Observation by UV-visible and NMR spectroscopy and theoretical confirmation of 4-isopropyltropolonate ion, 4-isopropyltropolone (Hinokitiol), and protonated 4-isopropyltropolone in acetonitrile

Protonated 4-isopropyltropolone (H₂L⁺) was produced by the addition of methanesulfonic or trifluoromethanesulfonic acid to 4-isopropyltropolone (hinokitiol: HL) dissolved in a protophobic aprotic solvent, acetonitrile, with poor solvation of anions and cations. The formation was confirmed by UV-visible and ¹³C NMR spectroscopy as well as theoretical calculations using the Gaussian03 quantum chemistry package. The ¹³C chemical shifts of all C-atoms in the seven-membered ring were properly assigned for H₂L⁺ and HL as well as for the deprotonated species, L^-. The influence of residual water in the organic solvent was regarded as negligible because such a water is in the "dihydrogen ether" condition.