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.

The gas-phase rotational spectrum of (cyanomethylene)cyclopropane, (CH 2 ) 2 CCHCN, generated by a Wittig reaction between the hemiketal of cyclopropanone and (cyanomethylene)triphenylphosphorane, is presented for the first time. This small, highly polar nitrile is a cyclopropylcontaining structural isomer of pyridine. The rotational spectra of the ground state and two vibrationally excited states were observed, analyzed, and least-squares fit from 130 to 360 GHz. Over 3900 R-, P-, and Q-branch, ground-state rotational transitions were fit to low-error, partial octic, A-and S-reduced Hamiltonians, providing precise determinations of the spectroscopic constants. The two lowest-energy vibrationally excited states, ν 17 and ν 27 , form a Coriolis-coupled dyad displaying small aand b-type resonances. Transitions for these two states were measured and least-squares fit to a two-state, partial octic, A-reduced Hamiltonian in the I r representation with nine Coriolis-coupling terms (G a , G a J , G a K , G a JJ , F bc , F bc J , F bc K , G b , and G b J). The observation of many resonant transitions and nine nominal interstate transitions enabled a very accurate and precise energy difference between ν 17 and ν 27 to be determined: ΔE 17,27 = 29.8975453 (33) cm −1 . The spectroscopic constants presented herein provide the foundation for future astronomical searches for (cyanomethylene)cyclopropane.

The rotational spectrum of 1-cyanocyclobutene from 130 to 360 GHz has been observed, assigned, and least-squares fit for the ground state and the two lowest-energy vibrationally excited states. Synthesis by UV photochemical dimerization of acrylonitrile and subsequent base-catalyzed dehydrocyanation affords a highly pure sample, yielding several thousand observable rotational transitions for this small organic nitrile. Over 2500 a-type, R-branch transitions of the ground state have been least-squares fit to low error with partial-octic A- and S-reduced Hamiltonians, providing precise determinations of the corresponding spectroscopic constants. In both reductions, computed spectroscopic constants are in close agreement with their experimentally determined counterparts. Two vibrationally excited states (ν27 and ν17) form a Coriolis-coupled dyad, displaying many a-type and b-type local resonances and related nominal interstate transitions. Somewhat unexpectedly, despite the very small permanent b-axis dipole moment, a number of b-type transitions could be observed for the ν17 state; this is explained in terms of state mixing by the Coriolis perturbations. Over 2200 transitions for each of these states have been least-squares fit to a low-error, two-state, partial-octic, A-reduced Hamiltonian with nine Coriolis-coupling terms (G a , Ga J , G a K , G a JJ , F bc , F bc K , G b , G b J , and F ac ). The availability of so many observed rotational transitions, including resonant transitions and nominal interstate transitions, enables a very accurate and precise determination of the energy difference (ΔE 27,17 = 14.0588093 (43) cm–1) between ν27 and ν17. The spectroscopic constants presented herein provide a starting point for future astronomical searches for 1-cyanocyclobutene.

Four cyanobutadiene isomers of considerable interest to the organic chemistry, molecular spectroscopy, and astrochemistry communities were synthesized in good yields and isolated as pure compounds: (E)-1-cyano-1,3-butadiene ( E-1), (Z)-1-cyano-1,3-butadiene ( Z-1), 4-cyano-1,2-butadiene (2), and 2-cyano-1,3-butadiene (3). A diastereoselective synthesis was developed to generate (E)-1-cyano-1,3-butadiene (1) (10:1 E/Z) via tandem SN2 and E2′ reactions. The potential energy surfaces of the E2′ reactions leading to (E)- and (Z)-1-cyano-1,3-butadiene (1) were analyzed by density functional theory calculations, and the observed diastereoselectivity was rationalized in the context of the Curtin–Hammett principle. The preparation of pure samples of these reactive compounds enables measurement of their laboratory rotational spectra, which are the critical data needed to search for these species in space by radioastronomy.

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.