Nucleus-Independent Chemical Shifts from Semiempirical Calculations

Abstract

A recently developed special MNDO parameterization for NMR chemical shifts is used to compute the nucleus-independent chemical shifts (NICS) for a wide range of organic molecules, including [n]annulenes, polycyclic hydrocarbons, heterocycles, cage molecules, fullerenes, and pericyclic transition states. The results are compared with published NICS data from ab initio and density functional calculations. In general, there is reasonable agreement. The semiempirical NICS values tend to be smaller in absolute value than their ab initio counterparts, but they often show similar trends. The aromatic or antiaromatic character of a given system can normally be assigned correctly on the basis of the MNDO NICS values

Using three major criteria to evaluate aromaticity of five-member C-containing rings and their Si-, N-, and P-substituted aromatic heterocyclics

“…at a ring or cage center, 0.5 Å [19], 0.6 Å [20], 0.8 Å [21], 1.0 Å [22], and etc, above the plane of the rings. Comparing with other criteria, NICS does not require reference standards, increment schemes and calibrating (homodesmic) equations for evaluation; and more important, in several sets of related molecules, NICS correlates well with other aromaticity indexes based on geometric, energetic, and other magnetic criteria [23].…”

Carbon‐rich Compounds: Computational Considerations

“…Nucleus-independent chemical shifts, introduced by Schleyer et al as a measure for aromaticity [20], can also be computed at the GIAO-MNDO level of theory, making this useful tool also available to much larger aromatic structures such as graphenes and fullerenes [21]. The ab initio reference data are generally reproduced well, but four-membered rings are still problematic.…”

Structural and Molecular Orbital Properties of Some Boroxine Derivatives-A Theoretical Study

“…NICS may be a useful indicator of aromaticity that usually correlates well with the other energetic, structural and magnetic criteria for aromaticity. 42,[54][55][56][57] Resonance energies and magnetic susceptibilities are measures of the overall aromaticity of a polycycle, but do not provide information about the individual rings. However, NICS is an effective probe for local aromaticity of individual rings of polycyclic systems.…”