Precise equilibrium structure of thiazole (c-C₃H₃NS) from twenty-four isotopologues

The pure rotational spectrum of thiazole (c-C₃H₃NS, C_s) has been studied in the millimeter-wave region from 130 to 375 GHz. Nearly 4800 newly measured rotational transitions for the ground vibrational state of the main isotopologue were combined with previously reported measurements and least-squares fit to a complete sextic Hamiltonian. Transitions for six singly substituted heavy-atom isotopologues (¹³C, ¹⁵N, ³³S, ³⁴S) were observed at natural abundance and likewise fit. Several deuterium-enriched samples were prepared, which gave access to the rotational spectra of 16 additional isotopologues, 14 of which had not been previously studied. The rotational spectra of each isotopologue were fit to A- and S-reduced distorted-rotor Hamiltonians in the I^r representation. The experimental values of the ground-state rotational constants (A₀, B₀, and C₀) from each isotopologue were converted to determinable constants (A₀″, B₀″, and C₀″), which were corrected for effects of vibration-rotation interactions and electron-mass distributions using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The moments of inertia from the resulting constants (A_e, B_e, and C_e) of 24 isotopologues were used to determine the precise semi-experimental equilibrium structure (r_e ^SE) of thiazole. As a basis for comparison, a purely theoretical equilibrium structure was estimated by an electronic structure calculation [CCSD(T)/cc-pCV5Z] that was subsequently corrected for extrapolation to the complete basis set, electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born-Oppenheimer correction. The precise r_e ^SE structure is compared to the resulting "best theoretical estimate" structure. Some, but not all, of the best theoretical r_e structural parameters fall within the narrow statistical limits (2σ) of the r_e ^SE results. The possible origin of the discrepancies between the best theoretical estimate r_e and semi-empirical r_e ^SE structures is discussed.