (or with the reverse sign). This does not give an identical result [Pg.533]

Even for frequency scale measurements, the v term requires due care e.g., the shift of [Co(NH3)6] CI3 is 8,173 when K3[Co(CN)g] is the reference but the shift of K3[Co(CN)6] is —8,107 ppm when [Co(NH3)e] CI3 is taken as the reference. Because of the magnitudes of shifts for these nuclei, the true reference frequency should be used as denominator, but this may not always have been done. Further, the automatic data reduction on some multinuclear FT spectrometers uses the observing frequency not the reference frequency. In such cases, the safe rule is to reset the frequency to the reference frequency before data reduction. [Pg.533]

The main problem arises with the conversion of results reported to a different reference. Here the equation 5 = 5(ref ) -I- 5 v,r/v, should be used [primes refer to the old scale with reference, ref, which has a shift of 5(ref) on the new scale]. The term Vr /Vr can, of course, be replaced by 1 -l-5(ref)x 10 .  [Pg.533]

Nowadays, the shift of a metal nucleus is almost never measured with the reference substance in the same tube but rather it is a separate sample measured separately. Either the field is assumed to be the same for both measurements or more commonly the H frequency of the solvent is used to ensure that they are (subject to differences in solvent which can be easily corrected for). In the latter case, the H frequency of TMS may be measured or calculated so that. sfmetal nucleus) can be determined. Because of the sensitivity of transition metal shifts to conditions, especially concentration and temperature, considerable variations are likely in frequencies measured for the metal references on different occasions. The H frequencies of solvents and (particularly) the H frequency of TMS are much less susceptible to variation and furthermore avoid diamagnetic corrections as they are internal. Thus rather than measure the reference substance on many occasions and in each laboratory, more consistent results will be obtained from a single well-determined value of E for the reference, from which may be calculated the reference frequency at a particular field (e.g., given H frequency and solvent). The consequence of this approach is that it is not necessary for the reference substance to be readily available or easily measured but merely for someone to have obtained a reliable value. [Pg.533]


The experiment is used for solving simple structural problems and for the evaluation of chemical shifts. This experiment is usually combined with the DEPT experiment (see 3.3.2.2) for additional information and for signal assignments. [Pg.55]

Srinivasan, P.R. and Lichter, R.L., Nitrogen-15 nuclear magnetic resonance spectroscopy evaluation of chemical shift references, J. Magn. Reson., 28, 227, 1977. [Pg.433]

Gdrardin, C., Henry, M., and Taulelle, F. (1993) Evaluation of Chemical Shifts in Solid-state NMR by Electronegativity Equalization Principle, in J.A. TosseU (ed.), in Nuclear Magnetic Shieldings and Molecular Structure, NATO-ASl Series C, 386, 566, Kluwer, Dordrecht. [Pg.330]

Fyfe CA, Feng Y, Grondey H. Evaluation of chemical shift-structure correlations from a combination of X-ray diffraction and 2D MAS NMR data for highly sihceous frameworks. Microporous Mater 1993 1 393-400. [Pg.184]

Numerous X-ray investigations have unravelled the solid state structure of contact and solvent-separated ion pairs. It was therefore considered to be of interest to evaluate also the potential of solid state NMR as a tool for the investigation of this structural problem. In addition to the study of chemical shifts discussed above (Section II.B), the quadrupole coupling constant of the nuclide Li, x( Li), was expected to be an ideal sensor for the bonding situation around the lithium cation because, due to its dependence on the electric field gradient, the quadrupolar interaction for this spin-3/2 nucleus is strongly influenced by local symmetry, as exemplified in Section II.C.3. This is also shown with some model calculations in Section ILF. [Pg.179]

The chemical shift <5 of a given nucleus in a molecule contains stereochemical information which, however, might often be uninterpretable because the corresponding chemical shift of another stereoisomer is not available for comparison. Thus, the evaluation of an isolated chemical shift is possible only if the contribution of the stereochemical influence can be identified unequivocally among other influences. This is, for example, the case when substituent effects on chemical shifts (SCS substituent chemical shift), instead of chemical shifts themselves, are determined, since some of these values are known to possess a well-defined stereochemical dependence. An SCS is defined as the chemical shift of a given nucleus in a substituted molecule (R—X) relative to the unsubstituted parent compound (R H) ... [Pg.296]

Another way of evaluating 13C chemical shifts in oximes, especially if only one diastereomer is available, is a comparison with the corresponding ketone. For example, a comparison of 2-butanone and the Z- and E-oximes (the <5 values in parentheses are differences relative to the ketone) shows that the s>n-oriented a-carbon is affected much more strongly than the other. [Pg.325]

The NMR observable most commonly exploited in studies of solid acidity is the chemical shift. While some NMR observables (e.g., dipolar couplings) lend themselves to a more or less direct quantitative evaluation, the chemical shift must be interpreted. Changes in the 13C or 15N isotropic shifts of adsorbates are observed upon complexation with Brpnsted sites, and the same is true of the H shift of the Brpnsted site, but one is hard pressed to interpret such changes quantitatively in terms of a detailed structure of the adsorption complex or even the extent of proton transfer. [Pg.120]

