Isotropic shielding calculations were performed across finely spaced two- and three-dimensional grids positioned through and around a wide range of molecules. These magnetic shielding calculations were used to investigate aromaticity, antiaromaticity and a variety of chemical bonding features.
This technique was found to be incredibly sensitive and able to distinguish between bonds of different order as well as bonds of the same order but in different environments. The shielding along the whole bonding region, as well as 1 Å above the bond and cross-sections through the bond, can be used to provide detailed information about the nature of the chemical bonding and the conjugation with the rest of the system.
Regions of deshielding have been found around unsaturated nuclei and these areas can be used to determine relative aromaticities as well as degrees of conjugation. The same is true of shielding features found at 1 Å above the molecular plane. Unsaturated heavy atoms also display these deshielded surroundings, but they can be harder to observe.
Antiaromatic systems exhibit a dumbbell shaped region of deshielding at the ring centre as well as significantly bent bonding regions which have been found to be a result, primarily, of the antiaromaticity rather than ring strain.
H-bonding can also be studied with this technique and it has been found that the shielding on the atoms involved is most informative. In the case of substituted malonaldehydes, the oxygen shieldings were used to determine relative aromaticities in the pseudo rings and, therefore, H-bond strength.
The sensitivity and information-rich nature of this technique has proven far superior to existing methods, such as the commonly used nucleus-independent chemical shift (NICS) technique, and therefore has great scope for future applications.
