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

Abstract

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.

Low temperature IR spectroscopy and photochemistry of matrix-isolated α-pyridil

“…This result is in consonance with the well-known large conformational flexibility around the C C central bond in ␣-dicarbonyls [1][2][3][4] and also with the previously reported photochemistry of this type of compounds, where the intercarbonyl bond cleavage appears as a prevalent pathway [3,47]. (d) The C 1 C 5 and C 3 C 6 bonds, connecting the O C C O group to the pyridyl rings (149.5-150.7 pm) are longer than in the previous ␣-dicarbonyl compounds studied in our laboratory, in particular, benzil, 1-phenyl-1,2-propanedione and ␣-furil (148.7, 148.6 and 145.6-145.8 pm, respectively [2][3][4]). This result indicates that the conjugation between the pyridyl substituents and the dicarbonyl moiety is less important in ␣-pyridil than between this group and the phenyl or furanyl substituents in the remaining molecules.…”

“…This range of values is typical for a non-conjugated C C single bond and follows the trend previously observed for other ␣-dicarbonyls, like diacetyl, benzil, 1-phenyl-1,2-propanedione and ␣-furil (155.7, 154.4, 155.0 and ca. 154.7 pm, respectively) [1][2][3][4]. As discussed in detail elsewhere [4], the long intercarbonyl bond length in ␣-dicarbonyls results essentially from the balance between the relatively weak -electron delocalization within the O C C O fragment and the more important -electron system repulsion due to the interaction between the positively charged carbonyl carbon atoms.…”

“…154.7 pm, respectively) [1][2][3][4]. As discussed in detail elsewhere [4], the long intercarbonyl bond length in ␣-dicarbonyls results essentially from the balance between the relatively weak -electron delocalization within the O C C O fragment and the more important -electron system repulsion due to the interaction between the positively charged carbonyl carbon atoms. This result is in consonance with the well-known large conformational flexibility around the C C central bond in ␣-dicarbonyls [1][2][3][4] and also with the previously reported photochemistry of this type of compounds, where the intercarbonyl bond cleavage appears as a prevalent pathway [3,47].…”

“…As discussed in detail elsewhere [4], the long intercarbonyl bond length in ␣-dicarbonyls results essentially from the balance between the relatively weak -electron delocalization within the O C C O fragment and the more important -electron system repulsion due to the interaction between the positively charged carbonyl carbon atoms. This result is in consonance with the well-known large conformational flexibility around the C C central bond in ␣-dicarbonyls [1][2][3][4] and also with the previously reported photochemistry of this type of compounds, where the intercarbonyl bond cleavage appears as a prevalent pathway [3,47]. (d) The C 1 C 5 and C 3 C 6 bonds, connecting the O C C O group to the pyridyl rings (149.5-150.7 pm) are longer than in the previous ␣-dicarbonyl compounds studied in our laboratory, in particular, benzil, 1-phenyl-1,2-propanedione and ␣-furil (148.7, 148.6 and 145.6-145.8 pm, respectively [2][3][4]).…”

“…In general, simple ␣-dicarbonyl compounds show interesting structural, spectroscopic and photophysical properties, including photorotamerism, due to conformational changes associated with the internal rotation around the flexible O C C O intercarbonyl torsional coordinate in the ground or excited electronic states [1][2][3][4][5][6][7][8]. Because of that, these compounds have received particular attention concerning their potential use for the development of molecular devices [9][10][11][12][13][14].…”