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.

Three cyanobutadiene isomers have been synthesized and their rotational spectra analyzed in the 130–375 GHz frequency range. These species, which are close analogues of known interstellar molecules and are isomers of the heterocyclic aromatic molecule pyridine (C5H5N), offer the opportunity of revealing important insights concerning the chemistry in astronomical environments. The s-trans conformers of E-1-cyano-1,3-butadiene and Z-1-cyano-1,3-butadiene are observed, while both the anti-clinal and syn-periplanar conformers of 4-cyano-1,2-butadiene are evident in the rotational spectra. Over 1000 transitions for s-trans-Z-1-cyano-1,3-butadiene and for syn-periplanar-4-cyano-1,2-butadiene are fit to an octic, distorted-rotor Hamiltonian with low uncertainty (<50 kHz). Although neither s-trans-E-1-cyano-1,3-butadiene nor anti-clinal-4-cyano-1,2-butadiene can be fully treated with a distorted-rotor Hamiltonian in this frequency range, we provide herein minimally perturbed, single-state least-squares fits of over 1000 transitions for each species, yielding sets of spectroscopic constants that are expected to enable accurate prediction of high-intensity transitions at frequencies up to 370 GHz for both isomers. The assigned transitions and spectroscopic constants for these cyanobutadienes have already enabled the identification of two isomers in harsh reaction environments and should be sufficient to enable their identification in astronomical environments by radio astronomy.

The pure rotational spectrum of diketene has been studied in the millimeter-wave region from ∼240 to 360 GHz. For the ground vibrational state and five vibrationally excited satellites (ν, 2ν, 3ν, 4ν, and ν), the observed spectrum allowed for the measurement, assignment, and least-squares fitting a total of more than 10 000 distinct rotational transitions. In each case, the transitions were fit to single-state, complete or near-complete sextic centrifugally distorted rotor models to near experimental error limits using Kisiel's ASFIT. Additionally, we obtained less satisfactory least-squares fits to single-state centrifugally distorted rotor models for three additional vibrational states: ν + ν, ν, and 5ν. The structure of diketene was optimized at the CCSD(T)/ANO1 level, and the vibration-rotation interaction (α) values for each normal mode were determined with a CCSD(T)/ANO1 VPT2 anharmonic frequency calculation. These α values were helpful in identifying the previously unreported ν and ν fundamental states. We obtained a single-crystal X-ray structure of diketene at -173 °C. The bond distances are increased in precision by more than an order of magnitude compared to those in the 1958 X-ray crystal structure. The improved accuracy of the crystal structure geometry resolves the discrepancy between previous computational and experimental structures. The rotational transition frequencies provided herein should be useful for a millimeter-wave or terahertz search for diketene in the interstellar medium.

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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