Analysis of aromatic delocalization: Individual molecular orbital contributions to nucleus-independent chemical shifts

Thomas Heine, Paul V. Ragué Schleyer, Clémence Corminboeuf, Gotthard Seifert, Roman Reviakine, Jacques Weber

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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 N6H6 2+ 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.

Original languageEnglish
Pages (from-to)6470-6475
Number of pages6
JournalJournal of Physical Chemistry A
Issue number33
Publication statusPublished - 2003 Aug 21


All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry

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