Quantifying the conceptual problems associated with the isotropic NICS through analyses of its underlying density

Abstract

The isotropic Nucleus Independent Chemical Shift (NICS_iso) is widely considered to be a suitable descriptor for aromaticity based on the correlations it exhibits with other aromaticity descriptors. To gain more insight into the origin of these correlations, we establish causal relations between the NICS_iso and the underlying current density patterns by resolving the NICS_iso into its underlying density. Our results indicate that the origin of the behavior of the NICS_iso can be radically different from what is generally assumed. Not only does this bring into question the robustness of applying the NICS_iso beyond the realms of where good correlations with other measures of aromaticity have been established, it also points to an inherent weakness in all interpretations of NICS_iso values that are not based on additional data.