A theoretical (DFT, GIAO-NMR, NICS) study of the carbocations and oxidation dications from azulenes, homoazulene, benzazulenes, benzohomoazulenes, and the isomeric azulenoazulenes

Protonation of parent azulene (1), homoazulene (8), representative isomeric benzazulenes (9, 9A, and 9B), and benzohomoazulenes (10, 10A, and 10B) as well as the mono- and diprotonation of isomeric azulenoazulenes (11–16) were studied by DFT at the B3LYP/6-31G(d) level. The most likely carbocations were identified based on relative protonation energies. For comparison, complete experimental ¹³C NMR data were obtained for parent azulenium ion 1H⁺++++ and guaiazulenium ion 2H⁺+++ in TFA.

The oxidation dications derived from benzazulenes (9, 9A, and 9B), benzohomoazulenes (10, 10A, and 10B) and azulenoazulenes (11–16) were also investigated. For azulenoazulene dications the singlet and triplet states are both minima, but except for 11²⁺2+2+ and 13²⁺, the triplet states are higher in energy.

Structural/geometrical changes in the carbocations were examined. GIAO-NMR, NPA charges (and changes in charges), and NICS (and ΔNICS) were employed to compute the NMR chemical shifts (Δδ¹³C values) in order to derive charge delocalization maps and to gauge relative aromaticity/antiaromaticity in the energetically most favored carbocations and oxidation dications. The emerging trends are discussed and compared.

Creation of tropylium or homotropylium entities in the carbocations (monoprotonated) and carbodications (diprotonated) is identified as an important driving force governing the protonation outcomes.

Consideration of the AM1-derived ΔΔH_f values (and the DFT-derived ΔΔG values) suggests that the two-electron oxidation of azulenoazulenes should be experimentally feasible. Given their antiaromatic (paratropic) character, azulenoazulene dications represent interesting targets for NMR study. GIAO-derived charge delocalization mapping allows the most likely sites for nucleophilic attack on the dications to be identified.