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

Abstract

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.

Hunting for Complex Organic Molecules in the Interstellar Medium: The Role of Accurate Low-Cost Theoretical Geometries and Rotational Constants

“…However, an accuracy between 0.0005 and 0.001 Å for bond lengths (in the range from 1 to 2 Å) and of about 0.1° for valence angles can be considered a reasonable guess for an accuracy of 0.1% on rotational constants . While this target is reachable by state-of-the-art composite wave function methods, ,, the 0.01% goal would require consideration of additional effects (e.g., diagonal Born–Oppenheimer approximation and finite nuclear models) and also places essentially insurmountable demands on the basis set selection and correlation treatments . Based on these premises, we will analyze the perspectives of a low-cost model able to approach the 0.1% accuracy target for large molecules.…”

Rotational Spectroscopy of 1-Cyano-2-methylenecyclopropane (C5H5N)─A Newly Synthesized Pyridine Isomer

“…The rotational spectrum of 1-cyano-2-methylenecyclopropane ( 7 ) was collected from 235 to 500 GHz using a previously described millimeter-wave spectrometer in a continuous flow at room temperature, with a sample pressure of 12 mTorr. − The complete spectrum from 235 to 500 GHz was obtained using automated data collection software in less than 2.5 days with the following experimental parameters: 0.6 MHz/s sweep rate, 10 ms time constant, and 50 kHz AM and 500 kHz FM modulation in a tone-burst design. The separate spectral segments were combined into a single broadband spectrum using Kisiel’s Assignment and Analysis of Broadband Spectra (AABS) software. , Pickett’s SPFIT/SPCAT was used for least-squares fitting and spectral predictions, along with the PIFORM program for analysis .…”

Precise Equilibrium Structure of Benzene

“…The motivation behind combining experimental and computed values is that the experimental B 0 constants can be determined with remarkable accuracy and precision and that the computational equilibrium geometry (r e ) and potential energy function are close enough to their real counterparts (particularly at presently practical levels of theory) that the small errors in the computed sums of the vibration-rotation interaction constants can be neglected. This assumption has proven useful to the structure determination of a variety of molecules, including the r e SE geometries of several prototypical organic molecules, e.g., ethylene, 19 propene, 20 acetylene, 21,22 butadiene, 19 formaldehyde, 8 ketene, 23,24 phenyl radical, 16,25 and benzene. 26 These precise structures provide important benchmarks for the fundamental C−H bond distances and angles for sp-, sp 2 -, and sp 3 -hybridized carbon atoms.…”

“…This assumption has proven useful to the structure determination of a variety of molecules, including the r e SE geometries of several prototypical organic molecules, e . g ., ethylene, propene, acetylene, , butadiene, formaldehyde, ketene, , phenyl radical, , and benzene . These precise structures provide important benchmarks for the fundamental C–H bond distances and angles for sp-, sp 2 -, and sp 3 -hybridized carbon atoms.…”