Structure of Furan from 44 Isotopologues─Resolving Differences in the Theoretical (re) and Semi-Experimental Equilibrium (reSE) Structures

Abstract

The rotational spectra of furan and numerous isotopologues were recorded and analyzed between 6 and 750 GHz. High-temperature, high-pressure, base-catalyzed H/D exchange was employed to create a mixture of all possible furan-d_x isotopologues, allowing for the measurement and analysis of the rotational spectra of 44 furan isotopologues. The experimental rotational constants of these isotopologues, combined with CCSD(T)/cc-pCVTZ corrections for the effects of vibration–rotation interaction and the electron-mass distribution, were used to determine a semi-experimental equilibrium (r_e^SE) structure of furan. The large number of isotopologues used, including 35 isotopologues observed for the first time, allowed for the determination of the r_e^SE structure to a high level of accuracy and precision (≤0.0003 Å uncertainties in bond distances and ≤0.02° in bond angles). The current study additionally corrects for differences in principal axis rotation between the structure at which the vibration–rotation interaction constants were evaluated (CCSD(T)/cc-pCVTZ) and the r_e^SE structure. The difference in rotation between these two structures had been previously reported, but not addressed. The “best theoretical estimate” (BTE) for the structure of furan was determined using a CCSD(T)/cc-pCV5Z geometry, with additional corrections addressing finite basis set, higher-level electron correlation, and relativistic effects, as well as the diagonal Born–Oppenheimer correction. The BTE structure of furan is compared to the r_e^SE structure of furan, which is also compared to the previously determined r_e^SE structure of its sulfur-containing analogue, thiophene.