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

S. LOPES, A. GOMEZ-ZAVAGLIA, R. FAUSTO

Estambul, Turquía

Congreso; European Congress on Molecular Spectroscopy EUCMOS 2006; 2006

EUCMOS

a-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 and the two C4H3O-C(=O) fragments nearly planar. The three conformers differ in the orientation of the furan rings relatively to the carbonyl groups: the most stable conformer (form I; C2 symmetry; O=C-C=O intercarbonyl dihedral equal to 153.1 degrees) has both furan rings oriented in such a way that one of their b-hydrogen atoms approaches the oxygen atom of the most distant carbonyl group, forming two H-C=C-C-C=O six-membered rings; the second most stable conformer (form II; C1 symmetry; O=C-C=O intercarbonyl dihedral equal to 126.9 degrees) has one furan ring orientated that way, while the second furan group is rotated by ca. 180 degrees (resulting in an energetically less favorable H-C=C-C=O five-membered ring); in the third conformer (form III; C2 symmetry; O=C-C=O dihedral equal to 106.2 degrees), both furan rings assume this last orientation relatively 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 latter are high enough to prevent observation of conformational isomerization for the matrix isolated compound. The three possible conformers of a-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 vapor onto a 10 K cooled suitable substrate. On the other hand, in consonance with the available X-ray data [S.C. Biswas, S. Ray and A. Podder, Chem.Phys.Lett., 134 (1987) 541)], the IR spectra obtained for the neat low temperature crystalline state reveals that, in this phase, a-furil exists uniquely in its most stable conformational state, I.