Is nucleus-independent chemical shift scan a reliable aromaticity index for planar and neutral A2B2 clusters?

Nucleus-independent chemical shift (NICS) scan has been employed in order to explain the aromaticity in eleven planar, neutral and tetrameric mixed clusters of trans-A2B2 type. The results suggest that for seven A2B2 clusters, although very high negative NICS values are obtained while performing the NICS scan, yet the typical minimum is not present in the scanning curve. The NICS scanning procedure thus fails to account for the aromatic behavior in those four-membered A2B2 clusters. Amongst them, interestingly it has been observed that for four such clusters the scan shape looks like the mirror image of a typical scan of an antiaromatic system. Based on the NICS scan, at the most, those four clusters can be said to possess nonaromatic behavior. In case of the remaining four clusters, the NICS scan shape passes through a distinct minimum and successfully explains their aromatic behavior. Hence, the overall investigation indicates that NICS scan alone cannot be used as a reliable aromaticity index for planar and neutral A2B2 clusters.