Chemical shift dependencies

NMR The H NMR signals for the hydroxyl protons of phenols are often broad and their chemical shift like their acidity lies between alcohols and carboxylic acids The range is 8 4-12 with the exact chemical shift depending on the concentration the solvent and the temperature The phenolic proton m the H NMR spectrum shown for p cresol for example appears at 8 5 1 (Figure 24 4)... 

One criterion of aromaticity is the ring current, which is indicated by a chemical shift difference between protons, in the plane of the conjugated system and those above or below the plane. The chemical shifts of two isomeric hydrocarbons are given below. In qualitative terms, which appears to be more aromatic (Because the chemical shift depends on the geometric relationship to the ring current, a quantitative calculation would be necessary to confirm the correctness of this qualitative impression.) Does Hiickel MO theory predict a difference in the aromaticity of these two compounds ... 

The difference in resonance NMR frequency of a chemically shielded nucleus measured relative to that of a suitable reference compound is termed chemical shift [164,165], and is a measure of the immediate electromagnetic environment of a nucleus. While the chemical shift depends on the Bo field, J does not. Chemical shifts, which cover a range of about 10 ppm for protons (i.e. 600 Hz in case of a 14.1 kG magnetic field) and 250 ppm for 13C, are the substance of NMR. 

The impact of combinations of functional groups on CF2 chemical shifts depends on how they are arranged. If they are consecutive, then the closest one largely determines the chemical shift (Scheme 4.40). 

In addition to the influence of neighbors on 29Si chemical shifts, the geometrical effects (such as Si-O-T angles) already described above are also evident of mixed frameworks with elements other than Si on tetrahedral positions. This is reflected by the broadness of the bars shown in Fig. 1. Multinuclear NMR investigations on a large set of sodalite structures with various framework compositions show that T-O-T bond angle (T = Si, Al, Ga) and dTT distance chemical shift dependences exist, and mutual correlations between chemical shift of these NMR nuclei can be observed [68],... 

In the case of ionic species, which may be represented by the ammonium ion, the nitrogen chemical shift depends upon the nature of the counter ion as well as the concentration. 

Experiments were performed at 5°C in order to arrest the cis-trans isomerization of the protonated Schiff base. Spectra with one equivalent of acid and different mixing times showed one NOE cross-peak between H15 of the retinal molecule and the proton on the counterion, as shown for a mixing time of 0.4 s in Figure 10. The strong chemical shift dependence of the H15 resonance on the concentration of the acid dictated the use of less than one equivalent of the protonating formic acid, and therefore an incomplete protonation (>80%) of the retinal, in order to avoid an overlap between the formate and the H15 peaks in the spectrum. This should not affect the observed result since an average chemical shift, between those of HI 5 of the retinal in its nonprotonated and protonated... 

A fair amount of H nuclear magnetic resonance (NMR) data for various 1,4-oxazines exist, but the observed chemical shifts depend heavily on the substitution pattern as well as the number of ring double bonds. Representative data for most of the known types of 1,4-oxazines and dihydro-l,4-oxazines are given in Table 4. 

Furthermore, in flexible linear peptides the chemical shifts are typical of random structures similar to nonfolded proteins. Deviation from these random shifts sometimes identifies specific conformational preferences. NH-proton chemical shifts depend strongly on external influences (solvent, temperature, concentration, specific sequence). Random coil shifts for these protons correlate less well than chemical shifts of the a-protons or a-carbonsJ19-261 Not only are the shift differences of different heterotopic protons similar, but also those of diastereotopic P-protons. A preferred side-chain conformation is normally only found when there is also a preferred backbone conformation. 

Diselenoether complexes. There is a ring contribution 59 to the 7TSe NMR chemical shifts dependent upon the chelate ring size. 

Chemical shifts depend on the strength of the applied magnetic field B0 according to eq. (1.8 c) when measured on the frequency scale. Coupling constants remain constant when the strength of the magnetic field BQ changes. 

Because of intramolecular mobility (rotations, inversions) and intermolecular interactions, chemicals shifts depend on temperature, solvent, and concentration. Coupling constants, however, for the most part do not depend on these conditions. 

The chemical shift dependence of the carbonyl resonances on ring size in cycloalkanones is particularly remarkable In the series of cycloalkanones, cyclopentanone is found to have the largest carbonyl shift (219.6 ppm). The CO signals of cyclobutanone and cyclohexanone are both observed at higher field (x 209 ppm). The carbonyl carbons of cy-clooctanone and cyclononanone are much more deshielded than those of cyclohexanone, cycloheptanone, cyclodecanone and cycloundecanone. The carbonyl resonances of the twelve to seventeen membered ring ketones occur at S values similar to those of acyclic ones [282, 288]. 

C chemical shifts depend, in part, on the amount of electron density around the 13C nucleus. Since benzene ring substituents perturb the electron density at selected carbons around the ring, one might expect these substituents to exert a noticeable effect on the chemical shifts of these nuclei. 

If the frequency of the radiation source is fixed, the precise field at which resonance occurs depends upon the chemical environment of the nucleus under scrutiny. Under low resolution - where the effects of spin-spin interaction (see below) are not observable - the spectrum should show one peak for each chemically-distinct nucleus of the type under scrutiny the intensity of a peak (measured by its area) is proportional to the number of equivalent nuclei encompassed by its envelope. We can thus count the number of nuclei of each chemically-distinct type in the sample. The chemical shift depends on the extent to which the nucleus is shielded from the applied field by electrons. Detailed, quantitative... 

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