Lopes Susy, Gómez-Zavaglia Andrea, Fausto Rui
Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal.
Phys Chem Chem Phys. 2006 Apr 21;8(15):1794-806. doi: 10.1039/b516164a. Epub 2006 Mar 7.
Alpha-furil [C(4)H(3)O-C(=O)-C(=O)-C(4)H(3)O] 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 C(4)H(3)O-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 (C(2) symmetry; O=C-C=O intercarbonyl dihedral equal to 153.1 degrees), has both furan rings orientated in such a way that one of their beta-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, II (C(1) symmetry; O=C-C=O intercarbonyl dihedral equal to 126.9 degrees ), has one furan ring orientated as in I, while the second furan group is rotated by ca. 180 degrees (resulting in an energetically less favourable H-C=C-C=O five-membered ring); in the third conformer, III (C(2) symmetry; O=C-C=O dihedral equal to 106.2 degrees ), 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 alpha-furil were also found to be present in CCl(4) 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, alpha-furil exists uniquely in its most stable conformational state, I.
α-糠偶酰[C(4)H(3)O-C(=O)-C(=O)-C(4)H(3)O]已在氩气和氙气基质中分离出来，并通过傅里叶变换红外光谱（FTIR）进行研究，同时辅以密度泛函理论（DFT）(B3LYP)/6-311++G(d,p)计算。所得光谱已完全归属，并揭示了基质中存在三种不同的构象异构体，它们在羰基间键周围均呈现扭曲构象，两个C(4)H(3)O-C(=O)片段近乎平面。这三种构象异构体在呋喃环相对于羰基的取向上有所不同：最稳定的构象异构体I（C(2)对称性；羰基间二面角O=C-C=O等于153.1度），其两个呋喃环的取向使得它们的一个β-氢原子接近最远羰基的氧原子，形成两个H-C=C-C-C=O六元环；第二稳定的构象异构体II（C(1)对称性；羰基间二面角O=C-C=O等于126.9度），其中一个呋喃环的取向与I相同，而第二个呋喃基团旋转了约180度（形成一个能量上不太有利的H-C=C-C=O五元环）；在第三种构象异构体III（C(2)对称性；二面角O=C-C=O等于106.2度）中，两个呋喃环相对于二羰基采取后一种取向。理论计算预测，两种较高能量形式的能量分别比最稳定形式高5.85和6.22 kJ mol(-1)，构象互变的能垒高于40 kJ mol(-1)。这些能垒足够高，以防止观察到基质隔离化合物的构象异构化。α-糠偶酰的三种可能构象异构体也存在于CCl(4)溶液中，以及由其蒸汽快速冷凝到合适的10 K冷却基质上制备的化合物的低温纯非晶相中。另一方面，可以得到X射线数据[S. C. Biswas, S. Ray and A. Podder, Chem. Phys. Lett., 1987, 134, 5][541]，低温纯晶态的红外光谱表明，在此相中，α-糠偶酰仅以其最稳定的构象状态I存在。
