Matrix isolation and low temperature solid state FTIR spectroscopic study of α-furil

Abstract

α-Furil [C₄H₃O–C(O)–C(O)–C₄H₃O] has been isolated in argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. The obtained spectra were fully assigned and revealed the presence in the matrices of three different conformers, all of them exhibiting skewed conformations around the intercarbonyl bond with the two C₄H₃O–C(O) fragments nearly planar. The three conformers differ in the orientation of the furan rings relative to the carbonyl groups: the most stable conformer, I (C₂ symmetry; OC–CO intercarbonyl dihedral equal to 153.1°), has both furan rings orientated in such a way that one of their β-hydrogen atoms approaches the oxygen atom of the most distant carbonyl group, forming two H–CC–C–CO six-membered rings; the second most stable conformer, II (C₁ symmetry; OC–CO intercarbonyl dihedral equal to 126.9°), has one furan ring orientated as in I, while the second furan group is rotated by ca. 180° (resulting in an energetically less favourable H–CC–CO five-membered ring); in the third conformer, III (C₂ symmetry; OC–CO dihedral equal to 106.2°), both furan rings assume the latter orientation relative to the dicarbonyl group. The theoretical calculations predicted the two higher energy forms being 5.85 and 6.22 kJ mol⁻¹ higher in energy than the most stable form, respectively, and energy barriers for conformational interconversion higher than 40 kJ mol⁻¹. These barriers are high enough to prevent observation of conformational isomerization for the matrix isolated compound. The three possible conformers of α-furil were also found to be present in CCl₄ solution, as well as in a low temperature neat amorphous phase of the compound prepared from fast condensation of its vapour onto a suitable 10 K cooled substrate. On the other hand, in agreement with the available X-ray data [S. C. Biswas, S. Ray and A. Podder, Chem. Phys. Lett., 1987, 134, 541], the IR spectra obtained for the neat low temperature crystalline state reveals that, in this phase, α-furil exists uniquely in its most stable conformational state, I.