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Analysis of Aromatic Delocalization: Individual Molecular Orbital Contributions to Nucleus-Independent Chemical Shifts

2003; American Chemical Society; Volume: 107; Issue: 33 Linguagem: Inglês

10.1021/jp035163z

1520-5215

Thomas Heine, Paul von Ragué Schleyer, Clémence Corminbœuf, Gotthard Seifert, Roman Reviakine, Jacques Weber,

Fullerene Chemistry and Applications

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 contributions provide 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 results from the σ framework and from the π orbitals. The diatropicity of both these contributions decreases with the number 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 sulfur rings. MO-NICS contributions, which can be interpreted using London−Hückel theory, correlate with inverse ring size.