Aromatic molecules typically exhibit a variety of characteristics, including bond length averaging, high delocalization energies, structural stability, and the capacity to generate integral induced ring currents in the presence of magnetic fields. To put it succinctly, the delocalization of π electrons stabilizes aromatic molecules. The term "anti-aromaticity" refers to the feature that, for particular compounds, the delocalization of electrons causes the compound to be unstable. The nuclear independent chemical shift (NICS), which was created in 1996 as a computational tool to evaluate aromaticity, is the most often used indicator of aromaticity.
Alfa Chemistry offers its customers a professional NICS calculation service to help them identify and quantify the aromaticity of their compounds, solving their research problems by taking into account all design factors such as requirements, cost and delivery time. The NICS method is also one of several that fall under the standard of aromatic magnetism, meaning that it uses the response of the aromatic system to an external magnetic field to identify and quantify aromatic features.
Alfa Chemistry uses the Gaussian program to calculate the magnetic shielding tensor of the system. The isotropic shielding value can be easily calculated from the magnetic shielding tensor. The basic procedure for calculating the NICS values includes:
- Optimization of the molecular structure.
- Setting the sites for which the NICS values need to be calculated based on the optimized structures.
- Calculation of the magnetic shielding tensor using the NMR keyword.
- Analysis of the results.
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Our NICS methods includes, but is not limited to:
NICS Methods for Monocyclic Systems
- Single-point NICS Methods
This NICS method refers to the method of assigning and quantifying aromaticity using a single NICS probe. The methods included in this classification are represented as follows:
D. NICS(r)π (LMO-NICS; CMO-NICS; The σ-Only Model)
- Multi-point NICS Methods
The method uses multiple NICS probes. These can usually be divided into 1D and 3D methods. The former uses NICS probes along a line, while the latter uses NICS probes in the form of a grid.
NICS Methods for Polycyclic Systems
The practice of doing NICS calculations on a single ring of a polycyclic system spread throughout the early 2000s. This way of describing polycyclic systems is still widely used today, and in many instances, these values are crucial for mapping the magnetic behavior of these systems. Sometimes individual values are also summed to provide global values.
Fig 1. NICS calculations on (a) benzene, (b) coronene, (c) circumcoronene, and (d) dicircumcoronene. (Forse A. C, et al. 2015)
The NICS value determines the presence or absence of aromaticity, but not the strength of aromaticity. NICS stands for the negative magnetic shielding value at a deliberately chosen site outside the nucleus. At the beginning of its use, nics is typically taken at the geometric center of the heavy atom on the ring, and the NICS at this position is known as NICS (0). However, for particular compounds, a location taken 1 above or below the plane, known as NICS(1), which mostly reflects the contribution of electrons, provides a better evaluation of the aromaticity. The system is aromatic when the NICS value is negative, and it is anti-aromatic when the NICS value is positive.Later, it was further argued that the NICS value should be examined only for the zz value (z-axis perpendicular to the ring plane), which best reflects π-aromaticity, and this was referred to as NICS(1)zz.
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- Forse A. C, et al. (2015). "Nuclear Magnetic Resonance Studies of Ion Adsorption in Supercapacitor Electrodes (Doctoral thesis)."