Hydration and Conformation of 2-Ethylfuran Explored by Microwave Spectroscopy

Rotational spectra of one conformer of a 2-ethylfuran···H₂O complex and two conformers of the isolated 2-ethylfuran molecule have been recorded by chirped-pulse Fourier transform microwave spectroscopy. The species were probed while entrained within a gas sample undergoing supersonic expansion. The spectra of five isotopologues of the complex have been analyzed to yield rotational (A₀, B₀, C₀) and centrifugal distortion constants (D_J, D_JK, d₁) allowing structural parameters to be determined by fitting to the experimentally determined moments of inertia. Quantum chemical calculations have been performed to support the interpretation of the experimental results and gain further insights. 2-Ethylfuran is shown to adopt C₁ symmetry within the observed conformer of 2-ethylfuran···H₂O with the length of the hydrogen bond, r(H_b···O1), which connects H₂O with 2-ethylfuran determined to be 2.0950(42) Å in the r₀ geometry. The geometry of the hydrogen bonding interaction deviates from linearity such that the ∠(O_w–H_b···O1) angle (where O_w and O1 are the oxygen atoms of water and furan, respectively) is 167.69(16)° in the r₀ geometry. The experimental and theoretical results thus imply the presence of a weak interaction between the oxygen of H₂O and the ethyl group within the observed conformer of 2-ethylfuran···H₂O. Evidence is presented to suggest that the C_s conformer of the isolated 2-ethylfuran molecule is lower in energy than the C₁ conformer implying that the energy ordering of the two lowest-energy conformers of 2-ethylfuran reverses when the isolated molecule is hydrated by a single H₂O molecule.