Aromatic molecules magnetic properties

The magnetic properties of crystals composed of aromatic polynuclear molecules may give information on the relative orientations of the benzene rings to each other. Thus, Clews and Lonsdale (1937) concluded from the magnetic anisotropy of crystals of o-diphenylbenzene that the planes of the o-phenyl groups are inclined at 50° to the plane of the main ring. 

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... 

PBEO and B3LYP functionals) because it seems to be the best choice for a number of properties geometrical parameters, dipole moments, and magnetic properties. To demonstrate the accuracy that can be obtained in A and g tensor calculation, we selected 27 radicals (aliphatic and aromatic), including a and 7i species (Fig. 6.1). The selected molecules are neutral, cationic, anionic doublet, triplet, quartet and localized and conjugated radicals. 

We have shown, in the first part of this contribution, how the ring current concept was classically associated with the notion of aromaticity. To be consistent with our earlier discussion, we have introduced the concept of potential strobilism to describe the phenomena which are measured by all the magnetic properties of matter in the case of cyclic molecules which are the seat of a ir-electron delocalization, that is of a ring current. This new term is not very pleasing phonetically but it has the merit of deriving from a striking image ... 

One is compelled to conclude that the study of the magneto-optical behaviour of strobilic molecules in solution does not seem to be directly uti-lizable as a criterion of strobilism. But the main interest of this conclusion is that it may be generalized to other magnetic properties of matter, and particularly to the N.M.R. field so many people have tried to measure the aromaticity of a molecule by N.M.R. studies in solution. 

A novel approach in the materials chemistry of zeotypes uses their highly organized pore structure for the preparation of advanced materials with potentially interesting magnetic, optical or electronic properties [20]. An illustrative example is the work of Caro, Marlow and coworkers, who prepared a material that exhibits strong second harmonic generation (SHG), a phenomenon in non-linear optics (NLO) that is important for laser applications. The material is based on the aromatic molecule -nitroaniline (pNA) [30-32]. This polar molecule also exhibits strong hyperpolarizability, an... 

From the Table it can be seen that aromatic hydrocarbons tend to be more easily reduced as the molecular size increases. However, even for simple aromatic molecules both molecular size and molecular symmetry must be analysed for a quantitative understanding 37). Detailed 3H and 13C NMR studies of reduced hydrocarbons enabled to correlate between the topology of each system, its magnetic properties, and antiaromaticity. These studies will be discussed in the following sections. 

Kleinpeter and Koch have recently introduced spatial magnetic properties as a measure of homoaromaticity. These are calculated as through-space NMR shieldings (TSNMRS) and visualized as iso-chemical shielding surfaces (ICSF). The structures, the intramolecular flexibility, and the and C NMR chemical shifts are calculated on the MP2/6-311 + G level of theory. TSNMRS of benzene and the cyclopropenylium cation as prototype aromatic molecules published earlier are used as references for homoaromaticity in a number of homoaromatic carbocations. The existence and the degree of homoaromaticity are deduced from the TSNMRSs by comparing the experimental and computed H and NMR NMR chemical shifts. 

Interest in X, -phosphinine systems has continued, the aromatic or ylidic state of these systems now having been evaluated using various magnetic property criteria. Much depends on the nature of the substituents at phosphorus, the more electron-withdrawing, the more aromatic is the molecule, but no evidence of d-orbital participation was found. A route to the X -diphosphinine (400) has been described. Studies of the protonation of the X. -triphosphinine (401) have also appeared. ... 

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