Nucleus-independent chemical shift index

NIGS index (= Nucleus-Independent Chemical Shift index)... 

NICS nucleus-independent chemical shift index... 

A theoretical evaluation of the aromaticity of the pyrones pyromeconic acid, maltol, and ethylmaltol along with their anions and cations was carried out at several levels (Hartree-Fock, SVWN, B3LYP, and B1LYP) using the 6-311++G(d,p) basis set <2005JP0250>. The relative aromaticity of these compounds was evaluated by harmonic oscillator model of aromaticity (HOMA), nucleus-independent chemical shifts (NICSs), and /6 indexes and decreases in the order cation > neutral molecule > anion. 

An interesting comparison between several aromaticity indices (harmonic oscillator model, nucleus-independent chemical shift, para-delocalization index, aromatic fluctuation index, multicenter indices, atoms-in-molecules theoretical indices and graph-theoretical indices) concluded that the most reliable ones are based on electron delocalization (08JCC1543). 

The electronic structure and the aromaticity of 1,2-azaphosphole, 1,2-oxaphosphole, 1,2-thiaphosphole, and 1,2-diphosphole have been theoretically investigated in terms of aromatic stabilization energies (ASEs), resonance energies (REs), magnetic susceptibility exaltations, and nucleus-independent chemical shift (NICS) indexes <2002JOC1333>, as well as based on isodesmic reactions <2003T1657>. 

In NMR experiments one can only measure the CSs in a limited set of points in space (the nuclei positions r ). However, the induced field and, consequently, the nuclear shielding tensor, are defined and can be computed for aU points in space such that one can drop the index i from Equation 12.4 and calculate the shielding tensor at any arbitrary position r, yielding the so-called nucleus-independent chemical shifts (NICS) [24]. NICS values are, thus, not referenced with respect to any nucleus and that is why they are nucleus independent. Also, and similarly to site-specific shieldings, rather than dealing with the shielding itself, one frequently reports the isotropic NICS shift (5j, j,5) defined as follows ... 

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,... 

Seal, R, Is nucleus-independent chemical shift scan a reliable aromaticity index for planar and neutral A2B2 clusters J. Mol. Struct. (THEOCHEM) 2009, 893, 31. 

Aromatic compounds are important in industry as well as in living systems and clusters are important in many areas such as photography, catalysis, and quantum dots. Apart from the basic aspects of aromaticity, experts discuss the structures, properties, reactivity, stability, and other consequences of aromaticity of a variety of metal clusters in this volume. The Foreword was written by Professor Paul v. R. Schleyer, an authority in this subject and the originator of the nucleus-independent chemical shift (NICS) index, the most widely anployed measure of aromaticity. This book will be useful for graduate students as well as researchers in this field. 

---