Anisotropy diamagnetic

Since the magnetic susceptibility is a second-rank tensor, its principal components may differ from each other. For example, in the benzene molecule the anisotropy parameter 

Diamagnetic anisotropy in single crystals may even be enhanced. For example, the diamagnetic mass susceptibility of graphite, with fields either parallel or perpendicular to the hexagonal c axis, is Xu = —264 and x l = —3.8 (in units of 10-9 m3 kg-1). 

Which of the underlined protons (or sets of protons) has the greater chemical shift (that is, the higher frequency signal) q q 

Without referring to Table 15.1, label the proton or set of protons in each compound that gives the signal at the lowest frequency a, at the next lowest b, and so on. 

The chemical shifts of hydrogens bonded to carbons are at higher frequencies than one would predict from the electronegativities of the carbons. For example, a hydrogen bonded to a benzene ring appears at 6.5 to 8.0 ppm, a hydrogen bonded to the terminal s] carbon of an alkene appears at 4.7 to 5.3 ppm, and a hydrogen bonded to a carbonyl carbon appears at 9.0 to 10.0 ppm (Table 15.1). 

The unusual chemical shifts associated with hydrogens bonded to carbons that form tt bonds are due to diamagnetic anisotropy. This term describes an environment in which different magnetic fields are found at different points in space. (Anisotropic is Greek for different in different directions. ) 

The magnetic field induced by the TT electrons of a benzene ring in the vicinity of the protons attached to the sf carbons has the same direction as the applied magnetic field. As a result, these protons sense a larger effective magnetic field, so their signals appear at higher frequencies. 

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Values of the diamagnetic anisotropy of benzene and other aromatic hydrocarbon molecules are calculated on the basis of the assumption that the p, electrons (one per aromatic carbon atom) are free to move from carbon atom to adjacent carbon atom under the influence of the impressed fields. When combined with the assumed values for the contributions of the other electrons (-2.0X 10-6 for hydrogen, —4.5 X10 c for aromatic carbon, — 6.0XlO-6 for aliphatic carbon) these lead to principal diamagnetic susceptibilities of molecules in approximate agreement with the available experimental data. The diamagnetic anisotropy of graphite is also discussed. 

CERTAIN diamagnetic crystals, such as bismuth and graphite, show pronounced diamagnetic anisotropy, the susceptibility being of normal magnitude in the basal plane and abnormally large in the direction perpendicular to this plane. A similar phenomenon is shown by... 

Simultaneous weight-change and diamagnetic-anisotropy measurements upon adsorption and desorption of bromine in PG have been carried out (M4). They showed that final susceptibility values are reached well before full bromine saturations, and remain until 80% desorption is attained. 

Since the first report of the nonequivalence phenomenon, approximately 40 chiral substances have been reported to induce enantiomeric nonequivalence in the NMR spectra of a host of solutes. These CSAs are encountered in subsequent discussions. Two qualities considered to be essential in the design of the first reported experiment (3) are evident in nearly all CSA-solute combinations. In all cases, the CSA and the solute have the common feature of complementary functionality, which permits their interaction. Both are in general hydrogen bond donors or acceptors the CSAs are acids, amines, alcohols, sulfoxides, or cyclic compounds such as cyclodextiins, crown ethers, or peptides, which attractively interact with appropriate enantiomeric solutes, engendering different spatial environments for their nuclei. In nearly every case the CSA contains a group of high diamagnetic anisotropy near its asymmetric center, a feature... 

The H-NMR spectrum of the parent compound (4) 9,20,30.59 analyzed as an AA BB CC system the ratioJ j/Jj g = 0.73 is as expected when compared both with hetero analogs and with benzene and 1,3-cyclohexadiene. Calculations on the ring current contributions to diamagnetic anisotropy and chemical shifts have been reported. H-NMR data for other benzo[c]furans have been published.66.70,71.109,111.114,132... 

H and, especially for phosphine complexes, 31PNMR spectroscopy have proved a useful means of characterizing Mo2+ complexes and assigning their stereochemistry. The diamagnetic anisotropy induced by Mo—Mo quadruple bonds has been discussed with special reference to the 31PNMR spectra of [MozCLCPRs). ] complexes.139... 

Some diamagnetic crystals (graphite, bismuth, naphthalene and other aromatic substances) show prohounced diamagnetic anisotropy. The observed anisotropy of crystals of benzene derivatives correspond to the molar diamagnetic susceptibility —54 X 10 with the field direction perpendicular to the plane of the benzene ring and —37 X lO"6 with it in the plane. This molecular anisotropy has been found to be of some use in determining the orientation of the planes of aromatic molecules in crystals.1... 

The calculations are based on ring current and atomic diamagnetic anisotropy contributions (56) based on the intercalation overlap geometry depicted in the text. The overlap geometry corresponds to intercalation of dT-dA site. 

The overlap of proflavine with adjacent base pairs was varied until there was approximate agreement between the experimental upfield complexation shifts (Table V) and those calculated from ring current and atomic diamagnetic anisotropy contributions from the base pairs (56). The calculated upfield shifts are somewhat smaller than the experimental complexation shifts at the proflavine protons in the synthetic DNA complex (Table V). This... 

Diamagnetic anisotropies have also been evaluated109 for the different tautomeric forms of purine. They, too, fall into two groups ... 

We point out again that an appreciation of the concepts of electronegativity (inductive effects) and of electron delocalization—combined with an understanding of diamagnetic anisotropy—permits both rationalization and prediction of approximate chemical shift. Several examples make the point ... 

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