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Abstract

We prepared and characterized a series of mono- and dicarbaldehydes of 2,6-dihydroxynaphthalene that bear potential resonance-assisted hydrogen bonding (RAHB) unit(s). X-ray crystal structures of selected compounds revealed that each salicylaldehyde moiety forms an intramolecular hydrogen bond and that the introduction of formyl groups into either the alpha- or beta-position causes a considerable difference in geometry, which was interpretative from a conventional scheme of resonance hybrids including ionic state. Analyses on NMR chemical shifts suggested that the compounds in solution are present as an equilibrium mixture between closed and open forms with respect to RAHB units. Ab initio calculations indicated that the formation of an intramolecular hydrogen bond strikingly influences the aromaticity of the individual local six-membered ring of naphthalene. The trend of the change in aromaticity was analyzed in connection with the extra stabilization energy of RAHB. In the UV-vis spectra, the beta-formyl derivatives specifically showed a substantial red shift compared to alpha-formyl derivatives. The absorption features were successfully reproduced by TD-DFT calculation, and those data were consistently explained from the effects of RAHB on electronic state of the naphthalene's pi-system. Finally, we pointed out a similarity in the electronic state between RAHB-bearing molecules and cata-condensed aromatic hydrocarbons.