Fourier transform microwave spectroscopy of the ¹³C- and ¹⁸O-substituted tropolone. Proton tunneling effect for the isotopic species with the asymmetric potential wells

Fourier-transform microwave spectroscopy has been applied for the ¹³C/¹⁸O-substituted tropolone to observe tunneling-rotation transitions as well as pure rotational transitions. The tunneling-rotation transitions were observed between the ¹³C-4 and -6 forms as well as between ¹³C-3 and -7, between ¹³C-1 and -2, and between ¹⁸O-8 and -9 (we denote these tunneling pairs as ¹³C-46, etc., below) although they have an asymmetric tunneling potential due to the difference in the zero point energy (ZPE). From the observed tunneling splittings ΔE_ij (0.9800–1.6824 cm⁻¹), the differences in ZPE Δ_ij for the ¹³C-46, -37, -12, and ¹⁸O-89 species are derived to be −0.1104, 0.5652, −1.3682, and 1.3897 cm⁻¹ to agree well with the DFT calculation. The state mixing ratio of the tunneling states decreases drastically from (44%:56%) to (8.7%:91.3%) for ¹³C-46 and ¹⁸O-89 with an increase in the asymmetry Δ_ij of the tunneling potential function. The observed tunneling-rotation interaction constants F_ij decrease from 16.001 to 9.224 cm⁻¹ as the differences in ZPE Δ_ij increase, while the diagonal tunneling-rotation interaction constants F_u increase from 1.767 to 13.70 cm⁻¹, explained well by the mixing ratio of the tunneling states.