Precise equilibrium structure determination of thiophene (<i>c</i>-C4H4S) by rotational spectroscopy—Structure of a five-membered heterocycle containing a third-row atom

Abstract

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 I^r 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 (A_e, B_e, and C_e) for 24 isotopologues. A precise semi-experimental equilibrium (r_e ^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 r_e ^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 r_e ^SE structure is compared to the resulting "best theoretical estimate" structure. Several 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 for the computed parameters that fall outside the statistical uncertainties is discussed.