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 (reSE) 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 reSE 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 reSE 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 reSE results. The possible origin of the discrepancies for the computed parameters that fall outside the statistical uncertainties is discussed.

Thiophene (C 4 H 4 S, C 2v symmetry, µ a = 0.55 D) is the sulfur analog of furan. With the intent of improving its gas-phase structure determination, its rotational spectrum was collected from 8 -360 GHz, and 21 deuterium containing isotopologues were synthesized and their rotational spectra were collected from 130 -360 GHz. The heavy atom 13 C, 34 S, and 33 S isotopologues were observable in the rotational spectra of the normal isotopologue and several deuterium containing forms at natural abundance. The resultant determinable rotational constants (A , B , C ) were computationally corrected for vibration-rotation interactions and electron mass with CCSD(T) calculations and 24 total isotopologues were least-squares fit to afford the semi-experimental equilibrium structure (r e SE ). For comparison, theoretical structures were determined at several levels of theory up to CCSD(T)/cc-pCV5Z. The quintuple zeta structure was further refined to account for extrapolation to the complete basis set limit, residual electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born-Oppenheimer correction. The resultant r e SE structure and "best" theoretical structure are compared.

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