| Abstract
| - Individual molecular orbital (MO) contributions to the magnetic shielding of atoms as well as to the nucleus-independent chemical shifts (NICS) of aromatic compounds can be computed by the widely used gauge-including atomic orbital (GIAO) method. Detailed analyses of magnetic shielding MO-NICS contributionsprovide interpretive insights that complement and extend those given by the localized MO (“dissected NICS”,LMO-NICS) method. Applications to (4n + 2) π-electron systems, ranging from [n] annulenes to Dnh S3, S5,and N6H62+ rings as well as to D2h cyclobutadiene, show the extent to which their diatropic character resultsfrom the σ framework and from the π orbitals. The diatropicity of both these contributions decreases with thenumber of nodes of the wave function around the ring. The highest-energy orbitals can become paratropic.This is generally the case with the σ orbitals, but is found only for “electron-rich” π systems such as sulfurrings. MO-NICS contributions, which can be interpreted using London−Hückel theory, correlate with inversering size.
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