Precise equilibrium structure determination of thiophene (c-C4H4S) by rotational spectroscopy-Structure of a five-membered heterocycle containing a third-row atom.

The rotational spectrum of thiophene (c-C4H4S) has been collected between 8 and 360 GHz. Samples of varying deuterium-enrichment were synthesized to yield all possible deuterium-substituted isotopologues of thiophene. A total of 26 isotopologues have been measured and least-squares fit using A- and S-reduced distorted-rotor Hamiltonians in the Ir representation. The resultant rotational constants (A0, B0, and C0) from each reduction were converted to determinable constants (A″, B″, and C″) to remove the impact of centrifugal distortion. The computed vibrational and electron mass corrections [CCSD(T)/cc-pCVTZ] were applied to the determinable constants to obtain semi-experimental equilibrium rotational constants (Ae, Be, and Ce) for 24 isotopologues. A precise semi-experimental equilibrium (re SE) structure has been achieved from a least-squares fit of the equilibrium moments of inertia. The combination of the expanded isotopologue rotational data with high-level computational work establishes a precise re SE structure for this sulfur-containing heterocycle. The CCSD(T)/cc-pCV5Z structure has been obtained and corrected for the extrapolation to the complete basis set, electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born-Oppenheimer correction. The precise re SE structure is compared to the resulting "best theoretical estimate" structure. Several of the best theoretical re structural parameters fall within the narrow statistical limits (2σ) of the re SE results. The possible origin of the discrepancies for the computed parameters that fall outside the statistical uncertainties is discussed.