Contributions from orbital–orbital interactions to nucleus-independent chemical shifts and their relation with aromaticity or antiaromaticity of conjugated molecules

The out-of-plane components of the nucleus-independent chemical shifts (NICS) for a group of aromatic and antiaromatic [n]-annulenes have been separated into several contributions on the basis of the gauge-including atomic orbital (GIAO) method. The analysis of the orbital interactions responsible of the NICS(π)zz values shows that the large and positive values found for antiaromatic compounds are due to the predominance of π → π ∗ rotational transitions that overcome the diamagnetic and gauge contributions. However, NICS(π)zz for aromatic compounds are dominated by contributions arising from the gauge transformation and no significant contributions are found from occupied–unoccupied orbital mixing. This analysis has been compared with previous work about ring currents in the same compounds.