Similarly, Lippmaa and coworkers evaluated the relative acidities of linear and branched carboxylic acids from the variation with degree of protonation of the measured 13C NMR shifts.23 The method was then extended to secondary deuterium IEs, evaluated from the variation with degree of protonation of the measured 13C NMR shifts of a mixture of isotopologues.24 The data were fit by nonlinear least squares to Equation (17), where 6H and 6D are the observed chemical shifts of undeuterated and deuterated isotopologues, 6H and <5d are those chemical shifts in the deprotonated form, < >]) and are those chemical shifts in the protonated form, R = K /K, and n is the degree of protonation of the undeuterated material. This is the same equation as Equation (15), but adapted to deuteration, and again n is evaluated from chemical shifts as (hH — <5h)/(<5h - h)-... [Pg.129]

The presence of contact relaxation indicates that a given moiety is covalently bound to a paramagnetic metal ion and provides an estimate of the absolute value of A (Eqs. (3.26) and (3.27)). Sometimes the contact coupling constant can be evaluated by chemical shift measurements, and it is therefore possible to predict whether the contact relaxation contributions to R m, Rim or both, are negligible or sizable. [Pg.106]

Importantly, Lindquist et al.53 also document that ascidians can exhibit chemical differences between defensive secondary metabolites among adults and larvae. For example, larvae from colonies of Sigillina cf. signifera contained more tambjamine C, less tambjamine E, and no tambjamine F as compared to adults.65 Moreover, larvae of Trididemnum solidum contain only four of the six didemnins found in adults.53 This could be the result of different selective pressures during planktonic vs. benthic life history phases. In contrast, Lucas et al.44 found no differences in the saponin chemical defenses of the embryos, larvae, and adults of the sea star Acanthaster planci. Clearly, additional studies are needed to expand the evaluation of ontogenetic shifts in defensive chemistry in marine organisms. [Pg.201]

Figure 2 shows a comparison of nitrogen adsorption isotherms at 77 K for the organic-inorganic hybrid materials developed for adsorption of mercury ions. The shape of the nitrogen adsorption isotherms indicates that the mesostructure was preserved after chemical modification of the surface. Evaluation of the shift of the capillary condensation step and reduction of its height provide information about formation of the chemically bonded layer inside mesopores. [Pg.330]

Carbon-13-NMR is a routine means of characterizing organometallic compounds, as well as a mechanistic probe [52-56]. The sensitivity of C-NMR is reflected in the wide range of chemical shifts and scalar coupling constants, Jc-x that allows for evaluation of subtle changes in the organometallic species at the M—C bond. This sensitivity has allowed organometallic chemists to evaluate fluxional processes and electron delocalization in... [Pg.111]

Applications of the GIAO approach at the SCF level are now relatively routine, but correlated calculations are more difficult because the most convenient implementations of this approach require the analytic evaluation of the second derivatives. Therefore, correlated studies using GIAO basis functions are effectively limited to levels of theory for which analytic second-derivative methods are available. Although the number of calculations thus far carried out on chemical shifts is far too small to give us a clear picture of basis set and correlation effeas, the initial results of GIAO-MBPT(2) calculations suggest that correlation is indeed important for these phenomena. In Table 31 are results from a few representative calculations of chemical shifts. [Pg.158]

Solvent accessibility of native and mutant HiPIPs has been determined by multinuclear NMR methods, in particular, by use of the H/ exchange rates of backbone amide protons, evaluated by HSQC experiments, and from the isotopic perturbation of chemical shifts of labeled native and mutant HiPIPs (43, 149). [Pg.332]

Besides C, the spectra of [118, 280] and b [281] were found as useftil to study the changes in surface chemistry and heteroatom arrangements present on the carbon surface. Especially valuable for this purpose is phosphorus. Large chemical shift for P allows to easy distinguish between various phosphorous containing compounds present on the carbon surface. The study of chemical shift for B was foimd to be extremely useful to evaluate the incorporation of boron atoms to the carbon structure and boron atoms environment (boron to boron connected configurations or isolated boron atoms) [281]. [Pg.202]


See other pages where Evaluation of Chemical Shifts is mentioned: [Pg.531]    [Pg.531]    [Pg.329]    [Pg.225]    [Pg.226]    [Pg.467]    [Pg.162]    [Pg.129]    [Pg.549]    [Pg.217]    [Pg.223]    [Pg.122]    [Pg.212]    [Pg.226]    [Pg.239]    [Pg.305]    [Pg.68]    [Pg.34]    [Pg.63]    [Pg.458]    [Pg.45]    [Pg.225]    [Pg.904]    [Pg.223]    [Pg.261]    [Pg.131]    [Pg.1113]    [Pg.57]    [Pg.149]    [Pg.314]    [Pg.538]    [Pg.301]   


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