Rotational Spectroscopy and Partial Semi-Experimental Equilibrium (r_e^SE) and Substitution (r_s^SE) Structures of Cyclobutenone (c‑C₄H₄O)

Rotational constants (<i>B</i>₀) for the six isotopologues were converted to equilibrium values (<i>B</i>_<i>e</i>) using CCSD(T)/cc-pCVTZ vibration-rotation interaction and electron-mass corrections, enabling the determination of partial <i>r</i>_<i>e</i>^SE and <i>r</i>_<i>s</i>^SE structures that define all of the heavy-atom parameters of cyclobutenone. The two different sets of semi-experimental structural parameters (<i>r</i>_<i>e</i>^SE and <i>r</i>_<i>s</i>^SE) are compared to the “best theoretical estimate” (BTE <i>r</i>_<i>e</i>) values, which are derived from the CCSD(T)/cc-pCV5Z <i>r</i>_<i>e</i> structure with additional corrections addressing finite basis set, higher-level electron correlation, and relativistic effects, as well as incorporating the diagonal Born–Oppenheimer correction.

Additionally, transitions of the four lowest-energy vibrationally excited states have been assigned, measured, and least-squares fit to A-reduced, sextic centrifugally distorted-rotor Hamiltonians in the I^<i>r</i> representation. While ν₂₁ was well-modeled using a single-state Hamiltonian, 2ν₂₁, ν₁₄, and ν₂₀ form an anharmonic- and Coriolis-coupled triad and were treated by a three-state Hamiltonian model.