Chemical isomeric shift

Chemical Shifts and Isomeric Shift Differences of (All-f) and (13-Z) Astaxanthin... 

The isomerization shifts conform to those that are described in literature (Englert and Vecci 1980, Englert 1995). All chemical shifts and isomeric shift differences of the olehnic region are listed in Table 4.1. 

JJor chemists interested in modem theories of chemical bonding, the most useful data obtainable by the Mossbauer technique are the magnitude and sign of the electric quadrupole field gradient tensor and the magnitude of the shift, 8, (which we prefer to call the chemical isomeric. Cl, shift), of the center of the Mossbauer spectrum relative to some standard absorber. Although a considerable amount of chemical and structural information is potentially available from quadrupole data on iron compounds, relatively little use has been made of such data in the literature, and we will not discuss this parameter here. We will instead restrict ourselves to two main points review of the explanations put forth to explain Cl shift data in iron compounds, and a survey of some of the correlations and generalizations which have been found. 

To use Equation 2 to determine s electron density diflFerences, it must be "calibrated —i.e., source-absorber or absorber-absorber combinations must be found for which the 5 electron density diflFerence is known. The most common method for calibrating the isomeric shift formula is to measure isomeric shifts for absorbers with diflFerent numbers of outer shell 5 electrons—e.g., by using compounds with the absorbing atoms in different valence states. The accuracy of this method depends on how much is known about the chemical bonds in suitably chosen absorber compounds, in particular about their ionicity and their hybridization. t/ (0) 2 can be obtained for an outer 5 electron from the Fermi-Segre formula or preferably from Hartree-Fock calculations. 

Calibration of the Isomeric Shift Scale with Isomeric and NMR Chemical Shift Data. The alkali iodide isomeric shifts (5) relative to ZnTe are displayed in Table I and in Figure 3. 

Ill the presence of hydrogenation catalysts, unsaturated compounds undergo not only hydrogenation but also isomerization—shift of double bonds, or stereochemical transformations—which also affects physical and chemical properties.)... 

In order to determine the quantum yields of isomerization via the Tj state, the quantitative analysis, by HPLC, of time-dependent changes in the compositions of the starting and the product isomers becomes necessary. When a new isomer is found, its structure needs to be determined by 1H and 13C NMR spectroscopy using changes in the 1H and 13C chemical shifts in reference to those of the all-trans isomer ( isomerization shifts ) and the 1H-1H NOE correlations. 

Table 2. H chemical shifts (ppm) for the M-trans isomer and isomerization shifts for cis isomers in CDCI3... 

Thus, the H chemical shifts and the isomerization shifts are very useful to identify the all-trans or a cis configuration, but measurements of nuclear Over-hauser effects (NOE) are necessary to determine the configurations precisely. 

The complex [Fe(phen)2(CN)2] shows a smaller chemical isomer shift (0-18 mm s at 300 K) than does [Fe(phen)3](C104)2 (0-31 nun s" ) because of the greater 7i-bonding capabilities of the cyanide. The spectrum of so-called /ra 5 -[Fe(phen)2(CN)2] shows Mossbauer parameters identical to those of the cw-complex and from the arguments in Section 7.4 it can be concluded that the difference is not one of m-/ra j-isomerism [47]. 

The terms chemical shift, chemical isomer shift, isomeric shift, and center shift are also occasionally used. 

If the y -radiation emitted by the radiation source strikes an absorber (the substance to be investigated), the atoms of which are present in the same chemical compound and in the same crystal lattice as are the atoms of the source, resonance occurs when source and absorber have the relative velocity zero. This is no longer the case when the nuclei in the absorber belong to atoms which exist in a state of chemical combination different from that of the atoms of the source. In order to fulfil the condition for resonance it is then necessary, as a rule, to move the absorber relative to the radiation source with a certain velocity and thus to change the energy of the quanta received in such a way as to correspond to the excitation energy of the nuclei in the absorber substance. This velocity relative to the zero velocity of an arbittary radiation source is called the line shift 6. It is made up of the isomeric shift and the temperature shift. 

The basic skeleton of isoprenoids may be modified by the introduction of a wide variety of chemical groups, by isomerization, shift of double bonds, methyl groups, etc. Hence a bewildering number of chemical structures arises. In addition compounds derived from other biogenic pathways may contain isoprene residues. For instance the K vitamins (D 8.1), ubiquinones (D 8.3), chlorophylls (D 10.1), plastoquinones, and tocopherylquinones (D 22.4) have isoprenoid side chains with up to ten isoprene units. Polyketides (D 3.3), alkaloids (D 8.4.2), and coumarins (D 22.2.2) may be substituted by dimethylallyl groups. The terpene residues are attached to nucleophilic sites, such as active methylene groups and phenolic oxygen atoms. 

Isomeric shift phys chem Shift in the Mossbauer resonance caused by the effect of the valence of the atom on the interaction of the electron density at the nucieus with the nuclear charge. Also known as chemical shift. [r-sajmer-ik shift) isomerism chem The phenomenon whereby certain chemicai compounds have structures that are different although the compounds possess the same elemental composition. I T sam-3,riz-am I... 

NMR Chemical shift differences m their H NMR spectra aid the structure deter mmation of esters Consider the two isomeric esters ethyl acetate and methyl propanoate As Figure 20 9 shows the number of signals and their multiplicities are the same for both esters Both have a methyl singlet and a triplet-quartet pattern for their ethyl group... 

The conformational characteristics of PVF are the subject of several studies (53,65). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. C-nmr chemical shifts are in agreement with these predictions (66). The stiffness parameter (5) has been calculated (67) using the relationship between chain stiffness and cross-sectional area (68). In comparison to polyethylene, PVF has greater chain stiffness which decreases melting entropy, ie, (AS ) = 8.58 J/(molK) [2.05 cal/(molK)] versus... 

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

The same arguments can be applied to other energetically facile interconversions of two potential reactants. For example, many organic molecules undergo rapid proton shifts (tautomerism), and the chemical reactivity of the two isomers may be quite different It is not valid, however, to deduce the ratio of two tautomers on the basis of subsequent reactions that have activation energies greater than that of the tautomerism. Just as in the case of conformational isomerism, the ratio of products formed in subsequent reactions will not be controlled by the position of the facile equilibrium. 

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 ... 

There was never any doubt that the major tautomer in the case of in-dazole 27 is the IH-one 27a (Scheme 16), and C chemical shifts compared with those of the two V-methyl derivatives confirmed that this is the case in DMSO-dg (770MR716). NMR has been used to determine the equilibrium isomeric composition of N-CHROH, N-C(CH3)20H, N-COCH3 and N-Si(CH3)3 derivatives for azoles and benzazoles, the conclu-sipn being that this composition parallels the annular tautomeric composition [78JCS(P2)99]. The tautomerism of 1-hydroxybenzotriazole was also studied... 

While the comparison of the OMTS and the (CH2)12 spectra helped to learn something about the kind of information solid state chemical shifts can provide, we can obtain much more detailed data about the correlation of chemical shifts and the rotational isomeric states from the spectra of larger cycloalkanes. Usually conformational shift variations are discussed by (i) the so called y-gauche effect and (ii) the vicinal gauche effect, Vg 15) ... 

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