ABSTRACT
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.
ABSTRACT
The pure rotational spectrum of diketene has been studied in the millimeter-wave region from â¼240 to 360 GHz. For the ground vibrational state and five vibrationally excited satellites (ν24, 2ν24, 3ν24, 4ν24, and ν16), the observed spectrum allowed for the measurement, assignment, and least-squares fitting a total of more than 10â¯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: ν24 + ν16, ν23, and 5ν24. The structure of diketene was optimized at the CCSD(T)/ANO1 level, and the vibration-rotation interaction (αi) values for each normal mode were determined with a CCSD(T)/ANO1 VPT2 anharmonic frequency calculation. These αi values were helpful in identifying the previously unreported ν16 and ν23 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.
