Nucleus-independent chemical shift diatropic

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... 

One of the well-accepted magnetic criteria of aromaticity is the nucleus independent chemical shift (NICS) which is defined as the negative value of the absolute magnetic shielding calculated at some selective points. The NICS(O) and NICS(l) values are computed at the centre and lA above the aromatic ring respectively. Aromatic systems possess negative NICS values as that indicates the presence of an induced diatropic ring current. Positive NICS values refer to paratropicity and are present in antiaromatic... 

More recently, a new and widely used index of aromaticity, the nucleus-independent chemical shift (NICS), has originally been proposed by Schleyer and coworkers [44,45]. It is defined as the negative value of the absolute shielding computed at a ring center or at some other interesting points of the system. Negative NICS values correspond to aromaticity, while positive values are associated with antiaromaticity. There is a whole family of NICS methods, which include NICS computations at ring centers [43], NICS(O) and above [46], NICS(I) and NICS(2), dissected NICS values, i.e., the total NICS at a particular point in space may be dissected into paratropic and diatropic components, which mainly arise from the C-C cr and jt multiple bonds, respectively [46,47] and MO contributions to NICS,... 

Figure 18. Patterns in computed NICS values (nucleus-independent chemical shifts, HF/DZP level) for neutral fullerenes. Diatropic and paratropic rings are depicted in red and blue, respectively (from the data given in ref 234).

Magnetic effects result from aromatic diatropic and paratropic antiaromatic ring currents and are reflected in H NMR chemical shifts, magnetic susceptibility exalation, and nucleus-independent chemical... 

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