1,8-Chalcogen-bridged naphthalenes.: Strong carbon bases in the gas phase

High-level density functional theory computations have been used to estimate the gas-phase proton affinities of the complete series of 1,8-chalcogen-bridged naphthalene derivatives. Our estimates show the 1,8-naphthalenediylbis(oxy) to be an oxygen base as strong as the so-called proton sponges. All the remaining chalcogen-bridged naphthalene derivatives are predicted to be among the strongest known carbon bases in the gas phase, the para carbon with respect to the most electronegative chalcogen atom being the most basic site. This enhanced basicity reflects the electron-donating ability of sulfur, selenium and tellurium, and the aromatization of the five-membered ring in the protonated form. Protonation at the heteroatoms is always disfavored with respect to ring protonation, but the energy gap between the carbon-protonated and chalcogen-protonated species decreases significantly on going from sulfur to tellurium derivatives. When the two chalcogen atoms involved are different the basicity of the system is controlled by the amount of charge transfer from the less electronegative chalcogen atom to the most electronegative one through the X-Y bond. This effect is maximum in OTe, which accordingly becomes the strongest base of the whole series.