Chemical shifts and relative intensities

A Comparison of the Chemical Shifts and Relative Intensities in the Silicon NMR Spectra of ZK-4, Na-X and Gallium Sodalite... 

Fig. 18. The plots of chemical shifts and relative intensities for two signals of PDPhSM against temperature in a wide range of temperatures from 2TC to 226 C. (O low frequency peak, Q high frequency peak, intensity of low frequency peak and intensity of high frequency peak)...

Figure 9. Representation of the chemical shifts and relative intensities in the NMR spectra of selected basic binary arachno borane fragments. For numbering and structures see Figure 12. B3H and B9Hi are fluxional in solution and therefore have simpler spectra than implied by their static molecular synMnetries.

The NMR results for three sodium faujasites and a gallium sodalite are shown in Figs. 1 and 2. The chemical shifts and relative peak intensities are given in Tables III and IV. The relative peak intensities are normalized to E I(Si-nT) = 1.0. 

Molecular structures may be deduced from the chemical shifts and integrated intensities. These shifts are relatively constant, except when they are influenced by hydrogen bonding, and may, to some extent, be predicted in terms of short- and long-range shielding by other parts of the molecule. 

Chemical shifts and relative area intensities of the methine carbon... 

Each type of Si( Al) n=0,1,2,3,4) species gives a characteristic Si MAS NMR signal in a well-defined range of chemical shifts (Fig. 8) [18,20,53]. The composition of the zeolite framework influences the relative intensities of the Si MAS NMR signals of the above-mentioned Si(nAl) species. The relative signal intensities are directly related to the relative concentrations of the various Si(nAl) units present in the zeolite structure. Consequently, from a careful analysis of the chemical shifts and line intensities, the specific types and relative population of the distinct Si(nAl) units present in a zeolite can be determined. The relative intensity of the lower frequency peaks increase with an increase of the zeolite Si/Al ratio (Fig. 9). Hence, the Si/Al ratio of the lattice can be calculated from the spectral intensities. 

Little difference was noted when peak heights were used. The error in the T data is less than + 10%. Nuclear Overhauser enhancement factors (q) were obtained by measuring the integrated intensity of peaks in a difference spectrum from one with enhancement minus one with no enhancement and dividing that value by the integral from the one with no enhancement i.e. n ( nOe no nOe / (I nOe" Accuracy should be 10% or better. Linewidtns were measured at half heights, and chemical shifts are relative to TMS. 

Exercise 23-8 The 19F spectrum of 4,4-difluoroazacyclohexane in acetone solution at 25° is a sharp, narrowly spaced 1 4 6 4 1 quintet at —60° it is a broad quartet with a chemical-shift difference of 960 Hz and J of 235 Hz, and at—90° it is a pair of overlapping quartets with chemical-shift differences and relative intensities of 1050 Hz (75%) and 700 Hz (25%), both with J of 235 Hz. Account for these changes in the 19F spectra with temperature. Review Section 9-10C.3... 

Based on chemical shifts and peak multiplicities, the on-flow HPLC-NMR characterisation of the majority of the components in the mixture of 27 tripeptides was achieved and demonstrated that this approach is likely to be an effective method for compound mixtures. The elution positions of all of the alanyl-containing peptides were determined, with the exception of A-M-M-NH2, which may have co-eluted with another peptide or may have been synthesised in a much smaller quantity. The only other tripeptides for which assignments have not been obtained are the MY2-NH2 isomers and two of the three M2Y-NH2 isomers. These eluted towards the end of the gradient run and are not as well resolved under these HPLC conditions. Additionally, with changes in the relative chemical shifts of the solvent signals, the intensities of the non-TV-terminal a-CH protons and the methionyl [3-methylene signals from these peptides may have been reduced by the effects of the solvent suppression irradiation of the water and acetonitrile resonances, respectively. With further optimisation of the elution conditions, it is possible that all 27 analytes could have been resolved and characterised. 

As the ratio of A 8v (the difference in chemical shift between two coupled nuclei) to J decreases, the relative intensities of the lines in a multiplet deviate further from first-order (e.g., Pascal triangle) ratios. Inner lines (those facing the coupled multiplet) increase in intensity, while outer lines lose intensity. This slanting of the multiplets is one type of second-order effect. At very small values of A Sv/J, not only may extra lines appear in the multiplets but also apparent line positions and spacings may not equate with true chemical shifts and coupling constants (e.g., deceptive simplicity and virtual coupling). 

The centers of these two quartets—designated (ab)+ and (ab) —are separated by V2 Jax + Jbx I-The X portion of the spectrum consists of three pairs of lines symmetrically placed around vx. The two strongest lines are separated by Jax + Jbx l which is just twice the separation of the centers of the (ab)+ and (ab) quartets. The spacings and relative intensities of the other lines depend on the chemical shifts and coupling constants, as described in Appendix B. 

In this experiment proton NMR spectroscopy is nsed in evalnating the equilibrium composition of various keto-enol mixtures. Chemical shifts and spin-spin splitting patterns are employed to assign the spectral features to specific protons, and the integrated intensities are used to yield a quantitative measure of the relative amounts of the keto and enol forms. Solvent effects on the chemical shifts and on the equilibrium constant are investigated for one or more j8-diketones and j8-ketoesters. 

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