Rotational Spectroscopy and Partial Semi-Experimental Equilibrium (reSE) and Substitution (rsSE) Structures of...

Partial semi-experimental equilibrium (reSE) and substitution (rsSE) structures of cyclobutenone (c-C4H4O, Cs, μa = 3.4 D) have been determined for the first time via rotational spectroscopy. The rotational spectrum of this reactive species, which contains a highly strained four-membered ring, has been measured from 9 to 735 GHz. Over 3800 transitions for the ground vibrational state of the normal isotopologue of cyclobutenone were measured, assigned, and least-squares fit to sextic distorted-rotor A- and S-reduced Hamiltonians in the Ir representation, providing a complete set of quartic and sextic centrifugal distortion parameters. Rotational transitions for each of the singly substituted heavy-atom isotopologues were measured at natural abundance. Rotational constants (B0) for the six isotopologues were converted to equilibrium values (Be) using CCSD(T)/cc-pCVTZ vibration-rotation interaction and electron-mass corrections, enabling the determination of partial reSE and rsSE structures that define all of the heavy-atom parameters of cyclobutenone. The two different sets of semi-experimental structural parameters (reSE and rsSE) are compared to the "best theoretical estimate" (BTE re) values, which are derived from the CCSD(T)/cc-pCV5Z re 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 Ir 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.