The Aromaticity and Möbius Characteristics of Carbenohetero-[8]-annulenes.

Christian J. Kastrup, Steven Oldfield and Henry S. Rzepa*


Department of Chemistry, Imperial College of Science Technology and Medicine, London, SW7 2AY.
Summary Computational investigation of the novel 8-p carbenohetero-[8]-annulene ring system 1 reveals that its Hückel highly antiaromatic nature as a planar system can be attenuated or even reversed by a C2 symmetric distortion to Möbius aromatic species, the degree of which is highly sensitive to the substituents and the heteroatoms.
Graphical Abstract

Supplemental Information

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Table 1. Ab initio calculated properties for 1 and 5.
1 Energy, Hartree (B3LYP/6-31G*) NICS, ppm r1,r2,r3,r4,f Point Group Symmetry
Z=C:, X=NH, R=H -303.5375a 21.1 1.360,1.428,1.337,1.468, 14.3 C2
Z=C:, X=NH, R=H -303.6241b 23.6 1.352,1.425,1.331,1.467, 0.0 C2vc
Z=C:, X=NH, R=H -303.6243b 19.4 1.354,1.425,1.331,1.465, 18.4 C2
Z=C:, X=NH, R=H, triplet -303.5792b -8.3 1.375,1.370,1.417,1.373, 0.0 C2v
Z=C:, X=NH, R=F -700.4496 1.2 1.367,1.410,1.366,1.450,33.189 C2
5, X=NH -610.8548 8.9 1.360,1.426,1.409,1.487, 35.7 C2
Z=C:, X=O, R=H -343.3315b 15.2 1.333,1.407,1.325,1.456, 0.0 C2vd
Z=C:, X=O, R=H -343.3340b 7.7 1.332,1.405,1.326,1.452, 24.9 C2
Z=C:, X=O R=H -343.2339e 8.8 1.345,1.405,1.333,1.456, 23.7 C2
Z=C:, X=O, R=F -740.1387 -1.3 1.353,1.379,1.334,1.448, 29.0 C2
5, X=O -650.5536 6.3 1.343,1.410,1.400,1.474, 32.2 C2
Z=C:, X=HC-, R=H -270.0042 77.7 1.362 (1.530), 1.531 (1.380), 1.366 (1.436), 1.445, 0.0 Csf
Z=C:, X=HC-, R=H -270.0083 24.7 1.354 (1.506), 1.526 (1.372), 1.391 (1.464), 1.423, 18.8 C1
Z=C:, X=HC-, R=F -865.5182 -3.5 1.337 (1.430), 1.459 (1.362), 1.353 (1.424), 1.402, 25.5 C1
Z=CH+, X=NH, R=H -303.9659 21.0 1.321,1.445,1.331,1.474, 21.4 C2
Z=CH+, X=O, R=H -343.6329 14.6 1.273,1.447,1.325,1.468, 15.4 C2
Z=CH+, X=HC-, R=H -270.8684 79.4 1.404 (1.393), 1.452 (1.465), 1.356 (1.350), 1.500, 25.4 C1
Z=CH+, X=HC-, R=F -965.5308 -7.5 1.384, 1.421, 1.347, 1.450, 48.4 C2
Z=C:, X=PH, R=H -876.6275 41.3 1.673,1.660,1.480,1.340, 0.0 Cs
Z=C:, X=PH, R=H -876.6920 7.1 1.707,1.830,1.348,1.469,35.7 C2
Z=C:, X=S, R=H -989.1832 18.4 1.654,1.820,1.340,1.470, 0.0 C2v
Z=C:, X=S, R=F -1386.0821 2.3 1.654,1.826,1.3381.459,38.4 C2
Z=Si:, X=NH, R=H -555.01480 14.0 1.748,1.414,1.344,1.464,23.69 C2
Z=Si:, X=O, R=H -594.77315 7.4 1.678,1.375,1.341,1.462,22.9 C2
Z=Si:, X=PH, R=H -1128.1269 29.5 2.183, 1.807, 1.348, 1.474, 0.0 C2vg
Z=Si:, X=PH, R=H -1128.1596 4.7 2.256,1.828,1.348,1.465, 46.9 C2
Z=Si:, X=PH, R=H -1128.0819 25.5 2.254,1.670,1.470,1.340, 0.0 Cs
Z=Si:, X=S, R=H -1240.6793 5.2 2.178,1.790,1.330,1.460, 50.0 C2
aEnergy of Z=CH, X=N,NH -303.5846 Hartree. b6-311G(3d) basis. c1 negative Hessian root, corresponding to 63.1i cm-1. d1 negative Hessian root, corresponding to 109.1i cm-1. e MP2/6-31G(d) value -342.2132 NICS 6.8. Triplet ROB3LYP/6-311G(3d) energy -343.2753 of Cs symmetry corresponding to carbene to p* excitation. f2 negative Hessian roots, corresponding to 329.3i cm-1 and 128.4i cm-1 corresponding to twisting distortions of the ring. g3 negative Hessian roots, corresponding to 502.3i cm-1 (C2 on PH), 305.8 i cm-1 (Cs), 88.4i cm-1 (C2 on ring).
Figure 1..
HOMO-3 for
Z=C:, X=O, R=F		 HOMO for
Z=CH+, X=HC-, R=F		 HOMO-1 HOMO-2 HOMO-3
Möbius orbital Möbius orbital Möbius orbital Möbius orbital Möbius orbital