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

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-dx 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 (reSE) structure of furan. The large number of isotopologues used, including 35 isotopologues observed for the first time, allowed for the determination of the reSE 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 reSE 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 reSE structure of furan, which is also compared to the previously determined reSE structure of its sulfur-containing analogue, thiophene.