Improved semi-experimental equilibrium structure and high-level theoretical structures of ketene.

The millimeter-wave rotational spectrum of ketene (H2C=C=O) has been collected and analyzed from 130 to 750 GHz, providing highly precise spectroscopic constants from a sextic, S-reduced Hamiltonian in the Ir representation. The chemical synthesis of deuteriated samples allowed spectroscopic measurements of five previously unstudied ketene isotopologues. Combined with previous work, these data provide a new, highly precise, and accurate semi-experimental (reSE) structure for ketene from 32 independent moments of inertia. This reSE structure was determined with the experimental rotational constants of each available isotopologue, together with computed vibration-rotation interaction and electron-mass distribution corrections from coupled-cluster calculations with single, double, and perturbative triple excitations [CCSD(T)/cc-pCVTZ]. The 2σ uncertainties of the reSE parameters are ≤0.0007 Å and 0.014° for the bond distances and angle, respectively. Only S-reduced spectroscopic constants were used in the structure determination due to a breakdown in the A-reduction of the Hamiltonian for the highly prolate ketene species. All four reSE structural parameters agree with the "best theoretical estimate" (BTE) values, which are derived from a high-level computed re structure [CCSD(T)/cc-pCV6Z] with corrections for the use of a finite basis set, the incomplete treatment of electron correlation, relativistic effects, and the diagonal Born-Oppenheimer breakdown. In each case, the computed value of the geometric parameter lies within the statistical experimental uncertainty (2σ) of the corresponding semi-experimental coordinate. The discrepancies between the BTE structure and the reSE structure are 0.0003, 0.0000, and 0.0004 Å for rC-C, rC-H, and rC-O, respectively, and 0.009° for θC-C-H.

Synthesis, Purification, and Rotational Spectroscopy of 1-Cyanocyclobutene (C5H5N).

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 (Ga , GaJ, GaK, GaJJ, Fbc , FbcK, Gb , GbJ, and Fac). 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 (ΔE27,17 = 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.

Millimeter/Submillimeter Spectroscopic Detection of Desorbed Ices: A New Technique in Laboratory Astrochemistry.

A new laboratory technique has been developed that utilizes gas-phase, direct-absorption millimeter and submillimeter spectroscopy to detect and identify desorbed species from interstellar and cometary ice analogues. Rotational spectroscopy is a powerful structure-specific technique for detecting isomers and other species possessing the same mass that are indistinguishable with mass spectrometry. Furthermore, the resultant laboratory spectra are directly comparable to observational data from far-infrared and millimeter telescopes. Here, we present the proof-of-concept measurements of the detection of thermally desorbed H2O, D2O, and CH3OH originating in a solid film created at low temperature (∼12 K). The surface binding energy of H2O is reported and compared to results from traditional techniques, including mass spectrometry and quartz-crystal microbalance measurements of mass loss. Lastly, we demonstrate that this technique can be used to derive thermodynamic values including the sublimation enthalpy and entropy of H2O.