Abstract
Nucleus Independent Chemical Shift (NICS) values, pioneered by Schleyer, provide detailed insights into electronic structures of transition states. These show that the [2+2+2]-cycloaddition transition states, studied early by Schleyer and by us, have aromatic transition structures and that fused cyclopropanes are aromatic in the transition structure, while fused four-membered rings are antiaromatic. The nucleophilic ring-opening of 1- and 2-cyanobicyclo[1.1.0]butanes, studied earlier by Hoz, and ring openings of cyanocyclopropane, cyanocyclobutane, and 2-cyanobicyclo[2.2.0]hexane by hydroxide were investigated at the B3LYP/6-31+G* level. Orbital interactions through bonds explain relative facilities of ring opening.
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Sawicka, D., Houk, K. Aromaticity and Antiaromaticity in Small Ring Transition States, Assessed by NICS Values and Energetics. J Mol Model 6, 158–165 (2000). https://doi.org/10.1007/s0089400060158
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DOI: https://doi.org/10.1007/s0089400060158