Aromatic molecules susceptibility

Ruthenium is excellent for hydrogenation of aliphatic carbonyl compounds (92), and it, as well as nickel, is used industrially for conversion of glucose to sorbitol (14,15,29,75,100). Nickel usually requires vigorous conditions unless large amounts of catalyst are used (11,20,27,37,60), or the catalyst is very active, such as W-6 Raney nickel (6). Copper chromite is always used at elevated temperatures and pressures and may be useful if aromatic-ring saturation is to be avoided. Rhodium has given excellent results under mild conditions when other catalysts have failed (4,5,66). It is useful in reduction of aliphatic carbonyls in molecules susceptible to hydrogenolysis. 

Since the magnetic susceptibility anisotropy Ax is a characteristic attribute of aromatic molecules (66MI1 75MI2), its value could play the role of an aromaticity index... 

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 molecular magnetic susceptibility anisotropies for the pyranones have been determined by microwave techniques (71JA5591, 73JA2766). Values for the parameter A, which represents the out-of-plane minus the average in-plane molecular magnetic susceptibilities, were obtained for benzene, furan, 2- and 4-pyranone, and tropone. A may be separated into local and non-local contributions with the aid of known local group contributions for non-aromatic molecules. The results are presented in Table 12. 

The magnetic properties used to probe aromaticity arise from the presence of a diatropic ring current which tends to push an aromatic molecule out of a magnetic field (calculated property magnetic susceptibility, /). and which exerts NMR shielding on a proton at or above the ring center (calculated property nucleus-independent chemical shift, NICS). NICS values are obtained from the... 

To address the question of the respective importance of pathways in heterogeneous photocatalysis, a molecule susceptible to yield different primary products, depending on the initial attack by an OH radical or a hole, has been used. Six-membered aromatic carbon cycles do not fulfil this condition. For example, from a substituted benzene, the monohydroxycyclohexadienyl radical can be formed either from addition of the hydroxyl radical or from the capture of a hole, followed by the hydration and deprotonation of the resulting radical cation. 

Fig. 1.2. The field induced magnetic moment is depicted schematically in this drawing. This effect is most pronounced in aromatic molecules such as fluorobenzene, where comparatively strong electron ring currents may be induced, leading to a field induced, molecular magnetic dipole moment which opposes the exterior field. Trying to align the induced moment, the exterior field will exert a torque ind X H on the molecule and will thus perturb the overall rotation. This perturbation is seen as a splitting in the rotational spectra. Since there will be a torque only in the case that md and H are not aligned, i.e., if Jg is anisotropic, only the anisotropies of the molecular susceptibility tensor can be obtained from the splittings of the rotational lines...

The most troublesome consequence of using concentrated sulfuric acid is in its sulfonating property. An aromatic molecule is likely to be more easily oxidized to a cation radical if it contains electron-donor substituents. But these substituents also make the molecule more susceptible to electrophilic attack, i.e. to sulfonation. This is the case, for example, with aromatic ethers and diarylsulfides. For example, 1,4-dimethoxybenzene undergoes 70-90% sulfonation within a few minutes in concentrated sulfuric acid, and only a low concentration of the cation radical can be obtained (Nishinaga et al.,... 

The earliest semiempirical methods were well in advance of ab initio methods partly because of the limits on computing power. The first of these is the ir-molecular orbital (MO) theory of conjugated and aromatic molecules proposed by Hiickel " in 1931. Most succeeding semiempirical theories are direct descendants of Hiickel s original approach wherein one or more of the approximations has been improved. It is interesting to note that Hiickel theory in its original form still sees infrequent application to properties such as electric susceptibilities, soliton dynamics, and others. 

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