Improved Accuracy in Semi-Experimental Structure Determination by Resolving Problems Associated with Rotation of...

The rotational spectrum of the normal isotopologue of 1,3-oxazole (c-C₃H₃NO) was observed from 43 to 750 GHz. Over 3900 transitions for the ground vibrational state are measured, assigned, and least-squares fit to sextic centrifugally distorted-rotor Hamiltonians. The measured frequencies and resulting spectroscopic constants from this extended spectral range, combined with previous measurements of the nuclear quadrupole coupling constants, will facilitate astronomical searches for oxazole across the majority of the range of modern radiotelescopes. Spectra for a set of 30 oxazole isotopologues, which include multiple isotopic substitutions of each atom, are used to determine the first semi-experimental equilibrium (r_e^SE) structure and semi-experimental substitution structure (r_s^SE), each using CCSD(T) computed values for the vibration-rotation interaction and electron-mass corrections. The large number of isotopologues, including 21 isotopologues observed for the first time, and the redundant substitutions of each atom provide sufficient spectroscopic information to determine the r_e^SE structure with the expected high level of accuracy and precision (0.0001 or 0.0002 Å in bond distances and 0.013 to 0.025° in bond angles). In the course of this study, we analyzed a known issue for some r_e^SE structure determinations of near-oblate asymmetric tops in which inclusion of individual isotopologues degrades the structure determination. We demonstrate that this problem primarily arises from the difference in the values of the computed vibration-rotation interaction corrections as evaluated at the computed r_e geometry vs the r_e^SE geometry of the "real" molecule. Our solution to this problem substantially improves the r_e^SE structure of oxazole and likely can be generalized to many other molecules.