Shiraz University
13th Iranian Physical Chemistry Seminar
Shiraz University of Technology
Conformation, Structure, Intramolecular H-bonding, and Vibrational Assignment of
1-(2-thienyl)-4,4,4-trifluorobutane-1,3-dione
Abdo-Reza Nekoei a,, Sayyed Faramarz Tayyari b and Mohammad vakili
a
c
Department of Chemistry, Shiraz University of Technology, Shiraz, 71555-313, Iran (E-mail: nekoei@ymail.com)
95H
b
c
Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, 91775-1436, Iran
School of Chemistry, Damghan University of Basic Sciences, Damghan, P.O. Box 36715-364, Iran
Introduction
The Intramolecular hydrogen bond (IHB) strength of a -dicarbonyl plays very important roles in stabilities of their different
conformers and also in their chemical, physical and biological properties. Over the years, the keto-enol tautomeric equilibrium,
the structure of both keto and enol forms, and the nature of the strong intramolecular OHO hydrogen bond in the enol
forms of -dicarbonyls, and also their spectra properties were the subjects of intensive studies using a large variety of different
methods, including IR, Raman, microwave, and NMR spectroscopies, X-ray, electron and neutron diffraction measurements, and
quantum-chemical calculations using variety of techniques [1,2].
In the present work, 1-(2-thienyl)-4,4,4-trifluorobutane-1,3-dione, known as thenoyltrifluoroacetone (TTFA, see Fig. 1)
compound which has a wide range of applications in extracting lanthanides, actinides, and transition elements and also in
medicine, has been studied as a -dicarbonyl with two -substituted groups having different electron-withdrawing, steric, and
resonance effects. In this molecule, as other -dicarbonyls, the electronic characteristics of the substituents, temperature, and
the nature of the surrounding media have essential influences on IHB strength and the position of the keto-enol equilibrium [1].
Conformational stabilities in gaseous and different solution media, geometrical structure, rotational motion of the thienyl group,
the barrier height for proton transfer and the IHB strength of TTFA have been theoretically investigated. The vibrational
spectroscopy has been used as the empirical witness since it is a highly sensitive technique to experimentally examine the
hydrogen bonding and interaction with solvents. Also a full and clear vibrational assignment of TTFA is presented by considering
the theoretical calculations and IR and Raman spectra of the compound and its deuterated analogue.
Method of Analysis
All quantum calculations were carried out with the GAUSSIAN 03 software package, applying B3LYP method of the modern
density functional theory (DFT) and MP2 method, using variety of basis sets. Besides, since the stable conformers in solution
may differ from gaseous and solid states, the self-consistent Onsager reaction field model was used to study the solvent effects
on the four most stable chelated cis-enol tautomers of TTFA at B3LYP/6-311++G** level of DFT theory. The rotational barrier
of the thienyl ring and the frequencies calculations were performed at the B3LYP level using 6- 311++G** and 6-311G** basis
sets, respectively. The assignment of the calculated wavenumbers is aided by the animation option of GaussView 3.0 graphical
interface for Gaussian programs.
Results and Discussion
Among the forty different tautomers of TTFA, only four cis-enol conformers have the six-member chelated ring of the IHB,
which are illustrated in Fig. 1. The calculated relative energies clearly suggest that these chelated cis-enol conformers are so
stable that the presence of other conformers (non-chelated enol and keto forms) in significant amounts is unlikely. Table 1
shows that all chelated A and B conformers of TTFA have nearly the same stabilities in the gaseous phase at B3LYP, but MP2
calculations indicate that the B2 conformer is at least 1.1 kcal/mol more stable than the other chelated conformations, which is
consistent with the X-ray experiment [3]. However, these results are not reliable in solution states with the solvent of different
polarities. Fig. 2 shows that the energy differences of the chelated forms, which are negligible in their gaseous states, become
quite noticeable as the medium polarity increases. This causes the contents of the B conformers to be underestimated compare
to the A conformers in the solution. For instant, the relative energies of A1, B2, and B1 chelated forms with respect to A2 in
CH3CN and DMSO are about 1.3, 2.5, and 3.3 kcal/mol, respectively.
The barrier of the thienyl group rotational was also estimated for A1ļA2 interconversions. The barriers for internal rotation
are calculated to be 7.396 kcal/mol for A1ĺA2 and 7.457 kcal/mol for A2ĺA1. The high potential energy barrier, regardless of
steric effect, confirms the contribution of the thienyl ring’s -electrons in resonance conjugation with the chelated ring.
The barrier heights for proton transfers from A1ĺB1 and from A2ĺB2, calculated at B3LYP/6-31G**, are 2.01 and 1.89
kcal/mol, respectively. The corresponding barrier heights for AA and HFAA are 1.71 and 2.45 kcal/mol [4], respectively. These
results confirm that the IHB strength in TTFA conformers is stronger than that in HFAA, but weaker than that in AA.
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