Magnetic properties and aromaticity of o-, m-, and p-benzyne.
The relative aromaticities of the three singlet benzyne isomers, 1,2-, 1,3-, and 1,4-didehydrobenzenes have been evaluated with a series of aromaticity indicators, including magnetic susceptibility anisotropies and exaltations, nucleus-independent chemical shifts (NICS), and aromatic stabilization energies (all evaluated at the DFT level), as well as valence-bond Pauling resonance energies. Most of the criteria point to the o-benzyne<m-benzyne<p-benzyne aromaticity order, whereas the relative aromaticity of each isomer with respect to benzene depends on the aromaticity criterion. An additional aromaticity evaluation involved the transition state of the Bergman cyclization of (Z)-hexa-1,5-diyn-3-ene which yields p-benzyne. Dissected NICS calculations reveal an aromatic transition state with a larger total NICS but a smaller NICS(pi) component and thus lower aromaticity than benzene.
De Proft
Frank
F
Eenheid Algemene Chemie, Vrije Universiteit Brussel Faculteit Wetenschappen, Pleinlaan 2, 1050 Brussel, Belgium.
von Ragué Schleyer
Paul
P
van Lenthe
Joop H
JH
Stahl
Frank
F
Geerlings
Paul
P