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Title

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

Authors
Schleyer, Paul von Ragué
Seifert, Gotthard
Reviakine, Roman
Published in Journal of Physical Chemistry. A. 2003, vol. 107, no. 33, p. 6470-6475
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 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 D[nh] S[3], S[5], and N[6]H[6[2+]] rings as well as to D[2h] 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.
Keywords Chemical shiftsAromaticityDelocalizationAnnulenesLondo-Hückel
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Research group Groupe Weber
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HEINE, Thomas et al. Analysis of Aromatic Delocalization: Individual Molecular Orbital Contributions to Nucleus-Independent Chemical Shifts. In: Journal of Physical Chemistry. A, 2003, vol. 107, n° 33, p. 6470-6475. https://archive-ouverte.unige.ch/unige:3702

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Deposited on : 2009-10-06

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