Abstract
Significant C−C bond length alternation (Δr = 0.047 A), very low aromatic stabilization energy (ASE = −5.1 kcal/mol), magnetic susceptibility anisotropy (χanis = −83.1 ppm cgs), and exaltation (Λ = −13.8 ppm cgs), as well as the nucleus-independent chemical shifts (NICS(0), −3.7 ppm) demonstrate borepin to be much less aromatic than the tropylium ion. The B3LYP/6-311+G** relative stabilities of annelated borepin positional isomers isoelectronic with azulene (1a) and the benzotropylium cation (1b), e.g., the boraazulene anion (2c > 2b > 2a) and the benzo-fused borepins (5c > 5b > 5a), are in accord with the topological charge stabilization rule. However, the aromaticity orderings (2a > 2b > 2c; 5a > 5c > 5b) are opposite, as deduced from the Δr, χanis, and NICS values. All the aromaticity criteria support 4X > 3X ordering for the heterole-fused borepin positional isomers (3X, 4X; X = NH, S, O), in agreement with their relative stabilities. The NICS(1) values (i.e., computed 1 A above the ring centers) indi...
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